Electrophysiological comparison of 5-Hydroxytryptamine1A receptor antagonists on dorsal raphe cell firing

Single-unit recording studies were undertaken in chloral hydrate-anesthetized rats to compare the effects on dorsal raphe cell firing of several putative 5-hydroxytryptamine (HT)1A receptor antagonists, including WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide), p-MPPI (4-(2-methoxyphenyl)1-[2'-[N-(2"-pyridinyl)-p-iodobenzamido]ethyl] pip erazine), and two newly described 5-HT1A receptor antagonists, NDL-249 [(R)-3-(N-propylamino)-8-fluoro-3, 4-dihydro-2H-1-benzopyran-5-carboxamide] and NAD-299 [(R)-3-N, N-dicyclobutylamino-8-fluoro-3, 4-dihydro-2H-1-benzopyran-5-carboxamide]. Consistent with a 5-HT1A receptor antagonist profile, pretreatment with an approximately equimolar (0.02-0.03 micromol/kg) i.v. dose of each compound caused a significant rightward shift in the dose-response curve for 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin]. Antagonist potency was clearly highest for NAD-299 and WAY 100635, which caused shifts roughly 3 times greater than those for either p-MPPI or NDL-249 (ED50 for 8-OH-DPAT, 1.3 +/- 0.3 microg/kg; after NAD-299, 18.2 +/- 1.0 microg/kg; after WAY 100635, 16.9 +/- 2.9 microg/kg; after NDL-249, 6.0 +/- 1.2 microg/kg; after p-MPPI, 4.7 +/- 1.1 microg/kg). In separate studies, each of the antagonists was administered alone in increasing cumulative doses to evaluate whether they possessed intrinsic agonist activity in this system. At doses below 0.01 micromol/kg, none of the drugs altered firing by more than +/-20% basal rates. At higher doses (>0.1 micromol/kg), WAY 100635, NDL-249, and NAD-299 caused a dose-dependent suppression of dorsal raphe cell firing (ED50 = 0.6 +/- 0.2, 0.7 +/- 0.3, and 0. 9 +/- 0.4 micromol/kg, respectively). However, the ED50 values for inhibition by these drugs were roughly 30 times higher than the doses that antagonized effects of 8-OH-DPAT. Moreover, the inhibition by all three antagonists (but not 8-OH-DPAT) was readily reversed by d-amphetamine (3.2 mg/kg i.v.), a releaser of norepinephrine, suggesting that these effects were likely due to alpha adrenergic receptor blockade rather than to 5-HT1A receptor agonism. Thus, it was concluded that WAY 100635, NAD-299, NDL-249, and p-MPPI all fulfill criteria as 5-HT1A receptor antagonists lacking intrinsic efficacy in the dorsal raphe system. The newly described compound NAD-299 exhibits antagonist potency comparable to that of WAY 100635 in this electrophysiological assay.     

Full and partial 5-HT1A receptor agonists disrupt learning and performance in rats

As a means of characterizing the role of 5-hydroxytryptamine (5-HT1A) receptors in learning, a full 5-HT1A receptor agonist, 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), was administered both alone and in combination with two partial agonists (buspirone and 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl] piperazine hydrobromide (NAN-190)) and a 5-HT1A receptor antagonist (p-MPPI) to rats responding under a multiple schedule of repeated acquisition and performance of response sequences. In addition, the effects of another 5-HT1A receptor agonist, (LY228729), were also studied under this same procedure. When administered alone, both 8-OH-DPAT (0.1-3. 2 mg/kg) and LY228729 (0.32-3.2 mg/kg) dose dependently decreased overall response rate and increased the percentage of errors in the acquisition and performance components. At the doses of each drug tested, both buspirone (0.32 or 1 mg/kg) and NAN-190 (1 or 3.2 mg/kg) also decreased overall response rate and increased the percentage of errors. However, the effects of these drugs differed across behavioral components and dependent measures. The effects of buspirone and NAN-190 on rate and accuracy were also different when they were administered in combination with 8-OH-DPAT. In contrast, p-MPPI (3.2 or 10 mg/kg) had little or no effect when administered alone and antagonized the effects of 8-OH-DPAT; shifting the dose-effect curves for both response rate and the percentage of errors in both components to the right. Taken together, these results indicate that complex behaviors in rats are sensitive to disruption by drugs with both full and partial 5-HT1A receptor agonist properties, and that the effects of partial 5-HT1A receptor agonists on learning may be different depending on their efficacy at pre- and postsynaptic 5-HT1A receptors.     

Activation of 5-HT1A receptors potentiates the clonidine inhibitory effect in the locus coeruleus

Using in vivo extracellular recordings, we have examined the effect of the application of the prototypical 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), on the firing rate of locus coeruleus neurons. 8-OH-DPAT (1 microgram/kg, i.v.) did not modify the basal activity of the locus coeruleus but shifted to the left the dose-response curve for the clonidine induced inhibition of firing rate and reduced the corresponding ED50 by 77%. 2-[2-[4-(o-methoxyphenyl)piperazin-1-yl]ethyl]-4,4-dimethyl-1,3(2H ,4H)-isoquinolinedione (ARC 239; 75 micrograms/kg, i.v.), and chlorpromazine (75 micrograms/kg, i.v.) also shifted to the left the dose-response curve for clonidine and reduced by 38 and 46%, respectively, the ED50, while slightly increasing the basal firing rate. The results indicated that 5-HT1A receptors may modulate the responses mediated by alpha 2A-adrenoceptors in the locus coeruleus.     

Evidence for specific involvement of 5-HT1A and 5-HT2A/C receptors in the expression of patterns of spontaneous motor activity of the rat

The 5-HT1A and the 5-HT2A/C receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.006-0.4 mg kg-1 s.c.) and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.05-4.0 mg kg-1 s.c.), respectively, produced a similar stereotyped forward locomotion in rats, although the intensity of the behavioral change was considerably less with DOI. The stereotyped forward locomotion was accompanied by a slight decrease in total activity, suppression of rearing behavior and an increased activity in the periphery of the open-field arena. In support of receptor specificity, the effects of 8-OH-DPAT and DOI could be antagonised by pretreatment with the 5-HT1A/B and the 5-HT2A/C receptor antagonists (-)-pindolol (2 mg kg-1 s.c.) and ritanserin (2 mg kg-1 s.c.), respectively. In addition, (-)-pindolol, but not the selective beta-adrenoceptor antagonist betaxolol, markedly enhanced the behavioral effects produced by DOI. The nature of these specific actions and interactions in terms of pre- and post-synaptic serotonergic mechanisms remains an important question.     

The 5-HT1A agonist 8-OH-DPAT increases the number of spike-wave discharges in a genetic rat model of absence epilepsy

The effects of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) on the epileptiform activity has been investigated in adult WAG/RIJ rats. Either intraperitoneal (0.1-0.5 mg/kg) or intracerebroventricular (2-20 microg/rat) administration of 8-OH-DPAT caused marked, dose-dependent increases in the number and mean cumulative duration of spike-wave discharges. These effects were attenuated by NAN-190, a 5-HT1A receptor antagonist. These data indicate that serotonergic system regulates the epileptiform activity in this genetic model of human absence epilepsy.     

Serotonin depresses excitatory synaptic transmission and depolarization-evoked Ca2+ influx in rat basolateral amygdala via 5-HT1A receptors

The actions of serotonin on rat basolateral amygdala neurons were studied with conventional intracellular recording techniques and fura-2 fluorimetric recordings. Bath application of 5-hydroxytryptamine (5-HT or serotonin) reversibly suppressed the excitatory postsynaptic potential in a concentration-dependent manner without affecting the resting membrane potential and neuronal input resistance. Extracellular Ba2+ or pertussis toxin pretreatment did not affect the depressing effect of 5-HT suggesting that it is not mediated through activation of Gi/o protein-coupled K+ conductance. The sensitivity of postsynaptic neurons to glutamate receptor agonist was unaltered by the 5-HT pretreatment. In addition, the magnitude of paired-pulse facilitation was increased in the presence of 5-HT indicating a presynaptic mode of action. The effect of 5-HT was mimicked by the selective 5-HT1A agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and was blocked by the selective 5-HT1A antagonist 1-(2-methoxyphenyl)-4[4-(2-phthalimido)butyl]piperazine oxadiazol-3-yl]methyl]phenyl]-methanesulphonamide. In contrast, the selective 5-HT2 receptor antagonist ketanserin failed to affect the action of 5-HT. The effects of 5-HT and 8-OH-DPAT on the high K+-induced increase in [Ca2+]i were studied in acutely dissociated basolateral amygdala neurons. High K+-induced increase in [Ca2+]i was blocked by Ca2+-free solution and Cd2+ suggesting that Ca2+ entry responsible for the depolarization-evoked increase in [Ca2+]i occurred through voltage-dependent Ca2+ channels. Application of 5-HT and 8-OH-DPAT reduced the K+-induced Ca2+ influx in a concentration-dependent manner. The effect of 5-HT was completely abolished in slices pretreated with Rp-cyclic adenosine 3',5'-monophosphothioate (Rp-cAMP), a regulatory site antagonist of protein kinase A, suggesting that 5-HT may act through a cAMP-dependent mechanism. Taken together, these results suggest that functional 5-HT1A receptors are present in the excitatory terminals and mediate the 5-HT inhibition of synaptic transmission in the amygdala.     

Head and whole-body jerking in guinea pigs are differentially modulated by 5-HT1A, 5-HT1B/1D and 5-HT2A receptor antagonists

The effects of a serotonin (5-HT) releasing drug, p-chloroamphetamine, on plasma glucose levels were investigated in rats. p-Chloroamphetamine elicited a significant hyperglycemia. The hyperglycemic effects of p-chloroamphetamine were completely prevented by the 5-HT synthesis inhibitor, p-chlorophenylalanine. Prior adrenodemedullation abolished the hyperglycemia elicited by p-chloroamphetamine. p-Chloroamphetamine-induced hyperglycemia was prevented by methysergide, which blocks the 5-HT1 and 5-HT2 receptor, the 5-HT1A/1B/2C receptor antagonist, (-)-propranolol, the selective 5-HT1A receptor antagonist, 4-(2'-methoxyphenyl-1-[2'-n-2"pyridinyl)-p-iodobenzamido]-ethyl-pi perazine (p-MPPI), the 5-HT2A/2B/2C receptor antagonists, ritanserin and 4-isopropyl-7-methyl-9-(2-hydroxy-1-methyl-propoxycarbonyl)-4,6A,7 ,8,9,10,10A-octahydro-indolo[4,3-FG]quinolone maleate(LY 53857). However, the 5-HT3 and 5-HT4 receptor antagonist, tropisetron, the 5-HT4 receptor antagonist, 2-methoxy-4-amino-5-chloro-benzoic acid 2-(diethylamino) ethyl ester (SDZ 205-557), and the 5-HT2A receptor antagonist, ketanserin, did not affect the p-chloroamphetamine-induced hyperglycemia. These results suggest that p-chloroamphetamine-induced hyperglycemia is elicited by an enhanced 5-HT release and facilitated adrenaline release. Moreover, our results indicate that p-chloroamphetamine-induced hyperglycemia is mediated by 5-HT1A and 5-HT2B/2C receptors.     

Facilitation and inhibition of male rat ejaculatory behaviour by the respective 5-HT1A and 5-HT1B receptor agonists 8-OH-DPAT and anpirtoline, as evidenced by use of the corresponding new and selective receptor antagonists NAD-299 and NAS-181

1. Ejaculatory problems and anorgasmia are well-known side-effects of the SSRI antidepressants, and a pharmacologically induced increase in serotonergic neurotransmission inhibits ejaculatory behaviour in the rat. In the present study the role of 5-HT1A and 5-HT1B receptors in the mediation of male rat ejaculatory behaviour was examined by use of selective agonists and antagonists acting at these 5-HT receptor subtypes. 2. The 5-HT1A receptor agonist 8-OH-DPAT (0.25-4.00 micromol kg(-1) s.c.) produced an expected facilitation of the male rat ejaculatory behaviour, and this effect was fully antagonized by pretreatment with the new selective 5-HT1A receptor antagonist (R)-3-N,N-dicyclobutylamino-8-fluoro-3,4-dihydro-2H-1-benzopyran-5 -carboxamide hydrogen (2R,3R) tartrate monohydrate (NAD-299) (1.0 micromol kg(-1) s.c.). NAD-299 by itself (0.75-3.00 micromol kg(-1) s.c.) did not affect the male rat ejaculatory behaviour. 3. The 5-HT1B receptor agonist anpirtoline (0.25-4.00 micromol kg(-1) s.c.) produced a dose-dependent inhibition of the male rat ejaculatory behaviour, and this effect was fully antagonized by pretreatment with the 5-HT1B receptor antagonist isamoltane (16 micromol kg(-1) s.c.) as well as by the new and selective antagonist (R)-(+)-2-(3-morpholinomethyl-2H-chromene-8-yl)oxymethylmorphol ino methansulphonate (NAS-181) (16 micromol kg(-1) s.c.). Isamoltane (1.0-16.0 micromol kg(-1) s.c.) and NAD-181 (1.0-16.0 micromol kg(-1) s.c.) had no, or weakly facilitatory effects on the male rat ejaculatory behaviour. The non-selective 5-HT1 receptor antagonist (-)-pindolol (8 micromol kg(-1) s.c.), did not antagonize the inhibition produced by anpirtoline. 4. The present results demonstrate opposite effects, facilitation and inhibition, of male rat ejaculatory behaviour by stimulation of 5-HT1A and 5-HT1B receptors, respectively, suggesting that the SSRI-induced inhibition of male ejaculatory dysfunction is due to 5-HT1B receptor stimulation.     

Chronic alnespirone-induced desensitization of somatodendritic 5-HT1A autoreceptors in the rat dorsal raphe nucleus

The effects of long-term (7, 14 or 21 days) administration of the 5-HT1A receptor agonist alnespirone [5 mg/(kg day), i.p.] on the binding characteristics of 5-HT1A, 5-HT2A and 5-HT3 receptors, and the functional status of 5-HT1A autoreceptors were assessed using biochemical and electrophysiological approaches in rats. Whatever the treatment duration, the specific binding of [3H]8 hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT), [3H]trans,4-[(3Z)3-(2-dimethylaminoethyl) oxyimino-3(2-fluorophenyl) propen-1-yl] phenol hemifumarate ([3H]SR 46349B), and [3H]S-zacopride to 5-HT1A, 5-HT2A and 5-HT3 receptors, respectively, were unaltered in all the brain areas examined. In contrast, in vitro electrophysiological recordings performed 24 h after the last injection of alnespirone showed that the potency of the 5-HT1A receptor agonist, 8-OH-DPAT, to depress the firing of serotoninergic neurons in the dorsal raphe nucleus, was significantly reduced after a 21-day treatment with alnespirone. However, no changes were noted after a 7-day or 14-day treatment. These data indicate that desensitization of somatodendritic 5-HT1A autoreceptors is a selective but slowly developing adaptive phenomenon in response to their chronic stimulation in rats.     

5-HT1A receptor-mediated inhibition of long-term potentiation in rat visual cortex

We investigated the effect of 8-hydroxy-2-(N,N-dipropylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, on the induction of long-term potentiation in rat visual cortex slices. Perfusion of 8-OH-DPAT (0.1-10 microM) did not affect layer II/III field potentials evoked by test stimulation of layer IV, but significantly reduced long-term potentiation induced by tetanic stimulation. The inhibitory effect of 8-OH-DPAT was blocked by the 5-HT1A receptor antagonist, pindolol (10 microM), but not by the 5-HT2,7 receptor antagonist, ritanserin (100 microM), nor by the 5-HT3,4 receptor antagonist, MDL72222 (100 microM). These results suggest that the rat visual cortex long-term potentiation is inhibited by 5-HT1A receptor stimulation.     

Effect of testosterone and its metabolites upon the level of vasopressin messenger ribonucleic acid in the hypothalamus of the hyperosmotically stimulated male rat

We examined whether the selective 5-hydroxytryptamine 1A (5-HT1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) injected systemically can act directly on sympathoexcitatory neurons located in the rostral ventrolateral medulla (RVLM) to cause the hypotensive effect of this agent in rats. Microinjections of 8-OH-DPAT and buspirone into the RVLM produced a dose-dependent decrease in blood pressure. Microinjections of spiperone and pindolol, 5-HT1A antagonists, into the RVLM inhibited the depressor response to 8-OH-DPAT intravenously injected or injected into the RVLM. Microiontophoretic application of 8-OH-DPAT onto RVLM sympathoexcitatory neurons inhibited the firing of RVLM sympathoexcitatory neurons and the inhibition of unit activity by 8-OH-DPAT was blocked by microiontophoretic spiperone. Intravenous administration of 8-OH-DPAT also inhibited the firing of these neurons. Microiontophoretic application of spiperone onto the RVLM sympathoexcitatory neurons reversed the inhibitory response to intravenous 8-OH-DPAT. These results are consistent with the hypothesis that 8-OH-DPAT may exert a portion of its hypotensive effect through a direct inhibition of RVLM sympathoexcitatory neurons in rats. The receptor involved is probably the 5-HT1A type.     

The 5-HT1-like receptors mediating inhibition of sympathetic vasopressor outflow in the pithed rat: operational correlation with the 5-HT1A, 5-HT1B and 5-HT1D subtypes

1. It has been suggested that the inhibition of sympathetically-induced vasopressor responses produced by 5-hydroxytryptamine (5-HT) in pithed rats is mediated by 5-HT1-like receptors. The present study has re-analysed this suggestion with regard to the classification schemes recently proposed by the NC-IUPHAR subcommittee on 5-HT receptors. 2. Intravenous (i.v.) continuous infusions of 5-HT and the 5-HT1 receptor agonists, 8-OH-DPAT (5-HT1A), indorenate (5-HT1A), CP 93,129 (5-HT1B) and sumatriptan (5-HT(1B/1D)), resulted in a dose-dependent inhibition of sympathetically-induced vasopressor responses. 3. The sympatho-inhibitory responses induced by 5-HT, 8-OH-DPAT, indorenate, CP 93,129 or sumatriptan were analysed before and after i.v. treatment with blocking doses of the putative 5-HT receptor antagonists, WAY 100635 (5-HT1A), cyanopindolol (5-HT(1A/1B)) or GR 127935 (5-HT(1B/1D)). Thus, after WAY 100635, the responses to 5-HT and indorenate, but not to 8-OH-DPAT, CP 93,129 and sumatriptan, were blocked. After cyanopindolol, the responses to 5-HT, indorenate and CP 93,129 were abolished, whilst those to 8-OH-DPAT and sumatriptan (except at the lowest frequency of stimulation) remained unaltered. In contrast, after GR 127935, the responses to 5-HT, CP 93,129 and sumatriptan, but not to 8-OH-DPAT and indorenate, were abolished. 4. In additional experiments, the inhibition induced by 5-HT was not modified after 5-HT7 receptor blocking doses of mesulergine. 5. The above results suggest that the 5-HT1-like receptors, which inhibit the sympathetic vasopressor outflow in pithed rats, display the pharmacological profile of the 5-HT1A, 5-HT1B and 5-HT1D, but not that of 5-HT7, receptors.     

Differential inhibition of stress-induced adrenocortical responses by 5-HT1A agonists and by 5-HT2 and 5-HT3 antagonists

The present studies were designed to determine the effects of 5-HT1A receptor agonists and 5-HT2A/2C and 5-HT3 antagonists on adrenocortical responses to a variety of stress paradigms in conscious male rats. The following stressors were examined: acoustic stress (105 dB for 2 min); foot shock (0.2 mA, five shocks over 5 min); conditioned fear (animals placed in the foot shock chamber for 5 min, 24 h after foot shock); restraint (Plexiglas restrainer for 5 min); injection of recombinant human interleukin-1 alpha (IL-1, 20 micrograms/kg, IP); injection of cocaine hydrochloride (20 mg/kg, IP). Drug treatments consisted of intracerebroventricular (ICV) or intraperitoneal (IP) injections of the 5-HT1A agonists, 8-OH-DPAT and ipsapirone (0.1 pmol, ICV), the 5-HT2A/2C antagonist, ketanserin (2 mumol/kg, IP), and the 5-HT3 antagonist, MDL-72222 (20 nmol, ICV). The plasma corticosterone (CS) responses to foot shock and restraint stress were not affected by any of the serotonergic drugs tested. The 5-HT1A agonist, 8-OH-DPAT, was able to attenuate the adrenocortical responses to acoustic stimulation, conditioned fear, IL-1 alpha, and cocaine administration, with ipsapirone also being effective in reducing the responses to acoustic stimulation and cocaine injection. The 5-HT2 antagonist, ketanserin was able to reduce the adrenocortical response in the conditioned fear paradigm and the response to IL-1 alpha injection. The 5-HT3 antagonist, MDL-72222 was only effective in reducing the response to acoustic stimulation. Thus, adrenocortical responses to each of the applied stressors were differentially affected by the 5-HT receptor ligands tested. The results of this study indicate that 5-HT1A agonists may be efficient stress response-reducing agents. However, their efficacy depends on the lack of a somatosensory component to the applied stressor and their agonist properties suggest that their action may not involve direct effects on serotonergic pathways mediating the observed responses. In contrast, the specificity of the 5-HT2 and 5-HT3 antagonists in blocking adrenocortical responses to certain stressors suggests that these drugs exert their effects by blocking serotonergic neurotransmission in pathways mediating the adrenocortical responses to specific stimuli.     

5-HT1A receptor activation: short-term effects on the mRNA expression of the 5-HT1A receptor and galanin in the raphe nuclei

Systemic administration of the 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)-tetralin (8-OH-DPAT; 0.3 mg/kg, s.c.) was used to explore the effects of activation of 5-HT1A receptors on expression of mRNA coding for 5-HT1A receptor, tryptophan hydroxylase (TPH) and galanin in the ascending raphe nuclei. 8-OH-DPAT increased the hybridization signal of the 5-HT1A receptor by 105% in the dorsal raphe nucleus (B7) 30 min after the injection. No effects were seen at the later time points (2-8 h). In the median raphe nucleus (B8) and the B9 cell group in the medial lemniscus, 8-OH-DPAT induced a marked decrease in labeling 30 min after injection. At 8 h following 8-OH-DPAT injection, the effect had shifted to an increase in 5-HT1A receptor labeling by 68% in the B8 area. Importantly 8-OH-DPAT had no significant effects on the expression of mRNA coding for TPH and galanin. The results suggest an important and differential mechanism for the regulation of 5-HT1A receptor mRNA levels in the dorsal and median raphe nuclei. This regulation may be of importance for the differential control of the activity of the ascending 5-HT neurons, and hence for mood regulation. The results also indicate a dissociation between the effects mediated by 5-HT1A receptor functions and those regulating the coexisting peptide galanin in the dorsal raphe.     

Analysis of the 5-HT1A receptor involvement in passive avoidance in the rat

1. The effects of the 5-HT2A/2C agonist DOB, the selective 5-HT1A agonist NDO 008 (3-dipropylamino-5-hydroxychroman), and the two enantiomers of the selective 5-HT1A agonist 8-OH-DPAT (R(+)-8-OH-DPAT and S(-)-8-OH-DPAT) were studied in a step-through passive avoidance (PA) test in the male rat. 2. The 5-HT1A agonists injected prior to training (conditioning) produced a dose-dependent impairment of PA retention when examined 24 h later. R(+)-8-OH-DPAT was four times more effective than S(-)-8-OH-DPAT to cause an impairment of PA retention. Both NDO 008 and the two enantiomers of 8-OH-DPAT induced the serotonin syndrome at the dose range that produced inhibition of the PA response, thus, indicating activation of postsynaptic 5-HT1A receptors. 3. Neither NDO 008 nor R(+)-8-OH-DPAT induced head-twitches, a behavioural response attributed to stimulation of postsynaptic 5-HT2A receptors. In contrast, DOB induced head-twitches at the 0.01 mg kg(-1) dose while a 200 times higher dose was required to produce a significant impairment of PA retention. 4. The impairment of PA retention induced by both NDO 008 and R(+)-8-OH-DPAT was fully blocked by the active S(+)- enantiomer of the selective 5-HT1A antagonist WAY 100135 and the mixed 5-HT1A/beta-adrenoceptor antagonist L(-)-alprenolol. In contrast, the mixed 5-HT2A/2C antagonists ketanserin and pirenperone were found to be ineffective. Moreover, the beta2-adrenoceptor antagonist ICI 118551, the beta-antagonist metoprolol as well as the mixed beta-adrenoceptor blocker D(+)-alprenolol all failed to modify the deficit of PA retention by NDO 008 and R(+)-8-OH-DPAT. None of the 5-HT1A or 5-HT2A/2C receptor antagonists tested or the beta-blockers altered PA retention by themselves. 5. A 3 day pretreatment procedure (200+100+100 mg kg(-1)) with the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA) did not alter PA retention and did not prevent the inhibitory action of the 5-HT1A agonists, indicating that their effects on PA do not depend on endogenous 5-HT. 6. The effects of NDO 008 on PA were also studied using a state-dependent learning paradigm. NDO 008 was found to produce a disruption of PA when given either prior to training or retention or both prior to training and retention but it failed to affect PA retention when given immediately after training. .7 These findings indicate that the deficit of passive avoidance retention induced by the 5-HT1A agonists is mainly a result of stimulation of postsynaptic 5-HT1A receptors but not 5-HT2A receptors. The 5-HT1A receptor stimulation appears to interfere with learning processes operating at both acquisition and retrieval.     

Contrasting effects of chronic paroxetine on 5-HT1A control of dorsal raphe cell firing and 5-HT release

To test the role of 5-HT1A receptors in the action of antidepressants, we investigated the effect of chronic paroxetine (10 mg/kg, p.o. for 21 days) on functional assays of 5-HT1A sensitivity. We constructed cumulative concentration response curves to the selective 5-HT1A agonist (+)-8-OH-DPAT on both extracellular recordings of 5-HT neurones and electrically stimulated 5-HT release in dorsal raphe brain slices. Chronic paroxetine desensitized the 5-HT1A receptors controlling firing, with an increase in EC50 from 10.7 nM to 46.2 nM 8-OH-DPAT. Chronic paroxetine did not, however, desensitize the 5-HT1A receptors controlling 5-HT release but increased the 8-OH-DPAT Emax from 54.9% to 79.2% inhibition of 5-HT release. These data suggest that there are either two distinct populations of 5-HT1A receptors or separate second messenger systems, one controlling 5-HT release and another influencing firing. Furthermore chronic paroxetine treatment can differentially modulate these different populations.     

Pharmacological characterization of in vivo properties of putative mixed 5-HT1A agonist/5-HT2A/2C antagonist anxiolytics. I. Antipunishment effects in the pigeon

A conflict procedure in pigeons was used to characterize the antipunishment effects of the putative mixed 5-hydroxytryptamine (5-HT)1A agonist/5-HT2A/2C antagonists WY 50,324, CGS 18102A, LEK 8804 and FG 5974 and to further investigate interactions between the antipunishment effects of the 5-HT1A agonists buspirone and 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] administered in combination with the mixed 5-HT2A/2C antagonist ritanserin and the alpha 1 antagonist prazosin. The 5-HT1A agonists, buspirone and 8-OH-DPAT, which lack affinity for 5-HT2A/2C receptors, produced dose-related increases in punished responding. Of the compounds with a mixed binding profile, only WY 50,324 showed effects that were comparable to those observed after 8-OH-DPAT, whereas FG 5974 and CGS 18102A exhibited limited effects on punished responding, and LEK 8804 was ineffective. Administration of a relatively low, behaviorally active dose of ritanserin (0.16 mg/kg) significantly enhanced the potency of 8-OH-DPAT and buspirone to increase punished responding from 8 to 50-fold without altering their effects on unpunished responding. Importantly, ritanserin failed to increase the number of doses of 8-OH-DPAT that significantly increased punished responding. In contrast, prazosin (2.5 mg/kg) significantly enhanced the potency and increased the number of doses of buspirone exerting significant effects on punished responding, but did not alter the effects of 8-OH-DPAT. Taken together, the results neither explain the suggested greater efficacy in producing anxiolytic effects of compounds with putative mixed 5-HT1A agonist and 5-HT2A/2C antagonist properties, nor confirm a proposed interaction between alpha1 adrenoreceptors and 5-HT1A agonists in preclinical tests of anxiolytic activity.     

Differential effects of serotonin (5-HT) lesions and synthesis blockade on neuropeptide-Y immunoreactivity and 5-HT1A, 5-HT1B/1D and 5-HT2A/2C receptor binding sites in the rat cerebral cortex

The present study was aimed at comparing the effects of serotonin (5-HT) synthesis blockade using chronic administration of p-chlorophenylalanine (PCPA) and 5,7-dihydroxytryptamine injections of variable volume (3 vs. 6 microl) on the density of NPY immunoreactive (Ir) neurons and binding of [3H]8-OH-DPAT, S-CM-G[125I]TNH2 and [125I]DOI to 5-HT1A, 5-HT1B/1D, and 5-HT2A/2C receptors in rat cortical regions. Three weeks after large but partial (89% depletion in 5-HT tissue concentration) lesions of 5-HT neurons no changes in neither NPY immunoreactivity nor 5-HT receptor binding were detected. The complete 5,7-DHT lesions produced increases in the number of NPY-Ir neurons in the upper regions of the cingular (134%), frontal (140%) and parietal cortex (48%) and corresponding decreases in 5-HT2A/2C binding (16-26%). No changes in 5-HT1A and 5-HT1B/1D binding were observed after lesions of this kind. After PCPA treatment, decreases in NPY-Ir neurons density (22-40%) and increases in 5-HT1A and 5-HT1B/1D receptor binding sites (20-50%) were distributed in both upper and deeper cortical regions. The lack of effect of the partial lesion suggests that spared 5-HT neurons may exert compensatory mechanisms up to a large extent. The changes in NPY immunoreactivity and 5-HT2A/2C binding detected in the upper regions of the cortex after complete 5-HT lesions probably result from local cellular rearrangements, whereas blocking 5-HT synthesis has more widespread influence on NPY neurons and on 5-HT1A and 5-HT1B/1D receptor subtypes. Moreover, decreases in DOPAC concentrations detected only after complete lesions suggest that the involvement of catecholaminergic transmission may also differentiate 5,7-DHT and PCPA treatments. Altogether, these data suggest that different receptor subtypes might be involved in 5-HT-NPY relationships.     

Anxiolytic 5-hydroxytryptamine1A agonists suppress firing activity of dorsal hippocampus CA1 pyramidal neurons through a postsynaptic mechanism: single-unit study in unanesthetized, unrestrained rats

Recent behavioral studies indicate that conditioned fear response to contextual stimuli is reduced effectively by anxiolytic 5-hydroxytryptame (5-HT)1A agonists. Since the hippocampus seems to play an essential role in associative fear memories evoked by context, it is important to assess the effect of 5-HT1A agonists on pyramidal cell activity in the hippocampus. We examined the effects of 5-HT1A agonists on the spontaneous firing rate of hippocampal CA1 pyramidal neurons in unanesthetized, unrestrained rats. Systemic administration of selective 5-HT1A agonists, 8-hydroxy-2-(di-n-propylamino)tetralin, buspirone, ipsapirone, and flesinoxan produced a dose-dependent inhibition of neuronal activity. Putative 5-HT1A antagonists NAN-190 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine and (-)-pindolol did not change neuronal activity of CA1 pyramidal neurons. The suppression of neuronal activity by buspirone was antagonized by NAN-190 but not by (-)-pindolol. Lack of antagonistic activity of (-)-pindolol for the suppression of pyramidal neurons via a postsynaptic mechanism is consistent with the results of recent electrophysiological experiments in anesthetized rats. Pretreatment with parachlorphenylalanine did not change the spontaneous firing rates of hippocampal CA1 pyramidal neurons or abolish the suppressant effects of buspirone on these neurons. Taken together, the present results strongly suggest that suppression of the hippocampal CA1 pyramidal neuronal activity by anxiolytic 5-HT1A agonists in awake rats is mediated by postsynaptic 5-HT1A receptors located on pyramidal neurons.     

Electrophysiological evidence for a large receptor reserve for inhibition of dorsal raphe neuronal firing by 5-HT1A agonists

Previous studies [Meller et al. (1990) Mol. Pharmacol., 37:231-237] have shown that a large receptor reserve exists for the inhibition of serotonin synthesis in rat cortex and hippocampus by the 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), whereas little or no reserve exists for the lower efficacy agonists ipsapirone and BMY 7378. The current studies were undertaken to determine if the above drugs exhibit similar relative efficacies and receptor reserves in an electrophysiological model of 5-HT1A receptor activation, i.e., the inhibition of dorsal raphe cell firing. Intravenous dose-response curves were constructed in untreated control rats, or in rats which received an injection of the irreversible receptor inactivator N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 6 mg/kg, s.c.) 24 hours before recording. All three drugs fully inhibited dorsal raphe cell firing in control rats (ED50's: 1.5 micrograms/kg, 8-OH-DPAT; 30.0 micrograms/kg, ipsapirone; 17.5 micrograms/kg, BMY 7378). However, unlike effects on serotonin synthesis, EEDQ treatments caused no depression of the maximal inhibitory response for any of the agonists, although all dose-response curves were shifted to the right (ED50's: 10.1 micrograms/kg, 6.7-fold shift, 8-OH-DPAT; 139.9 micrograms/kg, 4.7-fold shift, ipsapirone; 53.8 micrograms/kg, 3.1-fold shift, BMY 7378). Although the order of agonist efficacies was similar for both inhibition of serotonin synthesis and dorsal raphe cell firing (8-OH-DPAT > ipsapirone > BMY 7378), a large (> 50%) receptor reserve was estimated for all three drugs in this electrophysiological system.(ABSTRACT TRUNCATED AT 250 WORDS)