Benzamide derivatives provide evidence for the involvement of a 5-HT4 receptor type in the mechanism of action of serotonin in frog adrenocortical cells

Functional Dimerization of Serotonin Receptors: Role in Health and Depressive Disorders

Understanding the neurobiological underpinnings of depressive disorder constitutes a pressing challenge in the fields of psychiatry and neurobiology. Depression represents one of the most prevalent forms of mental and behavioral disorders globally. Alterations in dimerization capacity can influence the functional characteristics of serotonin receptors and may constitute a contributing factor to the onset of depressive disorders. The objective of this review is to consolidate the current understanding of interactions within the 5-HT receptor family and between 5-HT receptors and members of other receptor families. Furthermore, it aims to elucidate the role of such complexes in depressive disorders and delineate the mechanisms through which antidepressants exert their effects.

Translating the promise of 5HT4 receptor agonists for the treatment of depression

Animal experimental studies suggest that 5-HT4 receptor activation holds promise as a novel target for the treatment of depression and cognitive impairment. 5-HT4 receptors are post-synaptic receptors that are located in striatal and limbic areas known to be involved in cognition and mood. Consistent with this, 5-HT4 receptor agonists produce rapid antidepressant effects in a number of animal models of depression, and pro-cognitive effects in tasks of learning and memory. These effects are accompanied by molecular changes, such as the increased expression of neuroplasticity-related proteins that are typical of clinically useful antidepressant drugs. Intriguingly, these antidepressant-like effects have a fast onset of their action, raising the possibility that 5-HT4 receptor agonists may be a particularly useful augmentation strategy in the early stages of SSRI treatment. Until recently, the translation of these effects to humans has been challenging. Here, we review the evidence from animal studies that the 5-HT4 receptor is a promising target for the treatment of depression and cognitive disorders, and outline a potential pathway for the efficient and cost-effective translation of these effects into humans and, ultimately, to the clinic.

Synthesis and structure-activity relationship studies in serotonin 5-HT4 receptor ligands based on a benzo[de][2,6]naphthridine scaffold

A small series of serotonin 5-HT4 receptor ligands has been designed from flexible 2-methoxyquinoline compounds 7a,b by applying the conformational constraint approach. Ligands 7a,b and the corresponding conformationally constrained analogues 8a-g were synthesized and their interactions with the 5-HT4 receptor were examined by measuring both binding affinity and the ability to promote or inhibit receptor-G protein coupling. Ester derivative 7a and conformationally constrained compound 8b were demonstrated to be the most interesting compounds showing a nanomolar 5-HT4R affinity similar to that shown by reference ligands cisapride (1) and RS-23,597-190 (4). The result was rationalized by docking studies in term of high similarity in the binding modalities of flexible 7a and conformationally constrained 8b. The intrinsic efficacy of some selected ligands was determined by evaluating the receptor-G protein coupling and the results obtained demonstrated that the nature and the position of substituents play a critical role in the interaction of these ligands with their receptor.

Serotonin directly stimulates cortisol secretion from the interrenals in goldfish

While serotonin (5-HT) can stimulate the hypothalamic-pituitary-interrenal stress axis in fish, the specific site(s) of 5-HT action are poorly understood. In this study, goldfish (Carassius auratus) were injected intraperitoneally with either saline or the 5-HT1A/7 receptor agonist 8-OH-DPAT at a dose of 100 or 400 μg/kg body weight and sampled 1.5 and 8 h post-injection. Relative to unhandled controls, the saline and 100 μg/kg 8-OH-DPAT treatments elicited similar transient 5- to 7-fold increases in plasma cortisol and the 400 μg/kg 8-OH-DPAT dosage resulted in a sustained 16-fold increase in cortisol levels. Although the 5-HT1A receptor is expressed in the brain preoptic area (POA), the pituitary and the head kidney, neither the saline nor the 8-OH-DPAT treatments affected the mRNA abundance of POA corticotropin-releasing factor and pituitary pro-opiomelanocortin or plasma adrenocorticotropic hormone (ACTH) levels. To assess the direct actions of 5-HT on cortisol secretion relative to those of ACTH, head kidney tissue were superfused with 10(-7)M 5-HT, ACTH or a combined 5-HT/ACTH treatment. Overall, the ACTH and 5-HT/ACTH treatments resulted in higher peak cortisol and total cortisol release than in the 5-HT treatment but the response time to peak cortisol release was shorter in the combined treatment than in either the 5-HT or ACTH alone treatments. Both 8-OH-DPAT and cisapride, a 5-HT4 receptor agonist, also stimulated cortisol release in vitro and their actions were reversed by selective 5-HT1A and 5-HT4 receptor antagonists, respectively. Finally, double-labeling with anti-tyrosine hydroxylase and anti-5-HT revealed that the chromaffin cells of the head kidney contain 5-HT. Thus, in goldfish, 5-HT can directly stimulate cortisol secretion from the interrenals via multiple 5-HT receptor subtypes and the chromaffin cells may be involved in the paracrine regulation of cortisol secretion via 5-HT.

Dogmas and controversies in the handling of nitrogenous wastes:5-HT2-like receptors are involved in triggering pulsatile urea excretion in the gulf toadfish,Opsanus beta

When injected arterially, serotonin (5-hydroxytryptamine; 5-HT) has been shown to elicit naturally sized urea pulse events in the gulf toadfish, Opsanus beta. The goal of the present study was to determine which 5-HT receptor(s) was involved in mediating this serotonergic stimulation of the pulsatile excretion mechanism. Toadfish were surgically implanted with caudal arterial catheters and intraperitoneal catheters and injected with either 8-OH-DPAT (1 μmol kg–1), a selective 5-HT1A receptor agonist, α-methyl-5-HT (1 μmol kg–1), a 5-HT2 receptor agonist, or ketanserin, a 5-HT2 receptor antagonist (0.01, 0.1, 1 and 10 μmol kg–1) plus α-methyl-5-HT. 8-OH-DPAT injection did not mediate an increase in urea excretion, ruling out the involvement of 5-HT1A receptors in pulsatile excretion. However, within 5 min,α-methyl-5-HT injection caused an increase in the excretion of urea in>95% (N=27) of the fish injected, with an average pulse size of 652±102 μmol N kg–1 (N=26). Withα-methyl-5-HT injection there was no corresponding increase in ammonia or [3H]PEG 4000 permeability. Urea pulses elicited byα-methyl-5-HT were inhibited in a dose-dependent fashion by the 5-HT2 receptor antagonist ketanserin, which at low doses caused a significant inhibition of pulse size and at higher doses significantly inhibited the occurrence of pulsatile excretion altogether. However, neither 8-OH-DPAT nor α-methyl 5-HT injection had an effect on plasma cortisol or plasma urea concentrations. These findings suggest the involvement of a 5-HT2-like receptor in the regulation of pulsatile urea excretion.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

Localization and role of serotonin in the adrenal gland of Podarcis sicula (Reptilia, Lacertidae)

The occurrence of serotonin (5-hydroxytryptamine; 5-HT) in the chromaffin cells of Podarcis sicula adrenal gland was demonstrated by immunocytochemical techniques: ABC and immunogold methods. At LM and EM levels, antiserum against 5-HT revealed serotonin immunoreactivity prevalently in noradrenalin (NA) cells, on and around secretory vesicles; adrenalin (A) cells appeared scarcely stained. The role of serotonin in the regulation of adrenal gland activity was studied in vivo using LM and EM techniques coupled to a specific radioimmunoassay for adrenocorticotropic hormone (ACTH) and corticosterone. 5-HT (0.7 mg/100 g body wt)/day for 4 days increased ACTH and corticosterone release; at LM and EM level clear signs of stimulation in the steroidogenic tissue were observed, as evidenced by the variations of lipid/cytoplasm ratio. In the chromaffin tissue, LM observations evidenced a variation of the numeric NA/A cell ratio; at EM level, chromaffin tissue showed intermediate cells with A, NA, and very clear granules with granular elements. The occurrence of these cells might be the result of a process of resynthesis following serotonin-stimulated catecholamine release. These data suggested that serotonin might be involved in the modulation of Podarcis pituitary-adrenal axis, and act as a paracrine factor to modulate corticosteroid production.

Role of 5-HT in the regulation of the brain-pituitary-adrenal axis: effects of 5-HT on adrenocortical cells

Serotonin (5-HT) plays a pivotal role in the regulation of the brain-pituitary-adrenal axis. In particular, 5-HT has been shown to control the activity of hypothalamic CRF neurons and pituitary corticotrope cells through activation of 5-HT1A and (or) 5-HT2A/2C receptor subtypes. 5-HT, acting through 5-HT2 receptors, can also trigger the renin-angiotensin system by stimulating renin secretion and consequently can enhance aldosterone production. At the adrenal level, 5-HT produced locally stimulates the secretory activity of adrenocortical cells through a paracrine mode of communication. The presence of 5-HT in the adrenal gland has been demonstrated immunohistochemically and biochemically in various species. In the frog, rat, and pig adrenal gland, 5-HT is synthesized by chromaffin cells, while in the mouse adrenal cortex, 5-HT is contained in nerve fibers. In man, 5-HT is present in perivascular mast cells. In vivo and in vitro studies have shown that 5-HT stimulates corticosteroid secretion in various species (including human). The type of receptor involved in the mechanism of action of 5-HT differs between the various species. In frogs and humans, the stimulatory effect of 5-HT on adrenocortical cells is mediated through a 5-HT4 receptor subtype positively coupled to adenylyl cyclase and calcium influx. In the rat, the effect of 5-HT on aldosterone secretion is mediated via activation of 5-HT7 receptors. Clinical studies indicate that 5-HT4 receptor agonists stimulate aldosterone secretion in healthy volunteers and in patients with corticotropic insufficiency and primary hyperaldosteronism. Local serotonergic control of corticosteroid production may be involved in the physiological control of the activity of the adrenal cortex as well as in the pathophysiology of cortisol and aldosterone disorders.Key words: HPA axis, renin-angiotensin system, adrenal gland, corticosteroid secretion, serotonergic receptors.

Rat glomerulosa cells express functional 5-HT7 receptors

It has been previously demonstrated that serotonin (5-HT) is a potent stimulator of aldosterone secretion in amphibians and mammals. The aim of the present study was to characterize the type of serotonergic receptor involved in the action of 5-HT on rat glomerulosa cells. The effects of 10 serotonergic receptor agonists and 12 receptor antagonists on aldosterone secretion from perifused rat adrenocortical slices were investigated. Correlation analysis between the potencies of the different compounds in our model and those previously reported for transfected 5-HT receptors showed that the rat adrenal 5-HT receptor exhibits the pharmacological profile of the 5-HT7 receptor. RT-PCR amplification with specific primers for the 5-HT7 receptor confirmed the presence of 5-HT7 receptor mRNA in the adrenal cortex. Western blot analysis using antibodies against the 5-HT7 receptor revealed the occurrence of a protein with an apparent molecular mass of 66 kDa in the zona glomerulosa. In glomerulosa cells, 5-HT induced a concentration-dependent increase of cAMP formation. These data demonstrate that rat adrenal glomerulosa cells express functional 5-HT7 receptors positively coupled to adenylyl cyclase.

Selective serotonin reuptake inhibitors and neuroendocrine function

Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are effective drugs for the treatment of several neuropsychiatric disorders associated with reduced serotonergic function. Serotonergic neurons play an important role in the regulation of neuroendocrine function. This review will discuss the acute and chronic effects of SSRIs on neuroendocrine function. Acute administration of SSRIs increases the secretion of several hormones, but chronic treatment with SSRIs does not alter basal blood levels of hormones. However, adaptive changes are induced by long-term treatment with SSRIs in serotonergic, noradrenergic and peptidergic neural function. These adaptive changes, particularly in the function of specific post-synaptic receptor systems, can be examined from altered adrenocorticotrophic hormone (ACTH), cortisol, oxytocin, vasopressin, prolactin, growth hormone (GH) and renin responses to challenges with specific agonists. Neuroendocrine challenge tests both in experimental animals and in humans indicate that chronic SSRIs produce an increase in serotonergic terminal function, accompanied by desensitization of post-synaptic 5-HT1A receptor-mediated ACTH, cortisol, GH and oxytocin responses, and by supersensitivity of post-synaptic 5-HT2A (and/or 5-HT2C) receptor-mediated secretion of hormones. Chronic exposure to SSRIs does not alter the neuroendocrine stress-response and produces inconsistent changes in alpha2 adrenoceptor-mediated GH secretion. Overall, the effects of SSRIs on neuroendocrine function are dependent on adaptive changes in specific neurotransmitter systems that regulate the secretion of specific hormones.

Pharmacological and molecular characterization of 5-hydroxytryptamine(7) receptors in the rat adrenal gland

Serotonin (5-hydroxytryptamine; 5-HT) is a potent stimulator of aldosterone secretion in the rat adrenal gland but the type of receptor involved in the mechanism of action of 5-HT remains unknown. The aim of the present study was to determine the pharmacological profile and to clone the receptor responsible for the corticotropic effect of 5-HT in rat glomerulosa cells. A series of 10 serotonergic receptor agonists and 12 receptor antagonists was used to characterize the receptor mediating the effect of 5-HT on aldosterone secretion from perifused rat adrenocortical slices. Correlation analysis between the potencies of the different compounds in our model and those previously reported for various recombinant 5-HT receptors showed that the rat adrenal 5-HT receptor exhibits the same pharmacological profile as the 5-HT(7) receptor transiently expressed in COS-7 cells (r = 0.82 for agonists, p <.05; r = 0.83 for antagonists, p <.01). Polymerase chain reaction with specific primers revealed the expression of 5-HT(7) receptor mRNA in the rat adrenal gland. Cloning of the polymerase chain reaction product confirmed that the amplified DNA corresponded to the 5-HT(7) receptor cDNA sequence. Western blot analysis showed the presence of a protein with an apparent molecular mass of 66 kDa in the adrenal cortex but not in the medulla. Taken together, these data demonstrate that the rat adrenal glomerulosa expresses functional 5-HT(7) receptors. Rat glomerulosa cells will thus provide a robust and sensitive bioassay for future studies on native 5-HT(7) receptors.

Nitrogen-containing heteroalicycles with serotonin receptor binding affinity: development of gastroprokinetic and antiemetic agents

To obtain gastroprokinetic agents with more potent and selective activity than metoclopramide and cisapride, a series of N-(4-benzyl-2-morpholinylmethyl)benzamides were designed and prepared. Their synthesis and structure-activity relationships were described. As a result, mosapride was selected as a promising candidate for potent gastroprokinetic activity with selective 5-HT4 receptor agonistic activity. As an extension to this project, the novel benzamide and the carboxamide derivatives having 1-benzyl-4-methylhexahydro-1,4-diazepine ring in the amine moiety were prepared and evaluated for 5-HT3 receptor antagonistic activity. DAT-582 was identified as an antiemetic agent in cancer chemotherapy. The asymmetric synthesis of DAT-582 and the SAR studies were briefly reviewed. In further modifications of the N-(1-benzyl-4-methylhexahydro-1,4-diazepin-6-yl)benzamides, the novel nicotinamides with 1-ethyl-4-methylhexahydro-1,4-diazepin ring were found to have potent 5-HT3 and dopamine D2 and D3 receptor antagonistic activities and to show weak central nervous system depression and extrapyramidal syndrome. After extensive SARs, AS-8112 was selected as a broad antiemetic agent.

Effect of prolonged administration of the serotonin4 (5-HT4) receptor agonist cisapride on aldosterone secretion in healthy volunteers

In man, serotonin (5-HT) has been shown to stimulate aldosterone secretion through activation of 5-HT4 receptors, In particular, we have observed that oral administration of a single dose of the 5-HT4 receptor agonist cisapride (10 mg) induces a 5-fold increase in plasma aldosterone levels in healthy volunteers. Surprisingly, the usual disorders associated with hyperaldosteronism, i.e. hypertension and hypokalemia, have never been reported during chronic treatment with cisapride. In the present study, we have investigated the effect of prolonged oral administration of cisapride (10 mg, 3 times/day during 7 days) on aldosterone secretion in 12 healthy volunteers, in a simple blind fashion versus placebo. On day 1 of the treatment, cisapride induced a significant increase in plasma aldosterone levels (PAL) which returned to the values observed after placebo treatment within 10 hrs. On days 2 and 3, PAL were similar in cisapride- and placebo-treated subjects. Urinary aldosterone, kalemia and reninemia were not influenced by cisapride during the 7 days of the treatment. The present study shows that cisapride only exerts a transient stimulatory effect on aldosterone secretion in healthy volunteers. These data explain why long-term administration of 5-HT4 agonists does not affect blood pressure in man. They also indicate that prolonged stimulation of adrenal 5-HT4 receptors in vivo yields to a rapid desensitization phenomenon, as previously observed in vitro.

Pharmacological profile of serotonergic receptors in the adrenal gland

The secretory activity of the adrenal gland is mainly regulated by peptidergic hormones (ACTH, angiotensin II) and ions. However, there is now increasing evidence that local factors, including neuropeptides and neurotransmitters, can also participate in the control of adrenocortical cells. In particular, serotonin (5-HT), produced by adrenochromaffin cells in frog and rat as well as by mast cells in the adrenal gland of rat and human, stimulates corticosteroid secretion. In both frog and human adrenal gland, the benzamide derivative (R,S)-zacopride induces a robust increase in corticosteroid release suggesting that the effect of 5-HT on steroidogenesis is mediated through activation of 5-HT4 receptors. In contrast, in rat, the stimulatory effect of 5-HT on aldosterone secretion is clearly not mediated by 5-HT4 receptors. In all three species, incubation of adrenocortical fragments with 5-HT induces a significant increase in cAMP formation. Our data suggest that 5-HT, released within the adrenal cortex, may act as a paracrine factor to stimulate steroid secretion. Although the corticotropic effect of 5-HT has been conserved from amphibians to primates, the type of receptors involved in the action of 5-HT markedly differs across species.

Effect of cisapride, a serotonin-4 receptor agonist, on aldosterone secretion: absence of age-related change

Twenty healthy volunteers (age ranging from 19 to 72) were studied for their aldosterone response to oral administration of 10 mg cisapride, a serotonin-4 receptor agonist. The participants were studied after minimum of 4-hour ambulation (high basal renin) and a repeated test after overnight bed rest (low basal renin). Oral administration of cisapride caused a two-fold increase in the plasma aldosterone levels (182.6 +/- 28.0% in high renin state and 203.4 +/- 38.6% in low renin state; P = 0.659 between the means of two states) without affecting the plasma renin levels, Scatterplot of percentile aldosterone increment against age distribution demonstrated that the R-square were very low (0.039 for high renin and 0.009 for low renin state). Our data suggest that the age does not alter the serotonergic mediated effect on aldosterone secretion. The cisapride effect is also independent of the different basal renin states. Cisapride, as a single oral dose, could serve as a simple test to assess the functional capacity of aldosterone secretion.

Regulation of plasma aldosterone levels by metoclopramide: a reappraisal of its mechanism from dopaminergic antagonism to serotonergic agonism

It has been thought, since the late 1970s, that dopamine exerts a tonic suppression of plasma aldosterone levels in human subjects. This action, however, had not been established directly using dopamine and dopamine mimetic drugs, which do not, in fact, affect the aldosterone levels. Rather, the conclusion was arrived at indirectly, based on the increase in aldosterone levels seen with dopamine receptor blockers; metoclopramide in particular, considered at the time of its discovery in the 1960s to be a new generation dopamine antagonist. However, metoclopramide is not a pure drug and in fact, shows intermediate affinity at certain serotonin receptor subtypes. Studies have been recently carried out in human subjects on the role of serotonergic transmission in mediating the metoclopramide as an aldosterone secretagogue effect. Here we briefly review this work and attempt to reassess the action of metoclopramide as an aldosterone secretagogue, from dopamine D2 antagonism to serotonin 5-HT4 partial agonism.

5-HT4 receptors improve social olfactory memory in the rat

Serotonin (5-HT) is involved in a large variety of physiological functions and it appears now that it could play a role in cognitive processes through the activation of 5-HT4 receptors. The present study was conducted to determine the effect of BIMU1, a mixed 5-HT4 agonist/5-HT3 antagonist on social olfactory recognition in rats, a behaviour test which has previously been shown to access short-term memory and to be sensitive to cholinergic drugs. This test is based on the investigation of an unfamiliar juvenile by an adult rat during two distinct 5-min presentations. At a 30-min delay after each presentation adults recognized the juvenile, whereas after a 2-hr delay all the adults had forgotten it. When administered intraperitoneally immediately after the first presentation, BIMU1 (10 mg/kg) enhanced short-term memory (i.e. recognition of the juvenile after a 2-hr delay). Ondansetron (10 and 100 micrograms/kg injected intraperitoneally), a 5-HT3 antagonist, had no significant effect on this form of memory. The effect of BIMU1 was antagonized by intraperitoneal injection of GR 125487, a very selective and potent 5-HT4 antagonist. The antagonistic effect was obtained at 1 and 10 mg/kg of GR 125487, but not at 0.1 mg/kg. It is certainly a specific effect on brain 5-HT4 receptors, since we determined a brain concentration of GR 125487 equal to 3.8 x 10(-7) M after the intraperitoneal injection of 10 mg/kg of this drug. This GR 125487 concentration is certainly sufficient to occupy all the 5-HT4 brain receptors (Kd = 10(-10) M) but not to occupy 5-HT3 receptors (Kd > 10(-6) M). The 5-HT4 specificity of the blockade by GR 125487 is further demonstrated by the fact that a 10-fold lower dose of GR 125487 (1 mg/kg) is also effective to inhibit the BIMU1 effect.

Serotonin increases interleukin-6 release and decreases tumor necrosis factor release from rat adrenal zona glomerulosa cells in vitro

Interleukin-6 (IL-6) and tumor necrosis factor (TNF) are secreted by rat adrenal zona glomerulosa cells. Serotonin increases the release of aldosterone, corti-costerone, and cortisol from the adrenal cortex. Therefore, the effects of serotonin on IL-6 and TNF release from rat adrenal zona glomerulosa cells were investigated. Cultures of rat adrenal zona glomerulosa cells were enzymatically prepared and cultured for 4-6 d. The cells were then exposed to serum-free RPMl-1640 medium containing vehicle (RPMl medium alone), serotonin, and/or endotoxin, interleukin-1β, or adrenocorticotrophic hormone (ACTH). Following a 5-h incubation, medium was removed from the cells, and IL-6 and TNF content of this medium determined with bioassays. Serotonin (1-1000 nM) increased basal IL-6 release from zona glomerulosa cells, but inhibited basal TNF release from these cells. Endotoxin and interleukin-1β (IL-1β) increased IL-6 and TNF release from zona glomerulosa cells. Serotonin potentiated IL-6 release stimulated by endotoxin and IL-1β, but inhibited TNF release stimulated by these agents. Serotonin potentiated ACTH-stimulated IL-6 release. Serotonin had no effect on IL-6 release from rat anterior pituitary cells. Because IL-6, TNF, and serotonin modify the release of aldosterone and glucocorticoids from adrenal cells, the stimulatory effects of serotonin on aldosterone and glucocorticoid release may be mediated in part by the effects of serotonin on IL-6 and TNF release from adrenal cells.

Pharmacological comparison between [3H]GR 113808 binding sites and functional 5-HT4 receptors in neurons

5-HT4 receptors positively coupled to adenylyl cyclase and possessing unique pharmacological properties were first described in mouse colliculi neurons using functional studies. The recent introduction of a radiolabeled 5-HT4 receptor antagonist, [3H]GR 113808 [1-[2-(methylsulphonylamino)ethyl]4-piperidinyl]methyl-1-methyl-in dole-3 carboxylate] having high specificity and affinity allowed the pharmacological comparison between the specific binding sites identified with this compound and the functional 5-HT4 receptors in the same preparation, the colliculi neurons. We show here that [3H]GR 113808 binding is saturable in this preparation and reveals a homogeneous population of sites with a pKd value of 9.5 +/- 0.2 and a Bmax of 75 +/- 23 fmol/mg protein. Seventeen agonists and six antagonists with molecules structurally related either to indoles, benzamides or benzimidazolones and previously known as 5-HT4 receptor ligands, were tested for their ability to compete with [3H]GR 113808 binding sites and to stimulate or inhibit 5-HT-stimulated adenylyl cyclase activity. Highly significant correlations were obtained between the affinities of either agonists or antagonists for [3H]GR 113808 binding sites and their potencies for functional 5-HT4 receptors (r = 0.87 and 0.99, respectively). In addition, we also found good correlations between the Kd of several 5-HT4 receptor ligands determined in cell membranes of mouse colliculi neurons and their Kd determined in previous studies in guinea-pig striatum (0.95) and in human caudate (0.97). [3H]GR 113808 binding studies demonstrated that the 50% decrease in 5-HT-stimulated cAMP accumulation which followed a 5 min exposure period with 5-HT (10 microM) was not accompanied by any significant decrease in the number of binding sites. Longer exposure periods with 5-HT resulted in a decrease in [3H]GR 113808 binding sites which started to be significant after 30 min.

Preparation and in vitro pharmacology of 5-HT4 receptor ligands. Partial agonism and antagonism of metoclopramide analogous benzoic esters

Alicyclic ester analogues of the gastroprokinetic benzamide metoclopramide (1) and its ester congener SDZ 205557 (2), a 5-HT4 receptor antagonist, were prepared by O-alkylation of 4-amino-5-chloro-2-methoxybenzoate with N-(2-chloroethyl) substituted alicyclic amines. The bromo and iodo analogue of compound 13b (2-(1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoate) were obtained by halogenation of dechloro-13b with N-halogenated succinimides. The series was evaluated in functional in vitro assays with regard to affinity for serotoninergic 5-HT4, 5-HT3 and muscarinic M3 receptors. The affinities for 5-HT3 and M3 receptors were below 6.0 (pKB or pA2). On 5-HT4 receptors in guinea-pig ileal longitudinal muscle and rat oesophagus, the majority of compounds revealed partial 5-HT4 receptor agonism susceptible to blockade by SDZ 205557, a reference 5-HT4 receptor antagonist (pKB = 7.25-7.73 (guinea-pig ileum) and 7.09-7.43 (rat oesophagus)). The relative agonist potency was in the range of 5-303% (5-HT: 100%). Compound 13b and its bromo analogue 17 were the most potent esters of the series. The enantiomers of 13g ((R)- and (S)-2-(2-methyl-1-piperidinyl)ethyl 4-amino-5-chloro-2-methoxybenzoate) interacted stereoselectively with 5-HT4 receptors and displayed enantiomeric potency ratios (R)/(S) of 4.3-8.7. There was an excellent correlation between (a) antagonist affinity on guinea-pig ileum and rat oesophagus, (b) relative agonist potency on guinea-pig ileum and rat oesophagus, and (c) between antagonist affinity and relative agonist potency within each assay (r2 > 0.91). The new compounds may serve as academic tools in evaluating the functional role of 5-HT4 receptors. The selective partial 5-HT4 receptor agonists presented in this paper may be useful to restore physiological motility and secretion in the gut with reduced or absent propensity to elicit tachycardia and desensitization of the intestinal target receptor.

Immunohistochemical localization of bioactive peptides and amines associated with the chromaffin tissue of five species of fish

Biogenic peptides and amines associated with the chromaffin tissue in Atlantic cod (Gadus morhua), rainbow trout (Oncorhynchus mykiss), European eel (Anguilla anguilla), spiny dogfish (Squalus acanthias) and Atlantic hagfish (Myxine glutinosa) were identified utilizing immunohistochemical techniques. Within the posterior cardinal vein (PCV) in cod, trout and eel, a subpopulation of chromaffin cells displayed immunoreactivity to tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (D beta H) but not to phenylethanolamine-N-methyltransferase (PNMT). TH-like immunoreactivity was observed within cells in hagfish hearts. Nerve fibres displaying vasoactive intestinal peptide (VIP) immunoreactivity and pituitary adenylyl cyclase activating peptide (PACAP) immunoreactivity innervated cod, trout and eel chromaffin cells. In eel, neuropeptide Y (NPY)-like and peptide YY (PYY)-like immunoreactivity was located within cells in the PCV, including chromaffin cells. Serotonin-like immunoreactivity was observed within eel and cod chromaffin cells and in hagfish hearts. In the dogfish axillary bodies, nerves displaying TH-like, VIP-like, PACAP-like, substance P-like and galanin-like immunoreactivity were observed. These results are compared with those of other vertebrates, and potential roles for these substances in the control of catecholamine release are suggested.

Effect of a series of 5-HT4 receptor agonists and antagonists on steroid secretion by the adrenal gland in vitro

We have previously shown that serotonin (5-hydroxytryptamine, 5-HT) stimulate corticosterone and aldosterone secretion from perifused frog adrenal gland in vitro through activation of 5-HT4 receptors. In the present study, we have used this model to investigate the effect of newly discovered 5-HT4 receptor agonists and antagonists on corticosteroid secretion. Serotonin, the benzamide derivatives (R,S)-zacopride ((R,S)-4-amino-N-(1- azabicyclo[2.2.2]oct-3-yl)-5-chloro-2-methoxybenzamide, HCI) and its enantiomers, the azabicycloalkyl benzimidazole derivatives BIMU 1 (endo-N- (8-methyl-8-azabicyclo-[3.2.1]oct-3-yl)-2,3-dihydro-3-ethyl-2-oxo- 1H-benzimidazole-1-carboxamide, HCl) and BIMU 8 (endo-N-(8-methyl-8- azabicyclo-[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl)ethyl-2-oxo-1H- benzimidazole-1-carboxamide, HCl) were all capable of enhancing corticosterone and aldosterone secretion in a dose-dependent manner. Serotonin was the most potent stimulator of steroidogenesis (EC50 = 1.5 x 10(-7) M) while the potency of the benzamide and the benzimidazolone derivatives was approximately 10 times lower. The rank order of efficacy of the different 5-HT4 receptor agonists was: (S)-zacopride > BIMU 8 = (R,S)-zacopride > BIMU 1 = (R)-zacopride = 5-HT. The stimulatory effects of 5-HT and the benzimidazolone derivatives on corticosteroid secretion were not additive, suggesting that they activated the same receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a serotonin receptor endogenous to frog melanophores

The response of a cell line of Xenopus laevis melanophores to serotonin was examined. Serotonin increased intracellular levels of cAMP and induced pigment dispersion in the cells. The responses depended on both the concentration of serotonin applied and on the time for which the cells were exposed to serotonin. Using a recently described, microtiter-plate-based bioassay, a series of serotonin receptor ligands were evaluated as agonists or antagonists at the melanophore serotonin receptor. The pharmacological profile suggests the presence of a receptor which shares some properties with but appears different from other previously described serotonin receptors.

The guinea-pig distal colon--a sensitive preparation for the investigation of 5-HT4 receptor-mediated contractions

1. Experiments were designed to characterize pharmacologically the contractile responses to 5-hydroxytryptamine (5-HT) in the guinea-pig isolated distal colon longitudinal muscle-myenteric plexus preparation (LMMP). 2. In the presence of methiothepin (100 nM) and granisetron (1 microM), 5-HT (10 pM-10 nM) produced concentration-dependent contractile responses of the guinea-pig distal colon LMMP, with a pEC50 of 9.2 +/- 0.08. 3. Responses to 5-HT were mimicked by a series of tryptamine analogues, with the following rank order of potency; 5-HT > 5-MeOT >> 5-CT > tryptamine > 2-Me-5-HT. All were found to be full agonists. 4. Responses to 5-HT were also mimicked by a series of substituted benzamide analogues. Their rank order of potency was 5-HT > renzapride > cisapride > (S)-zacopride > (R)-zacopride > metoclopramide. All were full agonists relative to 5-HT. 5. The benzimidazolone derivatives, BIMU 1 and BIMU 8 were approximately equipotent partial agonists (intrinsic activities of 0.8 +/- 0.07 and 0.5 +/- 0.08 respectively) in the guinea-pig distal colon. 6. Tropisetron produced a rightward displacement of the 5-HT concentration-effect curve, yielding an apparent pA2 of 6.4 +/- 0.1. The slope of the Schild plot (1.3 +/- 0.1) was significantly greater than unity. 7. SDZ 205,557 produced a concentration-dependent shift to the right of the 5-HT concentration-response curve, yielding an estimated pA2 of 7.8 +/- 0.1 and a slope which did not significantly deviate from unity. SDZ 205 557 produced similar pKB estimates (7.3-7.9) when tested against 5-MeOT,renzapride and 5-CT, indicating a common site of action.8. The pharmacological profile of the 5-HT-evoked contractions of the guinea-pig distal colon LMMPare consistent with activity at the 5-HT4 receptor. Furthermore, of the models of this receptor described in the literature, the guinea-pig distal colon appears to be the most sensitive model to date, making it a useful tool in the investigation of 5-HT4 receptor-mediated events.

The action of SDZ 205,557 at 5-hydroxytryptamine (5-HT3 and 5-HT4) receptors

1. The interaction of the novel antagonist, SDZ 205,557 (2-methoxy-4-amino-5-chloro benzoic acid 2-(diethylamino) ethyl ester), at 5-HT3 and 5-HT4 receptors has been assessed in vitro and in vivo. 2. In guinea-pig hippocampus and in the presence of 0.4 microM 5-carboxamidotryptamine, 5-HT4-mediated stimulation of adenylyl cyclase was competitively antagonized by SDZ 205,557, with a pA2 value of 7.5, and a Schild slope of 0.81. In rat carbachol-contracted oesophagus, 5-HT4-receptor mediated relaxations were surmountably antagonized by SDZ 205,557 with a similar pA2 value (7.3). This value was agonist-independent with the exception of (R)-zacopride, against which a significantly lower value (6.4) was observed. 3. In functional studies of 5-HT3 receptors, SDZ 205,557 exhibited an affinity of 6.2 in guinea-pig ileum compared with 6.9 at binding sites labelled by [3H]-quipazine in NG108-15 cells. In the anaesthetized, vagotomized micropig, SDZ 205,557 produced only a transient blockade of 5-HT4-mediated tachycardia. This contrasted with tropisetron, which was active for over 60 min after administration. The half-lives for the inhibitory responses of SDZ 205,557 and tropisetron were 23 and 116 min, respectively. 4. In conclusion, SDZ 205,557 has similar affinity for 5-HT3 and 5-HT4 receptors. The apparent selectivity observed in guinea-pig is due to the atypical nature of the 5-HT3 receptor in this species. The short duration of action of this novel antagonist may complicate its use in vivo. SDZ 205,557 should, therefore, be used with appropriate caution in studies defining the 5-HT4 receptor.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Agonist action of indole derivatives at 5-HT1-like, 5-HT3, and 5HT4 receptors in vitro

1. The potency of indole analogues has been studied, in vitro, at 5-hydroxytryptamine4 (5-HT4) receptors mediating contractions of guinea-pig ileum and relaxation of rat oesophagus. These have been compared to other 5-HT receptors in canine saphenous vein (5-HT1-like), rabbit aorta (5-HT2), and guinea-pig ileum (5-HT3). 2. At receptors mediating 5-HT4 responses in ileum and oesophagus, the rank orders of potency were similar. These rank orders differed from those observed at 5-HT1-like, 5-HT2, and 5-HT3 receptors. In particular, 5-hydroxy N,N, dimethyltryptamine but not 5-methoxy N,N, dimethyltryptamine acted as agonists at 5-HT4 receptors. At 5-HT1-like, 5-HT2 and 5-HT3 receptors these compounds were both active. 3. The 5-HT receptors mediating contractions of canine cephalic vein exhibited a rank order profile similar to that observed at receptors mediating contractions of canine saphenous vein, suggesting stimulation of a 5-HT1-like receptor. 4. The rank order of potency of the substituted indoles differed at 5-HT receptors mediating responses in canine saphenous vein, rabbit aorta and guinea-pig ileum (determined in the presence of 5-methoxytryptamine to desensitize 5-HT4 receptors), suggesting the presence of three distinct receptors. Indeed, at 5-HT3 receptors in the ileum, only three agonists (5-HT, 2-methyl-5-HT and 5-hydroxy N,N, dimethyltryptamine) elicited a response, while all remaining compounds were inactive. 5. It is concluded that rank orders of indole potency can prove useful in the delineation of 5-HT subtypes and together with differential antagonist affinities support the existence of four 5-HT receptor subtypes.

Serotonin-induced stimulation of cortisol secretion from human adrenocortical tissue is mediated through activation of a serotonin4 receptor subtype

The occurrence of serotonin in the human adrenal gland was demonstrated both by immuno-histochemical and biochemical approaches. Using specific polyclonal antibodies to serotonin, the presence of numerous immunoreactive cells was revealed by means of the peroxidase-antiperoxidase technique. These cells exhibited the morphological characteristics of mast cells. Combination of high performance liquid chromatography and electrochemical detection showed the presence of substantial amounts of both serotonin and its metabolite 5-hydroxyindolacetic acid in adrenocortical extracts. The role of serotonin in the regulation of steroidogenesis from human adrenocortical slices was studied in vitro using a perifusion system technique coupled to a specific radioimmunoassay for cortisol. Graded doses of serotonin (from 10(-8) M to 3 x 10(-7) M) increased cortisol production in a dose-dependent manner. Prolonged exposure of adrenal fragments to serotonin (10(-7) M) induced a biphasic response, i.e. a rapid and transient increase in cortisol secretion followed by a plateau phase, suggesting the existence of a desensitization phenomenon. The stimulatory effect of serotonin (10(-7) M) was not altered during infusion of the serotonin1 and/or serotonin2 receptor antagonists methysergide (10(-6) M) and ketanserin (10(-6) M), respectively. In contrast, ICS 205 930 (10(-6) M), a non-selective serotonin3/serotonin4 antagonist, totally abolished the response of adrenal slices to serotonin (10(-7) M). The benzamide derivative zacopride, considered as a serotonin4 agonist, induced a robust stimulation of cortisol secretion. In addition, the corticotropic effects of serotonin (10(-7) M) and zacopride (10(-6) M) were not additive. Incubation of adrenocortical fragments with zacopride (10(-6) M) or serotonin (10(-6) M) caused a significant increase in cAMP formation. Taken together, these data suggest that serotonin, locally released by intra-adrenal mast-like cells, may act as a paracrine factor to stimulate cortisol secretion in man. Our results also indicate that serotonin-induced corticosteroid production is mediated through activation of a serotonin4 receptor subtype positively coupled to adenylate cyclase.

Interactions between vasotocin and other corticotropic factors on the frog adrenal gland

The adrenocortical cells of the amphibian interrenal (adrenal) gland are controlled by multiple factors including neuropeptides and classical neurotransmitters. In particular, it has recently been shown that vasotocin (AVT), the amphibian counterpart of vasopressin, is a potent stimulator of frog corticosteroidogenesis. In the present study, we have investigated the possible interactions between AVT and other regulatory factors on frog interrenal tissue. When AVT (10(-9) M) and serotonin (10(-6) M) were infused together, a strict addition of the individual effects was observed. Similar results were obtained with concomitant infusion of AVT and vasoactive intestinal peptide or AVT and ACTH. In contrast, when AVT (10(-9) M) and acetylcholine (5 x 10(-5) M) were added together, the increase in corticosteroid secretion was less than additive. Dopamine induced a significant reduction of AVT-evoked stimulation of corticosterone production. These results indicate that regulatory peptides or classical neurotransmitters which participate in the control of adrenal steroidogenesis may interact on their target cell to modulate the activity of their congeners.

Serotonin synthesis in adrenochromaffin cells

The inter-renal (adrenal) gland of amphibians is composed of chromaffin and steroidogenic cells which can interact through a paracrine mode of communication. We have previously shown that serotonin is present in secretory granules of frog adrenochromaffin cells; concurrently, we have demonstrated that serotonin is a potent stimulator of corticosterone and aldosterone secretion by adrenocortical cells. The aim of the present study was to determine the origin of the amine contained in frog chromaffin cells. Using 3H-labelled tryptophan as a precursor, we observed the formation of substantial amounts of serotonin and its metabolite 5-hydroxyindoleacetic acid by frog inter-renal slices. Newly synthesized serotonin was secreted into the incubation medium and the release process was enhanced by depolarizing concentrations of KCl. Fluoxetine, and inhibitor of serotonin uptake, caused an increase of 3H-labelled serotonin in the incubation medium, suggesting that the indoleamine was taken up again by adrenal chromaffin cells. The capacity of the frog inter-renal gland to synthesize serotonin was also demonstrated by incubating inter-renal slices with non-labelled tryptophan or 5-hydroxytryptophan. In these conditions, we observed that the rate of synthesis was higher when 5-hydroxytryptophan was used as a a precursor, rather than tryptophan. Taken together, these results indicate that chromaffin cells, which have the capacity for synthesizing and releasing serotonin, behave like authentic serotonergic paraneurons. As far as is known, these data provide the first evidence for the occurrence of tryptophan-5-hydroxylase activity within the adrenal gland.

The novel antidepressant, tianeptine, reduces stress-evoked stimulation of the hypothalamo-pituitary-adrenal axis

The possible effect of tianeptine, a novel antidepressant agent, on the neuroendocrine response to stress was investigated in adult male rats. Tube restraint stress for 30 min induced a marked increase of plasma ACTH and corticosterone. A single i.p. injection of tianeptine (10 mg/kg), 120 min before stress caused a significant decrease of ACTH and corticosterone levels. In order to investigate the kinetics of the effect of tianeptine, the drug was injected at various times (from 15 min to 12 h) before restraint stress. The inhibitory effect of tianeptine on stress-induced elevations of plasma ACTH and corticosterone occurred from 1 to 3 h after the injection. Administration of increasing doses of tianeptine revealed that only the highest doses (10 and 20 mg/kg) had a significant effect on stress-evoked stimulation of ACTH and corticosterone secretion. These results show that the antidepressant, tianeptine, reduces the activation of the hypothalamo-pituitary-adrenal (HPA) axis induced by restraint stress. Since depressed patients generally exhibit an elevated cortisol level, the present data suggest that part of the therapeutic properties of tianeptine could be accounted for by the effect of this antidepressant to modulate the activity of the HPA axis.