The pharmacotherapeutic potential of neuropeptide Y for chronic pain

Abstract

Chronic pain is a major medical problem that requires new therapeutic options. Discovered by Victor Mutt in 1982, neuropeptide Y (NPY) is rapidly emerging as a master regulator of pain relief. Genetic knockdown of NPY or pharmacological inhibition of its receptors demonstrates that NPY signaling tonically inhibits indices of chronic inflammatory and neuropathic pain. Primary targets of NPY analgesia include neurons in the dorsal horn of the spinal cord and the parabrachial nucleus of the brain that express the Npy1r (Y1) receptor. NPY signaling is enhanced following injury, and endogenous analgesic synergy between Y1 receptors and mu opioid receptors maintain chronic pain sensitization in a latent state of remission. We propose that disruptions to endogenous NPY analgesia may mediate pathological transitions from acute to chronic pain, which could be treated by CNS administration of Y1 agonists or Npy2r (Y2) agonists or antagonists, depending on the pain state. Chemogenetic manipulations or targeted ablations in rodent models of chronic inflammation or peripheral nerve injury establish that spinal Y1‐interneurons are necessary and sufficient to elicit behavioral signs of both the sensory and affective dimensions of pain. Transcriptomic and in situ hybridization studies revealed three primary subpopulations of spinal Y1‐interneurons that are conserved in higher order mammals, including non‐human primates and humans. Spinally directed (intrathecal) administration of Y1‐selective pharmacological agonists inhibit pronociceptive neurons that co‐express Y1 and gastrin‐releasing peptide to inhibit neuropathic pain. To circumvent highly invasive administration routes, ongoing studies are leveraging the intranasal route for delivery of NPY into the brain.

Introduction

Chronic pain is a major medical problem that requires new therapeutic options

Physiological pain is an evolutionarily conserved alarm system that serves to minimize tissue damage in the face of noxious stimuli [1, 2]. A successful reflex withdrawal behavior to a potentially noxious mechanical (touch) or thermal (hot or cold) stimulus minimizes the duration of pain to a transient period (transient physiological pain). More complex behaviors associated with acute pain after tissue injury, such as guarding or avoidance, facilitate recovery. Acute pain is adaptive and normally subsides during healing. If pain continues in a manner that is grossly disproportionate to any underlying pathology [3, 4], for example, transitions from acute to chronic pain, then it can be considered a disease rather than a symptom [5]. Chronic pain is prevalent worldwide and impacts up to 30% of adults [6]. This problem is compounded by a severe lack of safe and efficacious treatment options. Mild analgesics such as aspirin, acetaminophen, and ibuprofen are relatively safe but ineffective. Powerful analgesic drugs that mimic the actions of endogenous opioid peptides, such as morphine, hydrocodone, and oxycodone [7, 8], which are notorious for their adverse physiological effects and high addictive potential, and can even make pain worse due to the phenomenon of opioid‐induced hyperalgesia [9]. As such, there is an urgent need for discovery of new targets that can revolutionize the pharmacotherapy of chronic pain.

Preclinical pain research has leveraged behavioral, molecular, and physiological assays to reveal new pharmacotherapeutic targets

Basic pain research has made great strides in the past few decades through behavioral, molecular, and physiological analyses in animal models of inflammatory, postsurgical, and neuropathic (related to nerve damage) pain. Behavioral assays to measure response thresholds to various noxious stimuli include the use of von Frey filaments calibrated to bend at specific forces to assess mechanical (touch) sensitivity, hotplate and Hargreaves’ tests for heat sensitivity, and coldplate and acetone to assess cold sensitivity. More complex behavioral analyses such as conditioned place preference/aversion are also commonly leveraged to assess the affective and cognitive dimensions of ongoing or spontaneous (unrelated to external stimuli) pain. These are important endpoints because clinical pain patients present not only with stimulus‐evoked sensory components of pain but also spontaneous and affective components of pain [10, 11]. Together, these behavioral assays can provide valuable information about stimulus‐dependent and stimulus‐independent responses to noxious stimulation in rodents. In addition, immunohistochemistry and fluorescent in situ hybridization (FISH) provide valuable information about the cellular content of mRNA and protein in defined pain‐related regions in the nervous system, in addition to in situ measures of neuronal activity. For example, neuronal activity is tightly associated with the stimulus‐evoked expression of the immediate early gene Fos and the phosphorylated form of extracellular signal‐regulated kinase (pERK). This review also describes electrophysiological methods to access more precise information about neuronal activity.

This review will focus on the neuronal neuropeptide Y (NPY) receptor subtypes that modulate pain, namely, Npy1r (Y1) and Npy2r (Y2) [12, 13, 14]. Since our reviews on the function of NPY in 2005 and 2007 and on Y1‐expressing interneurons (Y1‐INs) in the spinal cord published 5 years ago [13], new groundbreaking studies have yielded critical details on the role and mechanisms by which NPY and its receptors modulate chronic pain. Here, we provide an update and extension that considers not just spinal NPY and Y1 receptors but also recent advances in the mechanism of analgesia action of endogenous NPY itself and its cognate Y2 receptor [15]. We focus on the role of NPY in pain modulation throughout the nervous system (neurons in the dorsal root ganglion, dorsal horn of the spinal cord, and numerous brain regions) in injured states, though we also discuss important findings in uninjured states as well.

NPY and its receptors

NPY is a 36 amino acid peptide discovered by Tatemoto and Mutt in 1982 [16, 17]. NPY is highly evolutionarily conserved across the animal kingdom [18]. In mammals, NPY acts at four G protein coupled receptor (GPCR) subtypes (Y1, Y2, Y4, Y5) to regulate diverse physiological and cognitive processes involved in obesity and metabolic diseases, stress, anxiety, substance misuse, gastrointestinal dysfunction, and cancer [19]. Y1 and Y2 have been most thoroughly studied in the context of pain [12]. All NPY receptors couple to inhibitory G proteins (G_i/o) [20], meaning that their activation dampens intracellular signaling. Cryo‐electron microscopy has recently revealed the structural bases of NPY‐G protein interactions for both the Y1 [21] and Y2 receptors [22] and their associated G_i/o proteins. Two notable signaling pathways engaged by G_i/o‐coupled NPY receptors include (1) inhibition of adenylyl cyclase [20, 23], leading to decreased production of cyclic adenosine monophosphate (cAMP) and cAMP‐dependent cellular signaling; and (2) activation of G protein‐gated inwardly rectifying potassium (GIRK) channels [24], leading to membrane hyperpolarization.

NPY and its receptors are expressed throughout the nervous system in regions critical to pain, including the pseudounipolar dorsal root ganglia (DRG) neurons that extend axons to both the peripheral tissues and to the central nervous system (Fig. 1). Primary afferent neurons—responsible for the detection of noxious stimuli—terminate within the superficial dorsal horn of the spinal cord, where many Y1‐expressing and NPY‐expressing interneurons (Y1‐INs and NPY‐INs, respectively) reside. Y2‐immunoreactivity is also expressed in the dorsal horn of rodents, but rhizotomy [25] and transcriptomic studies [13, 26] indicate that this staining exclusively represents presynaptic sites of the central terminals of primary afferent neurons (Fig. 1). Some spinal projection neurons extend axons to the lateral parabrachial nucleus (lPBN). The parabrachial nucleus (PBN) is a rich source of Y1‐expressing neurons with NPYergic inputs from brain regions that modulate autonomic and other functions. The expression of NPY and its receptors at these and many other pain‐related regions [27] makes the NPY system well positioned to modulate pain.

Fig. 1.

Neuronal expression of NPY, Y1, and Y2 in peripheral tissues, sensory neurons, and spinal cord dorsal horn. Npy1r (Y1) and Npy2r (Y2) receptors are expressed by sensory neurons in the dorsal root ganglion (DRG). Y1 is transported distally to peripheral nerve endings, and to a limited degree proximally to the dorsal horn of the spinal cord. Y2 is transported to both peripheral nerve endings as well as central terminals in lamina I and II. Y1 but not Y2 is expressed in dorsal horn interneurons, particularly in lamina II but also deeper laminae (Y1‐INs, blue). Y1‐INs may form synapses with vertical cells (green) that in turn form synapses with lamina 1 projection neurons (red). NPY interneurons are a subset of GABAergic interneurons (NPY‐INs, grey) within lamina i‐II of the dorsal horn. These NPY‐INs release NPY to inhibit the release of glutamate and substance P from sensory neurons that express NPY receptors, as well as glutamate from Y1‐INs. Sensory neuron expression of NPY is dramatically up‐regulated after nerve injury (not shown). NPY, neuropeptide Y. Source: Created in BioRender. Nie, A. (2025) https://BioRender.com/al3xyxi.

Endogenous and exogenous NPY decreases inflammatory and neuropathic pain

Early transgenic technologies allowed body‐wide manipulations of NPY or Npy1r expression in rodent pain models. These were leveraged to determine the region‐independent actions of NPY in nociceptive and neuropathic pain. Following an initial study reporting that germline deletion of NPY did not change nociceptive pain [28], a strong set of subsequent studies in the setting of nerve injury revealed that the NPY‐Y1 system clearly exerts pain‐inhibitory actions. First, rats overexpressing NPY exhibited reduced behavioral hypersensitivity after spinal nerve ligation model of neuropathic pain [29]. Second, global knockdown of NPY in mice using doxycycline in NPY^tet/tet mice increased mechanical and thermal hypersensitivity in the spared nerve injury (SNI) model of neuropathic pain, in which the tibial and common peroneal branches of the sciatic nerve are ligated [30, 31]. Third, germline Npy1r knockout mice displayed exaggerated allodynia after plantar injection of capsaicin [32] or complete Freund's adjuvant (CFA, which contains heat‐killed Myobacterium tuberculosis bacteria)‐induced inflammation [31]. Together, these studies indicate an important antinociceptive function of endogenous NPY‐Y1 signaling.

Antihyperalgesic actions of NPY in the spinal cord: Intrathecal injection studies

To engage specific sites of NPY action, drugs can be injected into the peripherally tissues to access the peripheral terminals of nociceptor, directly into the DRG, into the intrathecal space to access the spinal cord, into the intracerebroventricular (ICV) ventricles to access the whole brain, and locally into the brain parenchyma. The intrathecal route has been particularly valuable towards our understanding of the spinal mechanisms of NPY analgesia.

Exogenous NPY acts at spinal Y1 receptors to reduce neuropathic pain

In two rat models of traumatic injury to the sciatic nerve (SNI or chronic constriction injury, CCI), intrathecal administration of NPY or the Y1 selective agonist [Leu³¹, Pro³⁴]‐NPY, but not the Y2 selective agonist PYY_3–36, dose‐dependently reduced mechanical and cold hypersensitivity and the expression of non‐noxious, light brush‐evoked Fos [33, 34, 35] and pERK [36]. Notably, these effects were lost in spinal cord‐conditional Y1 deletion mutant mice, but not primary afferent‐conditional Y1 deletion mutant mice. This indicates that the antihyperalgesic effects of intrathecal [Leu³¹, Pro³⁴]‐NPY are mediated by Y1 receptors on spinal cord interneurons rather than the central terminals of primary afferents. Intrathecal NPY also reduced hind paw withdrawal latency to heat in CD1 mice following partial sciatic nerve ligation [31]. Coadministration studies with the Y1‐selective antagonist BIBO3304 indicate that Y1 mediates NPY inhibition of neuropathic pain‐like behavior and Fos and pERK expression [35, 36].

Exogenous NPY acts at spinal Y1 receptors to reduce inflammatory and postsurgical pain

Studies have also leveraged intrathecal delivery of NPY receptor agonists in models of inflammatory and postsurgical pain. Intrathecal NPY or [Leu³¹, Pro³⁴]‐NPY produced antihyperalgesic effects in rat and mouse intraplantar CFA models of inflammatory pain [31, 37, 38], the rat intraplantar formalin model of ongoing pain [39], rat and mouse hindpaw plantar incision models of postsurgical pain [40, 41, 42], the rat intra‐knee joint formalin model of joint inflammation [43], and a mouse model of cancer‐induced bone pain (CIBP) [44]. BIBO3304 prevented these antihyperalgesic effects, and the inhibitory effects of intrathecal NPY on mechanical and heat hyperalgesia following CFA injection were lost in Npy1r knockout mice [31]. Unlike BIBO3304, the Y2 selective antagonist BIIE0246 failed to prevent the antihyperalgesic effects of NPY in these pain models [37, 39], with the exception of CIBP. Furthermore, intrathecal administration of PYY_3–36 did not alleviate mechanical or heat hypersensitivity following plantar incision in mice [34].

Endogenous NPY tonically acts at Y1 receptors in the spinal cord to mask latent pain sensitization

Injury sensitizes CNS neurons, but remarkably, this does not end after the resolution of acute inflammatory or postsurgical pain. Rather, it silently persists within a state of latent sensitization (LS) that is masked by endogenous analgesic signaling [45]. For example, when administered after initial pain resolution, intrathecal administration of BIBO3304 reinstated pain‐like behaviors (such as decrease in mechanical threshold and conditioned place avoidance) and neuron activity (assessed as an increase in stimulus‐evoked expression of Fos or pERK) [30, 46]. Control studies repeatedly demonstrate that BIBO3304 does not change thresholds in mice with sham surgery. Thus, compensatory engagement of endogenous NPY receptor systems, along with other pain‐inhibitory GPCRs, contributes to endogenous analgesia that maintains LS in a state of remission. (Possible mechanisms of endogenous NPY analgesia are further discussed in “Tonic inhibition of latent chronic pain sensitization by NPY” section). We have speculated that disruptions to endogenous analgesia could be a contributing factor to the episodic nature of some forms of chronic pain. As LS operates in humans [47, 48], we propose that endogenous NPY inhibition of chronic pain associated with LS could potentially be treated with spinal delivery of Y1 agonists. Indeed, we found that supplemental administration of NPY, 52 days after the induction of inflammation, reversed NPY knockdown‐induced mechanical hypersensitivity [30].

Antinociceptive and antihyperalgesic actions of NPY in the brain

A recent review [27] details NPY and NPY receptor expression throughout dozens of pain‐related brain region, behavioral consequences of NPY delivery to various brain regions on acute pain, and future directions for NPY‐focused pain research in the brain. Here, we focus on the behavioral pharmacology of NPY in various brain regions following injury and highlight upcoming chemogenetic studies of Y1‐expressing neurons in the PBN.

Intracerebroventricular administration of NPY attenuates pain‐like behaviors

In uninjured rodents, ICV injection of either NPY, a Y1‐selective agonist, or a Y5‐selective agonist reduced withdrawal responses to noxious heat in the hotplate test [49] and electrical tail stimulation [50], as well as pain‐like behaviors in the acetic acid test of visceral pain [51], the intraplantar formalin test of acute ongoing chemical pain [50], and during withdrawal from repeated morphine administration [52, 53]. Effects on morphine withdrawal symptoms as well as the effects on acetic‐acid‐induced pain‐like behaviors were not observed when the Y2‐selective agonist NPY_13–36 was injected instead of NPY [51, 53]. ICV NPY also attenuated pain‐like behaviors (limb use and weight‐bearing ratio) in a mouse model of CIBP. These effects were blocked by both Y1 and Y2 receptor antagonists. Together, these ICV data indicate that NPY acting at Y1, Y2, and/or Y5 receptors is predominantly antinociceptive throughout the brain.

Administration of NPY into various brain regions attenuates pain‐like behaviors

Specific brain regions involved in NPY‐mediated pain modulation have been targeted with local intraparenchymal injection of pharmacological agents. For example, microinjection of NPY into the rat rostral ventral medulla—a final site of convergence of descending pathways from the brain to the dorsal horn of the spinal cord—inhibited mechanical and cold hypersensitivity following CFA, SNL [54], or SNI [55]. These effects were dose‐dependent and could be blocked with BIBO3304, indicating a Y1‐specific action [55]. Microinjection of NPY into the nucleus accumbens of uninjured rats reduced mechanical and heat hypersensitivity [56]. Similarly, microinjection of NPY into the arcuate nucleus of the hypothalamus in uninjured rats or rats with carrageenan‐induced inflammation decreased mechanical and heat hypersensitivity [57]. Likewise, microinjection of NPY into the periaqueductal grey decreased heat and mechanical hypersensitivity in uninjured rats [58], in mice with carrageenan‐induced inflammation [59], and in rats with mononeuropathy [60]. Furthermore, microinjection of NPY into the lPBN in mice attenuated the development of inflammatory nociception in the formalin model [61]. The effects of NPY in each of these brain regions were blocked with coadministration of the Y1 antagonists BIBO3304 or NPY_28–36. By contrast, injection of the Y2 agonist NPY_3–36 into the arcuate nucleus failed to reverse carrageenan‐induced mechanical or heat hypersensitivity [57]. Together, microinjection data indicate that NPY acts at Y1 receptors to exert antinociceptive and antihyperalgesic effects when injected into a wide range of brain regions.

Does endogenous NPY‐Y1 signaling in the brain provide homeostatic regulation of pain in the context of competing need states?

Intracerebral injection of NPY agonists may mimic regulatory functions of endogenous NPY release into the brain. For example, the analgesic effects of hunger were blocked by injection of BIBO3304 into the lPBN [61] in the mouse formalin model of pain. This result indicates that endogenous NPY in the PBN mediates hunger‐induced analgesia. Furthermore, a recent preprint suggests that intracranial injection of BIBO3304 into the lPBN similarly blocks inhibition of inflammatory pain symptoms by additional homeostatic threats such as fear and thirst (https://www.biorxiv.org/content/10.1101/2024.02.26.582069v1). BIBO3304 did not affect pain‐like behaviors in the absence of competing needs. These results suggest that diverse competing need states cause NPY release at the lPBN to inhibit pain transmission to higher centers. Indeed, the authors identified distinct NPYergic projections to the lPBN that were activated by need states that include hunger, thirst, and fear. These early results indicate that endogenous NPY‐Y1 signaling via these projections is antinociceptive in the lPBN.

Pronociceptive actions of NPY?

The administration of NPY or Y1 agonist decreases behavioral and molecular signs of persistent pain across a wide range of regions within the spinal cord and brain. However, here we present three notable exceptions of pronociceptive actions, the first of which is highly dependent on injury state: (1) at the central terminals of Y2‐expressing primary afferent neurons (only in the setting of acute injury); (2) at the peripheral terminals and cell bodies of primary afferent neurons; or (3) at second order neurons of the dorsal column/medial lemniscus tract located in the brainstem dorsal column nuclei (DCN). Here we summarize results indicating that exogenous and endogenous NPY exert pronociceptive effects through actions at Y2 receptors in the periphery and Y1 receptors in the DCN. This is in stark contrast with the antinociceptive and antihyperalgesic actions of NPY in the spinal cord and most other brain regions.

Pronociceptive actions of NPY‐Y2 signaling at sensory neurons

NPY acts at Y2 receptors on the central terminals of sensory neurons to suppress transient nociception and latent sensitization—but to facilitate injury‐induced acute pain

Discrepancies in the behavioral consequences of intrathecal injection of the Y2‐antagonist BIIE0246 to target central terminals of sensory neurons—reports of either pronociception or antinociception—have now been resolved (Table 1). In the uninjured state, intrathecal administration of BIIE0246 elicits ongoing nociception, hypersensitivity to sensory stimulation, and aversion [34, 62]. Conversely, in the acute phase of nerve injury and inflammation, intrathecal BIIE024 reduced mechanical and thermal hypersensitivity as well as the affective dimension of pain measured by a conditioned place preference paradigm. When administered in chronic pain models of LS, BIIE0246 profoundly reinstated pain‐like behaviors. These results led to our proposal that tissue or nerve injury induces a switch from G_i/o to G_q coupling to Y2 receptors [15], which eventually reverts back from G_q to G_i/o during the development of the LS state [15]. Thus, when Y2 is coupled to G_i/o, Y2 agonism inhibits transient withdrawal reflexes to noxious stimulation in the absence of injury, as well as LS. When Y2 is coupled to G_q, Y2 agonism facilitates injury‐induced hypersensitivity. This model clarifies the pharmacotherapeutic potential of Y2 research and implicates Y2 antagonists as potential new non‐opioid therapeutic agents for chronic pain in patients who do not develop LS.

Table 1.

State‐dependent effects of intrathecal neuropeptide Y (NPY) receptor agonists and antagonists.

	No treatment	Y1 agonist	Y1 antagonist	Y2 agonist	Y2 antagonist
Normal	Normal sensitivity	No effect	No effect	No effect	↑ Hypersensitivity
Acute injury	Hypersensitivity	↓ Hypersensitivity	No effect	No effect	↓ Hypersensitivity
LS	Normal sensitivity	No effect	Reinstatement	No effect	Reinstatement

Abbreviation: LS, latent sensitization.

Source: Created in BioRender. Nie, A. (2025) https://BioRender.com/b75jvdd.

Upward arrow indicates an increase. Downward arrow indicates a decrease.

NPY acts at Y2 receptors at peripheral terminals and cell bodies of sensory neurons to facilitate nociception

Evidence for pronociceptive actions at the peripheral terminals and cell bodies of primary afferents come from hindpaw and DRG NPY injection studies. Injection of NPY into these peripheral regions exacerbated nerve injury‐induced mechanical hypersensitivity [29, 63, 64]. Codelivery of Y2 but not Y1 antagonists into the DRG [65] or hindpaw [66] blocked the effects of NPY. It is important to note that these studies have not been repeated in uninjured animals. As such, it remains possible that in the absence of injury, NPY does not exert pronociceptive effects at peripheral terminals and cell bodies of sensory neurons. If this is the case, the proposed switch in G‐protein coupling described in “NPY acts at Y2 receptors on the central terminals of sensory neurons to suppress transient nociception and latent sensitization—but to facilitate injury‐induced acute pain” section could also occur in these regions.

Studies of DRG neurons following nerve injury suggest a pronociceptive role of endogenous NPY in the periphery. In the absence of injury, primary afferent neurons of the DRG express virtually no NPY. However, nerve injury induces a robust upregulation of NPY in the somata of medium‐ and large‐diameter myelinated DRG neurons [67]. Administration of Y2 antagonists into the DRG attenuated behavioral signs of neuropathic pain [65]. This result is consistent with the proposal that following nerve injury, NPY release from large primary afferents may mediate cross excitation of nociceptors, a phenomenon in which transmitter release from cell bodies within the DRG excites neighboring primary afferents [12] (but see “Nerve injury‐induced de novo expression of NPY in sensory neurons does not affect neuropathic pain” section).

Pronociceptive actions of NPY at the nucleus gracilis

Primary afferent nociceptors form synapses with second order neurons in the superficial dorsal horn that send contralateral projections to the brain. In contrast, heavily myelinated primary afferent neurons—in which nerve injury induces NPY upregulation—send ipsilateral projections directly to the brain along the dorsal column‐medial lemniscus system. Injection of NPY into the nucleus gracilis induced mechanical hypersensitivity; this was blocked by coadministration of the Y1‐selective antagonist BIBO3304 [63]. Administration of NPY antiserum—the nonselective competitive antagonist NPY_18–36—or BIBO3304 into the nucleus gracilis attenuated behavioral signs of neuropathic pain, leading to the suggestion that nerve injury elicits the release of NPY that maintains neuropathic pain [63] (but see “Nerve injury‐induced de novo expression of NPY in sensory neurons does not affect neuropathic pain” section).

Nerve injury‐induced de novo expression of NPY in sensory neurons does not affect neuropathic pain

To further interrogate the hypothesis that nerve injury‐induced NPY expression in sensory neurons facilitates neuropathic pain through its release from somata or from the central terminals in the DCN [63], as suggested above, we used a conditional knockout (cKO) approach. By crossing Pirt^Cre (a marker for primary afferent neurons) with NPY^lox/lox, we generated primary afferent neuron‐specific NPY knockout mice (Pirt‐NPY). Nerve injury‐induced upregulation of DRG NPY expression was abolished in Pirt‐NPY mice, whereas NPY expression in CNS neurons (arcuate nucleus of the hypothalamus) was not affected [68]. As expected, NPY expression in the spinal cord was slightly reduced, likely due to knockout from axon collaterals from neurons that primarily innervate the DCN. Strikingly, cKO of NPY from primary afferent neurons did not affect mechanical or cold hypersensitivity following SNI (nor tSNI, a modified SNI model that causes transient and submaximal hypersensitivity). Furthermore, Pirt‐NPY mice exhibited no difference in gabapentin‐induced conditioned place preference assay, indicating no effect of primary afferent‐expressed NPY on the affective component of ongoing pain. Finally, following remission of tSNI‐induced hypersensitivity, NPY cKO did not affect BIBO3304‐induced reinstatement of pain‐like behaviors. Taken together, these results indicate that NPY expressed by primary afferents following nerve injury does not affect neuropathic pain or latent neuropathic pain sensitization.

How can this lack of effect be reconciled with the evidence for a pronociceptive role of endogenous NPY expressed by primary afferents as detailed in “Pronociceptive actions of NPY” section? One potential explanation for this discrepancy is that opposing effects of NPY expressed by primary afferents at different sites of release could result in no net effect; NPY release from primary afferent terminals at the spinal cord dorsal horn could exert antinociceptive effects that neutralize pronociceptive effects of NPY release from the peripheral terminals, cell bodies within the DRG, or at terminals in the DCN. Regardless of mechanism, the net zero effect of NPY expressed by primary afferents on neuropathic pain implies another role for this NPY, such as nerve regeneration.

Y1‐expressing neurons in the spinal cord and parabrachial nucleus are predominately pronociceptive

Immunohistochemical and transcriptomic characterization of spinal Y1‐INs

In the spinal cord, Y1 is densely expressed on interneurons in the superficial dorsal horn (Fig. 1). In rat immunohistochemical studies, these interneurons (Y1‐INs) extensively coexpress excitatory, glutamatergic neuron markers, including somatostatin, calbindin, and calretinin [69, 70]. The lack of an effective, specific antibody for Y1 in mice is problematic, but Y1‐Cre x tdTomato reporter mouse lines indicate colocalization with excitatory but not inhibitory markers, including the developmental fate transcription factors Tlx3 (excitatory neuron development) and Pax2 (inhibitory neuron development) [71]. Additionally, transcriptomic studies demonstrate that Y1 is selectively enriched in excitatory neuron populations, including the glutamatergic clusters Glut2, Glut8, and Glut9. These glutamatergic clusters reside primarily in the superficial dorsal horn, where nociceptive signaling is concentrated, and express the immediate early gene Arc (a marker of neuronal activity like Fos) following noxious stimuli [26]. As such, these Glut2, Glut8, and Glut9 clusters that express Y1 are heavily implicated in pain. Furthermore, Npy1r overlaps greatly with Nmur2 [26], the gene for neuromedin U receptor 2. Support for a contribution of Nmur2 to pain comes from both behavioral [72, 73] and electrophysiological [74] studies.

Injury increases excitability and activity in spinal Y1‐INs

Electrophysiological studies have leveraged mice that express enhanced green fluorescent protein (eGFP) on the Npy1r promoter to reveal important physiological properties of Y1‐INs. In uninjured mice, most Y1‐INs that expressed eGFP (Y1eGFP) exhibited fast A‐type K⁺ currents and delayed, short‐latency firing (DSLF) [75]. Almost all Y1eGFP DSLF cells were rapidly adapting, and many exhibited rebound spiking, which is likely mediated by T‐type calcium channels [76].

Furthermore, electrophysiological studies of Y1eGFP neurons have reported that injury leads to significant changes in both passive and active membrane properties as well as synaptic activity in Y1‐INs recorded from both uninjured and nerve‐injured mice. Nerve injury increased the resting membrane potential of Y1‐INs to a value closer to the action potential firing threshold [36]. This is in contrast with randomly sampled dorsal horn interneurons in rats [77] or inhibitory interneurons in mice [78], which indicates a specific increase in Y1‐INs. Next, recordings were combined with 20pA current injection steps to interrogate active membrane properties. Nerve injury increased the number of action potentials in Y1‐INs in response to current injection steps, lowered rheobase (the minimum current required to elicit action potential firing), increased the number of neurons that exhibited rebound spiking, and increased action potential frequency [36]. Finally, to analyze potential changes in synaptic activity, spontaneous excitatory postsynaptic currents (sEPSCs) were recorded. Nerve injury increased both amplitude and frequency of sEPSCs, indicating an increase in both presynaptic inputs and postsynaptic responsiveness [36]. Together, these data indicate that nerve injury increases myriad components of excitability in Y1‐INs, and this may represent a mechanism of neuropathic pain.

Activation of spinal Y1‐INs causes pain‐like behavior

As NPY receptors are inhibitory, it follows that NPY binding to receptors expressed by pronociceptive neurons will yield analgesic activity by. Indeed, studies that directly activate NPY receptor‐expressing neurons confirm this. Chemogenetic manipulations using DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) have contributed greatly to neuroscience by allowing targeted manipulation of select neuron populations. Excitatory or inhibitory DREADDs can be selectively expressed in transcriptomically distinct neuron populations, such as those expressing Y1. DREADDs can be activated by administration of clozapine‐N‐oxide, typically by systemic or local (i.e., intrathecal or intracranial microinjection) routes. In uninjured mice, chemogenetic activation of spinal Y1‐INs induced robust mechanical and cold hypersensitivity and spontaneous nocifensive behaviors [36], as well as avoidance in a conditioned place aversion (CPA) paradigm. Optogenetic activation of Y1‐INs also induced mechanical and cold hypersensitivity in a stimulus frequency‐dependent manner [36]. These data provide strong evidence that activation of spinal Y1‐INs is sufficient to elicit behavioral signs of both the sensory and affective dimensions of pain.

Spinal Y1‐expressing neurons contribute to acute, inflammatory, and neuropathic pain

Neuronal ablation and inhibition studies indicate that (1) in injured states, Y1‐INs are necessary for sensory and affective dimensions of pain, but, in contrast, (2) in the absence of injury, Y1‐INs are necessary for the affective but not reflexive nocifensive responses to noxious stimuli.

Targeted ablation of spinal cord Y1‐INs with NPY‐saporin

Saporin is a potent ribosomal toxin that induces apoptosis [79] upon entry into cells. Intrathecal injection of saporin conjugated to NPY (NPY‐saporin) allows specific ablation of Y1‐INs while sparing Y1‐expressing DRG neurons [70, 80]. In uninjured animals, ablation of Y1‐INs with intrathecal NPY‐saporin decreased behavioral responses to noxious thermal stimuli thought to engage supraspinal pain modulatory systems. For example, NPY‐saporin reduced (1) hindpaw licking and guarding in a 44°C hotplate assay of affective pain; (2) ongoing nociception during both phases of the response to intraplantar injection of dilute formalin; and (3) aversion to a 10°C coldplate [80, 81]. However, NPY‐saporin did not change hindpaw withdrawal responses in the hot plate, Hargreaves, pin prick, or von Frey assays, indicating a lack of involvement in the protective reflexive nocifensive responses to noxious stimuli in the absence of injury [70, 81]. Together, these ablation data reveal that in the absence of injury, Y1‐INs mediate the affective dimension of acute pain but not its reflexive nocifensive behaviors.

NPY‐saporin studies indicate that Y1‐expressing neurons are also necessary for inflammatory and neuropathic pain. In rats with peripheral nerve injury (SNI), intrathecal NPY‐saporin dose‐dependently reduced the development of mechanical and cold hypersensitivity [70]. In rats with cutaneous inflammation (CFA), intrathecal NPY‐saporin reduced several operant and cognitive measures of allodynia, including responsiveness to cold in a thermal preference assay, feeding interference, and an escape task [80].

Reversible inhibition of spinal cord Y1‐INs with chemogenetics

Though ablation studies provide valuable insights into the functions of neuronal populations, their results can be confounded by toxicity or compensatory circuit rearrangements. To avoid these potential pitfalls and to test the results of the NPY‐saporin studies, we leveraged a chemogenetic approach that allowed more precise control of Y1‐INs. Chemogenetic inhibition of spinal Y1‐INs alleviated SNI‐induced mechanical and cold hypersensitivity along with light‐brush evoked pERK in the spinal cord dorsal horn [36]. It also produced a robust conditioned place preference, indicating alleviation of the affective dimension of neuropathic pain. Notably, chemogenetic inhibition of Y1‐INs did not affect mechanical thresholds or responses to acetone in uninjured mice. These data indicate that activity in Y1‐expressing neurons in the spinal cord are necessary for the full manifestation of neuropathic pain.

Y1‐expressing neurons in the parabrachial nucleus drive inflammatory pain and likely neuropathic pain as well

In the lPBN, chemogenetic activation of Y1‐expressing neurons in uninjured mice produced aversion in a CPA paradigm (https://www.biorxiv.org/content/10.1101/2024.02.26.582069v1). Activation of these neurons also evoked spontaneous behaviors often exhibited by animals following injury. Conversely, chemogenetic inhibition of lPBN Y1‐expressing neurons alleviated formalin‐induced inflammatory pain‐like behaviors and SNI‐induced mechanical hypersensitivity and cold allodynia (https://www.biorxiv.org/content/10.1101/2024.02.26.582069v1). Together, these data indicate that Y1‐INs in lPBN are pronociceptive, as their activation is sufficient to induce pain‐like behaviors in the absence of injury and their inhibition is sufficient to reduce pain‐like behaviors in injured animals.

Dorsal horn neurons that co‐express Y1 and GRP mediate neuropathic pain inhibition by NPY

It follows from “Endogenous and exogenous NPY decreases inflammatory and neuropathic pain” and “Y1‐expressing neurons in the spinal cord and parabrachial nucleus are predominately pronociceptive” sections that the potent antihyperalgesic effects of intrathecal NPY are mediated by inhibition of spinal Y1‐INs. However, Y1‐INs can be segregated into subpopulations based on their morphology, neurophysiology, and gene expression. Single cell transcriptomic studies in mouse spinal cord revealed three largely non‐overlapping subpopulations demarcated by coexpression of Y1 with Grp, Npff, or Cck [82]. We confirmed these results with in situ hybridization and further discovered that these subpopulations are conserved in higher order mammalian species, including non‐human primates and humans [82]. This heterogeneity raised the question of whether one of these subpopulations predominantly mediates neuropathic pain and NPY analgesia. To address these questions, we leveraged Cre transgenic approaches to selectively manipulate individual Y1‐IN subpopulations.

Contribution of Y1‐IN subpopulations to chronic pain

In the first set of studies, we capitalized on the fact that virtually all Npff‐expressing cells coexpress Y1 to test the hypothesis that the Npff‐expressing subpopulation of Y1‐INs (Npff/Y1‐INs) contributes to neuropathic pain with a simple chemogenetics approach. In Npff‐cre mice crossed with mice expressing an inhibitory DREADD, we reported that chemogenetic inhibition of Npff/Y1‐INs failed to reduce SNI‐induced mechanical or cold hypersensitivity. These data indicate that the Npff/Y1‐IN subpopulation does not contribute to neuropathic pain. On the other hand, nerve‐injured mice responded to light brushing of the hindpaw with increased Fos in the Grp/Y1‐IN subpopulation, but not the Npff/Y1‐IN or Cck/Y1‐IN subpopulations. This suggests that following nerve injury, mechanical stimuli predominantly activate Grp/Y1‐INs and promotes further studies to evaluate this subpopulation as a major contributor to neuropathic pain. However, although virtually all Npff‐expressing cells coexpress Y1, the same is not true for Grp Cck‐expressing cells. Therefore, future studies to determine the contribution of the Grp‐ (or Cck)‐expressing subpopulations of Y1‐INs are planned using Gcp‐Cre or Cck‐cre mice crossed with Y1 FlpO mice, with the predicted result being that selective elimination of the Grp subpopulation of Y1‐INs will reduce SNI‐induced mechanical or cold hypersensitivity [82].

Contribution of Y1‐IN subpopulations to NPY inhibition of chronic pain

To evaluate the contributions of Y1‐IN subpopulations to the inhibition of neuropathic hypersensitivity with intrathecal NPY, we leveraged an Npy1r‐cKO approach in the SNI mouse model. Conditional deletion of Npy1r from Grp/Y1‐INs but not Cck/Y1‐INs prevented the antiallodynic effects of the Y1‐selective agonist [Leu³¹, Pro³⁴]‐NPY. (Npff/Y1‐INs were not tested for antiallodynic effects of NPY because they do not contribute to neuropathic pain, as noted in “Contribution of Y1‐IN subpopulations to chronic pain” section). These results indicate that Y1 agonism at the spinal Grp/Y1‐IN subpopulation is responsible for intrathecal NPY‐mediated relief of neuropathic pain. Ongoing studies are evaluating the respective contributions of each Y1‐IN subpopulation towards the antihyperalgesic actions of intrathecal Y1 agonist in other models of acute and chronic pain, such as the plantar incision model of postsurgical pain. Preliminary results suggest patterns of involvement that are distinct from those observed in neuropathic pain.

Tonic inhibition of latent chronic pain sensitization by NPY

Tissue or nerve injury engages a long‐lasting endogenous analgesia system comprised of a constellation of inhibitory GPCRs that maintain LS within a state of remission [“Endogenous NPY tonically acts at Y1 receptors in the spinal cord to mask latent pain sensitization” section] and [30, 45, 83]. The past 10 years have yielded a plethora of new data that provides a more detailed understanding not only of the number of GPRCs involved (including Y1) and their interactions with each other (such as Y1 and the mu opioid receptor) but also the intracellular signaling events that mediate the LS that is masked by spinal NPY‐Y1 activity (Fig. 2).

Fig. 2.

Mechanistic model for the tonic inhibition of intracellular signaling cascades of latent pain sensitization by NPY agonism at spinal neurons that express Npy1r. (a) In the basal state, Y1‐INs are relatively inactive in terms of both action potential firing and intracellular excitatory signaling. (b) Injury leads to action potential firing and prolonged depolarization of Y1‐expressing neurons, which allows glutamate released by presynaptic neurons to open NMDARs. NMDARs allow Ca2+ influx, which activates adenylyl cyclase type 1 (AC1). A1C is responsible for the production of cAMP, which activates downstream signaling molecules, including PKA and Epac. These molecules activate mediators of nociception such as TRPV1 and TRPA1 (not pictured) to enhance neuronal excitability and nociceptive signaling in the long term. (c) NPY‐Y1 signaling following endogenous NPY release or NPY administration might oppose latent sensitization of Y1‐expressing neurons by direct inhibition of AC1 via Gi/o signaling or by activation of GIRK channels, leading to K⁺ efflux‐mediated hyperpolarization that reduces excitability and/or inhibits NMDAR activation. As such, NPY‐Y1 signaling maintains latent sensitization within a state of remission. cAMP, cyclic adenosine monophosphate; GIRK, gated inwardly rectifying potassium; NMDAR, n‐Methyl‐d‐aspartate receptors; NPY, neuropeptide Y; PKA, protein kinase A. Source: Created in BioRender. Nie, A. (2025) https://BioRender.com/reebso4.

Injury engages an NMDAR‐AC1‐cAMP‐PKA/Epac signaling pathway for latent sensitization that is opposed by NPY

The Y1 receptor couples to inhibitory G‐proteins α_i/o, leading to the inactivation of adenylyl cyclase and subsequent down regulation of cAMP [84]. Adenylyl cyclase type 1 (AC1) is found in superficial laminae of spinal cord and activated by Ca²⁺/calmodulin following n ‐Methyl‐d‐aspartate receptors (NMDAR) activation, contributing to inflammatory pain. Solway et al. were the first to demonstrate that, when conducted during the remission phase of LS, either genetic knockdown of NPY or pharmacological disruption of NPY‐Y1 signaling with BIBO3304 reinstated behavioral and molecular signs of spinal nociceptive transmission and pain [30]. Our subsequent studies reported that a spinal NMDAR‐mediated AC1 activation contributes to the LS masked by mu opioid receptor constitutive activity MOR_CA [45]. A similar mechanism drives the LS that is masked by tonic NPY release [46]. For example, we reported that the NMDAR blocker MK801 prevented BIBO3304‐induced reinstatement of inflammatory pain in the CFA model [85] as well as neuropathic pain in the tSNI model [46] (a milder variant of the SNI model that allows for the resolution of hypersensitivity within approximately 5 weeks). MK801 also blocked BIBO3304‐induced CPA in tSNI mice, indicating that NMDAR activation is necessary not only for the sensory but also the affective dimension of LS. Next, AC1 deletion mutant mice and an AC1 inhibitor, NB001, were utilized to determine whether AC1 contributed to the LS that is masked by NPY [46, 85]. As observed in earlier studies of endogenous opioid analgesia [45], AC1 deletion had little effect in sham control mice but prevented BIBO3304‐induced reinstatement of hypersensitivity after inflammation (CFA) [45] as well as hypersensitivity and the affective dimension of pain (CPA) after nerve injury (tSNI) [46, 85]. The effects of AC1 deletion were mimicked in CFA or tSNI mice with intrathecal injection of NB001. These data indicate the necessity of spinal AC1 to LS and suggest that spinal NPY‐Y1 signaling tonically inhibits NMDAR‐AC1 signaling to mask LS (Fig. 2).

AC1 activation produces cAMP, which acts as a second messenger to activate downstream signaling molecules, including protein kinase A (PKA) and exchange protein directly activated by cAMP (Epac). To investigate the role of PKA in LS, the PKA activator 6Bnz was administered intrathecally during LS (21 days after induction of inflammation) [85]. 6Bnz robustly reinstated mechanical hypersensitivity and increased brush‐evoked pERK expression in laminae 1–2 of the spinal cord dorsal horn. The PKA inhibitor H89 blocked this effect of 6Bnz and attenuated BIBO3304‐induced reinstatement, highlighting the necessity of PKA to LS. Given that PKA‐mediated phosphorylation of either TRPA1 and TRPV1 can sensitize them to inflammation [86, 87], these downstream receptors were examined in the context of LS. The TRPA1 antagonist HC030031 as well as the TRPV1 antagonist AMG9810 attenuated BIBO3304‐induced reinstatement in mice with CFA [85] or tSNI [46]. Furthermore, in NPY knockdown mice that displayed increased duration of tSNI‐induced hypersensitivity, HC030031 attenuated both mechanical and cold hypersensitivity. Together, these results suggest that NMDAR‐AC1‐PKA‐TRPA1/TRPV1 signaling mediates LS and is masked by endogenous NPY‐Y1 signaling.

To determine whether either of the two isoforms of Epac (Epac1 and Epac2) contribute to LS in parallel with PKA, the Epac activator 8cpt was first administered 21 days after CFA, at which point it reinstated mechanical hypersensitivity and increased brush‐evoked pERK expression in the superficial spinal cord dorsal horn. Furthermore, the Epac1/2 dual antagonists HJC0197 and ESI‐09 as well as the Epac2 selective antagonist HJC0350 attenuated BIBO3304‐induced reinstatement of hypersensitivity. These data indicate that cAMP engages both Epac1 and Epac2 to maintain the LS. Taken together, the above results indicate that the NPY‐Y1R axis maintains LS in a state of remission by inhibiting AC1, PKA, Epac1, and EPac2. An NMDAR‐AC1‐cAMP‐PKA/Epac signaling pathway mediates sensitization of nociceptive circuits. These results support further investigations toward the discovery of new treatments for chronic pain that would extinguish LS with pharmacological inhibition of AC1, PKA, or Epac.

Injury enhances NPY‐Y1 signaling in dorsal horn

Injury can increase NPY release, as well as its coupling with G proteins, both of which would increase Y1 activation and thus inhibition of pronociceptive Y1‐INs. For example, nerve injury increases NPY release in spinal cord dorsal horn as measured by Y1 receptor internalization [88]. Similarly, paw inflammation increases NPY mRNA and NPY‐immunoreactivity in spinal cord [89]. Given the net zero effect of primary afferent‐expressed NPY on neuropathic pain as discussed above [68], it should follow that NPY released by dorsal horn interneurons is responsible for tonic inhibition of both sensory and affective components of LS in the spinal cord. CFA‐induced inflammation as well as SNI increase coupling between Y1 receptors and G‐proteins to enhance inhibitory signaling [36, 38]. This phenomenon may function to increase NPY‐mediated inhibition of inflammatory or neuropathic pain. Alternatively, NPY‐mediated GIRK activation [24] could hyperpolarize Y1‐INs, leading to inhibition of NMDAR activation and subsequent inhibition of the entire signaling pathway (Fig. 2). These results support further investigations toward the discovery of new treatments for chronic pain that will increase or mimic endogenous NPY analgesia.

Endogenous analgesic synergy between Y1 and mu opioid receptors

We and others have reported that inflammation‐ or nerve injury‐induced LS is tonically opposed by a growing list of spinal Gi‐GPCR mechanisms that include (1) NPY release at Y1‐interneurons as detailed above; (2) μ‐opioid receptor constitutive activity (MOR_CA) as initially reported in Science [45, 90, 91]; and (3) kappa opioid receptors [91, 92] as well as delta opioid, CRF, and alpha‐2 adrenergic receptors [91, 93]. One might have predicted that each antagonist/inverse agonist would have produced a partial reinstatement of hyperalgesia, suggestive of an additive effect. However, the magnitude of reinstatement produced by each antagonist was as large as the magnitude of the initial injury‐induced hyperalgesia. Recent data focusing on Y1 now explain how each of many receptor mechanisms can be completely necessary for full manifestation of LS: Endogenous analgesic receptors can work synergistically with each other to maintain LS in remission, just as analgesic drugs can work synergistically (a drug interaction whereby two or more drugs cause a much greater analgesic effect than the sum of their individual effects) [94, 95]. To elucidate the interactions between Y1 and mu opioid receptors (MOR) in masking LS, BIBO3304 and the MOR antagonist CTOP were independently administered and then coadministered during the remission phase of LS. Remarkably, coadministration of BIBO3304 and CTOP reinstated mechanical hypersensitivity at remarkably low doses, reflected by a robust leftward shift in the dose response curve as compared to either drug given alone [42]. Combinatorial drug interaction analysis revealed a significant synergistic interaction. Synergy was blocked by the GluN2A‐preferring NMDAR antagonist, PEAQX, indicating that MOR and Y1 synergistically oppose LS that is mediated by NMDAR [42].

To test for Y1‐MOR synergy for the treatment of acute postsurgical pain, Y1 and MOR agonists were administered independently or together 2 days post incision. At this timepoint, intrathecal administration of [Leu³¹Pro³⁴]‐NPY and the MOR agonist DAMGO each caused dose‐dependent reversal of hypersensitivity. However, coadministration of these two agonists did not result in a synergistic interaction. These results suggest that Y1‐MOR synergy may take time to develop or might be limited to chronic pain states. Future studies might test for synergy between Y1 and opioid receptor agonists in models of chronic pain.

The molecular and cellular mechanisms that underlie endogenous Y1‐MOR synergy have yet to be elucidated. Notably, Npy1r and Oprm1 mRNA colocalize significantly in the spinal cord dorsal horn (Npy1r/Oprm1—51.96 ± 1.59%; Oprm1/Npy1r—32.28 ± 1.22%) [42]. As such, intracellular mechanisms of synergy, whereby GPCRs engage in mutual reinforcement, are possible. One such potential mechanism is heterodimerization of Y1 and MOR, which may potentiate signaling [96]. Alternatively, activation of one GPCR may cause redistribution in a shared pool of G‐proteins, which could increase G‐protein binding affinity for the other receptor [97]. This would allow higher levels of GPCR activation to be reached with lower ligand concentrations. Another potential mechanism is that downstream signaling of one GPCR could increase trafficking of the other to the cell membrane, thereby increasing sensitivity to ligands [98]. However, intercellular mechanisms between different cells that each express one receptor or the other could also mediate synergy [42]. Future studies could interrogate the mechanisms of GPCR synergy to increase our understanding of complex drug interactions.

Though MOR is the only opioid receptor for which synergy with Y1 has been formally studied, some data suggest the potential for interactions between Y1 and the other opioid receptors. FISH studies have established that 76.8% of inhibitory neurons that express mRNA for proenkephalin, the precursor for the endogenous delta opioid receptor (DOR) ligand enkephalin, coexpress NPY [99]. Furthermore, ∼40% of spinal DOR‐expressing neurons coexpress Npy1r mRNA [99]. These significant overlaps in peptide and receptor expression indicate the potential for pain‐related interactions between Y1 and DOR. Demonstration of Y1 synergy with opioid receptors may allow lower doses of opioids to be used without compromising analgesic efficacy.

Ongoing translational research on intranasal administration of NPY

Intrathecal and intracranial delivery of NPY and its analogs has shown great promise in behavioral and molecular signs of pain. However, these routes of administration risk injury and infection [100], making them problematic for translation to humans. Furthermore, neuropeptides do not readily cross the blood brain barrier, rendering systemic administration inefficacious for CNS delivery of [Leu³¹Pro³⁴]‐NPY. To overcome this barrier and noninvasively target CNS Y1‐INs for the treatment of postsurgical and neuropathic pain, we decided to leverage the intranasal route of administration, which has emerged as a promising approach for the brain delivery of neuropeptides to address psychiatric and neurological disease [101]. Notably, intranasal NPY has already been tested in humans for the treatment of major depressive disorder [102]. Ongoing studies in our laboratory are evaluating intranasal delivery of [Leu³¹Pro³⁴]‐NPY for the treatment of chronic pain.

Conclusions and future directions

New pharmacotherapeutics are urgently needed to address the chronic pain crisis. Over the past few decades, NPY receptors have emerged as a promising target for drug development. Endogenous NPY tonically inhibits LS of nociceptive circuits, allowing remission of pain following injury. We propose that disruptions to this compensatory NPY‐analgesia system may mediate pathological transitions to chronic pain, which could be treated by CNS‐targeted administration of Y1 agonists, spinal administration of Y1 agonists or Y2 antagonists, or peripheral administration of Y2 antagonists, depending on the pain state (Fig. 3). To circumvent highly invasive administration routes to target CNS sites (such as intrathecal injection), ongoing studies are leveraging the intranasal route for delivery of NPY into the brain.

Fig. 3.

Promising NPY‐based pharmacotherapies for the treatment of acute and chronic pain after tissue and/or nerve injury. (a) Intranasal administration of Y1 agonists may inhibit pronociceptive NPY‐receptor‐expressing neurons of the many brain regions that maintain persistent pain, such as the parabrachial nucleus. (b) Intrathecal administration of Y1 agonists may inhibit the central sensitization of pronociceptive spinal Y1‐INs to reduce chronic pain. Intrathecal administration of Y2 antagonists may inhibit the release of glutamate from Y2‐expressing sensory neuron terminals to reduce acute pain. (c) Peripheral administration (e.g., by transdermal patch) of Y2 antagonists could reduce pain after acute injury. Source: Created in BioRender. Nie, A. (2025) https://BioRender.com/64fwjka.

Current research is now elucidating the transcriptomic and functional identities of NPY receptor‐expressing neurons throughout the CNS. Spinal Y1‐INs are the most thoroughly studied and have been established to be a potently pronociceptive population, subject to inhibitory control by NPY. Furthermore, ongoing high‐profile studies using powerful modern neuroscience techniques are determining the integrative functions of NPY‐ and Y1‐expressing neurons in the PBN. These and other studies of endogenous NPY systems and NPY receptor‐expressing neurons will enhance the development of a new pharmacotherapy for chronic pain.

Conflict of interest statement

The authors declare no conflicts of interest.

Nie AA, Taylor BK. The pharmacotherapeutic potential of neuropeptide Y for chronic pain. J Intern Med. 2025;298:280–296.

Data availability statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.