Influence of intracisternal injection of somatostatin analog receptor subtypes 2, 3 and 5 on gastric acid secretion in conscious rats

Central actions of somatostatin-28 and oligosomatostatin agonists to prevent components of the endocrine, autonomic and visceral responses to stress through interaction with different somatostatin receptor subtypes

Somatostatin was discovered four decades ago and since then its physiological role has been extensively investigated, first in relation with its inhibitory effect on growth hormone secretion but soon it expanded to extrapituitary actions influencing various stressresponsive systems. Somatostatin is expressed in distinct brain nuclei and binds to five somatostatin receptor subtypes which are also widely expressed in the brain with a distinct distribution pattern. The last few years witnessed the discovery of highly selective peptide somatostatin receptor agonists and antagonists representing valuable tools to delineate the respective pathways of somatostatin signaling. Here we review the centrally mediated actions of somatostatin and related selective somatostatin receptor subtype agonists to influence the endocrine, autonomic, and visceral components of the stress response and basal behavior as well as thermogenesis.

Somatostatin modulates generation of inspiratory rhythms and determines asphyxia survival

Breathing and the activity of its generator (the pre-Bötzinger complex; pre-BötC) are highly regulated functions. Among neuromodulators of breathing, somatostatin (SST) is unique: it is synthesized by a subset of glutamatergic pre-BötC neurons, but acts as an inhibitory neuromodulator. Moreover, SST regulates breathing both in normoxic and in hypoxic conditions. Although it has been implicated in the neuromodulation of breathing, neither the locus of SST modulation, nor the receptor subtypes involved have been identified. In this study, we aimed to fill in these blanks by characterizing the SST-induced regulation of inspiratory rhythm generation in vitro and in vivo. We found that both endogenous and exogenous SST depress all preBötC-generated rhythms. While SST abolishes sighs, it also decreases the frequency and increases the regularity of eupnea and gasping. Pharmacological experiments showed that SST modulates inspiratory rhythm generation by activating SST receptor type-2, whose mRNA is abundantly expressed in the pre-Bötzinger complex. In vivo, blockade of SST receptor type-2 reduces gasping amplitude and consequently, it precludes auto-resuscitation after asphyxia. Based on our findings, we suggest that SST functions as an inhibitory neuromodulator released by excitatory respiratory neurons when they become overactivated in order to stabilize breathing rhythmicity in normoxic and hypoxic conditions.

Intracisternal antisense oligodeoxynucleotides to the thyrotropin-releasing hormone receptor blocked vagal-dependent stimulation of gastric emptying induced by acute cold in rats

Cold exposure increases TRH gene expression in hypothalamic and raphe nuclei and results in a vagal activation of gastric function. We investigated the role of medullary TRH receptors in cold (4-6 C, 90 min)-induced stimulation of gastric motor function in fasted conscious rats using intracisternal injections of TRH receptor (TRHr) antisense oligodeoxynucleotides (100 microg twice, -48 and -24 h). The gastric emptying of a methyl-cellulose solution was assessed by the phenol red method. TRH (0.1 microg) or the somatostatin subtype 5-preferring analog, BIM-23052 (1 microg), injected intracisternally increased basal gastric emptying by 34% and 47%, respectively. TRHr antisense, which had no effect on basal emptying, blocked TRH action but did not influence that of BIM-23052. Cold exposure increased gastric emptying by 64%, and the response was inhibited by vagotomy, atropine (0.1 mg/kg, i.p.), and TRHr antisense (intracisternally). Saline or mismatched oligodeoxynucleotides, injected intracisternally under similar conditions, did not alter the enhanced gastric emptying induced by cold or intracisternal injection of TRH or BIM-23052. These results indicate that TRH receptor activation in the brain stem mediates acute cold-induced vagal cholinergic stimulation of gastric transit, and that medullary TRH may play a role in the autonomic visceral responses to acute cold.

PYY-preferring receptor in the dorsal vagal complex and its involvement in PYY stimulation of gastric acid secretion in rats

1. Microinjection of peptide YY (PYY, 7-46 pmol) into the dorsal vagal complex (DVC) stimulated gastric acid secretion in urethane-anaesthetized rats. Using a variety of neuropeptide Y (NPY) and PYY derivatives, we characterized the pharmacological profile of the receptor mediating the acid secretory response to PYY. 2. [Pro34]rat(r)/porcine(p)PYY and [Pro34]human(h)PYY (23-117 pmol), microinjected unilaterally into the DVC resulted in a similar maximal increase in net acid secretion reaching 68+/-11 and 89+/-31 micromol 90 min(-1) respectively. 3. Rat/hNPY and pNPY (47 pmol) microinjected into the DVC induced a similar net gastric acid secretion (27+/-8 and 23+/-8 micromol 90 min(-1) respectively) and a higher dose (116 pmol) tended to reduce the response. 4. Pancreatic polypeptide (PP, 4-46 pmol), [Leu31,Pro34]r/hNPY (47 and 117 pmol) and the Y2 selective agonists, hPYY3-36, pNPY5-36 and PNPY13-36 (25-168 pmol) microinjected into the DVC failed to influence basal gastric acid secretion. 5. The rank order of potency of PYY > or = [Pro34]r/pPYY = [Pro34]hPYY> r/hNPY = pNPY to stimulate gastric acid secretion upon injection into the DVC and the ineffectiveness of PP, [Leu31,Pro34]NPY and C-terminal NPY/PYY fragments suggest that a PYY-preferring receptor subtype may be involved in mediating the stimulating effect.

Intracerebroventricular injection of somatostatin sst5 receptor agonist inhibits gastric acid secretion in rats

Somatostatin and its analogs act in the brain to influence gastric acid secretion. Five different somatostatin receptor subtypes have been characterized (sst1 to sst5). We studied the influence of somatostatin (0.18-0.6 nmol/rat) and selective sst2, sst3 and sst5 receptor ligands on basal gastric acid secretion in conscious rats equipped with chronic gastric and intracerebroventricular (i.c.v.) cannulae. Somatostatin-14 (0.36 nmol/rat), the sst2, sst3 and sst5 receptor agonist, Des-AA1,2,4,5,12,13-[D-Tryp8,D-Cys14]somatostatin (SMS 201-995) (0.18-0.36 nmol/rat) and the sst5 receptor agonist, BIM-23052, (0.8-1.2 nmol/rat) injected i.c.v. inhibited gastric acid secretion. Maximal inhibition reaching 42%, 60% and 42% was induced by somatostatin-14 (0.36 nmol/rat), SMS 201-995 (0.18 nmol/rat) and BIM-23052 (0.8 nmol/rat) respectively. The sst2 receptor agonist, DC 32-87 (0.2-0.8 nmol/rat) and sst3 receptor agonist, BIM-23056 (0.2-1.2 nmol/rat), did not modify gastric acid secretion, except the sst3 receptor agonist at 0.4 nmol/rat which increased acid output at 20 min post-injection. The sst2 receptor agonists (0.4 nmol/rat) co-injected i.c.v. with a subthreshold dose of sst5 (0.4 nmol/rat) inhibited gastric acid secretion. These results show that i.c.v. injection of somatostatin-14 inhibits basal gastric acid secretion in conscious rats through an action on sst5 receptor subtype which can be potentiated by sst2 receptor subtype.

SMS 201-995-induced stimulation of gastric acid secretion via the dorsal vagal complex and inhibition via the hypothalamus in anaesthetized rats

1. SMS 201-995, a somatostatin analogue which interacts with highest affinities at somatostatin receptor subtypes 5 > 2 > or = 3, was microinjected into selective brain sites and its influence on pentagastrin (10 micrograms kg-1 h-1, i.v.)-stimulated gastric acid secretion was investigated in rats anaesthetized with urethane. Gastric acid secretion was measured by flushing the stomach with saline through a gastric cannula every 10 min. 2. SMS 201-995 microinjected into the dorsal vagal complex (DVC, 7, 15, 30 and 60 ng) dose-dependently increased pentagastrin-stimulated gastric acid secretion. The peak acid response was reached within 20 min and returned to basal level 50 min post-injection. SMA 201-995 (30 ng) microinjected into the surrounding area or the central amygdala did not modify pentagastrin-stimulated acid secretion. 3. SMS 201-995 injected into the lateral ventricle (i.c.v., 100, 200, or 300 ng), paraventricular nucleus (PVN) or lateral hypothalamus (LH) (7.5, 15, or 30 ng) dose-dependently inhibited pentagastrin-stimulated gastric acid secretion. SMS 201-995 (30 ng) microinjected into the area surrounding the PVN or LH did not modify the acid secretion response to pentagastrin. 4. Vagotomy prevented the effects of SMS 201-995 (30 ng) microinjected into the DVC and LH. 5. Spinal cord transection abolished the inhibitory action of SMS 201-995 (30 ng) microinjected into the PVN but not the LH. 6. These results demonstrate that SMS 201-995 acts in the DVC to enhance and in the LH and PVN to inhibit pentagastrin-stimulated gastric acid secretion. The action is mediated through vagal (DVC, LH)or spinal (PVN) pathways. The site specific pattern of acid responses to SMS 201-995 may be linked to the distribution of receptor subtypes at these sites that convey the different biological actions of somatostatin.