An endogenous opioid alters neuronal plasticity to constrain cognitive flexibility

A recent study by Yang, Lalchandani Tuan and colleagues has updated our understanding on how an endogenous opioid, Dynorphin (Dyn), modulates synaptic plasticity and shapes cognitive flexibility [1].

Cognitive flexibility is the capacity to aptly change one’s behavior according to a changing environment [2]. The ability to do this affords greater resilience to negative life events, including stress [3] and can enhance creativity in adulthood [4]. Traditionally, the balance between dynorphin and the neuromodulator dopamine is key in regulation of flexible behavior. In the absence of dopamine, dynorphin predominates resulting in behavioral inflexibility disorders like Parkinson’s disease [5]. However, in the absence of dynorphin, there is excessive flexibility that can also be maladaptive. A recent study by Yang, Lalchandani Tuan and colleagues shows a powerful role for the endogenous opioid, Dyn, in modulating cognitive flexibility and long-term neuron plasticity in the striatum [1], an entryway to the basal ganglia, which is a circuit involved in fine motor learning and reward processing.

In the brain, long-term synaptic plasticity is a key cellular substrate for learning and behavioral adaptation. Synaptic plasticity is the ability of neuronal junctions otherwise known as synapses to strengthen or weaken over time in response to increases or decreases in their activity [6]. Disruptions in cognitive flexibility and synaptic plasticity are key outcomes in many brain disorders including major depressive disorder, schizophrenia, autism spectrum disorders (ASDs) and Parkinson’s disease. Long-term potentiation (LTP), a type of synaptic plasticity in traditionally excitatory glutamatergic synapses, occurs when neuronal synapses, are strengthened based on recent patterns of activity in response to experience, while long-term depression (LTD) occurs when efficacy of neuronal synapses are reduced or weakened. Both forms of long-term synaptic plasticity are fundamental properties of the nervous system and widely considered a primary mechanism underlying learning and memory [7]. Forms of excitatory synaptic plasticity occur in different brain areas and are modulated by other neurotransmitters and neuropeptides. The latter are a group of compounds that act as neurotransmitters and are short- chain polypeptides, including Dyn. The striatum is one brain area with dense glutamatergic inputs and various mechanisms of neuromodulation. The striatum is composed mainly of inhibitory projection neurons, termed spiny projection neurons (SPNs), classified into two subtypes based on their genetic machinery and axonal pathway projections [8], [9]. These two SPN subtypes form pathways in the basal ganglia known as direct or striatonigral and indirect or striatopallidal pathways. In addition to their projections, SPNs are distinct in receptor expression and signaling between these SPNs. The dopamine receptor D1 (D1R), and neuropeptides Dyn and substance P are enriched in direct pathway SPNs (dSPNs), while dopamine receptor D2 (D2R) and the neuropeptide enkephalin are enriched in indirect pathway SPNs (iSPNs) [10]. In this current study Yang, Lalchandani Tuan and colleagues [1] uncover a new role for the endogenous opioid, dynorphin, in dSPN synaptic plasticity and behavioral flexibility.

WHAT ARE DYNORPHINS AND WHERE ARE THEY LOCALIZED?

Goldstein and colleagues in 1979 described the opioid property of a short neuropeptide isolated from pig pituitary gland [11]. To symbolize its unusual potency, the natural peptide was named dynorphin. Dynorphins (Dyn) are now known to be a class of opioid peptides occurring from the precursor protein prodynorphin and cleavage of prodynorphin gives rise to Dyn A, Dyn B and Big Dyn. Dyns exert their effects primarily through the kappa-opioid receptor (KOR). Understanding the Dyn/KOR system has been a major challenge largely because of their ubiquitous cellular, subcellular, and regional expression patterns [12]. In addition, activation of KOR signaling may inhibit or excite groups of neurons depending on whether they are expressed on inhibitory or excitatory neurons [12]. Within the striatum, Dyn is preferentially expressed in dSPNs [8]. However, the endogenous function of Dyn in dSPNs is not well characterized. Yang, Lalchandani Tuan and colleagues in their recent study used a series of elegant pharmacological manipulations of KOR and a genetic mouse strain in which Dyn was deleted in the brain coupled with physiology and behavior to probe for the endogenous function of Dyn in dSPNs [1].

IMPLICATION OF DYN/KOR SIGNALING

Previous work by Atwood et al., showed endogenous opioid receptor activation induced robust LTD in the dorsal striatum [13]. Yang, Lalchandani Tuan and colleagues in their paper furthered this finding to show that KOR acts preferentially in dSPNs. They incubated striatal brain slices with a KOR agonist – activates KOR signaling similar to Dyn- or a KOR antagonist – prevents Dyn activation of KOR [1]. They observed that in the presence of the KOR agonist, LTP was abolished in dSPNs and remained unchanged in iSPNs. In contrast, LTP was enhanced in dSPNs in the presence of the KOR antagonist and unchanged in iSPNs. This indicated that Dyn/KOR signaling modulates LTP in dSPNs. A downside of using pharmacological manipulation is the inability to dissociate cell autonomous (direct) effects from multi-synaptic (indirect) effect. Therefore, to confirm the specificity of LTP modulation by KOR in dSPNs, Yang, Lalchandani Tuan and colleagues used a genetic mouse line, called Dyn cKO, in which Dyn was conditionally deleted in the cells that express a marker for striatal dSPNs. While this marker is also expressed in other brain regions including cortex, the authors found that Dyn cortical expression was unperturbed. [1]. In agreement with their pharmacological manipulations, they observed that LTP was enhanced in Dyn cKO mice. Taken together, Dyn activation of KOR signaling blunts LTP, while blocking Dyn activity on KOR or specific deletion of endogenous Dyn in dSPNs enhances LTP. This was a major finding by Yang, Lalchandani Tuan and colleagues because it demonstrates a unique effect of Dyn/KOR signaling within dSPNs. They also confirmed that Dyn/KOR activation and signaling in dSPNs does not affect other forms of synaptic transmission within the striatum.

Neurons express different types of receptors with differing intracellular signaling cascades that may eventually sway synaptic plasticity in favor of one cascade or the other [9]. dSPNs primarily express the dopamine D1 receptors (D1R) upon which activation favors LTP, while blockade results in LTD [14]. Considering dSPNs also express KORs, Yang, Lalchandani Tuan and colleagues tested the synergistic interaction of D1R and Dyn/KOR signaling in the regulation of LTP. They showed that in the presence of both D1R agonist and KOR agonist the outcome favored D1R activation of LTP suggesting that the D1R overrides KOR signaling and that Dyn/KOR modulation in striatum is most effective in the absence or with low levels of D1R activity [1]. These findings show a homeostatic balance between D1R and Dyn/KOR signaling in modulation of striatal plasticity in the direct pathway. This has implications for brain disorders with fluctuating dopamine levels, such as Parkinson’s Disease and substance use disorders and thus encourage the consideration of endogenous opioid modulation as an alternative to non-dopaminergic therapies.

HOW DO DYNORPHINS REGULATE COGNITIVE FLEXIBILITY?

LTP is associated with learning and memory, thus, in a final set of experiments, Yang, Lalchandani Tuan and colleagues investigated the behavioral relevance of striatal synaptic plasticity regulation by Dyn. Preclinical studies demonstrated that chronic KOR activation contributes to negative affective behaviors [15], in addition, Dyn/KOR dysregulation is implicated in drug seeking behaviors in substance use disorders [16]. The authors showed that Dyn cKO mice and control mice with intact Dyn performed comparable in tests that assay for anxiety, motor behavior, and working memory [1]. They then examined if Dyn/KOR signaling is necessary for cognitive flexibility. Yang, Lalchandani Tuan and colleagues used two types of discrimination reversal learning tasks to examine cognitive flexibility [1]. In these tasks, animals must learn to discriminate between cues in their environment. A correct choice of a cue results in the animal receiving a reward, such as a food reward. Once a high number of correct choices are made, the cues are reversed, such that the previously rewarded cue is incorrect, and the previously incorrect cue is now rewarded [17]. More specifically, Yang, Lalchandani Tuan and colleagues used the spatial reversal learning task and the 4-choice odor discrimination reversal learning task. In the former task, mice were trained to nose poke one of two holes to receive a food reward and upon reaching an 85% success rate, the correct and incorrect nose poke holes were reversed so that the mice must then learn to nose poke the new hole to obtain the food reward. In the 4-choice odor discrimination reversal learning task, mice were trained to dig for a food reward which is associated with an odor. Once reaching a high level of discrimination criteria, usually about 8 correct responses out of 10 trials, the food reward is paired with a different odor so that the mice must then learn to associate the new odor with a food reward. Interestingly, the authors observed that Dyn cKO mice consistently made fewer errors in both tasks and had faster adaptations to new reward pairings [1]. They further showed that this fast adaptation observed in Dyn cKO mice was not due to change in motivation to work for the reward suggesting that loss of striatal Dyn increased flexibility to adapt new reward cues while motivation was unaffected. To further support this finding, Dyn cKO mice were given a KOR agonist to mimic the effect of Dyn. In this experiment, Dyn cKO mice made more errors suggesting that Dyn/KOR impedes cognitive flexibility [1].

WHAT DOES THIS ALL MEAN?

The effect of Dyn/KOR regulation of dSPN circuitry is not simplistic, because in addition to potentially regulating LTP via a mechanism in dSPNs, the Dyn/KOR system has been shown to inhibit glutamate release from neurons that synapse onto dSPNs [18]. KORs expressed on SPNs also inhibit GABA (primary neurotransmitter of inhibitory neurons) release from other SPNs [18] and excitatory release onto other inhibitory neurons [19], which contributes to a reduction in inhibition. Therefore, the concerted effort of these effects of KOR activation in addition to this potential dSPN effect on striatal LTP described by Yang et al. may shape how the circuit works in complex ways.

Previous studies have provided evidence to suggest that the serotonergic system modulates KOR signaling to promote negative affective states [20]. However, Yang, Lalchandani Tuan and colleagues showed that in tests for anxiety, motor control and working memory, mice with specific deletion of Dyn from the brain including dSPNs, behaved comparably to the control mice [1]. Interestingly, Dyn cKO mice performed better in tests for cognitive flexibility. This would imply that in physiological conditions, endogenous Dyn signaling in the striatum would hinder cognitive flexibility. While this may suggest a disadvantageous effect of Dyn signaling, this could act as a “brake” to prevent rapid and excessive cognitive flexibility. Excessive flexibility may indeed increase the ability to acquire new memories but prevent consolidation of previously acquired memories therefore, Dyn may act as a modulator for cognitive flexibility. Flexibility deficits are signature symptoms observed in human neurodevelopmental and psychiatric conditions. Disorders like attention-deficit/hyperactivity disorder and autism spectrum disorders are characterized by excessive flexibilities, while cognitive inflexibility occurs in schizophrenia and brain disorders associated with cognitive decline such as Parkinson and Alzheimer diseases. Furthermore, Dyn is upregulated in striatal circuits in response to dysphoric states which are signature behaviors characterized by inability to adapt to task demands [21]. Thus, investigations into Dyn/KOR functioning in these brain disorders is warranted.

Finally, understanding the influence of Dyn/KOR regulation on sex specific behaviors is important as preclinical studies demonstrate sex specific behavioral outcomes when perturbing KOR signaling and sex chromosomes can impact the expression of the Dyn gene prodynorphin [22]. In line with this, a recent study in the Netherlands showed that women performed better in tests for memory, processing speed and flexibility during mid age [23], and while the study by Yang, Lalchandani Tuan and colleagues used both male and female mice, they did not analyze for sex differences. An interesting follow up will be the analysis of sex specific influences on the findings by Yang, Lalchandani Tuan and colleagues. In summary Yang, Lalchandani Tuan and colleagues have identified a new function of the endogenous opioid, Dyn, in striatal neuron reorganization and behavioral flexibility [1]. This study opens new avenues of research into how endogenous opioids in striatum may influence both typical cognitive functions and disrupted cognitive functions, which can impact detailed mechanistic understanding of a variety of brain disorders.