Serotonin Autoreceptors Research Articles

Studies on the mechanism of the sympatholytic effect of 8-OH DPAT: lack of correlation between inhibition of serotonin neuronal firing and sympathetic activity

Previous studies indicate that the selective 5-HT 1A agonist, 8-OH DPAT, acts in the central nervous system to inhibit sympathetic nerve activity. Based on the observations that: (1) 8-OH DPAT acts at serotonin (5-HT) autoreceptors to inhibit 5-HT neuronal firing; and (2) medullary 5-HT neurons provide a tonic excitatory input to sympathetic preganglionic neurons, we have hypothesized that 8-OH DPAT produces its sympatholytic effects by inhibiting medullary 5-HT neuronal firing and thereby removing an excitatory input to sympathetic preganglionic neurons. The present study was designed to critically test this hypothesis. The sympatholytic effects of 8-OH DPAT were compared in intact animals and in animals which received large electrolytic lesion in the midline area of the lower brainstem. These lesions extended from the obex rostral through the level of the facial motor nucleus and encompassed the brain stem from the dorsal to the ventral surface. The sympatholytic effect of 8-OH DPAT was identical in intact animals and in animals receiving the lesion. The inhibitory effects of 8-OH DPAT on activity recorded simultaneously from the inferior cardiac sympathetic nerve and from medullospinal 5-HT neurons were determined. Medullary 5-HT neurons were identified using criteria modeled after the electrophysiological and pharmacological characteristics previously described for dorsal raphe 5-HT neurons. Medullary 5-HT neuron activity was more sensitive to the inhibitory effects of 8-OH DPAT than was sympathetic activity. Indeed, low doses of 8-OH DPAT completely suppressed the firing of medullary 5-HT neurons but had little effect on sympathetic nerve activity. These data fail to support the hypothesis that inhibition of 5-HT neuronal firing is responsible for the central sympatholytic effects of 8-OH DPAT. Rather, the data suggest that 8-OH DPAT acts postsynaptically on 5-HT 1A receptors located on central sympathetic neurons to inhibit sympathetic nerve activity.

Facilitation of [ 3H]acetylcholine and [ 3H]5-hydroxytryptamine release from rat cerebral cortex synaptosomes by a factor extracted from the skin of the Australian frog Pseudophryne coriacea

A partly purified extract of the skin of the Australian frog Pseudophryne coriacea (PsC) evoked the release of [ 3H]acetylcholine ([ 3H]ACh) and of [ 3H]5-hydroxytryptamine ([ 3H]5-HT) from superfused rat cerebral cortex synaptosomes prelabeled with [ 3H]choline or [ 3H]5-HT, respectively. The PsC-evoked release of both transmitters was sensitive to tetrodotoxin and was strictly Ca 2+-dependent. The release of [ 3H]5-HT caused by PsC was unaffected by the 5-HT uptake inhibitor citalopram. Activation of muscarinic autoreceptors by ACh or of serotonin autoreceptors by 5-HT depressed the PsC-evoked release of [ 3H]ACh or of [ 3H]5-HT, respectively. It is concluded that PsC elicits a Ca 2+-dependent exocytotic-like transmitter release, possibly by opening Na + channels in the presynaptic membrane.

Phentolamine blocks presynaptic serotonin autoreceptors in rabbit and rat brain cortex

Possible antagonist effects of phentolamine at presynaptic serotonin autoreceptors were studied in slices of the occipito-parietal cortices of the rabbit and the rat. The slices were preincubated with 3H-serotonin and then superfused and stimulated electrically with single pulses or pulse trains. Nitroquipazine 1 mumol/l, a compound that inhibits the high affinity neuronal uptake of serotonin, was present in the superfusion medium in all one pulse-experiments as well as in experiments in which the effect of unlabelled serotonin was examined. In rabbit cortical slices, unlabelled serotonin reduced the single pulse-evoked overflow of tritium. Its concentration-response curve was not changed by the selective alpha 2-adrenoceptor antagonist idazoxan 1 mumol/l but was shifted to the right by phentolamine 1 and 10 mumol/l. Phentolamine 10 mumol/l also shifted to the right the concentration-inhibition curve of the selective 5-HT1-receptor agonist 5-carboxamidotryptamine. When the slices were stimulated by trains of 30 pulses at 3 Hz, phentolamine 1 and 10 mumol/l but not 0.1 mumol/l increased the evoked overflow of tritium, the maximal increase amounting to 178%; its effect was enhanced in the presence of nitroquipazine 1 mumol/l plus idazoxan 10 mumol/l (a drug combination that, when given alone, slightly increased the evoked overflow of tritium). The serotonin receptor antagonist metitepin at concentrations of 0.01-1 mumol/l also increased the overflow of tritium elicited by 30 pulses/3 Hz, the maximal increase amounting to 280%; its effect was potentiated in the presence of nitroquipazine 1 mumol/l plus idazoxan 10 mumol/l but was abolished or almost abolished in the presence of nitroquipazine 1 mumol/l plus phentolamine 10 mumol/l (a drug combination that, given alone, greatly increased the evoked overflow of tritium). When slices were stimulated by trains of 360 pulses at 3 Hz, there was no apparent antagonism of phentolamine 10 mumol/l against the inhibitory effect of unlabelled serotonin. In rat brain cortex slices, unlabelled serotonin reduced the overflow of tritium elicited by 4 pulses delivered at 100 Hz. Again, phentolamine 10 mumol/l shifted the concentration-response curve to the right. It is concluded that phentolamine blocks presynaptic serotonin autoreceptors in rabbit and rat brain cortex with pA2 values of 6.44 and 5.95, respectively. Previous failures to detect the antagonistic effect against exogenous agonists were probably due to stimulation conditions that led to marked endogenous autoinhibition of serotonin release.(ABSTRACT TRUNCATED AT 400 WORDS)

Different regulation of serotonin receptors following adrenal hormone imbalance in the rat hippocampus and hypothalamus

Adrenal influence on serotonin (5-HT) transmission in the hippocampal and hypothalamic areas was studied in adrenalectomized rats receiving or not corticosterone replacement. After adrenalectomy, the 5-HT presynaptic receptors were desensitized both in hippocampus and hypothalamus: a significant increase in 5-HT 1 and 5-HT 2 receptor binding numbers took place in membranes from the hippocampus, but not in hypothalamus, while no changes in affinity of receptors to radioligands were observed in either brain area. Corticosterone treatment restored the adrenalectomy-impaired 5-HT autoreceptor sensitivity in hippocampus and hypothalamus and 5-HT density receptor sites in the hippocampus. Serotonin autoreceptor down-regulation following adrenalectomy may increase 5-HT release to maintain the constancy of serotonergic transmission in the brain and 5-HT modulated CRH-ACTH release to compensate the plasma corticosteroid drop. Corticosterone seems to display a distinct tonic control on serotonin transmission in both hippocampus and hypothalamus, the diversity being due to the different roles played by the hormone in these brain regions.

Neuropharmacology of Serotonin: Vista International Hotel, New York City, NY, U.S.A., July 10–13, 1989

This monograph reviews all areas of serotonin neuropharmacology, from work on the serotonin autoreceptor and imipramine binding site, to the classification and identification of receptor sub-types and serotonin-mediated phosphoinosital breakdown. There are sections on the pharmacology of antidepressant, anti-anxiety, and hallucinogenic drugs, and in vivo monitoring of drug action by various techniques. There are also chapters on the effects of drugs on serotonin metabolism, serotonin-mediated behaviour, and on invertebrate pharmacology.

Phorbol‐12,13‐dibutyrate antagonizes while forskolin potentiates the presynaptic autoreceptor‐mediated inhibition of [3H]‐5‐hydroxytryptamine release in rat hypothalamic slices

In superfused rat hypothalamic slices prelabeled with [3H]-5-hydroxytryptamine [( 3H]-5-HT), the 5-HT autoreceptor agonists 5-methoxytryptamine and RU 24969 inhibited in a concentration-dependent manner the electrically evoked release of [3H]-5-HT. Exposure to phorbol-12,13-dibutyrate increased in a concentration-dependent manner the stimulation-evoked overflow of [3H]-5-HT and shifted to the right the 5-methoxytryptamine inhibition curve. Exposure to forskolin, a potent activator of adenylate cyclase, increased the stimulation-evoked [3H]-5-HT overflow and shifted to the left the 5-methoxytryptamine or RU 24969 inhibitory curves on transmitter release. A similar interaction was observed in the presence of 1-isobutyl,3-methylxanthine (IBMX), a phosphodiesterase inhibitor, or 8-bromo-cyclic AMP and the serotonin autoreceptor agonist 5-methoxytryptamine. In the presence of phorbol-12,13-dibutyrate or forskolin, the enhancing effect of the 5-HT autoreceptor antagonist methiothepin on the stimulation-evoked [3H]-5-HT overflow was significantly less pronounced than in the absence of these compounds. These results indicate that the presynaptic 5-HT autoreceptors that modulate the release of [3H]-5-HT in rat hypothalamic slices may be coupled to the phosphoinositide cycle and protein kinase C-dependent mechanisms. In addition, the increase in intracellular level of cyclic AMP by forskolin, IBMX, and 8-bromo-cyclic AMP potentiates the inhibitory effects of the autoreceptor agonist 5-methoxytryptamine on [3H]-5-HT release, although the mechanism of the interaction remains to be elucidated.

Evidence for G protein mediation of serotonin- and GABA B- induced hyperpolarization of rat dorsal raphe neurons

In vitro intracellular recording techniques in the rat brain slice preparation demonstrate that both serotonin (5-HT) and baclofen (a GABA B-receptor agonist) inhibit 5-HT neurons in the dorsal raphe nucleus by inducing a hyperpolarization of membrane potential and a decrease in apparent input resistance ( R in). Similar to previous results with 5-HT, baclofen-mediated inhibition of 5-HT neurons also shows an apparent reversal potential ( E rev) of approximately-90 mV, consistent with mediation by K channels. In slices from rats that had previously received a local injection of pertussis toxin (0.5 μg) immediately rostral to the dorsal raphe nucleus, there was a virtually complete blockade of inhibition induced by both the serotonin autoreceptor and the GABA B-receptor. Intracellular injection of the stable GTP analog (guanosine-5− O-(3-thiotriphosphate); GTPγS) mimicked the actions of both 5-HT and baclofen. The inhibitory actions of GTPγS were not additive with those of either 5-HT or baclofen, suggesting they share some common effector system. The stable cAMP analog (8-bromo-adenosine-3′, 5′-cyclic monophosphate (8-Br-cAMP) had no effect on membrane potential or apparent input resistance and did not block the inhibitory actions mediated by 5-HT or baclofen. The local injection of pertussis toxin (0.5 γg) caused a far greater blockade of 5-HT and baclofen-mediated inhibition than the intracerebroventricular (i.c.v.) injection of pertussis toxin (1.0 γg). In parallel sets of animals with i.c.v. and local injections, we measured the pertussis toxin-mediated ADP-ribosylation of G proteins in membranes prepared from dorsal raphe nucleus. These biochemical studies showed that sensitivities to 5-HT and baclofen correlated with the concentration of remaining non-ADP-ribosylated G proteins following in vivo pertussis toxin injection. In summary, these results provide evidence for the role of a G protein(s) in the mediation of the cAMP-independent increase in potassium conductance in 5-HT neurons of dorsal raphe nucleus induced by both 5-HT 1A and GABA B-receptors.

Evaluation of the role of pre- and postsynaptic serotonergic receptors in electroconvulsive shock therapy

Despite the well-established clinical efficacy of electroconvulsive therapy (ECT) in the treatment of affective disorders, there is no generally accepted theory regarding its mode of action. Pre- and postsynaptic serotonin (5-hydroxytryptamine, 5-HT) receptors were studied following treatment of rabbits with electroconvulsive therapy (8 shocks). No change was observed in the binding at the postsynaptic 5-HT 2 receptor in frontoparietal cortex, as indicated by an unchanged affinity and by the total number of receptors. In control rabbits, exogenously added serotonin and metitepin modulated the electrically evoked 3H-serotonin release, probably by acting on a presynaptic serotonin autoreceptor. Following electroconvulsive therapy, exogenous serotonin still inhibited the 3H-serotonin release, whereas metitepin had lost its enhancing effect. This observation provides evidence for a decreased amount of endogenous neurotransmitter present in the synaptic cleft at stimulation. The results indicate that postsynaptic receptor changes following antidcpressant treatments are related to the presynaptic autoreceptor changes.

Chronic lithium: desensitization of autoreceptors mediating serotonin release.

The effect of chronic lithium treatment on K+-induced release of preloaded [3H] serotonin [( 3H]5-HT) from brain slices and its regulation by the presynaptic serotonin autoreceptors were investigated in superfused cortical, hippocampal and hypothalamic brain slices. Three weeks of treatment with a lithium-containing diet increased stimulation-induced [3H]5-HT overflow in the three brain regions examined. The sensitivity of the inhibitory serotonin autoreceptors was tested by determining K+-elicited release inhibition or potentiation in response to exposure to the agonist, LSD or to the antagonist, methiothepin, respectively. A reduced maximal inhibitory response to LSD was obtained in lithium-treated animals. The potentiation by methiothepin was also markedly diminished in the treated animals. These results suggest that chronic lithium treatment induces a desensitization of serotonin autoreceptors which may result in increased serotonin release from the serotonin nerve terminals.

Protein kinase C: regulation of serotonin release from rat brain cortical slices

The effects of phorbol esters on serotonin release were examined in an attempt to investigate the role of protein kinase C in the regulation of serotonin release. Rat brain parietal cortical slices were incubated with [ 3H]5-HT in the presence of pargyline in order to label the serotonin stores. Potassium stimulated (30 s) release and spontaneous [ 3H]5-HT efflux were examined in slices during superfusion with Krebs-Ringer solution containing chlorimipramine. Repeated K + stimulations elicited reproducible responses with release ratios of approximately 1.0. Introduction of phorbol 12-myristate,13-acetate (PMA) or phorbol 12,13-dibutyrate (PDBu) 20 min prior to S 2, or S 3 resulted in dose-related increases in [ 3H]5-HT or [ 3H]NE release. PMA was slightly more potent (93% increase) than PDBu in potentiating K +-stimulated [ 3H]5-HT release. Phorbol and 4α-phorbol 12,13-didecanoate (4αPDD) which do not activate protein kinase C did not alter serotonin release. In contrast, basal [ 3H]5-HT and [ 3H]NE release were altered to a far lesser extent which was not always dose related. The response to the phorbol esters was reversible, Ca 2+-dependent and reached maximal effect after 20 min of superfusion. The putative protein kinase C inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) inhibited K +-induced [ 3H]5-HT release significantly (11%) but did not alter basal efflux. The PMA facilitation of serotonin release was, however, markedly prevented by the enzyme inhibitor. The effect of PMA on release was found not to be directly mediated through the prejunctional serotonin autoreceptors. This is suggested by (1) the similarity in PMA-induced release potentiation observed in the presence or absence of the serotonin agonist, 5-methoxytryptamine, and (2) by the inability of the kinase inhibitor, H-7, to alter K +-stimulated release by the serotonin receptor antagonist, methiothepin. These results on serotonin and norepinephrine release and those of previous workers dealing with other transmitters point to an important role of protein kinase C in the modulation of neurotransmitter release in the central nervous system.

(−)-Propranolol and (±)-cyanopindolol are mixed agonists-antagonists at serotonin autoreceptors in the hippocampus of the rat brain

The effects of (−)-propranolol, (+)-propranolol and (±)cyanopindolol on the release of [ 3H]5-hydroxytryptamine ([ 3H]5-HT) were investigated in synaptosomes from the hippocampus of the rat, depolarized in superfusion with 15mM KCl. (−)-Propranolol, but not (+)-propranolol, inhibited in a concentration-dependent way the K +-evoked release of [ 3H]5-HT. (±)-Cyanopindolol behaved similarly but was about 10 times more potent than (−)-propranolol. The inhibitory effects of (−)-propranolol and (±)-cyanopindolol were prevented by the autoreceptor antagonist methiothepin. Both β-adrenoceptor antagonists antagonized the inhibition by exogenous 5-HT of the K +-evoked release of [ 3H]5-HT. The data suggest that some β-adrenoceptor antagonists may behave as mixed agonists-antagonists at the 5-HT autoreceptor. Synaptosomes in superfusion appear to be particularly suitable to study separately agonistic compared to antagonistic activity of compounds having a mixed agonist-antagonist profile.

Serotonin autoreceptor in rat hippocampus: pharmacological characterization as a subtype of the 5-HT1 receptor.

The 5-hydroxytryptamine (5-HT) autoreceptors mediating inhibition of [3H]5-HT release in rat hippocampus have been characterized pharmacologically in terms of 5-HT receptor subtype by using superfused synaptosomes depolarized with 15 mM KCl. Exogenous 5-HT inhibited in a concentration-dependent way (pEC30 = 8.74) the K+-evoked release of [3H]5-HT. Methiothepin shifted the concentration-response curve of 5-HT to the right (pA2 = 8.62). The 5-HT2 receptor antagonists, ketanserin, methysergide or spiperone were ineffective against 5-HT. The 5-HT1 receptor agonist, 5-methoxy-3-[1,2,3,6-tetrahydropyridin-4-yl]-1H-indole (RU 24969) mimicked 5-HT and was equipotent as an inhibitor of the release of [3H]5-HT. In contrast, the putative 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was almost ineffective at 1 microM. Finally, (-)propranolol, used as a non-selective 5-HT1A/5-HT1B receptor antagonist, shifted to the right (pA2 = 7.91) the concentration-response curve of 5-HT whereas the 5-HT1C receptor antagonist mesulergine was ineffective. In conclusion, 5-HT nerve terminals of rat hippocampus possess autoreceptors which appear to belong to the 5-HT1B subtype.

Pharmacological characterization of release-regulating serotonin autoreceptors in rat cerebellum

The release of [ 3H]5-hydroxytryptamine ([ 3H]5-HT) evoked by 15 mM KCl in superfused rat cerebellum synaptosomes was inhibited by 5-HT (pEC 30=8.73). Methiothepin antagonized 5-HT (pA 2=9.28); ketanserin, methysergide, cinanserin and spiperone were ineffective. The receptors involved were activated (pEC 30=8.90) by the 5-HT 1 agonist 5-methoxy-3-[1,2,3,6-tetrahydropyridin-4-yl]-1H-indole (RU 24969)·(−)Propranolol shifted to the right (pA 2=8.05) the dose-response curve of 5-HT. The 5-HT 1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was ineffective. In conclusion, autoreceptors are present on 5-HT nerve endings in rat cerebellum and appear to belong to the 5-HT 1B subtype.

Autoregulation of fetal serotonergic neuronal development: Role of high affinity serotonin receptors

A microculture technique was used to study the factors regulating the development of fetal rat serotonergic neurons. Mesencephalic raphe cells from E14 rats co-cultured with hippocampal cells from E18 were grown for up to 4 days in the presence of various agents known to alter serotonergic function in the mature brain. Pargyline (a non-specific monoamine oxidase inhibitor) alone and with serotonin (10 −8 to 10 −6 M) both inhibited growth of serotonergic neurons as assessed by uptake of [ 3H]serotonin. 5-methoxytryptamine (5-MT), a serotonin agonist with selectivity for serotonin autoreceptors, inhibited growth at low concentrations, but this inhibition was overcome at higher concentrations. Using immunocytochemistry with a primary anti-serotonin antibody, 5-MT was observed to produce stunted processes, increase autoinnervation and lead to neuronal death. A model is proposed whereby high affinity serotonin receptors in fetal brainstem tissue and in fetal forebrain tissue regulate direction and extent of growth. We have confirmed the presence of these receptors using a direct binding assay.

Autoreceptors and alpha 2-adrenoceptors at the serotonergic axons of rabbit brain cortex.

Slices of the rabbit occipito-parietal cortex were preincubated with 3H-serotonin and then superfused and stimulated electrically (2 min at 3 Hz). In the absence of drugs, the stimulation-evoked overflow of tritium was approximately 3% of the tritium content of the tissue. Unlabelled serotonin and 5-carboxamido-tryptamine, when administered in the presence of 6-nitroquipazine, reduced the evoked overflow of tritium. Their effects were antagonized by metitepin (apparent pA2 value 8.1) and (+/-)-cyanopindolol (apparent pA2 value 6.4). Metitepin, but not cyanopindolol, increased evoked tritium overflow; the effect of metitepin was greater in the presence than in the absence of nitroquipazine. The evoked overflow of tritium was also depressed by clonidine, an effect antagonized by idazoxan (apparent pA2 value 7.0) but not by prazosin. Phenylephrine caused a decrease only at high concentrations that simultaneously accelerated basal tritium efflux. Prazosin and idazoxan did not change evoked tritium overflow, and phentolamine increased it significantly only when administered in the presence of (+)-oxaprotiline. Rauwolscine produced an inhibition that was prevented by metitepin. It is concluded that the serotonergic axons of the rabbit occipitoparietal cortex possess presynaptic, release-inhibiting serotonin autoreceptors and alpha 2-adrenoceptors. The receptors appear to receive an input of endogenous serotonin and, to a lesser extent, noradrenaline, under the conditions of these in vitro experiments.

Stimulation of the serotonin autoreceptor prevents the calcium-calmodulin-dependent increase of serotonin biosynthesis in rat raphe slices.

The role of the serotonin (5-hydroxytryptamine) autoreceptor in the regulation of the activity of tryptophan hydroxylase was investigated in rat raphe slices. The activity of tryptophan hydroxylase was estimated by measuring the accumulation of 5-hydroxytryptophan in the presence of inhibition of aromatic L-amino acid decarboxylase using 3-hydroxy-4-bromobenzyloxy-amine by HPLC with fluorescence detection. Serotonin and its agonists N,N-dimethyl-5-methoxytryptamine and 1-(m-chlorophenyl)-piperazine reduced the formation of 5-hydroxytryptophan to 50-60% at 10(-5) M. The effect of serotonin was reversed by 10(-5) M methiothepin, an antagonist of the serotonin autoreceptor. The calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5), dose-dependently reduced the basal formation of 5-hydroxytryptophan to 40-50% at 10(-6) and 10(-4) M, respectively. W-7 also reduced the activated formation by A-23187 or dibutyryl cyclic AMP in a dose-dependent manner. W-7 had no effect on 5-hydroxytryptophan formation reduced by serotonin at 10(-5) M. These results suggest that the role of the serotonin autoreceptor was related to the prevention of the calcium-calmodulin-dependent activation of tryptophan hydroxylase.

8-OH-DPAT-induced hyperphagia: Its neural basis and possible therapeutic relevance

The pharmacological and neurochemical bases of hyperphagia induced by the serotonin agonist 8-OH-DPAT were examined. In addition, the possible therapeutic potential of 8-OH-DPAT and related drugs in the treatment of anorexic pathology was assessed in an animal model of anorexia (as induced by acute immobilization stress). In normal rats 8-OH-DPAT elicited feeding after peripheral injection and after intracerebral application to the brainstem raphé nuclei. Feeding elicited by peripheral injection of the drug was attenuated by pretreatment with the serotonin synthesis inhibitor para-chlorophenylalanine. Following a hyperphagic dose of 8-OH-DPAT, brain serotonin metabolism was reduced, particularly in midbrain and pons-medulla. Our interpretation of these data is that 8-OH-DPAT elicits feeding via an agonist action on serotonin autoreceptors in the raphé nuclei. These receptors are probably of the 5-HT1A subtype as 8-OH-DPAT has a high affinity for this receptor and other putative 5-HT1A agonist (i.e. buspirone, TVX Q 7821) also elicit feeding. In contrast, putative 5-HT1B agonists (i.e. RU-24969 and quipazine) decrease feeding and cause anorexia. 8-OH-DPAT and other 5-HT1A agonists attenuated the anorexia and body weight loss caused by immobilization stress. Therefore, it seems possible that 5-HT1A agonists may be clinically useful in the treatment of anorexia.

Presynaptic serotonin receptor-mediated response in mice attenuated by antidepressant drugs and electroconvulsive shock.

Ligand-binding studies have demonstrated two types of serotonin (5-HT) receptor, 5-HT1 and 5-HT2, in the brains of rodents and there is additional evidence for the existence of 5-HT1 subtypes. Recently a new drug, 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT), has been identified which shows high selectivity for binding to 5-HT1 (possibly 5-HT1A) receptors and which binds to presynaptic serotonin autoreceptors in some regions of rat brain. We have shown previously, that this compound produces a hypothermic response in mice, probably via an agonist action at serotonin presynaptic receptors. Here we show that a wide range of antidepressant treatments decrease the hypothermic response to 8-OH-DPAT over a time course comparable to the onset of therapeutic action. Interestingly, repeated electroconvulsive shock (ECS) has the same effect. We propose that this change is relevant to the mechanism of action of antidepressant drugs.

Antidromic activation of dorsal raphe neurons from neostriatum: Physiological characterization and effects of terminal autoreceptor activation

Three types of neurons, distinguished on the basis of their spontaneous firing rates and patterns, extracellularly recorded waveforms and responses to neostriatal stimulation, were observed in the dorsal raphe nucleus in urethane-anesthetized rats. Type 1 neurons (presumed to be serotonergic) fired spontaneously from 0.1 to 3 spikes/s in a regular pattern, with initial positive-going bi- or triphasic action potentials. Type 1 cells exhibited long-latency antidromic responses to neostriatal stimulation (mean±S.E.M.24.9 ± 0.3ms) that sometimes occurred at discrete multiple latencies, and supernormal periods persisting up to 100 ms following spontaneous spikes. Type 2 cells fired spontaneously in an irregular, somewhat bursty pattern from 0 to 2 spikes/s with initial negative-going biphasic spikes, and were antidromically activated from neostriatal stimulation at shorter latencies than Type 1 cells (21.8 ± 0.9ms). Type 3 cells were characterized by initial positive-going biphasic waveforms and displayed a higher discharge rate (5–30 spikes/s) than Type 1 or Type 2 cells. Type 3 cells could not be antidromically activated from neostriatal stimulation. The relatively long conduction time to neostriatum of the Type 1 presumed serotonergic neuron is discussed with respect to previous interpretations of the synaptic action of serotonin in the neostriatum. In conjunction with these antidromic activation studies, the neurophysiological consequences of serotonergic terminal autoreceptor activation were examined by measuring changes in the excitability of serotonergic terminal fields in the neostriatum following administration of the serotonin autoreceptor agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). The excitability of serotonergic terminal fields was decreased by intravenous injection of 40 μg/kg 5-MeODMT, and by infusion of 10–50 μM 5-MeODMT directly into the neostriatum. These results are interpreted from the perspective of mechanisms underlying autoreceptor-mediated regulation of serotonin release.

Modulation of hippocampal serotonin (5-HT) release by endogenous adenosine

Slices of rabbit hippocampus were preincubated with [ 3H]serotonin then superfused continuously and stimulated twice electrically. The stimulation-evoked overflow of tritium was Ca 2+-dependent, tetrodotoxin-sensitive and subject to modulation by serotonin autoreceptors. It was decreased by various adenosine receptor agonists in an order of potency that was typical for A 1 − (R i − ) receptors: N 6-cyclohexyladenosine > (−)N 6-phenylisopropyladenosine >5′-N-ethylcarboxamideadenosine> (+)N 6-phenylisopropyladenosine = adenosine.The effects of the agonists were antagonized by 8-phenyltheophylline. The hypothesis that endogenous adenosine influences hippocampal serotonin release is supported by the following findings: both the adenosine receptor antagonists (theophylline or 8-phenyltheophylline (10 μM, each)) and the enzyme adenosine deaminase (10 μg/ml) increased, whereas R-E 244 (3 μM), an inhibitor of adenosine uptake, significantly decreased the evoked tritium overflow. When 8-phenyltheophylline was present throughout superfusion the effects of both R-E 244 and exogenous adenosine deaminase were abolished. It is concluded that serotonin release in the rabbit hippocampus is depressed by endogenous adenosine via A 1 − (R i − ) receptors.