Activation Of 5-HT1C Receptors Research Articles

Full-length and truncated Kv1.3 K+ channels are modulated by 5-HT1c receptor activation and independently by PKC.

In T-cells, the Shaker-related gene, Kv1.3 encodes the type n K+ channel, whereas the type l channel is a product of the Shaw. subfamily gene, Kv3.1. Both these genes are also expressed in the brain. We have used the Xenopus oocyte heterologous expression system to study the modulatory effects of serotonin (5-hydroxytryptamine, 5-HT) on both these cloned channels. In oocytes coexpressing the mouse 5-HT1c receptor and mouse Kv1.3 channel, addition of 100 nM 5-HT causes a complete and sustained suppression of Kv1.3 currents in approximately 20 min. In contrast, 5-HT has no effect on mouse Kv3.1 currents when coexpressed with 5-HT1c receptor. The 5-HT-mediated suppression of Kv1.3 currents proceeds via activation of a pertussis toxin-sensitive G protein and a subsequent rise in intracellular Ca2+, but Ca2+ does not directly block the channel. Protein kinase (PK) C activation is not part of the pathway linking 5-HT1c receptor to Kv1.3 channels. However, phorbol esters independently suppress Kv1.3 currents. Deletion of the first 146 amino acids from the NH2-terminal, containing putative tyrosine kinase and PKA phosphorylation sites, does not alter the time course of 5-HT-mediated suppression of Kv1.3 currents, indicating that these residues are not necessary for modulation. Treatment of oocytes with calmodulin or phosphatase inhibitors does not alter 5-HT-mediated modulation. Collectively, these experiments indicate that the mouse Kv1.3 channel is capable of being modulated by 5-HT via 5-HT1c receptor in a G protein and Ca(2+)-dependent manner, but the subsequent steps in the pathway remain elusive.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of housing, restraint and chronic treatments with mCPP and sertraline on behavioural responses to mCPP.

The effects of pretreatments on behavioural responses to activation of 5-HT1C receptors by m-chlorophenylpiperazine (mCPP) were investigated. The hypo locomotor and anxiogenic effects of mCPP (social interaction test) were influenced neither by previous housing (single versus grouped) nor by restraint (2 h, 24 h previously). In the absence of mCPP, 24 h group housing led to decreased social interaction and the restraint procedure led to significant decreases of feeding and locomotion. The hypophagic effect of mCPP was unaffected by previous restraint. However, chronic pretreatment with mCPP (2.5 mg/kg per day IP x 14) or with the antidepressant 5-HT reuptake inhibitor sertraline (5 mg/kg per day SC x 14) attenuated all three behaviours. The above findings are discussed with respect to published data on effects of pretreatments on responses to the activation of 5-HT1C receptors.

Characterization of functional responses in A9 cells transfected with cloned rat 5-HT1C receptors

Functional responses to stimulation of rat 5-HT1C receptors expressed in A9 cells were studied using whole cell voltage clamp and calcium recording techniques. Stimulation of 5-HT1C receptors evoked outward currents clamped at -50 mV. The outward currents were reduced when GTP was excluded from the intracellular recording solution or when GDP-beta-S was added. 8-Bromo cyclic AMP (5 mmol/l) neither produced an effect per se nor affected the 5-HT-induced outward current in A9 cells, thus excluding cAMP as a second messenger involved in 5-HT1C receptor activation. Phorbol myristic acetate (PMA; 10 mumol/l) did not affect the electrical activity of the transfected A9 cells but reduced the 5-HT-induced current amplitude to 71 +/- 9% of the control value (n = 12). This indicates that activation of protein kinase C does not play a direct role in the 5-HT-induced response in these cells. The 5-HT induced currents mainly involved potassium ions, although a small contribution of chloride ions was also observed. The 5-HT-induced current was inhibited by the K+ channel blocking agents tetraethylammonium (1 mmol/l), apamin (0,5 mumol/l) and 4-aminopyridine (5 mmol/l). The 5-HT-induced currents recorded at -50 mV were unaffected by removal of extracellular calcium, but inclusion of the calcium chelator BAPTA (5 mmol/l) in the intracellular solutions abolished the current. Measurement with the calcium indicator Fluo-3 revealed a 5-HT-induced increase in intracellular calcium which was not affected by removal of extracellular calcium but declined after repeated stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholecystokinin-induced satiety depends on activation of 5-HT1C receptors

To investigate the dependence of the satiating action of cholecystokinin on serotonergic function in rats, we examined the effects of systemic pretreatment with serotonin (5-HT) antagonists of varying selectivity for 5-HT receptor subtypes on suppression of food intake induced by systemic administration of cholecystokinin octapeptide (CCK-8). Mianserin, a 5-HT1C/2-selective antagonist, significantly attenuated the satiating action of CCK-8. Ketanserin, a 5-HT2 antagonist, and three 5-HT3 antagonists, MDL-72222, ICS 205-930, and ondansetron, however, had no effect on the satiating action of CCK-8. These results demonstrate that the satiating action of exogenous CCK depends on activation of 5-HT1 (probably 5-HT1C) receptors and that activation of 5-HT2 or 5-HT3 receptors is not required.

Effects of repeated mild stress and two antidepressant treatments on the behavioral response to 5HT1C receptor activation in rats

This study investigated the possible involvement of 5HT1C receptors in the development of depressive states and in the mode of action of antidepressants. The effects of repeated unpredictable mild stress (a regimen known to induce an anhedonic state in the rat) and of chronic administration of either of two recognized antidepressant treatments (sleep deprivation or inhibition of monoamine oxidase type A) in rats were studied on a 5HT1C receptor initiated response, i.e. mCPP-induced penile erection. A 3-week period of repeated, but unpredictable exposure to mild stressors induced a shift to the left of the dose-response curve for mCPP-induced penile erection. In contrast, 72-h REM sleep deprivation resulted in a shift to the right of the mCPP dose-response curve and 10-day administration of the monoamine oxidase type A inhibitor moclobemide (20 mg/kg IP bid) also resulted in a decreased number of mCPP-induced penile erections. These findings support the hypothesis that neuronal activities initiated via 5HT1C receptor stimulation may play a role in the pathophysiology and treatment of depression.

Serotonin agonists increase transferrin levels via activation of 5-HT1C receptors in choroid plexus epithelium

Choroid plexus epithelial cells are enriched in mRNA for proteins such as the iron carrier transferrin, which acts as a trophic factor in the brain. Choroid plexus epithelial cells also have a high density of 5-HT1C receptors linked to activation of the phosphoinositide (PI) hydrolysis second messenger system. The present studies show that the 5-HT1C/5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) potently increases PI hydrolysis and the levels of transferrin in primary cultures of rat choroid plexus epithelial cells. These effects are blocked by the 5-HT1C/5-HT2 receptor antagonists mesulergine and mianserin, but not by the 5-HT2 receptor-selective antagonist spiperone. Similarly, mesulergine and mianserin, but not spiperone, block the increases in transferrin levels and PI hydrolysis elicited by 5-carboxamidotryptamine (5-CT), a 5-HT1 receptor-selective agonist, and by serotonin. We conclude, therefore, that 5-HT1C receptor activation in the choroid plexus leads to an increase in the production of transferrin. By promoting transferrin synthesis in the choroid plexus, 5-HT may indirectly influence brain development and differentiation.

Activation of 5-HT1C-receptors suppresses excessive wheel running induced by semi-starvation in the rat.

Male Wistar rats were housed in cages linked to running wheels and fed on a schedule designed to reduce their body weight by 20-30%. During this period of semi-starvation the rats increased their daily running wheel activity (RWA) by up to 30 km/day. RWA could be kept at this level provided that body weight was kept constant. Different serotonin receptor (5-HT) agonists and antagonists were tested for their effects on RWA and it was found that RWA could be suppressed only by agonists with high affinity for the 5-HT1C receptor (TFMPP, mCPP, DOI and quipazine). Serotonin receptor agonists, which do not pass the blood-brain barrier, and 5-HT itself had no effect on RWA. The inhibitory effect of the agonists on RWA was prevented by pretreatment with antagonists that also had high affinity for 5-HT1C receptors (mianserin, metergoline and mesulergine). From these results we conclude that semi-starvation-induced hyperactivity can be blocked by 5-HT1C agonists. Furthermore we suggest that the animal model presented in this study might be a useful tool for in vivo studies on selective 5-HT1C receptor activation.

Behavioral pharmacology of antagonists at 5-HT 2/5-HT 1C receptors

The possible implication of 5-HT2 receptors in CNS disorders such as schizophrenia, anxiety and depression suggests that 5-HT2 antagonists may be useful in the treatment of these disorders. The present review examines behavioral procedures used to characterize 5-HT2 antagonist properties of compounds and behavioral models of clinical activity in which 5-HT2 antagonists have been reported to be active. The pharmacological profile of 5-HT2 receptors in part resembles that of 5-HT1C receptors. Responses that have been proposed to involve the activation of 5-HT1C receptors are examined for their usefulness to detect 5-HT1C antagonist properties of compounds; these responses would help to differentiate 5-HT2 from 5-HT1C antagonist activity.

Choroid plexus epithelial cells in primary culture: a model of 5HT1C receptor activation by hallucinogenic drugs.

Behavioral, electrophysiological and biochemical evidence suggest that the 5HT2 receptor plays a role in the action of hallucinogenic agents. Considering the structural and functional similarities between the 5HT2 and 5HT1C receptors, we hypothesized that the 5HT1C receptor may also be an important site of action of hallucinogens. The present manuscript evaluates this hypothesis by examining the properties of hallucinogens in the phenalkylamine and indolealkylamine classes at 5HT1C receptors. Epithelial cells isolated from the rat choroid plexus have a high density of 5HT1C receptors linked to phosphoinositide hydrolysis. Comparison of the actions of drugs in cultured cells and whole choroid plexus confirmed that the cell culture system can serve as an in vitro model of 5HT1C receptor activation. 2,5-Dimethoxy-4-bromoamphetamine (DOB), 2,5-dimethoxy-4-methylamphetamine (DOM), 2,5-dimethoxy-4-iodoamphetamine (DOI) and 3,4-methylenedioxyamphetamine (MDA) were evaluated. The rank order of potency to activate 5HT1C receptors [(-)DOB greater than (+/-) DOI greater than (+)DOB greater than (-)DOM much greater than (-)MDA greater than (+) MDA] was consistent with the rank order of effective behavioral doses in rats and humans. The indolealkylamine hallucinogen, 5-methoxy-N,N-dimethyltryptamine was also a 5HT1C receptor agonist, as is the primary amine, 5-methoxytryptamine. These data, combined with previous studies showing that (+)LSD potently activates 5HT1C receptors, suggest that future investigations of the mechanism of action of hallucinogens should consider the role of 5HT1C receptors in addition to the more commonly investigated 5HT2 receptors.

The antimigraine drugs ergotamine and dihydroergotamine are potent 5‐HT1C receptor agonists in piglet choroid plexus

1. Fozard & Gray (1989) proposed that migraine is mediated by stimulation of 5-HT1C receptors. We have examined the interaction of two effective anti-migraine agents, ergotamine and dihydroergotamine (DHE), with these receptors. Binding (inhibition of labelling by [3H]-mesulergine) and agonist activity (phosphoinositide hydrolysis) were measured in piglet choroid plexus, a tissue rich in 5-HT1C receptors. 2. The pKD for [3H]-mesulergine binding was 8.4. Ergotamine and DHE both inhibited [3H]-mesulergine binding with a pKD of 7.1. This was similar to the potency of m-chlorophenylpiperazine (m-CPP) (pKD 7.4) and rather less than that of 5-hydroxytryptamine (5-HT) (pKD 8.1). 3. Both ergotamine and DHE were full agonists (pEC50S 7.5 and 7.6 respectively) with potencies similar to that of 5-HT (pEC50 7.7) and greater than that of m-CPP (pEC50 7.1). Mesulergine 10(-7) M produced near-parallel rightward shifts of the concentration-response curves for all these agents of 1.8-2.2 log units, consistent with an action of the agonists at the same receptor. 4. There was no effect of prazosin, spiperone, mepyramine or atropine on the phosphoinositide hydrolysis induced by ergotamine, ruling out an action via alpha 1-adrenoceptors, 5-HT2, histamine H1, or muscarinic receptors. 5. It is concluded that, together with 5-HT, ergotamine and DHE are the most potent 5-HT1C agonists reported so far. These findings do not support the theory that 5-HT1C receptor activation causes migraine.

5-HT2 receptor antagonists and migraine therapy

5-Hydroxytryptamine (5-HT; serotonin) has been implicated in the pathophysiology of migraine, and several drugs with potent 5-HT2 receptor blocking activity (methysergide, pizotifen, cyproheptadine and mianserin) have been recognized as being clinically effective in migraine prophylaxis, although the selective 5-HT2 receptor antagonist ketanserin (the principal agent used to identify 5-HT2 receptor-mediated actions) seems to be ineffective in migraine. Pizotifen is the most widely used 5-HT2 receptor antagonist in migraine prophylaxis, because of its superior efficacy compared with cyproheptadine, and because the incidence and severity of adverse effects with pizotifen is lower compared with methysergide and mianserin. These agents have additional antagonistic effects at histamine H1, muscarinic cholinergic, alpha 1-adrenergic, alpha 2-adrenergic and dopamine receptors, but drugs which are selective for these non-5-HT receptors appear to be of no benefit in migraine. Actions mediated by 5-HT2 receptors which could be of relevance to migraine comprise cranial vasoconstriction, increased cranial capillary permeability and platelet aggregation, and some central nervous system effects and neuroendocrine functions. Although pizotifen, cyproheptadine and mianserin are considered to be relatively specific for 5-HT2 receptors, these agents and methysergide all share a high affinity for 5-HT1C binding sites; ketanserin, however, has little affinity for these sites, thus activation of 5-HT1C receptors may be an important step in the pathogenesis of migraine. It is not yet known which 5-HT1C receptor-mediated actions of 5-HT are relevant to migraine, but some behavioural actions and cranial vasodilatation via release of endothelium-derived relaxing factor may be involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Anxiogenic-like effect of infusing 1-(3-chlorophenyl) piperazine (mCPP) into the hippocampus

1-(3-Chlorophenyl) piperazine (mCPP) was previously shown to have an anxiogenic-like effect, i.e., it decreased total interaction in a rat social interaction test. Evidence indicated mediation by activation of 5-HT1C receptors with an ED50 of approximately 500 micrograms/kg IP (Kennett et al. 1989). A comparable effect is now shown on infusing 4 micrograms of the drug ICV or infusing 0.5 microgram into the hippocampus. Both responses were dose dependent. Infusion of 1 microgram mCPP into the amygdala had no effect. None of the above treatments significantly reduced locomotion. Results are consistent with the postulated role of the hippocampus in anxiety.

5-HT1C receptors in the serotonergic control of periaqueductal gray induced aversion in rats

The functional role of brain 5-HT and 5-HT receptor subtypes in periaqueductal gray (PAG) induced aversion has been investigated in rats. Antiaversive effects were found with the serotonin agonists TFMPP, mCPP and DOI but not with RU 24969 which was found to facilitate PAG aversion. The first three serotonin agonists share potent 5-HT1C activity while RU 24969 differs with a high 5-HT1A activity. Proaversive effects were found with the mixed 5-HT1C/5-HT2 antagonists cyproheptadine and ritanserin; this effect was already reported for the mixed 5-HT1C/5-HT2 antagonists metergoline and mianserin and is opposite to the effects of the selective 5-HT2 antagonists ketanserin, pirenperone, trazodone and spiperone. The antiaversive effects of mCPP (1 mg/kg) could be prevented by pretreatment of the animals with mianserin (1 and 10 mg/kg). These results suggest that 5-HT1C receptors play an important role in the serotonergic control of PAG aversion. 5-HT1C receptor activation seems to mediate antiaversive effects whereas acute 5-HT1C receptor blockade appears to facilitate PAG aversion. Functional interactions take place between several receptor types in the in vivo control of PAG aversion, where 5-HT1C receptors appear to play a predominant function.