Food intake, neuroendocrine and temperature effects of 8-OHDPAT in the rat

Effects of systemic 8-OH-DPAT on the feeding induced by hypothalamic NE infusion

Past research suggests that activating brain serotonin (5-hydroxytryptamine or 5-HT) systems can inhibit feeding induced by activating brain norepinephrine (NE) systems. To explore this interaction more fully, we tested the capacity of the endogenous 5-HT release inhibitor, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), to enhance feeding stimulated by infusing NE into the medial hypothalamus. All experiments were conducted using ad lib-fed adult male rats with indwelling cannulae aimed at the paraventricular nucleus (PVN). In the first study, proven PVN-NE responders were tested for 40-min food intake after receiving 20 nanomoles (nmol) 1-NE or saline in the PVN following subcutaneous (SC) pretreatment with 250 micrograms/kg 8-OH-DPAT or saline. Both drugs produced equivalent, reliable increments in feeding compared to PVN-saline. However, no additivity or synergy was seen when they were combined. Short-term water intake was unaffected by these treatments as was subsequent food or water intake over the next 22 hr. In a second study, additional proven PVN-NE responders were tested under two comparable conditions when 1) the 8-OH-DPAT dose was left at 250 micrograms/kg but the NE dose was lowered to 10 nmol, and 2) the 8-OH-DPAT dose was lowered to 120 micrograms/kg and the NE dose was increased to 40 nmol. In the first case, no reliable feeding was seen in response to either agent alone or combined. In the second case, NE alone enhanced feeding but 8-OH-DPAT did not. The combination of both produced the same enhanced feeding as seen with NE alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of serotonin antagonists on m-chlorophenylpiperazine-mediated responses in normal subjects

The serotonin (5HT) agonist, m-chlorophenylpiperazine (MCPP), has been used as a challenge agent to assess central 5HT receptor sensitivity in normal subjects and patients with panic disorder, obsessive-compulsive disorder, and major depression. Adrenocorticotropin, cortisol, and prolactin responses to MCPP were among the variables measured. MCPP's usefulness as a probe of 5HT receptors, however, hinges on its 5HT selectivity. To address MCPP's selectivity for 5HT, this study tested whether two different 5HT antagonists, methysergide (4 mg p.o.) and metergoline (4 mg p.o.), could block the hormonal and behavioral effects of MCPP (0.5 mg/kg p.o.) in 10 normal male subjects in comparison to placebo. Both 5HT antagonists abolished the prolactin release to MCPP. Metergoline, the antagonist with the more potent 5HT binding affinity, significantly blocked MCPP's effect on cortisol release as compared to placebo, and methysergide showed a nonsignificant trend to that effect. MCPP alone did not have a significant effect on behavioral variables, perhaps explaining why neither 5HT antagonist affected these measures. The findings from this study suggest that both MCPP-induced prolactin release and cortisol release are indeed 5HT-mediated effects.

Role of the adrenal gland in the metabolic and cardiovascular effects of 8-OH-DPAT in the rat

In the present study, the role of the adrenal gland in the hypoinsulinaemic and hyperglycaemic effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) vis-à-vis the cardiovascular effects were examined using adrenalectomized and sham-operated (with intact adrenals) Sprague-Dawley rats. Intravenous administration of 8-OH-DPAT (150 micrograms/kg) into conscious sham-operated animals caused significant increases in basal plasma glucose levels without any change in basal plasma insulin levels whereas glucose-stimulated plasma insulin levels were markedly inhibited. These changes were associated with significant and sustained falls in blood pressure (BP) and heart rate (HR) preceded by transient (less than 5 min) increases only in BP. In adrenalectomized animals, 8-OH-DPAT-mediated initial vasoconstriction, hyperglycaemia and inhibition of evoked plasma insulin levels were abolished. However, in these animals, 8-OH-DPAT produced falls in BP and HR that were comparable to those observed in sham-operated animals. Plasma corticosterone was increased 15 min after 8-OH-DPAT in sham-operated animals whereas it was undetectable in adrenalectomized animals. The data indicate that unlike the effects of 8-OH-DPAT to decrease BP and HR, the initial increase in BP and its effects on the metabolic parameters are clearly dependent on intact adrenals.

Investigation of the mechanism(s) of 8-OH-DPAT-mediated inhibition of plasma insulin in spontaneously hypertensive rats

1. Effects of the prototype selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-dipropylamino)tetralin (8-OH-DPAT), were studied on the glycaemia and insulinaemia in conscious spontaneously hypertensive (SH) rats concurrently with blood pressure (BP) and heart rate (HR); underlying mechanism(s) were investigated in anaesthetized and pithed SH rats and in the perfused rat pancreas. 2. Intravenous (i.v.) injections of 8-OH-DPAT (150 micrograms kg-1, i.v.) into fasted conscious but not anesthetized SH rats increased glycaemia; glucose-stimulated (i.v. glucose tolerance test) plasma insulin levels were significantly inhibited in both cases without significant changes in glucose tolerance. Metabolic changes were associated with prominent decreases in BP and HR. 3. No inhibitory effect of 8-OH-DPAT, 150 micrograms kg-1 i.v., on glucose-stimulated plasma insulin was observed in pithed SH rats; in contrast, clonidine (8 micrograms kg-1 i.v.), produced marked inhibition of insulin levels in association with glucose intolerance. Neither compound decreased BP; rather, pronounced vasopressor effects were observed. 4. In the isolated perfused pancreas of the rat, 8-OH-DPAT, at 10(-8) and 10(-7) M, concentrations known to activate 5-HT1A receptors in vitro, failed to modify glucose-stimulated insulin release. Inhibition (39 +/- 7%) was seen only at a high concentration of 10(-6) M. 5. The present data suggest that like the cardiovascular effects of 8-OH-DPAT, the inhibition of glucose-stimulated insulin release is mediated via the central nervous system. However, it is suggested that different mechanisms are involved in the cardiovascular actions and metabolic effects of 8-OH-DPAT in the SH rat; the latter are likely to reflect a consequence of activation of the hypothalamic-adrenal axis.

Mediation of ACTH and prolactin responses to 5-HTP by 5-HT2 receptors

Serotonin has a facilitatory role in the role of prolactin and adrenocorticotropin (ACTH) secretion. The serotonin precursor 5-hydroxy-L-tryptophan (5-HTP) dose dependently (30-100 mg/kg i.p.) increased plasma prolactin and ACTH in the male rat. Prolactin and ACTH responses to 5-HTP (100 mg/kg) were attenuated by pretreatment with the non-selective 5-HT receptor antagonist, metergoline (0.5 mg/kg), and by the selective 5-HT2 receptor antagonists, ritanserin (0.4 mg/kg), ketanserin (2.5 mg/kg), ICI (5.0 mg/kg) and spiperone (1.0 mg/kg). The 5-HT1 receptor antagonists, propranolol (40 mg/kg) and pindolol (4.0 mg/kg), failed to antagonize the prolactin and ACTH responses to 5-HTP (100 mg/kg), as did the selective 5-HT3 receptor antagonist, BRL 43694 (1.0 mg/kg). The results suggest that the prolactin and ACTH responses to 5-HTP in the male rat are mediated by 5-HT2 receptors.

The effects of gepirone on neuroendocrine function and temperature in humans

The effects of the selective 5-HT1A receptor agonist gepirone (10 and 20 mg orally) on neuroendocrine function and temperature were assessed using a single-blind cross-over design in 12 healthy male volunteers. Gepirone significantly increased plasma levels of ACTH, beta-endorphin, cortisol, prolactin and growth hormone. Following gepirone there was a significant decrease in body temperature and moderate increases in subjective reports of light-headedness, nausea and drowsiness. Our results are consistent with studies in rodents suggesting that 5-HT1A receptor agonists increase ACTH and prolactin secretion and decrease body temperature. Further investigations are needed to determine if the neuroendocrine and temperature effects of gepirone in humans are mediated by 5-HT1A receptors.

5-HT1A Receptors in major depression

The 5-HT(1A) receptor has been studied using quantitative tritium film auto radiography in post-mortem frontal cortex of 15 cases suffering from major depression and controls matched for age, sex, post-mortem delay and storage time. 5-HT(1A) receptor binding in the depressive cohort was not significantly different from control values. No consistent effects of antidepressant medication or mental state at death were seen on the results.

Effects of m-chlorophenylpiperazine in normal subjects: a dose-response study

m-Chlorophenylpiperazine (MCPP), a direct 5HT receptor agonist, was administered orally to 20 normal subjects in two doses (0.25 and 0.5 mg/kg) in a placebo-controlled design. Behavioral responses; ACTH, cortisol, prolactin and MCPP blood level; temperature and pulse rate were measured over a 210-min period after administration of tablets. Non-linear dose-response relationships between MCPP and ACTH, cortisol and prolactin response were found. On the higher dose, a significant increase in the number of physical symptoms was also noted and three subjects (15%) had a panic attack, while one subject (5%) had a panic attack on the lower dose. No effects on other behavioral variables, pulse rate and temperature were found using either dose. These findings attest to the usefulness of MCPP as a challenge agent to assess 5HT receptor hypersensitivity when given at a low oral dose (i.e. around 0.25 mg/kg), and to assess 5HT receptor hyposensitivity when given at higher oral doses (i.e. around 0.5 mg/kg).

Spiroxatrine augments fluoxetine-induced reduction of ethanol intake by the P line of rats

The present study was undertaken to determine if spiroxatrine, a reported 5-HT1A antagonist, could block the attenuating effects of fluoxetine (a 5-HT uptake inhibitor) on voluntary ethanol intake by the selectively bred alcohol-preferring P line of rats. Fluoxetine (10 mg/kg, IP) significantly reduced the intake of 10% ethanol by P rats approximately 50% during the 4-hour period of alcohol availability. Spiroxatrine (4 mg/kg, IP) was without effect on ethanol intake when given alone. However, when given 5 minutes before fluoxetine (10 mg/kg, IP), this dose of spiroxatrine augmented the reduction of ethanol intake to approximately 15% of control values after 4 hours. Similar experiments conducted with 1 mg/kg (IP) 8-hydroxy-2(di-N-propylamino) tetralin (DPAT) demonstrated that this 5-HT1A agonist also enhanced the attenuating effects of fluoxetine on ethanol intake. Likewise, spiroxatrine augmented the DPAT reduction of alcohol intake. Spiroxatrine enhanced the effect of DPAT and fluoxetine on food intake as it did on ethanol intake. The results suggest that spiroxatrine behaved as a partial agonist and/or modulator and not as an antagonist at 5-HT1A receptors under the present experimental conditions.

Selective activation of 5HT1A receptors induces lower lip retraction in the rat

The induction of lower lip retraction (LLR) by serotonergic (5HT) compounds and antagonism of LLR by compounds acting via a variety of receptor systems was investigated. LLR could be induced by subcutaneous injection of 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), buspirone, ipsapirone or RU 24969. Inactive were the putative 5HT1B,1C agonist 1-(3'chlorophenyl)-piperazine (mCCP), the 5HT2,1C agonist (dl)-1-(2,5 dimethoxy-4-iodophenyl)-2-aminopropane (DOI), the 5HT reuptake inhibitors citalopram and paroxetine and the 5HT-releasing compounds parachloroamphetamine (PCA) and fenfluramine. 5-Methoxy-N,N-dimethyltryptamine (5-MeODMT) induced lower lip retraction after pretreatment with metergoline, cyproheptadine or ritanserin but not by itself. 8-OH-DPAT-induced LLR could be antagonised by the direct and indirect 5HT agonists mCPP, DOI, 5-MeODMT, PCA, fenfluramine and high doses of paroxetine, but not by the 5HT antagonists metergoline, methysergide, mesulergine, GR38032F, xylamidine or pirenperone. The dopamine agonists apomorphine and pergolide antagonised 8-OH-DPAT-induced LLR, whereas SKF 38393 was weakly active. No significant antagonism was found with the dopamine antagonists haloperidol and spiperone, the alpha 2 agonist clonidine and the alpha 1 antagonist prazosin and the alpha 2 antagonist idazoxan. Also inactive were the antihistaminic mepyramine, the anticholinergic atropine, the opiate antagonist naloxone and the anxiolytic chlordiazepoxide. The results suggest that, in vivo, functional interactions take place between the various 5HT receptors. The hypothesis that lower lip retraction is induced by compounds directly and selectively stimulating 5HT1A receptors is discussed.

Marked increases in plasma catecholamine concentrations precede hypotension and bradycardia caused by 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in conscious rats

Plasma noradrenaline and adrenaline responses to 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), a selective putative 5-HT1A receptor agonist, have been studied in conscious, freely moving rats. Intravenously administered 8-OH-DPAT caused dose-related and sustained increases in plasma noradrenaline (2-fold) and adrenaline (11-fold) concentrations. Neither metergoline pretreatment (0.5 mg kg-1 i.v.) nor splanchnicectomy had any effect on the noradrenaline and adrenaline elevation caused by 8-OH-DPAT (250 micrograms kg-1 i.v.). The catecholamine responses peaked early but were still present during nadirs in blood pressure and heart rate. The discrepancy between plasma catecholamine and cardiovascular changes raises further questions about the mechanism of action of 8-OH-DPAT and supports other evidence suggesting a role for vagus stimulation in the cardiovascular effects caused by this drug.

Effect of selective serotonin (5-HT) agonists and 5-HT2 antagonist on prolactin secretion

The present study was undertaken to determine the involvement of serotonergic 5-HT1 and 5-HT2 receptor subtypes in stimulation of the secretion of prolactin. Several 5-HT agonists were administered, in a dose-response fashion, to conscious rats and the effect on the levels of prolactin in plasma was measured. The 5-HT1A + 5-HT1B agonist RU 24969 (5-methoxy-3[1,2,3,6-tetrahydropyridin-4-yl]-1H-indole succinate) and the 5-HT1 + 5-HT2 agonist MK-212 (6-chloro-2-[1-piperazinyl]pirazine) increased levels of prolactin in plasma in a dose-dependent manner. In contrast, the selective 5-HT1A agonists 8-OH-DPAT (8-hydroxy-2-[di-n-propylamino]tetralin) and ipsapirone (2-[4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl]-1,2-benzisothiazol-3 -(2H) one-1,1-dioxidehydrochloride) did not increase levels of prolactin in plasma at any dose. The 5-HT-releasing drug, fenfluramine, also increased the concentration of prolactin in plasma. Pretreatment with the selective 5-HT2 antagonist, LY53857 (6-methyl-1-[1-methylethyl]ergoline-8-carboxylic acid, 2-hydroxy-1-methyl propyl ester (Z)-2-butenedioate [1:1]), did not significantly diminish an increase in levels of prolactin in plasma, induced by injection of fenfluramine. The antagonist LY53857 inhibited, but did not block the MK-212- and RU 24969-induced increase in the levels of prolactin in plasma. By deduction, these data suggest that 5-HT1B receptors, or as yet undefined 5-HT receptor subtypes may be involved in the stimulation of the secretion of prolactin by endogenously released 5-HT, and that 5-HT2 receptors may play a minor role in the serotonergic regulation of the secretion of prolactin.

Prevention of the analgesic consequences of social defeat in male mice by 5-HT1A anxiolytics, buspirone, gepirone and ipsapirone

Behavioural and pharmacological studies have suggested that anxiety may be an important factor in the initiation of non-opioid analgesia in defeated male mice. In the present study, the effects of three 5-HT1A anxiolytics (buspirone, ipsapirone and gepirone) on basal nociception and defeat analgesia were examined. Results show that the analgesic consequences of social defeat were potently blocked by all three compounds, with a rank-order potency (minimum effective doses) of ipsapirone (0.05 mg/kg) greater than gepirone (0.1 mg/kg) greater than buspirone (0.5 mg/kg). These inhibitory effects on defeat analgesia were observed in the absence of intrinsic activity on basal nociception (tail-flick assay). When administered alone, (-)pindolol produced biphasic effects on defeat analgesia with enhancement at 0.5 mg/kg and inhibition at 5.0 mg/kg. Lower doses of (-)pindolol (0.05 and 0.25 mg/kg) which did not affect defeat analgesia when administered alone, totally blocked the inhibitory effects of ipsapirone (0.5 mg/kg). Data are discussed in relation to the involvement of 5-HT1A receptor mechanisms in this adaptive form of pain inhibition.

Comparative anxiogenic, neuroendocrine, and other physiologic effects of m-chlorophenylpiperazine given intravenously or orally to healthy volunteers

The serotonin agonist m-chlorophenylpiperazine (m-CPP) had greater anxiogenic and other mood and cognitive effects when administered intravenously (0.1 mg/kg) rather than orally (0.5 mg/kg) to healthy subjects. Nonetheless, similar elevations in peak plasma cortisol and prolactin concentrations were obtained with the two dosage regimens, and temperature elevations were greater after oral m-CPP. Plateau phase plasma concentrations of m-CPP at the times of the maximum neuroendocrine responses to intravenous and oral m-CPP were similar. Since all rodent and nonhuman primate studies have used parenterally administered m-CPP, and previous clinical investigations using intravenous rather than oral m-CPP have yielded somewhat discrepant results, our normative data should be useful for comparing results across different human studies and across species.

Differential effects of long-term antidepressant treatments on 8-OHDPAT-induced increases in plasma prolactin and corticosterone in rats

Intravenous administration of 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT) to rats produced increases in plasma prolactin and corticosterone concentrations. Long-term or short-term treatment with the MAO type A inhibiting antidepressant, clorgyline, or tricyclic antidepressants (imipramine and clomipramine), did not change baseline levels of either prolactin or corticosterone. Long-term but not short-term clorgyline treatment attenuated 8-OHDPAT's effect on plasma prolactin but not on corticosterone. On the other hand, long-term but not short-term treatment with clomipramine and to some extent imipramine also, accentuated 8-OHDPAT's effect on plasma prolactin but not on corticosterone. These findings demonstrate that long-term antidepressant treatment in rats produces a differential effect on 8-OHDPAT-induced increases in plasma prolactin and corticosterone, which is consistent with other clinical and animal studies demonstrating a differential effect of long-term antidepressant treatment on two different 5-HT-mediated neuroendocrine functions.