The effect of putative 5-HT1A receptor antagonists on 8-OH-DPAT-induced hypothermia in rats and mice

Exploring the role of 5-HT1A receptors in the regulation of prepulse inhibition in mice: implications for cross-species comparisons

Prepulse inhibition (PPI) is a model of sensorimotor gating, a sensory filtering mechanism which is disrupted in schizophrenia. Here, investigation of the role of the serotonin-1A (5-HT(1A)) receptor in the regulation of PPI in two mouse strains, C57Bl/6 and Balb/c, was used to address findings in the PPI literature on species and mouse strain differences that question the usefulness of PPI as a cross-species preclinical test. Although the full 5-HT(1A) receptor agonist, 8-OH-DPAT, induced markedly different strain-specific responses in PPI, other selective 5-HT(1A) receptor ligands with partial agonist or antagonist activity elicited similar effects across strains. Pretreatment with the serotonin precursor, 5-HTP, to increase serotonergic activity in the brain, unmasked a decrease in PPI caused by 8-OH-DPAT in C57Bl/6 mice. Pretreatment with the serotonin synthesis inhibitor, PCPA, to decrease serotonergic activity in the brain, unmasked an 8-OH-DPAT-induced increase in PPI in this strain. These studies show that the strain-dependent involvement of 5-HT(1A) receptors in PPI can be modulated by the type of 5-HT(1A) ligand used, or increasing or decreasing serotonin levels in the brain. These results help to clarify some of the mouse strain and species differences in PPI regulation and strengthen its usefulness as a cross-species measure of sensorimotor gating.

Serotonin 1A receptor agonist increases species- and region-selective adult CNS proliferation, but not through CNTF

Endogenous ciliary neurotrophic factor (CNTF)(1) regulates neurogenesis of the adult brain in the hippocampal subgranular zone (SGZ)(2) and the subventricular zone (SVZ)(3). We have previously shown that the cAMP-inhibiting D2 dopamine receptor increases neurogenesis by inducing astroglial CNTF expression. Here, we investigated the potential role of CNTF in the proliferative response to pharmacological stimulation of the serotonin 1A (5-HT1A)(4) receptor, which also inhibits cAMP, in adult mice and rats. Like others, we show that systemic treatment with the active R-enantiomer of the 5-HT1A agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)(5) induces proliferation in the SGZ in rats using unbiased stereology of 5-Bromo-2'-deoxyuridine (BrdU)(6) positive nuclei. However, despite the bioactivity of R-8-OH-DPAT, as also shown by a decrease in hippocampal nNOS(7) mRNA levels, it did not increase CNTF mRNA as shown by highly specific quantitative RT-PCR (qPCR)(8). Surprisingly, R-8-OH-DPAT did not cause an increase in SVZ proliferation in rats or in either the SVZ or SGZ of two different strains of mice, C57BL/6J, and 129SvEv, using acute or chronic treatments. There also were no changes in CNTF mRNA, and also not in mice treated with a widely used racemic mixture of 8-OH-DPAT, higher doses or after intracerebral injection, which reduced nNOS. In contrast to the others, we propose that the 5-HT1A receptor might be non-functional in mice with regards to regulating normal neurogenesis and has region-selective activities in rats. These species- and region-specific actions raise important questions about the role of the 5-HT1A receptor in human neurogenesis and its implications for the field of depression.

Effects of genetic background and null mutation of 5-HT1A receptors on basal and stress-induced body temperature: modulation by serotonergic and GABAA-ergic drugs

The stress-induced hyperthermia procedure, in which effects of drugs on basal (T(1)) and stress-induced body temperature (T(2)) are measured, predicts anxiolytic drug effect. Serotonergic drugs alter these responses and here, we studied the role of 5-HT(1A) receptors in stress-induced hyperthermia by using 5-HT(1A) receptor knockout mice. Three strains (129/Sv, Swiss Webster and C57Bl6) were used because genetic background can significantly modulate the null phenotype. We found that GABA-ergic drugs with an anxiolytic profile and stimulate alpha(2) subunit containing GABA(A) receptors, including diazepam and L838,417, result in reduced DeltaT (DeltaT=T(2)-T(1)). The alpha(1) subunit containing GABA(A) receptor was found to be primarily involved in regulation of basal body temperature T(1) and its stimulation can induce hypothermia. In addition, stimulation of 5-HT(1A) receptors by buspirone results in a reduced DeltaT, while stimulation of 5-HT(7) receptors primarily results in hypothermia. The null mutation of 5-HT(1A) receptors resulted in differences in drug-sensitivity that was further modulated by the genetic background. In particular, the null mutation on the SW and C57Bl6 backgrounds resulted in differential diazepam/L838,417 and 5-CT responses respectively. This indicates an interaction between the 5-HT(1A) receptor and genetic background and demonstrates the importance of selecting the background strain in a receptor knockout model.

A set-point model with oscillatory behavior predicts the time course of 8-OH-DPAT-induced hypothermia

Agonists for the 5-hydroxytryptamine (HT)(1A) receptor induce a hypothermic response that is believed to occur by lowering of the body's set-point temperature. We have developed a physiological model that can be used to predict the complex time course of the hypothermic response after administration of 5-HT(1A) agonists to rats. In the model, 5-HT(1A) agonists exert their effect by changing heat loss through a control mechanism with a thermostat signal that is proportional to the difference between measured and set-point temperature. Agonists exert their effect in a direct concentration-dependent manner, with saturation occurring at higher concentrations. On the basis of simulations, it is shown that, depending on the concentration and the intrinsic efficacy of a 5-HT(1A) agonist, the model shows oscillatory behavior. The model was successfully applied to characterize the complex hypothermic response profiles after administration of the reference 5-HT(1A) agonists R-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) and S-8-OH-DPAT. This analysis revealed that the observed difference in effect vs. time profile for these two reference agonists could be explained by a difference in in vivo intrinsic efficacy.

Enhanced sensitivity of postsynaptic serotonin-1A receptors in rats and mice with high trait aggression

Individual differences in aggressive behaviour have been linked to variability in central serotonergic activity, both in humans and animals. A previous experiment in mice, selectively bred for high or low levels of aggression, showed an up-regulation of postsynaptic serotonin-1A (5-HT(1A)) receptors, both in receptor binding and in mRNA levels, in the aggressive line [Brain Res 736 (1996) 338]. The aim of this experiment was to study whether similar differences in 5-HT(1A) receptors exist in individuals from a random-bred rat strain, varying in aggressiveness. In addition, because little is known about the functional consequences of these receptor differences, a response mediated via postsynaptic 5-HT(1A) receptors (i.e., hypothermia) was studied both in the selection lines of mice and in the randomly bred rats. The difference in receptor binding, as demonstrated in mice previously, could not be shown in rats. However, both in rats and mice, the hypothermic response to the 5-HT(1A) agonist alnespirone was larger in aggressive individuals. So, in the rat strain as well as in the mouse lines, there is, to a greater or lesser extent, an enhanced sensitivity of postsynaptic 5-HT(1A) receptors in aggressive individuals. This could be a compensatory up-regulation induced by a lower basal 5-HT neurotransmission, which is in agreement with the serotonin deficiency hypothesis of aggression.

Effect of the repeated administration of (+/-)-3,4-methylenedioxymethamphetamine on the behavioral response of rats to the 5-HT1A receptor agonist (+/-)-8-hydroxy-(di-n-propylamino)tetralin

In this study, we examined the effect of the subcutaneous administration (twice daily for 4 consecutive days) of 3,4-methylenedioxymethamphetamine (MDMA, 20 mg/kg) or saline (1 ml/kg) on the response of rats to the behavioral effects of the 5-HT1A agonist (+/-)-8-hydroxy-(di-n-propylamino)tetralin (8-OH-DPAT) 30 days after the last saline or MDMA treatment. The reciprocal forepaw treading elicited by the 0.25-mg/kg dose of 8-OH-DPAT was significantly lower in animals pretreated with MDMA compared to vehicle-treated animals. However, there were no significant differences between the MDMA- and vehicle-treated animals in flat body posture, locomotor activity and rectal temperature measured after the systemic administration of 8-OH-DPAT. Overall, our results suggest that the depletion of 5-HT levels by the repeated administration of MDMA does not produce a supersensitivity of central 5-HT1A receptors in the rat as determined via our approach.

Anxiolytic effects of flesinoxan in the stress-induced hyperthermia paradigm in singly-housed mice are 5-HT1A receptor mediated

In the stress-induced hyperthermia paradigm in singly-housed male mice, two sequential rectal temperature measurements reveal the basal temperature (T1) and, 10 min later, an enhanced body temperature (T2), due to the stress of the first rectal measurement. The difference T2 - T1 (deltaT) is the stress-induced hyperthermia and putatively reflects a stress-induced anxiogenic response. The full 5-HT1A receptor agonist flesinoxan ((+)-enantiomer), its (-)-enantiomer and the racemic mixture reduced stress-induced hyperthermia effects, indicating putative anxiolytic properties. The ratio of their potencies to reduce stress-induced hyperthermia was similar to their potency in receptor binding affinities for 5-HT1A receptors, supporting that the anti-hyperthermia effects are mediated by the 5-HT1A receptor. This was further substantiated when the 5-HT1A receptor antagonists WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexane carboxamine trihydrochloride) and DU 125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide, monomesylate) both were able to antagonize the anti-stress-induced hyperthermia effects of flesinoxan. The stress-induced hyperthermia paradigm in singly-housed mice represents a simple and robust paradigm to measure putative anxiolytic effects of drugs.

Effects of ipsapirone on plasma cortisol and body temperature in major depression

Major depressed patients have been reported to exhibit significantly attenuated hypothermic responses to ipsapirone, a serotonin (5-HT)-1A partial agonist, compared to normal controls. This study further investigated the cortisol and temperature responses to ipsapirone (0.5 mg/kg orally) and placebo in 20 normal volunteers and 12 major depressed patients. Both plasma cortisol and temperature were measured every 30 min before ipsapirone or placebo administration until 180 min post administration. Ipsapirone administration produced a significant increase in plasma cortisol levels as well as hypothermia. Major depressed patients showed significantly blunted ipsapirone-induced cortisol responses compared to normal controls. No significant differences in ipsapirone-induced hypothermic responses were found between major depressed patients and normal controls.

Are the cardiovascular effects and '5-HT syndrome' induced by MDL 73,975 and flesinoxan in the dog mediated by 5-HT1A receptors?

The cardiovascular effects of the 5-HT1A receptor agonists MDL 73,975 (8-[2-(2,3-dihydro-8-methoxy-1,4-benzodoxin-2-yl)methylaminol++ +]-ethyl]-8- azaspiro[4,5]decane-7,9-dione hydrochloride) and flesinoxan (10-300 micrograms/kg subcutaneously, s.c.), the 5-HT1A receptor antagonist NAN 190 (2-[4-[4-(2-methoxyphenyl)-1-piperazinyl]butyl]-1H-isoindole-1,3(2H)- dione,1,2-ethanedioate), and the alpha 1-adrenoceptor antagonist prazosin have been investigated in conscious normotensive and renal hypertensive dogs. In normotensive dogs the increases in heart rate and respiratory rate induced by both agonists were dose-related, as were the decreases in systolic and diastolic blood pressure induced by MDL 73,975. Both compounds caused a dose-related increase in the intensity of the '5-HT syndrome'. After pretreatment with NAN 190 (100 micrograms/kg s.c.) the increases in heart rate, respiratory rate and symptoms of the '5-HT syndrome' were significantly reduced but the decreases in systolic and diastolic pressure were additive. Pretreatment with prazosin (100 micrograms/kg s.c.) antagonized the '5-HT syndrome' and the increase in respiratory rate. Similar responses were evident in renal hypertensive dogs. Tolerance did not develop to the increases in heart rate, respiratory rate and manifestations of the '5-HT syndrome' in normotensive dogs during 5 days of treatment with MDL 73,975 or flesinoxan. In conclusion, MDL 73,975 and flesinoxan induced a 5-HT1A receptor-mediated fall in blood pressure but the changes in heart rate, respiratory rate and the '5-HT syndrome' are probably mediated by alpha 1-adrenoceptors.

Discriminative stimulus effect of flesinoxan: effect of 5-HT1A antagonists and PCPA

Rats were trained to discriminate 0.3 mg/kg (IP) flesinoxan from saline in a standard two-lever operant procedure and thereafter subjected to generalization and antagonism tests with the 5-HT1A receptor agonist ipsapirone and the beta-adrenergic/5-HT1 receptor antagonist pindolol. Ipsapirone (3.0 mg/kg) completely substituted for flesinoxan. Both the flesinoxan (0.3 mg/kg) and the ipsapirone cue (3.0 mg/kg) were dose-dependently blocked by (+/-)-pindolol. In a second group of rats, trained to discriminate 0.5 mg/kg (IP) of flesinoxan from saline, the putative 5-HT1A antagonist NAN-190 (in the dose range of 1.0 to 6.0 mg/kg) partially blocked the cue of flesinoxan. Generalization studies revealed that the flesinoxan cue could not be mimicked by NAN-190 (3.0 mg/kg). Finally, rats were pretreated with the 5-HT depletor parachlorophenylalanine (PCPA) and thereafter tested with the flesinoxan training dose (0.5 mg/kg). PCPA pretreatment did not significantly attenuate the recognition of the flesinoxan cue. The present results are in agreement with previous findings concerning the stimulus effect of flesinoxan and point to a mechanism that involves the activation of 5-HT1A receptors in the brain. Depletion of 5-HT did not significantly affect the stimulus effect of flesinoxan, suggesting that presynaptic 5-HT1A receptors do not play a crucial role in the mechanism underlying the stimulus effect of flesinoxan.

Differential cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, 5-methyl-urapidil and MDL 75,608A in conscious spontaneously hypertensive rats

The effects of intravenous (i.v.) administration of four agonists at central 5-HT1A receptors were investigated and compared. Acute iv injection of 0.1 mg/kg of 8-OH-DPAT induced a decrease in blood pressure both in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The maximal hypotensive effect was observed 15 and 10 min after injection, respectively, but the effect was greater and longer-lasting in the SHR. 8-OH-DPAT significantly decreased heart rate in WKY and, to a lesser extent, in SHR. The i.v. injection of 1 mg/kg of flesinoxan caused a similar fall in blood pressure and heart rate in SHR and WKY. The i.v. administration of 1 mg/kg of 5-methyl-urapidil or MDL 75,608A caused a fall in blood pressure which was significantly more pronounced in SHR than in WKY. 5-methyl-urapidil induced a significant tachycardia in WKY, but had little effect on heart rate in SHR. MDL 75,608A caused a short-lasting tachycardia in SHR and WKY. In conscious SHR, the intracerebroventricular (icv) injection of 10 micrograms of 8-OH-DPAT or 100 micrograms of either flesinoxan or MDL 75,608A caused a decrease in blood pressure and heart rate. The icv injection of 100 micrograms of 5-methyl-urapidil caused only a decrease in blood pressure. Chronic pre-treatment with these compounds, by daily i.v. injection, did not significantly influence the hypotensive or bradycardic effects in an acute experiment. The involvement of alpha 1-adrenoceptors in the effects of these compounds was studied by administering phenylephrine (1 microgram/i.v.) at 5-min intervals before and after the i.v. injection of the experimental compounds. The injection of phenylephrine reproducibly increased blood pressure by 35-40 mm Hg after saline pre-treatment, and these responses were not affected by the i.v. injection of 0.1 mg/kg of either 8-OH-DPAT or 1 mg/kg of flesinoxan. In contrast, the phenylephrine-induced pressor responses were markedly diminished at 5 min after treatment with 1 mg/kg of either 5-methyl-urapidil or MDL 75,608A, but slowly recovered thereafter. These results show that the 5-HT1A receptor agonists 8-OH-DPAT, flesinoxan, 5-methyl-urapidil and MDL 75,608A show antihypertensive properties in conscious SHR after iv or icv injection. However, the mechanism of action of the compounds differs: 8-OH-DPAT and flesinoxan may act predominantly as 5-HT1A receptor agonists, where as 5-methyl-urapidil and MDL 75,608A also seem to have an effect on peripheral alpha 1-adrenoceptors.

Effects of 7-hydroxy-N,N-di-n-propylaminotetralin on behaviour and blood pressure of spontaneously hypertensive rats

The in vivo effects of administration of the putative dopamine D3 receptor agonist 7-hydroxy-N,N-di-n-propylaminotetralin (7-OH-DPAT) were investigated in spontaneously hypertensive rats (SHR) and normotensive wistar-Kyoto controls (WKY). The i.p. injection of 7-OH-DPAT induced hyperactivity in WKY at 10 mg/kg, but only an inhibition of exploratory locomotor activity was observed in SHR at 1 mg/kg. In WKY and SHR with unilateral lesions of the nigrostriatal system, s.c. injection of 0.01-1 mg/kg of 7-OH-DPAT induced dose-dependent contralateral turning behaviour. This response was more pronounced in SHR than in WKY. The s.c. injection of 0.03, but not of 0.01 or 0.1 mg/kg, of 7-OH-DPAT induced yawning in WKY and SHR. The i.v. injection of 0.1 or 1 mg/kg of 7-OH-DPAT induced an immediate rise in blood pressure in both WKY and SHR. Pretreatment with the dopamine receptor antagonist haloperidol partially prevented this pressor response and, in addition, unmasked a late fall in blood pressure in SHR. The s.c. injection of 1 mg/kg of 7-OH-DPAT induced a decrease in body temperature, which was more pronounced in SHR than in WKY. This effect could be inhibited by pretreatment with haloperidol, but a residual hypothermia remained in SHR. These results suggest that 7-OH-DPAT induces a variety of effects in vivo, many of which may be mediated by dopamine D2 receptors or non-dopaminergic receptors. Thus, more selective dopamine D3 receptor agonists or -antagonists are needed to further explore the role of dopamine D3 receptors in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothermia induced by hyperbaric oxygen is not blocked by serotonin antagonists

Exposure to HBO causes hypothermia, bradycardia, head weaving, resting tremor, piloerection, and straub tail in rats. These physiological and behavioral responses can also be evoked by selective activation of serotonin1A (5-HT1A) receptors. The purpose of the current study was to determine if hypothermia caused by HBO is due to increased activation of 5-HT1A receptors. The levels of brain biogenic amines were measured in brain regions of Sprague-Dawley (SD) rats exposed to HBO. Exposure to HBO caused an increase in the levels of 5-hydroxyindoleacetic acid (5-HIAA) in the striatum (92%, p < 0.05) and occipital-temporal cortex (116%, p < 0.05), but not in other brain regions. Exposure to HBO did not change the levels of tryptophan, serotonin (5-HT), other biogenic amines, or their metabolites. It is hypothesized that the Fawn Hood (FH) rat, which is reported to be resistant to hypothermia induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), has an abnormality of 5-HT1A receptor activity. Although the FH rat was more resistant to hypothermia induced by HBO than the SD rat, we were not able to confirm that this rat was resistant to hypothermia induced by 8-OH-DPAT. The 5-HT receptor antagonists, 1-(1H-Indol-4-yloxy)-3-[(1-methylethyl)amino]-2-propanol (Pindolol), 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide (NAN-190), and methysergide, did not block hypothermia induced by HBO in SD rats. A series of control experiments were used to confirm that the antagonists blocked hypothermia induced by serotonin agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin 5-HT1A receptor-mediated hypothermia in mice: absence of spare receptors and rapid induction of tolerance

The mixed 5-hydroxytryptamine1A (5-HT1A) receptor agonist/antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol-[4.5]- decane-7,9-dione (BMY 7378) (5 mg/kg) did not significantly depress body temperature, but pretreatment with BMY 7378 blocked hypothermia induced by the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). In contrast, another partial 5-HT1A agonist, pindolol (10 mg/kg), slightly but significantly depressed body temperature by itself but did not attenuate hypothermia elicited by 8-OH-DPAT. Attempts to identify the synaptic locus of the receptor were unsuccessful because depletion of central serotonin (5-HT) by treatment with para-chlorophenylalanine (PCPA; 3 x 150 mg/kg) did not alter the hypothermic response to 8-OH-DPAT. Partial, irreversible 5-HT1A receptor inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (1 mg/kg) reduced the maximal hypothermic effect of 8-OH-DPAT (to 53% of control) without altering its ED50 (0.96 mg/kg). Analysis of the data indicated a linear relationship between 5-HT1A receptor occupancy and hypothermic response, that is, absence of receptor reserve. When groups of mice were treated with each of five different doses of 8-OH-DPAT (0.04, 0.16, 0.63, 2.5, and 10 mg/kg) 48 h apart, there was a significant reduction in hypothermic response after the second injection, but only at the three highest doses. The results demonstrate that 8-OH-DPAT-induced hypothermia in mice is mediated by a 5-HT1A receptor whose synaptic localization is uncertain but that has no receptor reserve. In addition, tolerance is observed after only a single agonist treatment.

Food intake and rumen motility in dwarf goats. Effects of some serotonin receptor agonists and antagonists

The serotonergic regulation of feeding behaviour has not so far been studied in ruminants. Therefore, the effects of some serotonin (5-HT) receptor agonists and antagonists on food intake and forestomach motility were studied in dwarf goats. Goats ate less food when treated intravenously (IV) with the 5-HT precursor 5-HTP (25 micrograms, 50 micrograms or 100 micrograms kg-1 min-1 over 15 min) than when they were treated with 5-HT (which does not pass the blood-brain barrier) or with saline. Accordingly, IV dexfenfluramine infusions (50 micrograms or 100 micrograms kg-1 min-1 over 15 min), which induces release of brain 5-HT, also led to dose-related reductions in food intake. In contrast, no anorectic effects were observed after IV infusions with the selective 5-HT reuptake inhibitor fluoxetine (100 micrograms kg-1 min-1 over 15 min), the selective 5-HT1A agonist 8-OH-DPAT (0.5 micrograms kg-1 min-1 over 15 min), or eltoprazine (4 or 8 micrograms kg-1 min-1 over 15 min), a mixed 5-HT1A/5HT1B receptor agonist. None of the 5-HT antagonists tested gave any increase in food consumption in this model. Interestingly, the non-selective 5-HT receptor antagonist methysergide (360 micrograms/kg IV) reduced food intake. This effect was most noticeable at 3 h after injection. The 5-HT3 receptor antagonist ondansetron (IV 10 micrograms kg-1 min-1 over 15 min) and the peripheral 5-HT2 receptor antagonist xylamidine (IV 100 micrograms kg-1 min-1 over 10 min) failed to modify food intake. These results provide evidence for central serotonergic involvement in the control of feeding. However, this control system differs markedly in goats and rodents. Dexfenfluramine, 5-HTP and eltoprazine administered at similar dose rates to those used in the food intake experiments induced some clinical signs including inhibition of forestomach contractions. These results, together with our earlier in vivo and in vitro observations, suggest that the inhibitory effects of serotonin receptor agonists on forestomach contractions are due to interactions with both peripheral and central serotonergic receptors. The change in smooth muscle tension, which leads to a change in the signals transmitted via vagal afferents to the central nervous system, appears not to modify feeding behaviour in dwarf goats.

Discriminative stimulus effects of 8-OH-DPAT in pigeons: antagonism studies with the putative 5-HT1A receptor antagonists BMY 7378 and NAN-190

Pigeons were trained to discriminate 0.3 mg/kg of the 5-HT1A receptor agonist 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) from saline. RU 24969 (5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole), at doses of 5.6-10 mg/kg, and eltoprazine (5.6 mg/kg), both mixed 5-HT1A/B agonists, substituted completely for 8-OH-DPAT, whereas 3.0-10 mg/kg of the 5-HT1B/C agonist TFMPP (1-(m-trifluromethylphenyl)piperazine) and 0.1-3.0 of the 5-HT3 antagonist MDL 72222 (3-tropanyl-3,5-dichlorobenzoate) yielded only saline-appropriate responses. Substitution for 8-OH-DPAT by eltoprazine and RU 24969, which does not occur in rats, provides in vivo support for the suggestion that the absence of a 5-HT1B receptor in the pigeon allows more complete expression of 5-HT1A-mediated effects. BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)]8-azaspirol-[4.5]- decane-7,9-dione) attenuated the 8-OH-DPAT stimulus at doses from 1.0 to 10 mg/kg but, when administered alone, also resulted in approximately 40% 8-OH-DPAT-appropriate responding at the highest dose. NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalamido)butyl)-piperazine (0.3-3.0 mg/kg) produced a dose-dependent and complete antagonism of the 8-OH-DPAT-discriminative stimulus; administered alone NAN-190 resulted only in saline-key responding. NAN-190 also reversed the rate-decreasing effects of higher doses of 8-OH-DPAT. The beta-adrenoceptor antagonist (+/-)-pindolol (5.6-17 mg/kg) antagonized the discriminative stimulus effects of lower 8-OH-DPAT doses but was unable to block the effects of higher doses of 8-OH-DPAT. Prazosin (1.0-10 mg/kg), which like NAN-190, is an alpha 1-antagonist, neither substituted for nor blocked the discriminative stimulus effects of 8-OH-DPAT. These results suggest that NAN-190 is an effective 5-HT1A receptor antagonist in this procedure with pigeons, with no indication of agonist actions, whereas BMY 7378 and pindolol are best characterized as partial 5-HT1A receptor agonists.

Evidence for postsynaptic mediation of the hypothermic effect of 5-HT1A receptor activation

1. The 5-HT1A ligand BMY 7378 (8-[2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]8-azaspirol [4,5]-decane-7,9-dione dihydrochloride, 0.032-2 mg kg-1, s.c.) caused hyperphagia, a response to the activation of presynaptic 5-HT1A receptors. 2. BMY 7378 (8 mg kg-1, s.c.) and the 5-HT1A agonist (8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), 0.10 and 0.25 mg kg-1 s.c.) also caused hypothermia. This was inhibited by (-)-pindolol (1-mg kg-1, i.p.) and not prevented by pretreatments with p-chlorophenylalanine which grossly depleted 5-hydroxytryptamine (5-HT) from terminal regions. The hypothermic effects are explicable by activation of postsynaptic 5-HT1A receptors. Infusion of BMY 7378 (8-64 micrograms) into the dorsal raphe was without convincing hypothermic effect. 3. BMY 7378 (8 mg kg-1, s.c.) inhibited another effect of activation of postsynaptic 5-HT1A receptors, i.e., the induction of components of the 5-HT syndrome by 8-OH-DPAT (0.5, 1.0 mg kg-1, s.c.) which suggests that BMY 7378 has antagonistic as well as agonistic effects at these sites. 4. Partial agonist properties of BMY 7378 at postsynaptic sites were also indicated by doses for hypothermia being much greater than those for hyperphagia i.e., ED50 (hypothermia) greater than 2 mg kg-1, ED50 (hyperphagia) = 0.010 mg kg-1. This contrasts with the similar ED50 values for both the hypothermic (ED50 = 0.08-0.10 mg kg-1) and hyperphagic (ED50 = 0.06-0.10 mg kg-1) effects of 8-OH-DPAT.5. The evidence obtained for mediation of the hypothermic response to 5-HTIA agonists by postsynaptic sites is relevant to the interpretation of the effects on it of antidepressant treatments and depressive illness.

Effects of sertraline and citalopram given repeatedly on the responsiveness of 5-HT receptor subpopulations

The effect of repeated treatment (5 and 10 mg/kg, po, twice daily, 14 days) with sertraline and citalopram (antidepressants which selectively inhibit the reuptake of 5-hydroxytryptamine (5-HT)) on the responsiveness of different 5-HT receptors to their agonists, was examined in rats and mice. Sertraline and citalopram (both at a dose 5 and 10 mg/kg) antagonized (the first one more potently) the hypothermia induced in mice by 8-OH-DPAT (a 5-HT1A agonist), but not the behavioural syndrome induced in rats by this substance. The m-chlorophenylpiperazine-induced hypothermia in mice (a 5-HT1B effect) was increased by sertraline and citalopram (only in a dose of 10 mg/kg). Both antidepressants, given repeatedly (as well acutely) attenuated exploratory hypoactivity induced in rats by m-chlorophenylpiperazine (a 5-HT1C effect). L-5-HTP-induced head twitches in mice (5-HT2 effect) were antagonized dose-dependently by both repeated sertraline and citalopram. Both antidepressants (citalopram only in higher dose) reduced the fenfluramine-induced hyperthermia in rats (5-HT2 effect). The results indicate that sertraline and citalopram given repeatedly decrease the responsiveness of 5-HT1A (presynaptic) and 5-HT2 receptors but increase the responsiveness of 5-HT1B receptors to respective agonists.