Inhibitory effects of tandospirone, a 5-HT1A agonist, on medial vestibular nucleus neurons responding to lateral roll tilt stimulation in rats

An electrophysiological study was performed using chloral hydrate-anesthetized rats to determine whether tandospirone, a 5-HT_1A agonist, affects neuronal activities of the medial vestibular nucleus (MVN), since serotonergic innervation and 5-HT_1A receptors are present in this nucleus. Tandospirone applied microiontophoretically at a current of 20–60 nA caused an inhibition of tilt-induced firing of α-type neurons, which showed increased and decreased firing with lateral tilt ipsilateral and contralateral to the recording site, respectively, along with that of β-type neurons which exhibited the reverse responses to ipsilateral and contralateral tilt stimulation. The inhibition was antagonized during simultaneous, iontophoretic application of WAY-100635 (20–60 nA), a 5-HT_1A receptor antagonist, although WAY-100635 alone rarely affected spontaneous or tilt-induced firing in either type of neurons. These results suggest that tandospirone acts on a 5-HT_1A receptor to inhibit transmission of otolith information to α- and β-type MVN neurons.     

Cholinergic and glutamatergic transmission in medial vestibular nucleus neurons responding to lateral roll tilt in rats

The responses of the medial vestibular nucleus (MVN) neurons to lateral tilt and the neurotransmitters mediating otolith information to MVN neurons were investigated using rats. A computer-operated goniometer was tilted 20° clockwise and counterclockwise at an angular speed of 5°/s and paused in the inclined positions for 10 s to record neuronal responses in the static phase. The 185 MVN neurons recorded were classified into eight types according to their responses to tilt (, β, γ, δ, , ζ, η and θ). A majority showed increased firing in response to ipsilateral tilting and decreased firing in response to contralateral tilting ( type: 31.4%) or exhibited the reverse pattern (β type: 36.8%). Further, other groups of neurons increased (γ type) or decreased (δ type) firing rates to either side tilting and increased ( and ζ type) or decreased (η and θ type) firing only on one side. Atropine or -glutamic acid diethyl ester hydrochloride (GDEE) applied microiontophoretically antagonized tilt-induced firing of type neurons in 58.8% or 60.0%, respectively, and of β type neurons in 66.7% or 58.3%, respectively. When the effects of atropine and GDEE were examined in the same neurons, antagonizing effects of both drugs on tilt-induced firing were obtained in 28.6% and 40.0% of and β type neurons, respectively. These results suggest that both acetylcholine and glutamate act as neurotransmitters in the transmission of otolith information to most MVN neurons.     

5-HT1A receptor-mediated inhibition of lateral vestibular nucleus neurons projecting to the abducens nucleus

Electrophysiological studies were performed using cats anesthetized with α-chloralose, to elucidate the 5-hydroxytryptamine (5-HT) receptor subtypes involved in the 5-HT-induced inhibition of the lateral vestibular nucleus (LVN) neurons projecting to or through the abducens nucleus. The effects of 5-HT receptor subtype agonists and antagonist were examined in polysynaptic neurons activated by stimulation of the ipsilateral abducens nucleus (IAN) antidromically, since these neurons are sensitive to 5-HT as shown in our previous study. Iontophoretic application of 5-HT and8-hydroxy-2-(di-n-propylamino)tetrain (8-OH-DPAT), a selective 5-HT_1A agonist, inhibited orthodromic spikes elicited by vestibular nerve stimulation in the majority of polysynaptic neurons activated by stimulation of ipsilateral IAN antidromically. There was a good correlation between the effects of 5-HT and 8-OH-DPAT. Iontophoretically applied 5-HT and 8-OH-DPAT also inhibited glutamate-induced firing in these neurons. Simultaneous application of 1-(2-methoxyphenly)-4-[4-(2-phthalimido)butyl]piperazine (NAN-190), a 5-HT_1A agonist/antagonist, significantly antagonized the 8-OH-DPAT-induced inhibition of glutamate-induced firing, although NAN-190 alone also caused weak suppression of glutamate-induced firing. Microiontophoretically applied 1-(3-chlorophenyl)piperazine (mCPP), a 5-HT_1B agonist inhibited the orthodromic spike elicited by vestibular nerve stimulation and glutamate-induced firing in only a small number of the LVN neurons. 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT₂ agonist, rarely affected these neurons. We postulate that postsynaptically located 5-HT_1A receptors are mainly involved in the 5-HT-induced inhibition of polysynaptic neurons projecting in the region of the IAN.     

Inhibition of lateral vestibular nucleus neurons by 5-hydroxytryptamine derived from the dorsal raphe nucleus

Electrophysiological studies were performed to elucidate the effect of 5-hydroxytryptamine (5-HT) originating in the dorsal raphe nucleus (DR) on neuronal activity in the lateral vestibular nucleus (LVN) neurons, using cats anesthetized with alpha-chloralose. LVN neurons were classified into monosynaptic and polysynaptic neurons according to their responses to vestibular nerve stimulation. Conditioning stimuli applied to the DR inhibited orthodromic spikes elicited by vestibular nerve stimulation predominantly in polysynaptic neurons of the LVN. The iontophoretic application of 5-HT also inhibited orthodromic spikes of the LVN neurons. A close correlation was observed between the effects of DR conditioning stimulation and iontophoretically applied 5-HT in the same neurons. These inhibitions with both treatments were antagonized during the application of methysergide, a 5-HT antagonist. In the majority of LVN polysynaptic neurons that responded to antidromic stimulation of the ipsilateral or contralateral abducens nucleus, orthodromic spikes elicited by vestibular nerve stimulation were inhibited by DR conditioning stimulation and the iontophoretic application of 5-HT. In contrast, LVN neurons that responded to antidromic stimulation of the vestibulospinal tract were rarely affected by these treatments. These results indicate that 5-HT derived from the DR inhibits the synaptic transmission of LVN polysynaptic neurons ascending to the abducens nucleus, and suggest that 5-HT derived from the DR is involved in the regulation of the vestibulo-ocular reflex.     

Glutamate responsiveness of medial vestibular nucleus neurons in aged rats

Disequilibrium, dizziness, vertigo and falls are vestibular system-related problems which are very common especially in older people. In order to clarify these age-related disorders one must understand first the age-related changes in the properties of vestibular neurons that are responsible for equilibrium. The responsiveness of medial vestibular nucleus (MVN) neurons to the NMDA and AMPA/kainate receptor agonists was investigated in slices prepared from young and aged rats using extracellular single cell recording techniques. In both young and aged rats bath application of NMDA and AMPA caused a reversible, dose-dependant increase in the spontaneous discharge of the MVN neurons. The excitatory effects of both NMDA and AMPA on the spontaneous activity of aged MVN neurons were similar to those of young MVN neurons. The spontaneous firing rates of the MVN cells were also similar in young and aged rats. These results suggest that the responsiveness of the NMDA and AMPA/kainate receptors and the excitability of the MVN neurons do not change with age.     

Target neurons of floccular middle zone inhibition in medial vestibular nucleus

Unitary activities of 288 neurons were recorded extracellularly in the medial vestibular nucleus (MV) in anesthetized cats. In 19 neurons, located in the rostral part of the MV adjacent to the stria acustica, floccular middle zone stimulation resulted in cessation of spontaneous discharges. Systematic microstimulation in the brainstem during recording of 16 of 19 target neurons of floccular middle zone inhibition revealed that the target neurons projected to the ipsilateral abducens nucleus (ABN), and not to the contralateral ABN nor the oculomotor nucleus. The conjugate ipsilateral horizontal eye movement elicited by middle zone stimulation may be mediated by this pathway to motoneurons and internuclear neurons in the ipsilateral ABN. In additional experiments, the MV neurons responding antidromically to ipsilateral ABN stimulation and orthodromically to ipsilateral 8 nerve stimulation were recorded extracellularly. In only 7 of 36 recorded neurons, middle zone stimulation depressed the orthodromic and spontaneous activities. Many neurons were free of floccular inhibition. As to the route of floccular inhibitory control over the vestibulo-ocular reflex (VOR) during visual-vestibular stimulation, we propose that the interaction of target and VOR relay neurons takes place at the ipsilateral ABN and modulates the VOR, in addition to well known Ito's proposal that the interaction of the floccular output and the VOR takes place at secondary vestibular neurons and modulates the VOR.     

Electrophysiological evidence for a functional interaction between 5-HT1A and 5-HT2A receptors in the rat medial prefrontal cortex: An lontophoretic study

In this study, we examined the interaction of 5-HT_1A and 5-HT_2A receptors in the rat medial prefrontal cortex (mPFc) using the techniques of extracellular single unit recording and microiontophoresis. The iontophoresis of the selective 5-HT_1A receptor agonist (±)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) produced a current-dependent suppression (2.5-20 nA) of the basal firing rate of spontaneously active mPFc cells. The iontophoretic (5-10 nA) and systemic administration (0.1-0.5 mg/kg, i.v. ) of the 5-HT_2A/5-HT_2C receptor antagonist ritanserin and the selective 5 HT_2A receptor antagonist MDL 28727 significantly potentiated and prolonged 8-OHDPATs suppressant action. In addition, the systemic administration of another selective 5-HT_2A antagonist MDL 100907, but not its less active enantiomer MDL 100009, also potentiated and prolonged 8-OHDPATs action. The potentiating effect of the 5-HT_2A receptor antagonists on the action of 8-OHDPAT is specific in that neither the iontophoresis of ritanserin nor MDL 28727 altered the suppressant action produced by the iontophoresis of the 5-HT₃ receptor agonist 2-methylserotonin onto mPFc cells. Moreover, the suppressant action of 8-OHDPAT was not altered by the systemic administration of the selective 5-HT₃ receptor antagonist granisetron (0.1-0.5 mg/kg, i.v.). On the other hand, the iontophoresis of a low current (0.5 nA) of the 5-HT_2A,2C receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) potentiated the excitation induced by the iontophoresis of 1-glutamate on quiescent mPFc cells. The iontophoresis of 8-OHD-PAT at a current that had no effect on the firing rate of 1-glutamate activated when administered alone significantly attenuated the excitatory action produced by the iontophoresis of DOI. Overall these results confirm and extend the hypothesis that there is an interaction between 5-HT_1A and 5-HT_2A receptors in the mPFc at the neuronal level. © 1994 Wiley-Liss, Inc.     

5-HT1A受体对双相呼气和吸气神经元电活动的调制

目的探讨5-HT1A受体对延髓脑片双相呼气神经元和吸气神经元电活动的影响。方法在新生大鼠延髓脑片上同步记录舌下神经根和双相呼气神经元／吸气神经元单位的放电活动，并在灌流的改良Kreb’s液中先后加以5-HT1A受体的特异性激动剂（＋／-）-8-hydroxy-2-（di-N-propylamino）tetralin hydrobromide（8-OH-DPAT）和特异性拮抗剂多次甲基多苯基多异氰酸酯[4-iodo-N-[2-[4-（methoxyphenyl）-1-piperazinyl]ethyl]-N-2-pyridynyl-benzamide hydrochloride]（PMPPI）观察对神经元单位放电的影响。结果给予5-HT1A受体激动剂8-OH-DPAT后，双相呼气神经元／吸气神经元的呼吸周期和呼气时程明显延长，积分幅度降低，单位放电峰频率显著性降低；给予特异性拮抗剂PMPPI后，对呼吸周期，呼气时程的作用相反，而积分幅度和单位放电峰频率无明显变化。结论5-HT1A受体可能通过影响双相呼气神经元的电活动参与了呼吸时相的转换，同时也可能介导了吸气神经元的抑制性突触输入。     

Comparison of the effects of NMDA antagonists on medial vestibular nucleus neurons in brainstem slices from labyrinthine-intact and chronically labyrinthectomized guinea pigs

The responses of ipsilateral medial vestibular nucleus (MVN) neurons in brainstem slices from guinea pigs compensated for a unilateral labyrinthectomy (UL), to the (NMDA) receptor/channel antagonists CPP and MK801, were compared with those of MVN neurons in brainstem slices from labyrinthine-intact guinea pigs observed in a previous study. The average resting activity of ipsilateral MVN neurons from compensated animals was significantly higher than that for MVN neurons from labyrinthine-intact animals; however, there were no significant differences in the average magnitude of the decrease in firing rate from baseline in response to CPP or MK801 and the only significant difference in the number of responses was to MK801, where fewer ipsilateral MVN neurons from compensated animals responded with a decrease in firing rate. These results suggest that vestibular compensation is not associated with an up-regulation or increased affinity of NMDA receptors in the MVN ipsilateral to the UL.     

receptors contribute to afferent synaptic transmission in the medial vestibular nucleus of young rats

In the medial vestibular nucleus (MVN), the (NMDA) subtype of ionotropic glutamate receptors has been reported as operating at synapses between ipsilateral vestibular afferents and neurons. In the present study, we addressed the question of whether or not NMDA receptors contribute to afferent synaptic transmission in the MVN and if so, to what degree. Using nystatin-perforated or conventional whole-cell patch clamp methods in brainstem slices of young rats (postnatal day 4–6), we found that NMDA receptors contribute to a substantial extent to afferent synaptic transmission in the MVN of young rats.     

5-HT1A and 5-HT1B receptors control the firing of serotoninergic neurons in the dorsal raphe nucleus of the mouse: studies in 5-HT1B knock-out mice

The characteristics of the spontaneous firing of serotoninergic neurons in the dorsal raphe nucleus and its control by serotonin (5-hydroxytryptamine, 5-HT) receptors were investigated in wild-type and 5-HT1B knock-out (5-HT1B-/-) mice of the 129/Sv strain, anaesthetized with chloral hydrate. In both groups of mice, 5-HT neurons exhibited a regular activity with an identical firing rate of 0.5-4.5 spikes/s. Intravenous administration of the 5-HT reuptake inhibitor citalopram or the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) induced a dose-dependent inhibition of 5-HT neuronal firing which could be reversed by the selective 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xane carboxamide (WAY 100635). Both strains were equally sensitive to 8-OH-DPAT (ED50 approximately 6.3 microgram/kg i.v.), but the mutants were less sensitive than wild-type animals to citalopram (ED50 = 0.49 +/- 0.02 and 0.28 +/- 0.01 mg/kg i.v., respectively, P < 0.05). This difference could be reduced by pre-treatment of wild-type mice with the 5-HT1B/1D antagonist 2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carbox yli c acid [4-methoxy-3-(4-methyl-piperazine-1-yl)-phenyl]amide (GR 127935), and might be accounted for by the lack of 5-HT1B receptors and a higher density of 5-HT reuptake sites (specifically labelled by [3H]citalopram) in 5-HT1B-/- mice. In wild-type but not 5-HT1B-/- mice, the 5-HT1B agonists 3-(1,2,5, 6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP 94253, 3 mg/kg i.v.) and 5-methoxy-3-(1,2,3, 6-tetrahydropyridin-4-yl)-1H-indole (RU 24969, 0.6 mg/kg i.v.) increased the firing rate of 5-HT neurons (+22.4 +/- 2.8% and +13.7 +/- 6.0%, respectively, P < 0.05), and this effect could be prevented by the 5-HT1B antagonist GR 127935 (1 mg/kg i.v.). Altogether, these data indicate that in the mouse, the firing of 5-HT neurons in the dorsal raphe nucleus is under both an inhibitory control through 5-HT1A receptors and an excitatory influence through 5-HT1B receptors.     

Treatment of cerebellar ataxia with 5-HT1A agonist

Effective, pharmacologic approaches to the treatment of cerebellar ataxia are lacking or inadequate. We recently reported preliminary evidence that tandospirone citrate (tandospirone), a 5-HT1A agonist, improved cerebellar ataxia in patients with Machado-Joseph disease (MJD). In the course of that study, we found that such treatment also alleviated the pain associated with cold sensations in the legs, insomnia, anorexia, and depression, all of which are thought to be mediated through activation of the 5-HT1A receptor. In this paper, we reviewed the few published clinical trials that involved the use of 5-HT1A receptor agonists for the treatment of cerebellar ataxia, and discussed the current theories regarding their mechanism of action. Cortical cerebellar atrophy (CCA) was reported, in a double-blind study, to be amenable to treatment with tandospirone. Other types of spinocerebellar degeneration (SCD) i.e., olivopontocerebellar atrophy (OPCA) and Machado-Joseph disease (MJD) have also been reported to respond to the drug, but these have been small studies. Responsive patients exhibited only mild ataxia. The doses of 5-HT1A agonists that have been used successfully ranged from 12.5 mg/day to 60 mg/day (or 1 mg/kg), and were well tolerated by most patients.     

Chronic paroxetine desensitises 5-HT1D but not 5-HT1B autoreceptors in rat lateral geniculate nucleus

The present study examined the effect of chronic paroxetine (10 mg/kg p.o., 21 days) on the 5-HT_1B and 5-HT_1D autoreceptors controlling 5-HT efflux in slices of rat ventrolateral geniculate nucleus. Electrically stimulated 5-HT efflux (10 pulses, 200 Hz, 0.1 ms, 10 mA) was measured using fast cyclic voltammetry. Peak 5-HT efflux was greater (P<0.01) after chronic paroxetine (22.2±1.4 nM, mean±S.E.M.) than water (15.8±1.4 nM). 5-HT efflux was inhibited by CP 93129 (1 nM–10 μM) and sumatriptan (1 nM–1 μM), agonists at 5-HT_1B and 5-HT_1D receptors, respectively. Chronic paroxetine did not affect the sensitivity of the 5-HT_1B autoreceptor but shifted the sumatriptan concentration-response curve to the right (P<0.05). These data suggest that chronic paroxetine increases evoked 5-HT efflux. This may be the result of desensitisation of 5-HT_1D but not 5-HT_1B autoreceptors.     

Pre- and post-synaptic effects of the 5-HT3 agonist 2-Methyl-5-HT on the 5-HT system in the rat brain

Microiontophoretic applications of 5-HT and of the 5-HT₃ agonist 2-methyl-5-HT produced a current-dependent suppression of firing activity of both hippocampal (CA₁ and CA₃) and cortical neurons in anesthetized rats. Concomitant microiontophoretic applications of the 5-HT₃ antagonists BRL 46470A and S-zacopride, as well as their intravenous injection, did not antagonize the inhibitory effect of 5-HT and 2-methyl-5-HT. In contrast, the 5-HT_1A antagonist BMY 7378, applied by microiontophoresis or administered intravenously, significantly reduced the inhibitory action of 5-HT and 2-methyl-5-HT. The firing activity of dorsal raphe 5-HT neurons was also reduced by 5-HT, 2-methyl-5-HT and the 5-HT_1A agonist 8-OH-DPAT applied by microiontophoresis. While BRL 46470A (0.1 and 1 mg/kg, i.v.) did not antagonize the inhibitory effect of the three 5-HT agonists on 5-HT neuronal firing activity, only that of 8-OH-DPAT was attenuated by the 5-HT_1A antagonist (+) WAY 100135. R-zacopride significantly reduced the duration of suppression of firing activity of CA₃ pyramidal neurons induced by the electrical stimulation of the ascending 5-HT pathway, and this reducing effect was prevented by the three 5-HT₃/5-HT₄ antagonists renzapride, S-zacopride and tropisetron, but not by BRL 46470A. Finally, in in vitro superfusion experiments, both BRL 46470A and S-zacopride antagonized the enhancing action of 2-methyld-HT on the electrically-evoked release of [³H]-5-HT in both rat frontal cortex and hippocampus slices. These findings suggest that, in vivo, the suppressant effect of 2-methyl-5-HT on the firing activity of dorsal hippocampus pyramidal, somatosensory cortical, and dorsal raphe 5-HT neurons is not mediated by 5-HT₃ receptors, but rather by 5-HT_1A receptors. The attenuating effect of R-zacopride on the effectiveness of the stimulation of the ascending 5-HT pathway is not mediated by 5-HT₃ receptors. In contrast, in vitro, the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [³H]5-HT in both frontal cortex and hippocampus slices is mediated by 5-HT₃ receptors. © 1995 Wiley-Liss, Inc.     

Autoradiographic analyses of 5-HT1A and 5-HT2A receptors after social isolation in mice

Social isolation of rodents is used to model human psychopathological processes. In the present study, the effects of intermediate and long term isolation housing on postsynaptic 5-HT_1A and 5-HT_2A receptors were analyzed in male mice housed in groups or isolation for 4 and 12 weeks. [³H]8-OH-DPAT and [³H]ketanserin were used to label 5-HT_1A and 5-HT_2A receptors. Four representative sagittal sections (planes 1–4) were scored by in vitro autoradiography. Whereas after 4 weeks of housing both receptor densities were lowered significantly in isolated mice, after 12 weeks of housing only marginal isolation effects were seen. Intermediate isolation reduced 5-HT_1A receptors especially in the lateral frontal, parietal and entorhinal cortex (−63%), in the lateral CA1–3 and dentate gyrus region of the hippocampus (−68%), in the basolateral, basomedial, central and medial amygdaloid nuclei (between −38 and −66%), and in the hypothalamus (−28%). 5-HT_2A receptors were strongly reduced in the frontal cortex (between −47 and −74%), in the hippocampus (between −47 and −95%), in the striatum (between −66 and −76%), and in the accumbens nucleus (between −59 and −73%) in comparison to group housed control mice. After 12 weeks of isolation in the hippocampus continuously decreased 5-HT_1A receptor densities were demonstrated (between −24 and −61%). But increased 5-HT_2A receptor densities were seen in the lateral striatum (+86%) compared to control mice. Age-dependent effects were also found. After 12 weeks of group housing the 5-HT_1A and 5-HT_2A receptor densities were decreased (between −28 and −54%) in all analyzed brain regions in comparison to 4 weeks of group housing. Isolated animals showed diminished 5-HT_1A receptor densities in the cortex (−14%) and hippocampus (−15%), but increased 5-HT_1A receptor densities in the amygdala (+33%) after 12 weeks. The 5-HT_2A receptor densities were increased in all analyzed regions (between +31 and +96%) after 12 weeks of isolation compared to 4 weeks. To explain these dynamic, time-dependent pattern of isolation-induced changes different regulation processes are supposed regarding 5-HT_1A and 5-HT_2A receptors. Besides metabolism-related adaptation processes also neurotransmitter and hormonal (e.g., glucocorticoid) interactions especially in limbic regions have to be considered.     

N-Methyl-d-aspartate receptors regulate 5-HT release in the raphe nuclei and frontal cortex of freely moving rats: differential role of 5-HT1A autoreceptors

The effects of infusing N-methyl-

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-aspartate (NMDA) into the raphe nuclei on release of 5-HT in this brain region and also the frontal cortex of the same animal were studied using in vivo microdialysis in freely moving rats. Infusion of 25 μM NMDA into the raphe led to a substantial decrease in dialysate 5-HT in this region and a prolonged increase in terminal 5-HT release in the frontal cortex. These effects were blocked by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (D-AP5; 100 μM). When 25 μM NMDA was co-infused into the raphe with the selective 5-HT_1A receptor antagonist (N-{2-{4-(2-methoxyphenyl)-1-piperazinyl}ethyl-N-(2-pyridinyl)cyclohexanecarboxamide) (WAY-100635; 1.0 μM) the effect of NMDA infusion was unaltered. WAY-100635 infused alone into the raphe did not alter local 5-HT or extracellular 5-HT in the cortex. Infusion of 100 μM NMDA into the raphe was followed by an increase in local dialysate 5-HT and a decrease in 5-HT release in the cortex. These changes were reversed by D-AP5. Following infusion of 100 μM NMDA with 1.0 μM WAY-100635 into the raphe local 5-HT release was still increased, however, the decrease in 5-HT observed in the frontal cortex was abolished. These data suggest that the degree of NMDA receptor activation leads to dramatically different outcomes with regard to serotonergic transmission to the frontal cortex. Furthermore, there appears to be a differential role of the 5-HT_1A autoreceptor in regulating these effects. These data are discussed in relation to other studies on the regulation of serotonergic transmission in ascending pathways.     

Modification of 5-HT neuron properties by sustained administration of the 5-HT1A agonist gepirone: electrophysiological studies in the rat brain

The sustained administration of the 5-HT1A agonist gepirone (15 mg/kg/day, s.c.) in the rat produced an initial decrease of the firing activity of dorsal raphe 5-HT neurons which was followed by a progressive recovery to normal after 14 days of treatment. At this point in time, the effect of intravenous lysergic acid diethylamide (LSD) on the firing activity of 5-HT neurons was markedly reduced, whereas those of 8-hydroxy-2-N,N-propylamino-tetralin (8-OH-DPAT) and of gepirone were unchanged; however, the responsiveness of 5-HT neurons to direct microiontophoretic application of 5-HT, LSD, 8-OH-DPAT, and gepirone, but not of GABA, was reduced. The responsiveness of postsynaptic dorsal hippocampus pyramidal neurons to 5-HT, 8-OH-DPAT, and gepirone was not altered by the 14-day gepirone treatment. The effectiveness of the electrical stimulation of the ascending 5-HT pathway in reducing pyramidal neuron firing activity was not significantly modified in rats treated with gepirone for 14 days. Furthermore, this treatment did not alter the function of the terminal 5-HT autoreceptor. It is concluded that the progressive restoration of the firing activity of 5-HT neurons, due to a desensitization of the somatodendritic 5-HT autoreceptor, combined with the direct activation of normosensitive postsynaptic 5-HT1A receptor by gepirone, should result in an augmented tonic activation of postsynaptic 5-HT1A receptors. The progressive appearance of this phenomenon would be consistent with the time course of the clinical anxiolytic, and possibly antidepressant, effects of gepirone.     

Differential modulation of I(h) by 5-HT receptors in mouse CA1 hippocampal neurons

CA1 pyramidal neurons of the hippocampus express various types of serotonin (5-HT) receptors, such as 5-HT(1A), 5-HT(4) and 5-HT(7) receptors, which couple to Galpha(i) or Galpha(s) proteins and operate on different intracellular signalling pathways. In the present paper we verify such differential serotonergic modulation for the hyperpolarization-activated current I(h). Activation of 5-HT(1A) receptors induced an augmentation of current-induced hyperpolarization responses, while the responses declined after 5-HT(4) receptors were activated. The resting potential of neurons hyperpolarized (-2.3 +/- 0.7 mV) after 5-HT(1A) receptor activation, activation of 5-HT(4) receptors depolarized neurons (+3.3 +/- 1.4 mV). Direct activation of adenylyl cyclase (AC) by forskolin also produced a depolarization. In voltage clamp, the Ih current was identified by its characteristic voltage- and time-dependency and by blockade with CsCl or ZD7288. Activation of 5-HT(1A) receptors reduced I(h) and shifted the activation curve to a more negative voltage by -5 mV at half-maximal activation. Activation of 5-HT(4) and 5-HT(7) receptors increased I(h) and shifted the activation curve to the right by +5 mV. Specific activation of 5-HT(4) receptors by BIMU8 increased membrane conductance and showed an increase in I(h) in a subset of cells, but did not induce a significant alteration in the activation curve. In order to verify spatial differences, we applied BIMU8 selectively to the soma and to the dendrites. Only somatic application induced receptor activation. These data are confirmed by immunofluorescence stainings with an antibody against the 5-HT(4) receptor, revealing receptor expression at the somata of the CA1 region. A similar expression pattern was found with a new antibody against 5-HT(7) receptors which reveals immunofluorescence staining on the cell bodies of pyramidal neurons.     

Sleep/waking effects of a selective 5-HT1A receptor agonist given systemically as well as perfused in the dorsal raphe nucleus in rats

Sleep/waking stages and behavior were studied following the selective 5-HT_1A agonist 8-OH-DPAT given subcutaneously (s.c.) (0.010–0.375 mg/kg) as well as perfused continuously (10 μM) for 6 h into the dorsal raphe nucleus (DRN) using microdialysis. Given systemically, 8-OH-DPAT at 0.375 mg/kg s.c. induced 5-HT behavioral syndrome, increased waking to 149% and reduced slow wave sleep (SWS) to 86%, transition to 76% and rapid eye movement (REM) sleep to 73%. The effect on deep SWS (SWS-2) was biphasic, with an increase after 2 h. 8-OH-DPAT at 0.010 mg/kg did not have any vigilance effects. 8-OH-DPAT perfusion in DRN produced a fourfold increase in REM sleep compared to perfusion of artificial cerebrospinal fluid. This is consistent with the hypothesis that reduced 5-HT neurotransmission following 5-HT_1A autoreceptor stimulation will disinhibit cholinergic REM-promoting mesopontine neurons and thereby lead to a REM sleep increase. The other sleep/waking stages were not significantly affected by 8-OH-DPAT perfusion in DRN.     

Chronic administration of the 5-HT1A receptor agonist 8-OH-DPAT differentially desensitizes 5-HT1A autoreceptors of the dorsal and median raphe nuclei

The present study investigated alterations of the regulation of serotonin (5-hydroxytryptamine; 5-HT) release by 5-HT_1A autoreceptors following single and repeated treatment with the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT). Rats were pretreated with 8-OH-DPAT (1.0 mg/kg, s.c.) for 1, 7, or 14 days. The ability of an acute challenge administration of 8-OH-DPAT (1.0 mg/kg, i.p.) to decrease 5-HT release in the ventral striatum and the ventral hippocampus of rats maintained under chloral hydrate anesthesia was examined 24 h after the last pretreatment injection using in vivo microdialysis. The decrease of 5-HT release in the striatum produced by the challenge dose of the 5-HT_1A receptor agonist was diminished following 7 and 14 days of pretreatment, but not after 1 day of pretreatment, with 8-OH-DPAT. In contrast, decreases of 5-HT release in the hippocampus by the 8-OH-DPAT challenge were not altered after 1 or 7 days of pretreatment, and only a trend for attenuation appeared after pretreatment for 14 days. The results of the present study indicate that desensitization of 5-HT_1A autoreceptors regulating 5-HT release in different brain regions by repeated treatment with 8-OH-DPAT occurs at different rates. Synapse 25:107–116, 1997. © 1997 Wiley-Liss, Inc.