Metabotropic glutamate receptor 5 antagonist-induced stimulation of hypothalamic-pituitary-adrenal axis activity: interaction with serotonergic systems

The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP) produces anxiolytic or antidepressant effects in several rodent models through incompletely described mechanisms. Anxiolytics and antidepressants share several neuroendocrine features, including acute activation of the hypothalamic-pituitary-adrenal (HPA)-axis, desensitization of neuroendocrine responses with repeated dosing, and desensitization of the HPA axis to 5-HT1A agonist stimulation. We characterized these neuroendocrine parameters in rats treated systemically with MPEP and compared them to those induced by the anxiolytic buspirone. Acutely, MPEP dose-dependently (0.1-10 mg/kg i.p.) increased plasma corticosterone concentrations. These responses were blocked by 50% with the 5-HT1A antagonist WAY100635. The corticosterone responses to both 3 mg/kg MPEP and buspirone were decreased by 80% after 5 days of twice-daily injections. Repeated injection with MPEP decreased HPA-axis sensitivity to buspirone challenge by 75%. This desensitization was not associated with changes in mGluR5 or 5-HT1A receptor binding properties, expression of G-protein subunits coupled to these receptors, or in 5-HT-stimulated binding of [(3)H]-GTPgammaS to membranes.We conclude that MPEP acutely disinhibits the HPA axis, in part through uncharacterized changes in serotonergic signaling. Desensitization of 5-HT1A responses after repeated MPEP administration may indicate that, like other anxiolytics and antidepressants, plasticity in 5-HT signal transduction pathways has occurred.     

Neurophysiological effects of buspirone and isapirone in the hippocampus: comparison with 5-hydroxytryptamine

The neurophysiological effects of two novel anxiolytic compounds, buspirone and isapirone (TVX Q 7821), were compared with those of 5-hydroxytryptamine (5-HT) on the population spike in the CA1 region of the rat hippocampal slice. Micromolar concentrations of the two drugs mimicked the inhibitory effect of 5-HT but unlike 5-HT they did not produce any significant excitation. In slices in which 5-HT was purely excitatory, there was a marked reduction in the inhibitory response to these agents. The effect of isapirone was antagonised by spiperone. These results suggest that buspirone and isapirone are agonists for 5-HT1A receptors in the hippocampus.     

The interaction of serotonin depletion with anxiolytics and antidepressants on reticular-elicited hippocampal RSA

Hippocampal rhythmical slow activity (RSA) can be elicited by stimulation of the midbrain reticular formation. Buspirone, chlordiazepoxide and imipramine are all anxiolytic and have all been shown to decrease the frequency of RSA. All these compounds have been suggested to affect, directly or indirectly, 5-HT metabolism and function. The present experiments tested the possibility that buspirone, chlordiazepoxide and imipramine reduce RSA frequency via 5-HT1A autoreceptors. Rats received buspirone (10 mg/kg), chlordiazepoxide (5 mg/kg) and imipramine (30 mg/kg) after 5-HT depletion with p-chlorophenylalanine (PCPA, 100 mg/kg/day for 3 days or 350 mg/kg/day for 2 days) or after pretreatment with 5-HTP (40 mg/kg, to replete 5-HT) as well as pCPA. The frequency-reducing effects produced by buspirone and chlordiazepoxide were unchanged by either dose of pCPA, whereas the frequency-reducing effect of imipramine was completely eliminated by the high dose of pCPA. Pindolol, but not beta-blockers (a combination of metoprolol and ICI118 551), was able to block the effect of imipramine on RSA frequency. Pindolol has been reported to block the effects of buspirone but not chlordiazepoxide. These data suggest that: (1) buspirone obtains its frequency-reducing effects via pre- or post-synaptic 5-HT1A receptors rather than 5-HT1A autoreceptors; (2) chlordiazepoxide obtains its frequency-reducing effect via benzodiazepine receptors and GABA with no direct or indirect involvement of 5-HT systems; and (3) imipramine obtains its frequency-reducing effect by increasing the availability of 5-HT at 5-HT1A receptors which are not autoreceptors.     

Pertussis toxin-sensitive guanine nucleotide-binding protein(S) couple adenosine A1 and 5-hydroxytryptamine1A receptors to the same effector systems in rat hippocampus: biochemical and electrophysiological studies

Distinct membrane receptors that elicit similar cellular responses may share elements of signal transduction. In the present study, rat hippocampal adenosine (AD) and 5-hydroxytryptamine (5-HT) receptors were chosen to test this possibility using biochemical and electrophysiological techniques. Responses elicited by the AD receptor that mediates the inhibition of forskolin-stimulated adenylyl cyclase activity in rat hippocampal membranes and hyperpolarization of resting membrane potential (RMP) in rat hippocampal pyramidal cells were characterized and compared, in the same preparation, with those analogous responses elicited by the 5-HT1A receptor. A series of AD agonists including the selective AD A1 agonist (R)-phenylisopropyladenosine [(R)-PIA] inhibited forskolin-stimulated adenylyl cyclase activity in rat hippocampal membranes in a concentration-dependent manner. Cyclopentyltheophylline (CPT), a selective AD A1 antagonist, was a potent, competitive antagonist of this response with a dissociation constant (Kb) of 6 nM (Schild analysis). The rank order of agonist EC50 values and antagonist Kb values, as well as stereoselectivity, are consistent with the classification of this receptor as the AD A1 receptor. Spiperone, a potent 5-HT1A antagonist, competitively antagonized 5-HT-mediated inhibition of forskolin-stimulated adenylyl cyclase activity in rat hippocampal membranes with a Kb value of 14 nM. Intracellular recording techniques revealed that AD, (R)-PIA, 5-HT, and 5-carboxyamidotryptamine (5-CT) elicited concentration-dependent hyperpolarization of RMP within the same hippocampal pyramidal cell. The maximal hyperpolarization obtained for the AD or 5-HT analogs was the same for individual pyramidal cells. CPT and spiperone antagonized the hyperpolarization by (R)-PIA and 5-CT, respectively. Saturating concentrations of spiperone failed to antagonize (R)-PIA-mediated responses and CPT did not block responses elicited by 5-HT in either the biochemical or electrophysiological preparations. The combination of saturating concentrations of 5-HT and (R)-PIA evoked nonadditive biochemical responses relative to those observed with (R)-PIA alone. Similarly, electrophysiological experiments conducted under voltage-clamp conditions demonstrated that maximally effective concentrations of AD and 5-CT exhibited nonadditive behavior. Because the amount of outward current elicited when these agonists were coperfused was significantly less than the algebraic sum of the currents evoked individually by these agents, we infer that a population of AD A1 and 5-HT1A receptors activates a common pool of guanine nucleotide-binding proteins.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparison of antidepressant activity in 4- and 40-week-old male mice in the forced swimming test: involvement of 5-HT1A and 5-HT1B receptors in old mice

RATIONALE: A recent study suggested that selective serotonin reuptake inhibitors were inactive in 40-week-old male mice in the mouse forced swimming test, possibly because of alteration of 5-HT1 receptors. OBJECTIVES: The present study was aimed at investigating the action of various antidepressant drugs in 4- and 40-week-old male mice using the mouse forced swimming test and determining the involvement of 5-HT1A and 5-HT1B receptors mediating the effects. METHODS: Different classes of antidepressants [imipramine (tricyclic), maprotiline (noradrenline reuptake inhibitor), venlafaxine (mixed serotonin and noradrenaline reuptake inhibitors), fluvoxamine and sertraline (selective serotonin reuptake inhibitor)] were tested in the same randomised experimental session, alone and in combination with 5-HT1A and 5-HT1B receptor agonists [buspirone (partial 5-HT1A agonist), anpirtoline (5-HT1B agonist)] in the mouse forced swimming test. RESULTS: All antidepressants were found to be active in the mouse forced swimming test in 4-week-old mice and 40-week-old mice, with the exception of fluvoxamine in the 40-week-old mice. The anti-immobility effect after antidepressant administration was higher in 4-week-old male mice than in 40-week-old male mice. Venlafaxine is the most active antidepressant drug in 40-week-old mice. Prior administration of buspirone (0.06 mg/kg, i.p.) or anpirtoline (1 mg/kg, i.p.) enhanced the antidepressant-like effects in 4-week-old mice (except in the case of sertraline, 8 mg/kg). In elderly mice, only prior administration of buspirone enhanced the antidepressant-like effects of fluvoxamine. A neurochemical study showed that significantly higher serotonin and dopamine concentrations were found in 40-week-old control mice brains than 4-week-old control mice brains but that the noradrenaline concentration is higher in 4-week-old mice. CONCLUSION: Tricyclic, noradrenaline reuptake inhibitors and serotonin reuptake inhibitors are more active in 4-week-old mice than 40-week-old mice. Our results suggested that 5-HT1B receptors may be more altered than 5-HT1A receptors in 40-week-old mice.     

Effects of DAU 6215, a novel 5-hydroxytryptamine3 (5-HT3) antagonist on electrophysiological properties of the rat hippocampus.

1. The aim of the present study was to test the effects of DAU 6215 (endo-N-(8-methyl-8-azabicyclo-[3.2.1]-octo-3-yl)-2,3-dihydro-2-ox o-1H- benzimidazole-1-carboxamide carboxamide hydrochloride), a newly synthesized, selective 5-hydroxytryptamine3 (5-HT3) antagonist, on the cell membrane properties and on characterized 5-HT-mediated responses of pyramidal neurones in the hippocampal CA1 region. 2. Administration of DAU 6215, even at concentrations several hundred fold its Ki, did not affect the cell membrane properties of pyramidal neurones, nor modify extracellularly recorded synaptic potentials, evoked by stimulating the Schaffer's collaterals. 3. Micromolar concentrations (15-30 microM) of 5-HT elicited several responses in pyramidal neurones that are mediated by distinct 5-HT receptor subtypes. DAU 6215 did not antagonize the 5-HT1A-induced membrane hyperpolarization and conductance increase, a response that was blocked by the selective 5-HT1A antagonist NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phtalamido)butyl- piperazine). Similarly, DAU 6215 did not affect the membrane depolarization and decrease in amplitude of the afterhyperpolarization, elicited by the activation of putative 5-HT4 receptors. 4. 5-HT increased the frequency of spontaneous postsynaptic potentials (s.p.s.ps) recorded in pyramidal neurones loaded with chloride. In agreement with previous observations, most of the s.p.s.ps were reversed GABAergic events, produced by the activation of 5-HT3 receptors on interneurones, because they persisted in the presence of the glutamate NMDA and non NMDA antagonists, D-aminophosphonovaleric acid (APV; 50 microM) and 6,7-dinitroquinoxaline-2,3-dione (DNQX; 25 microM), and were elicited by the selective 5-HT3 agonist, 2-methyl-5-HT (2-Me-5-HT, 50 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacological and functional analysis of a novel serotonin receptor in the rat hippocampus

Administration of serotonin (5-hydroxytryptamine, 5-HT) to pyramidal cells of the CA1 region of the hippocampus results in a hyperpolarizing response which is followed by a rebound depolarization and a decrease in the calcium-activated afterhyperpolarization (AHP). While the hyperpolarizing response has been previously shown to be mediated by receptors of the 5-HT1A subtype, the identity of the receptor(s) involved in the depolarizing response and decrease of the AHP have not been identified. In the present study the effectiveness of a series of 5-HT receptor antagonists in blocking the membrane depolarization and reduction of the AHP was assessed. While a variety of 5-HT1 and 5-HT2 antagonists were found to be ineffective, the substituted benzamide BRL 24924 was found to be a potent and selective antagonist of the 5-HT-induced depolarization and decrease in the AHP in this region. This effect however appeared unrelated to the ability of this compound to block 5-HT3 receptors, as ICS 205-930 and MDL 72222 were markedly less efficacious in blocking these effects of 5-HT. Upon blockade of 5-HT1A receptors, 5-HT elicits a depolarization which is accompanied by a marked increase in excitability. These effects were also dose-dependently antagonized by BRL 24924. The present results thus suggest the presence in the CA1 region of the hippocampus of a novel 5-HT receptor at which BRL 24924 functions as a selective antagonist and which is capable of mediating slow excitatory responses in central neurons.     

Thermoregulatory responses to serotonin (5-HT) receptor stimulation in the rat. Evidence for opposing roles of 5-HT2 and 5-HT1A receptors

The effects of serotonergic agonists and antagonists on the body temperatures of rats were investigated. The administration of the serotonin (5-HT) agonist 6-chloro-2(1-piperazinyl)-pyrazine (MK-212) produced a dose-related increase in body temperature. A maximal increase in body temperature of approx. 1.1°C was observed 30min after the administration of 3 mg/kg of MK-212. In contrast, administration of the putative 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) resulted in marked, dose-related hypothermic responses. Body temperatures were decreased approx. 3°C 30 min after an injection of 0.3 mg/kg of 8-OH-DPAT. Body temperatures were affected differentially by 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). Large doses (3–10 mg/kg) of 5-MeODMT elicited hyperthermic responses, whereas small doses (0.5–1.0 mg/kg) produced hypothermie responses. Treatment of rats with ketanserin (3 mg/kg) completely prevented the hyperthermic effects of 5-MeODMT, and, in fact, converted a hyperthermic response to 5-MeODMT into a marked hypothermie response. Ketanserin (0.1–1.0 mg/kg) selectively antagonized the hyperthermic response to MK-212 but did not alter the hypothermic effect of 8-OH-DPAT. Mianserin (10 mg/kg) and pirenperone (0.03 mg/kg) also selectively antagonized hyperthermia induced by MK-212. In contrast, pindolol (0.03–0.1 mg/kg) and methiothepin (10 mg/kg) selectively antagonized hypothermia induced by 8-OH-DPAT but did not alter hyperthermia induced by MK-212. Spiperone (0.1–3 mg/kg) and pizotifen (10 mg/kg) attenuated the effects of both 8-OH-DPAT and MK-212. Xylamidine, a peripheral 5-HT antagonist, had no significant effect on hyperthermia induced by MK-212 or hypothermia induced by 8-OH-DPAT. It also was found that treatment of rats with the 5-HT₂ antagonists ketanserin or pirenperone alone resulted in a decrease in body temperature, whereas the administration of pindolol produced an increase in body temperature. On the basis of the present findings, it is concluded that the hyperthermic responses following the administration of MK-212 are mediated by central 5-HT₂ receptors and that the hypothermic responses to 8-OH-DPAT involve the activation of central 5-HT_1A receptors. The differential effects of 5-MeODMT on body temperature suggest that this indoleamine can activate both subtypes of 5-HT receptor.     

The 5-HT(1A) receptor agonist F 13640 attenuates mechanical allodynia in a rat model of trigeminal neuropathic pain

It has been reported that the treatment with a tricyclic antidepressant imipramine induces an increase in the sensitivity of 5-HT(1A) receptors and a decrease in the sensitivity of 5-HT(4) receptors in the rat hippocampus. 5-HT(1A) receptor agonists and neuroleptics also affect 5-HT(1A) receptors in different brain areas; therefore, it was of interest to compare their effects on hippocampal 5-HT receptors with the influence of the well-established antidepressant imipramine. We studied the effects of repeated treatment with imipramine, the 5-HT(1A) receptor agonists 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) and buspirone, and the neuroleptics haloperidol and clozapine on the sensitivity of rat hippocampal CA1 neurons to 5-HT(1A)- and 5-HT(4) receptor activation. Imipramine was administered for 21 days (10 mg/kg p.o., twice daily), 8-OH-DPAT for 7 days (1 mg/kg s.c., twice daily) and buspirone for 21 days (5 mg/kg s.c., twice daily). The rats received haloperidol (1 mg/kg) and clozapine (30 mg/kg) for 6 weeks in drinking water. Hippocampal slices were prepared 2 days after the last treatment with imipramine, 8-OH-DPAT or buspirone, and 5 days after the last treatment with the neuroleptics. Using an extracellular in vitro recording, we studied changes in the amplitude of stimulation-evoked population spikes, induced by 5-HT, 8-OH-DPAT and the 5-HT(4) receptor agonist zacopride. Activation of 5-HT(1A) receptors decreased, while activation of 5-HT(4) receptors increased the amplitude of population spikes. Imipramine significantly enhanced the inhibitory effects of 5-HT and 8-OH-DPAT, and attenuated the excitatory effect of zacopride. No other treatment used in the present study changed the sensitivity of hippocampal CA1 neurons to 5-HT(1A) and 5-HT(4) receptors activation. These findings indicate that adaptive changes in the sensitivity of hippocampal neurons to 5-HT(1A) and 5-HT(4) receptors agonists are specific to imipramine and may thus-at least partly-mediate its effects.     

Serotonin 1A receptor activation and hypothermia in humans: lack of evidence for a presynaptic mediation.

The hypothermia produced by 5-HT1A agonists had initially been claimed to be caused by the activation of cell body 5-HT1A autoreceptors resulting in decreased 5-HT transmission in laboratory animals. In order to address this issue in humans, 12 healthy volunteers underwent a dietary tryptophan depletion paradigm to decrease 5-HT availability, under double-blind conditions, during which body temperature was monitored following oral administration of the 5-HT1A agonist buspirone (30 mg). In addition, plasma prolactin and growth hormone evaluations, two responses that are mediated via the direct activation of postsynaptic 5-HT1A receptors, were determined. The hypothesis was that if responses are mediated by decreased transmission at postsynaptic 5-HT1A receptors, resulting from dampened 5-HT release as a consequence of 5-HT1A autoreceptors activation, then responses to the exogenous 5-HT1A agonist should be attenuated when 5-HT availability has been markedly decreased beforehand. Buspirone produced the same significant increase in prolactin and growth hormone in the tryptophan-depleted state as in the control condition. Similarly, the degree of hypothermia produced by buspirone was not significantly different in the two experimental conditions. In conclusion, these results strongly suggest that the hypothermia and the increases in prolactin and growth hormone produced by buspirone are attributable to the enhanced activation of postsynaptic 5-HT1A receptors, and not to a decrease in 5-HT transmission resulting from the activation of the 5-HT1A cell body autoreceptors on 5-HT neurons.     

Prolonged corticosterone treatment alters the responsiveness of 5-HT1A receptors to 8-OH-DPAT in rat CA1 hippocampal neurons

Hippocampal 5-HT(1A) receptors have been shown to be suppressed by glucocorticoids in a variety of animal studies, however the molecular mechanism and the functional meaning of this effect are still not well understood. The present study was designed to investigate the impact of repeated administration of corticosterone (10 mg/kg s.c. twice daily for 7 days) on the functional consequences of 5-HT(1A) receptor stimulation measured electrophysiologically in hippocampal slices. Additionally, the effects of corticosterone on 5-HT(1A) receptor binding and on receptor mRNA levels in the hippocampus were studied. Prolonged, but not acute treatment with corticosterone attenuated (+/-)-8-hydroxy-2-di- N-propylamino)tetralin hydrobromide (8-OH-DPAT)-induced inhibition of population spikes, and 8-OH-DPAT-induced hyperpolarization in rat CA1 hippocampal neurons. Chronic, but not acute treatment with corticosterone also decreased 5-HT(1A) receptor binding in the CA1 region (in the ventral part only) and the dentate gyrus. A single dose of corticosterone increased [(3)H]8-OHDPAT binding in the dentate gyrus and in the CA3 and CA4 hippocampal regions. Only acute, but not prolonged treatment with corticosterone decreased the level of 5-HT(1A) receptor mRNA in the CA1 region and dentate gyrus of the hippocampus. 5-HT turnover in the hippocampus was not influenced by chronic corticosterone. It is concluded that a chronically elevated level of corticosterone can induce functional desensitization of 5-HT(1A) receptors in the CA1 area of the hippocampus, although this effect is not always followed consequently by decreases in 5-HT(1A) receptor synthesis in this or other areas of the hippocampus.     

Effect of pindolol on endocrine and temperature responses to buspirone in healthy volunteers

Ten healthy subjects received buspirone (30 mg orally) with and without pre-treatment with the 5-HT1A receptor antagonist, pindolol (80 mg over 3 days). Following pindolol treatment the growth hormone and hypothermic responses to buspirone were significantly decreased. There was also a delay in the onset of the prolactin response to buspirone but the total amount of prolactin secretion, calculated as area under the curve, was not significantly reduced. The data suggest that the growth hormone and hypothermic responses to buspirone in humans are mediated by 5-HT1A receptors, but an explanation founded on pharmacokinetic factors cannot presently be excluded. Both this latter possibility and the lack of selectivity of pindolol for 5-HT receptors indicate the need for the further neuroendocrine studies of the mode of action of buspirone, preferably with more selective 5-HT1A receptor antagonists.     

A distinct distribution of functional presynaptic 5-HT receptor subtypes on GABAergic nerve terminals projecting to single hippocampal CA1 pyramidal neurons

5-HT is known to modify the excitability of GABAergic interneurons projecting to hippocampal CA1 neurons. In this study we investigate the presence and functionally characterize the 5-HT receptor subtypes found on the presynaptic nerve terminals of these GABAergic neurons. Using conventional whole-cell patch recording, we confirmed that the 5-HT_1A agonist, 8-hydroxy-2-dipropylaminotetralin, presynaptically decreased electrically evoked GABA release while the 5-HT₃ agonist, m-chlorophenylbiguanide (mCPBG), presynaptically facilitated release. Using the ‘synaptic bouton preparation’, where CA1 neurons are acutely isolated with functional nerve terminals/boutons remaining adherent, we next showed that these receptor subtypes are found presynaptically. We next used the technique of focal stimulation of a single bouton in this preparation to further investigate the distribution of these 5-HT receptor subtypes. We found that all boutons contained inhibitory 5-HT_1A receptors while a subset of boutons showed both 5-HT_1A and excitatory 5-HT₃ receptors. No boutons were detected which contained only 5-HT₃ receptors. Our studies show that presynaptic 5-HT receptor subtypes are found presynaptically and are not uniformly distributed. This provides another potential mechanism whereby 5-HT can modulate GABA release and hence the excitability of hippocampal neurons.     

Suppression of the hypo- and hyperthermic responses to 5-HT agonists following the repeated administration of monoamine oxidase inhibitors

Hypo- and hyperthermic responses resulting from the activation of putative 5-HT_1A and 5-HT₂ receptors, respectively, were examined after the chronic treatment of rats with monoamine oxidase inhibitors. The treatment of rats for 4 or 7 days with nialamide (40 mg/kg, twice daily) resulted in a suppression of the hypothermic effect of the 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.05–0.25 mg/kg, SC). The decrease in body temperature elicited by a low dose of 5-methoxy-N, N-dimethyltryptamine (5MeODMT, 1 mg/kg) also was diminished in rats treated chronically with nialamide. The administration of a high dose of 5MeODMT (5 mg/kg) resulted in a hyperthermic response, which was also attenuated after the repeated administration of nialamide. The repeated administration of clorgyline (a selective inhibitor of type A MAO) or deprenyl (a selective inhibitor of type B MAO) failed to alter the hypothermic effect of 8-OH-DPAT. However, in animals treated chronically with both clorgyline and deprenyl, a suppressed response to 8-OH-DPAT was observed. In view of the concept that the hypo- and hyperthermic responses to 5-HT agonists are mediated by 5-HT_1A and 5-HT₂ receptor subtypes, respectively, it is concluded that the responsiveness of these 5-HT receptor subtypes involved in thermoregulatory responses is decreased following chronic treatment of rats with monoamine oxidase inhibitors. It appears that inhibition of both type A and B MAO is necessary for this desensitization process.     

Pharmacological analysis of the variation in behavioural responses to tryptamine in five strains of mice

The effects of tryptamine on behaviour were studied in five strains of mice. There were significant strain differences in the intensity of the 5-HT syndrome (head weaving and hindlimb abduction) and head twitch responses. The intensity of the 5-HT syndrome correlated significantly with the brain tryptamine content, although the occurrence of head twitch was unrelated. The 5-HT₂ receptor antagonist, ketanserin, antagonized the head twitch responses elicited by tryptamine without affecting the head weaving or hindlimb abduction. Metergoline, an antagonist of both the 5-HT₁ and 5-HT₂ receptors, strongly inhibited both the 5-HT syndrome and the head twitch responses in all five strains. [³H]5-HT and [³H]ketanserin binding in the frontal cortex of the five strains of mice was also studied. The number of [³H]5-HT binding sites did not differ among the five strains of mice although there was a noticeable difference in the number of [³H]ketanserin binding sites. These results suggest that the 5-HT syndrome and the head twitch responses induced by tryptamine are linked separately with the 5-HT₁ and 5-HT₂ receptors, respectively. The strain differences in the tryptamine-induced 5-HT syndrome can be explained by the different levels of brain tryptamine. The differences in the head twitch responses in the mice are probably derived from the differences in the 5-HT₂ receptor binding sites in the brain and the content of brain tryptamine.     

EEG effects of buspirone and pindolol: a method of examining 5-HT<Subscript>1A</Subscript> receptor function in humans

Rationale. An involvement of 5-HT_1A receptors is postulated in the pathophysiology of affective disorders and mechanism of action of antidepressants. Methods for studying their functional integrity in humans are, however, limited. Preliminary data suggests that activation of somatodendritic 5-HT_1A receptors cause a negative shift in the EEG frequency spectrum. Animal research suggests that pindolol is an agonist at these receptors but an antagonist at postsynaptic 5-HT_1A receptors. Objective. We postulated that while pindolol would antagonise known postsynaptic mediated neuroendocrine responses to the 5-HT_1A agonist buspirone, both drugs would have a similar effect on the EEG frequency spectrum. Methods. Fourteen healthy men were administered placebo or pindolol (20 mg orally) 90 min before placebo or buspirone (30 mg orally) in a double blind cross-over study. Plasma prolactin and growth hormone were assayed and EEGs recorded before and after drug administration. Results. A significant negative shift in the EEG frequency spectrum was found for both buspirone and pindolol, with the combination producing a similar effect to each drug alone. In contrast, the neuroendocrine response to buspirone was significantly attenuated by pindolol. Conclusions. The data obtained are consistent with the EEG effects of buspirone and pindolol being mediated by somatodendritic 5-HT_1A receptors, in contrast to the neuroendocrine response, which is known to be mediated by postsynaptic receptors. The development of this novel method of assessing somatodendritic 5-HT_1A receptors in humans is a potentially important advance which may allow the testing of hypotheses of its involvement in depression and response to antidepressants. Electronic Publication     

The effects of acute and repeated administration of T3 to mice on 5-HT1 and 5-HT2 function in the brain and its influence on the actions of repeated electroconvulsive shock

The effects of the administration of L-triiodothyronine (T₃) On the function of 5-HT in the CNS and its influence on the actions of electroconvulsive shock have been examined in mice. A single injection of T₃ (100 μg/kg) had no effect 24 hr later on either 5-HT_1A-mediated hypothermia, induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.5 mg/kg) or the 5-HT_1B-mediated locomotor response to 5-methoxy-3-(l,2,3,6-tetrahydropyridin-4-yl) 1-H-indole (RU 24969; 50 ng i.c.v.). This treatment increased 5-HT₂-induced head-twitches, produced by 5-methoxy-N,N'-dimethyltryptamine (5-MeODMT; 2 mg/kg), but did not alter 5-HT₂ receptors in the frontal cortex, suggesting that this potentiation was mediated indirectly through a modulatory neurotransmitter. One injection of T₃ had no effect on the concentrations of 5-HT in the forebrain or mid/hindbrain, but increased 5-HIAA in the latter region. Daily injections of T₃ for 10 days attenuated the responses to both 8-OH-DPAT and RU 24969. Furthermore, 5-MeODMT-induced head-twitches returned to control values and this was accompanied by a 10% decrease in 5-HT₂ receptors in the cortex. Repeated administration of T₃ increased levels of 5-HT in mid/hindbrain and concentrations of 5-HIAA both here and in forebrain. Hence, treatment with T₃ attenuated the function of 5-HT_1A and 5-HT_1B receptors, but increased 5-HT₂-mediated responses, although the time-courses for these effects were different. Triiodothyronine also enhanced the synthesis and turnover of 5-HT in the brain of the mouse. Repeated electroconvulsive shock (90 V, 1 sec) decreased the hypothermia induced by 8-OH-DPAT. However, 5-MeODMT-induced head-twitches were enhanced by acute and repeated electroconvulsive shock. Administration of T₃ together with electroconvulsive shock did not alter the effects of electroconvulsive shock on 5-HT_1A-mediated hypothermia, but markedly potentiated its actions on 5-HT₂-mediated responses. These findings provide possible pharmacological evidence for the suggested antidepressant effects of T₃ and the potentiation of antidepressant therapy by this thyroid hormone.     

Characterization of 5-hydroxytryptamine-induced depolarizations in rat isolated vagus nerve

An additional component of the depolarization induced by 5-hydroxytryptamine (5-HT) in the rat isolated vagus nerve has recently been attributed to activation of 5-HT₄ receptors. To confirm and extend this finding, extracellular recordings of D.C. potentials were made using the ‘grease-gap’ technique during continuous superfusion of the isolated nerve. Beginning at 1 nM, 5-HT induced small depolarizations that displayed a slow onset. At concentrations 1 μM, large depolarizations with rapid onset were elicited. In the presence of the 5-HT₃ receptor antagonists, granisetron or ondansetron, 5-HT responses were diminished and exhibited an increased latency to peak. These small, slow depolarizations were not reduced by 5-HT₁ or 5-HT₂ receptor antagonists, but were potently inhibited by the 5-HT₄ receptor antagonist GR 113808 (pA₂ = 9.3), and mimicked by 5-methoxytryptamine (pEC₅₀ = 5.3). 5-HT₄-mediated responses were larger at 37°C than at 31°C, but also showed marked diminution with repeated 5-HT applications at concentrations greater than 1 μM. Conversely, 5-HT₃ receptor responses were potentiated at lower temperatures (31°C). Consistent with the reported positive coupling of 5-HT₄ receptors to adenylyl cyclase, forskolin and 8-Br-cAMP produced slowly developing depolarizations which were qualitatively similar to 5-HT₄ receptor activation. Pre-depolarization of nerves with 10 μM forskolin or 300 μM 8-Br-cAMP diminished the effect of 5-HT₄ receptors. This study has confirmed the presence of 5-HT₄ receptors on the vagus nerve of the rat and defined some conditions that optimize their pharmacological isolation. The rat isolated vagus nerve constitutes a simple and robust preparation for studying 5-HT₄ receptors in the peripheral nervous system.     

The pharmacology of the behavioral and hypothermic responses of rats to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)

The hypothermic and motor behavioural responses to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) have been investigated in the rat. The dose-effect relationship showed that hypothermia appeared at a lower dose than a definite motor syndrome. The hypothermic response to 8-OH-DPAT was attenuated following depletion of 5-hydroxytryptamine (5-HT) by repeated intraperitoneal (IP) administration of parachlorophenylalanine (200 mg/kg) or by injection of 5,7-dihydroxytryptamine (5,7-DHT, 100 g) into the region of the third ventricle; the motor behavioural response produced simultaneously was not. Indeed, after 5,7-DHT, it was increased. Quipazine (1 mg/kg, IP) antagonised the hypothermic response and facilitated the motor behaviour. Clenbuterol (2.5 mg/kg, IP) increased both hypothermic and motor responses. (±)-propranolol was without effect on the simple hypermotility produced by 8-OH-DPAT, although it is known to antagonise the hypothermic and stereotyped motor responses. It is concluded that 8-OH-DPAT probably produces its hypothermic effects by actions at 5-HT receptors located presynaptically on 5-HT neurones, while the stereotyped components of the serotonin syndrome appear to be mediated by post-synaptic receptors.     

The selective serotonin (5-HT)1A receptor ligand, S15535, displays anxiolytic-like effects in the social interaction and Vogel models and suppresses dialysate levels of 5-HT in the dorsal hippocampus of freely-moving rats

RATIONALE: The benzodioxane, S15535, possesses low intrinsic activity and marked selectivity at 5-HT1A receptors, hippocampal populations of which are implicated in anxious states. OBJECTIVE: Herein, we examined its potential anxiolytic actions in relation to its influence upon extracellular levels of 5-HT in the dorsal hippocampus of freely-moving rats. Its effects were compared with those of other anxiolytic agents: the 5-HT1A agonists, buspirone and 8-hydroxy-2-(di-n-propylamino)-tetralin HBr (8-OH-DPAT), the 5-HT2C antagonist, SB206,553 and the benzodiazepine, diazepam. METHODS: Potential anxiolytic actions were evaluated in the Vogel conflict paradigm (increase in punished responses) and the social interaction (SI) test (increase in active SI) in rats. Extracellular levels of 5-HT were determined by microdialysis. RESULTS: In analogy to diazepam. S15535 increased punished responses in the Vogel test. This action was dose dependently expressed over a broad (16-fold) dose range. Buspirone and 8-OH-DPAT were likewise active, but yielded highly biphasic dose-response curves. SB206,553 was dose dependently active in this model. In the SI test, S15535 similarly mimicked the anxiolytic-like effect of diazepam and was active over a broad dose range. Buspirone and 8-OH-DPAT again showed biphasic dose-response curves, as did SB206,553. In both the Vogel and SI tests, the anxiolytic-like effects of S15535 were abolished by the selective 5-HT1A receptor antagonist, WAY100,635, which was inactive alone. S15535 exerted its anxiolytic-like effects with a more pronounced separation to motor-disruptive doses than the other drugs. Finally, S15535 suppressed dialysate levels of 5-HT in the dorsal hippocampus, an action abolished by WAY100,635. Buspirone, 8-OH-DPAT and diazepam, but not SB206,553, also reduced 5-HT levels. CONCLUSION: Likely reflecting its distinctive ability to selectively and preferentially activate pre- versus postsynaptic 5-HT1A receptors, S15535 suppresses hippocampal 5-HT release and displays marked anxiolytic-like effects over a broad dose range in the relative absence of motor perturbation.