Central somatostatin signaling and regulation of food intake

Abstract

The discovery of somatostatin (SST) in the hypothalamus implicated the peptide in the inhibition of growth hormone release. However, as observed for numerous neuropeptides, SST was neither restricted to this one brain site nor to this one function. Subsequent studies established a widespread but specific expression of SST in the central nervous system of rodents and humans along with the expression patterns of five receptors (sst_1–5). Among biological actions, the activation of central SST signaling induced a robust stimulation of food and water intake, which is mediated by the sst₂ as assessed using selective sst agonists. The past years witnessed the identification of brain SST circuitries involved using chemogenetic and optogenetic approaches and further established a physiological orexigenic role of brain SST signaling. The present review will discuss these recent findings.

Graphical abstract:

The current review will present the latest development on the food intake−modulating effects of central somatostatin signaling and brain circuitries involved.

Introduction

Somatostatin (SST)-14 is a 14-amino-acid peptide first detected in the hypothalamus and shown to induce a robust inhibition of pituitary growth hormone (GH) secretion.¹ Subsequently, SST-28 was identified in the intestine,² as the N-terminally extended SST-14 form derived from the same prohormone, pro-SST.³ Both, SST-14 and SST-28 bind to all five SST receptors (sst_1–5) with similar affinity.^{4, 5} SST receptors are members of the G protein–coupled seven transmembrane domain receptor family.⁶ In addition, few sst isoforms have been identified including the full-length sst_2a and the truncated form, sst_2b in rodents⁷ along with several truncated sst₅ variants in rodents and humans.^{8, 9} These isoforms can modulate sst₂ signaling¹⁰ and were shown to play a role in different types of cancer.^11–13

Besides its inhibitory action on GH secretion, numerous brain-initiated extrapituitary effects of SST receptors and/or agonists have been described, such as an increase in blood pressure,¹⁴ blood glucose, body temperature, gastric acid secretion,¹⁵ and gastrointestinal propulsive motor function^{16, 17} pointing towards a pleiotropic role of the peptide in line with its widespread brain distribution.¹⁸ Moreover, SST influences behavior increasing grooming,¹⁹ locomotor activity,²⁰ and anxiety.²¹ Brain SST was also early on reported to alter feeding behavior in rodents.²² Subsequent studies identified the receptor(s) involved using selective peptide sst agonists and antagonists²³ and distinguished between pharmacological and physiological effects.²⁴ The current review will present the latest development on the food intake–modulating effects of central SST signaling and brain circuitries involved.

Central expression of SST and its receptors

SST displays a wide distribution in the rodent brain with strong immunoreactive signals detected using antibodies recognizing both, Sst-14 and Sst-28.²⁵ Brain areas encompass those that have been implicated in feeding regulation namely the hypothalamic arcuate (Arc), ventromedial, periventricular, lateral, and paraventricular (PVN) nuclei, the tuberal nucleus (TN), and the nucleus of the solitary tract (NTS).^{18, 25–29} SST neurons are also localized in the basal forebrain and mapping of SST projections showed input to other brain areas influencing food intake, such as the lateral hypothalamic area (LHA).³⁰ Recent studies using a droplet-based single-cell RNA sequencing strategy allowed the identification of four distinct SST neuronal populations in the LHA, which are transcriptionally distinct.²⁹

Along with the ligand, are widely expressed in the rodent brain.⁶ In food intake–regulatory nuclei, sst expression was localized in the Arc (sst₁ = sst_2a = sst₃ > sst₄), ventromedial hypothalamic nucleus (sst₁ > sst₃ > sst₂), dorsomedial hypothalamic nucleus (sst₁ = sst₃), PVN (sst_2a = sst₃), NTS (sst₁ = sst₂ > sst₃), and the dorsal motor nucleus of the vagus (sst_2a/b = sst₄ > sst₅).^31–35 Expression of both ligand and receptor—predominantly sst₂ and sst₃—in major food intake–regulatory nuclei, such as the Arc, LHA, PVN, and NTS, provides support for an autocrine/paracrine mode of action.

Orexigenic effect of central SST signaling

Over three decades ago, reports indicated that SST-14 or the sst_2,3,5 agonist, octreotide injected intracerebroventricularly (i.c.v.), into the dorsal hippocampus or anterior piriform cortex stimulates food intake in rats.^36–40 More recently, the pan-SST agonist, ODT8-SST injected i.c.v. at a low (0.3 nmol) dose led to a pronounced stimulation of food intake not only in the light phase but also in the dark phase—where food intake is already stimulated by other transmitters/hormones—under ad libitum conditions in rats and mice.^41–43 While the stimulation of food intake was observed rapidly after i.c.v. injection of octreotide (within 10 min),³⁶ it was delayed in onset (within 60 min) and longer lasting following i.c.v. ODT8-SST.⁴¹ The i.c.v. ODT8-SST–induced orexigenic effect was associated with a sustained increase in the respiratory quotient and energy expenditure likely contributing to the observed body weight loss 24 h post injection.⁴¹ It is to note that in contrast to the orexigenic effect observed following injection of low doses, higher SST doses in the nmol range injected i.c.v. resulted in an inhibition of food intake in rats and mice^38,44–46 likely related to the induction of other behaviors, such as barrel rotations,⁴⁷ but also a leakage/transport into the peripheral circulation⁴⁸ where SST predominantly has an anorexigenic effect.⁴⁹

Further studies investigated the SST receptor(s) involved in this orexigenic action. As described above, the sst_2,3,5 agonist octreotide injected i.c.v. induced a robust increase of light phase food intake in rats^{36, 37} and mice.⁵⁰ Similarly, the selective sst₂ agonist, S-346–011 injected i.c.v. also stimulated food consumption in rodents, whereas the sst₁ agonist, S-406–062 and sst₄ agonist, S-315–297 did not.^{50, 51} Conversely, the orexigenic effect of ODT8-SST was abolished by the selective sst₂ antagonist, S-406–028⁴¹ clearly pointing towards an sst₂-mediated signaling.^{50, 51}

Analysis of the underlying food intake microstructure using an automated food intake monitoring system for solid food in undisturbed rodents showed that i.c.v. injection of the sst₂ agonist, S-346–011 increased the number of meals and reduced intermeal intervals during a 4-h period, whereas not altering meal size compared to controls.⁵⁰ This is indicative of the suppression of satiety, a mechanism causing a delayed onset of the next meal after completing a meal as reflected by longer intermeal intervals, whereas satiation, a mechanism causing the termination of a meal, was unaltered.⁵²

The stimulation of food intake by central activation of SST signaling is likely to be of physiological significance since the sst₂ antagonist, S-406–028 reduced dark phase food intake—when rodents show their highest food intake⁵³—by ~30% in rats.⁵¹ In contrast, when injected during the light phase under ad libitum feeding conditions, the sst₂ antagonist had no effect on food intake⁵¹ implying the recruitment of Sst–sst signaling to stimulate food intake during the dark photoperiod. This is also supported by a higher hypothalamic Arc⁵⁴ and reduced PVN⁵⁵ Sst mRNA expression in rats housed under conditions of a sustained dark photoperiod along with increased hypothalamic Sst release at the beginning of the dark phase.^56–58

A recent study provided convergent evidence that SST in the TN plays a physiological role to regulate food intake.²⁸ First, either fasting or intraperitoneal (i.p.) injection of ghrelin, a hormone released during the hunger state,⁵⁹ induced a robust expression of the immediate early gene c-Fos in TN SST neurons.²⁸ Second, chemogenetic studies further established that activation of SST neurons in the TN promoted a robust food consumption for the subsequent 3 h by enhancing eating time and frequency.²⁸ Conversely, optogenetic inhibition of SST neurons in the TN reduced cumulative eating time and frequency.²⁸ Lastly, selective ablation of these SST neurons resulted in a mild reduction of daily and dark cycle food intake and respiratory exchange ratio.²⁸

New developments also unraveled the implication of a population of lateral septum SST-neuronal pathways in facilitating food-seeking behavior.⁶⁰ Optogenetic gamma-frequency stimulation of SST-positive lateral septum cells projecting to the LHA induced a specific food-seeking behavior in mice as indicated by a shortened latency to reach the food zone, while not stimulating food intake per se.⁶⁰ Interestingly, when the LHA was directly stimulated using optogenetics food intake was increased.⁶⁰ These, at first sight, conflicting results have been explained by the differential pattern of activation. Optogenetic stimulation of the lateral septum to LHA projections stimulated the firing of food zone mismatch (cells preferentially active at a distance from the food zone) and less so of food zone match (LH neurons in the free-access feeding model matching the location of the food zone) cells. Conversely, direct activation of the LHA induced the opposite pattern (predominant firing of food zone match neurons).⁶⁰ Further chemogenetic approaches in Sst-Cre mice established that selective activation of SST neurons in LHA increases eating behavior.²⁹

Other optogenetic and adeno-associated viral tracing studies in mice discovered the involvement of basal forebrain Sst in mediating palatable food consumption.³⁰ The authors demonstrated that the activation of basal forebrain SST neurons induces selective consumption of high-calorie palatable food (high-fat and high-sucrose) without affecting normal food intake, suggesting that SST neurons in the basal forebrain are involved in hedonic feeding. By contrast, stimulation of projections from SST neurons in the ventral forebrain to the LHA selectively induced fat intake.³⁰ It is to note that the sst₂ antagonist had no modulating effect on tail pinch–induced food intake,⁶¹ a model to reliably and rapidly stimulate food consumption of regular chow.⁶² In view of recent evidence that SST signaling in the basal forebrain may be involved in increasing consumption of high-calorie food,³⁰ it will be relevant to establish whether sst₂ signaling will counteract the stress-related increase of comfort food consumption.^{63, 64}

To investigate downstream signaling pathways involved in the orexigenic effect of central SST several pharmacological studies have been performed. Peripheral injection of the μ-opioid receptor antagonist, naloxone⁶⁵ blocked the orexigenic effect of i.c.v. ODT8-SST in rats.⁴¹ Similarly, in chicks, β-funaltrexamine, another μ antagonist prevented the i.c.v. SST–induced orexigenic effect, while δ or κ antagonists did not.⁶⁶ The interaction between SST and opioid signaling might happen at the cellular level as the sst₂ was shown to heterodimerize with the μ-opioid receptor.⁶⁷ It is to note that functional consequences of this heterodimerization are still to be unraveled.

Moreover, neuropeptide Y (NPY) and orexin signaling are involved in mediating SST’s effect on food intake. I.c.v. injection of the NPY1 receptor antagonist, BIBP-3226⁶⁸ or the orexin type 1 receptor (OX1) antagonist, SB-334867⁶⁹ prevented the food intake–stimulating action of i.c.v. ODT8-SST.^{41, 43} The finding that the sst₂ antagonist, S-406–028 did not block the food intake stimulated by orexin-A⁴³ points towards an SST →sst₂ →OX1 route of signaling. In addition, NPY1 signaling in the Arc was shown to be involved in the downstream mediation of orexin A’s–induced food intake–stimulatory effects.⁷⁰ This was shown by the blunting of the orexigenic effect of orexin-A by an i.c.v. injected NPY1 antagonist.⁷¹ Moreover, there is neuroanatomical evidence that orexin-expressing neurons in the LHA project to NPY positive neurons in the Arc⁷² to activate these neurons.^{73, 74} Taken together, SST is likely to mediate its orexigenic action via an sst₂ → orexin-A-OX1 → NPY-Y1 pathway.

Dipsogenic effect of central SST signaling

In line with the observed orexigenic effect, the sst_2,3,5 agonist, octreotide was also shown to induce water intake. This increase was rapid in onset (within 10 min) and not subsequent to the stimulation of food intake since water intake was increased in the absence of food.^{75, 76} This observation is consistent with a previous study reporting that the dipsogenic response preceded the orexigenic effect.³⁶ The dipsogenic response was also established following i.c.v. injection of ODT8-SST, SST-14, and cortistatin,⁴² another endogenous sst₁₋₅ agonist.⁷⁷ While the endogenous ligands, SST-14 and cortistatin exerted short-lasting effects (10 min), ODT8-SST induced a long-lasting (60 min) dipsogenic response.⁴²

Similar to the orexigenic effect, the stimulation of water intake is likely mediated via the sst₂. This is based on the observation that the selective sst₂ agonist, S-346–011 injected i.c.v. increased water intake to a similar extent as i.c.v. ODT8-SST or cortistatin, while the sst₁ agonist, S-406–062 and the sst₄ agonist, S-315–297 had no effect.⁴² Conversely, i.c.v. injection of the sst₂ antagonist, S-406–028 blocked the dipsogenic effect of the pan-sst agonists, cortistatin and ODT8-SST.⁴² Sst₂ signaling has physiological relevance in the regulation of drinking behavior as the sst₂ antagonist, S-406–028 injected i.c.v. reduced dark phase—the photoperiod when rats usually drink even independently of food intake⁷⁸—water intake by ~50%.⁴² Lastly, SST-14 and cortistatin injected i.c.v. increased water intake to a similar extent observed during the first hours of the dark phase.⁴²

The sst₂-mediated dipsogenic response is likely to involve downstream angiotensin II–angiotensin-1 receptor (AT1) signaling as i.c.v. injected losartan, an AT1 antagonist, prevented the dipsogenic response to i.c.v. ODT8-SST, while an sst₂ antagonist did not alter the angiotensin II–induced dipsogenic response.⁴² In addition, octreotide was shown to stimulate the release of angiotensin I in the rat hypothalamus pointing towards a physiological cascade.⁷⁹

Another study showed the blockage of the dipsogenic response to ODT8-SST by i.c.v. injection of the OX1 antagonist, SB-334867 in the presence of food,⁴³ possibly linking the SST-induced downstream food intake- and water intake–stimulatory effect with orexin signaling. A possible dissociation should be further investigated in the absence of food.

Summary

SST is widely expressed in the brain at the level of the ligands as well as the receptors in specific brain areas regulating feeding behavior. The stimulation of food intake by SST has been described early on, a finding confirmed and expanded by independent groups. The effect is mediated by the sst₂ as assessed using selective agonists and antagonists. Converging evidence points towards the physiological relevance of central Sst–sst₂ signaling to contribute to the orexigenic and dipsogenic responses during the dark phase in rodents. Chemo- and optogenetic approaches along with adeno-associated viral tracing have delineated new SST circuitries in the ventral forebrain, TN and LHA involved in the orexigenic effect and high-calorie food consumption (Fig. 1). The recruitment of these novel SST signaling circuitries warrants further investigations to assess whether they are implicated under stress-related comfort food–seeking conditions.

Figure 1.

Mediation of brain SST’s orexigenic and dipsogenic effects. Abbreviations: ↑, increase; ↓, decrease; =, no change; μ, μ-opioid receptor; Arc, arcuate nucleus; AT1, angiotensin-II receptor 1; i.c.v., intracerebroventricular; Y₁, LHA, lateral hypothalamic area; LS, lateral septum; NPY1, neuropeptide Y receptor 1; OX1, orexin receptor 1; PVN, paraventricular nucleus; SON, supraoptic nucleus; sst₂, somatostatin receptor 2; TN, tuberal nucleus.