Ovarian steroid regulation of 5-HT1A receptor binding and G protein activation in female monkeys.

Serotonin 5-HT(1A) receptors play an important role in serotonin neurotransmission and mental health. We previously demonstrated that estradiol (E) and progesterone (P) decrease 5-HT(1A) autoreceptor mRNA levels in macaques. In this study, we questioned whether E and P regulate 5-HT(1A) binding and function and G(alpha) subunit protein expression. Quantitative autoradiography for 5-HT(1A) receptors and G proteins using [3H]8-OH-DPAT and [35S]GTP-gamma-S, respectively, was performed on brain sections of rhesus macaques from four treatment groups: ovariectomized controls (OVX), E (28 d), P (28 d), and E (28 d) plus P (the last 14 d) treated. Western blot analysis for G(alpha) subunits was performed on raphe extracts from cynomolgus macaques that were OVX or OVX treated with equine estrogens (EE, 30 months). In the hypothalamus, E or E + P but not P alone decreased postsynaptic 5-HT(1A) binding sites. In the dorsal raphe nucleus (DRN), E, P, and E + P treatments decreased 5-HT(1A) autoreceptor binding. The Kd values for 8-OH-DPAT were the same for each treatment group. Both the basal and the R-(+)-8-OH-DPAT stimulated [35S]GTP-gamma-S binding were decreased during hormone replacement whereas the coupling efficiency between the receptor and G proteins was maintained. Finally, EE treatment reduced the level of G(alphai3), but not G(alphai1), G(alphao), and G(alphaz) in the DRN. In conclusion, these observations suggest that ovarian hormones may increase serotonin neurotransmission, in part, by decreasing 5-HT(1A) autoreceptors, 5-HT(1A) postsynaptic receptors, and the inhibitory G proteins for intracellular signal transduction.     

Changes in estrogen receptors alpha and beta expression in the brain of mice exposed prenatally to bisphenol A

The expression of ERs alpha and beta and serotonergic neurons were evaluated in the brains of mice prenatally exposed to Bisphenol A, a known endocrine disrupting chemical (EDc). Bisphenol A was administered orally at a dose of 2ng/g body weight on gestinational days 11-17 to pregnant ICR mice. Newborn male offspring (Bis-A mice) were evaluated for the immunoreactivity of ERs alpha and beta, serotonin, and serotonin transporter positive cells in the dorsal raphe nucleus (DRN). The serum testosterone level was also evaluated. In the Bis-A mice, the expression of ERs alpha and beta at 5 and 13 weeks was increased compared with the controls (P<0.04), but this difference disappeared by the 9th week. The serotonin, serotonin transporter, and testosterone level differences between two groups did not reach significance. Exposure to bisphenol A may have changed the expression of ERs in the brain, but did not directly affect serotonin neurons in the DRN.     

Regulation of serotonin release by GABA and excitatory amino acids

Regulation of serotonin release by gamma-aminobutyric acid (GABA) and glutamate was examined by microdialysis in unanaesthetized rats. The GABA(A) receptor agonist muscimol, or the glutamate receptor agonists kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate or N-methyl-D-aspartate were infused into the dorsal raphe nucleus (DRN) while extracellular serotonin was measured in the DRN and nucleus accumbens. Muscimol produced decreases, and the glutamate receptor agonists produced increases in serotonin. To determine if these receptors have a tonic influence on serotonergic neurons, glutamate or GABA(A) receptor antagonists were infused into the DRN. Kynurenate, a nonselective glutamate receptor blocker, produced a small, 30% decrease in serotonin. A similar decrease was obtained with combined infusion of AP-5 and DNQX into the DRN. The GABAA receptor blocker bicuculline produced an approximately three-fold increase in DRN serotonin. In conclusion, glutamate neurotransmitters have a weak tonic excitatory influence on serotonergic neurons in the rat DRN. However, the predominate influence is mediated by GABA(A) receptors.     

Serotonin depletion in the dorsal and ventral hippocampus: effects on locomotor hyperactivity, prepulse inhibition and learning and memory

We present an overview of our studies on the differential role of serotonergic projections from the median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) in behavioural animal models with relevance to schizophrenia. Stereotaxic microinjection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the MRN or one of its main projections regions, the dorsal hippocampus, induced a marked enhancement of phencyclidine-induced locomotor hyperactivity and a disruption of prepulse inhibition (PPI) in rats. There was no enhancement of locomotor hyperactivity induced by amphetamine or MK-801 or after 5,7-DHT lesions of the DRN or ventral hippocampus. Rats with dorsal hippocampus lesions did not show significant changes in the Y-maze test for short-term spatial memory, the Morris water maze for long-term spatial memory, or in the T-maze delayed alternation test for working memory. These chronic lesion studies suggest a modulatory influence of serotonergic projections from the MRN to the dorsal hippocampus on phencyclidine effects and prepulse inhibition, but not on different forms of learning and memory. The results provide new insight into the role of serotonin in the dorsal hippocampus in aspects of schizophrenia.     

Gender differences in proenkephalin gene expression response to delta9-tetrahydrocannabinol in the hypothalamus of the rat

Chronic exposure to delta9-tetrahydrocannabinol (delta9-THC) produces an activation of preproenkephalin (PENK) gene expression in the rat hypothalamus. The levels of circulating gonadal steroids concurrently modulate this neuropeptide in male and female rats. However, whether gonadal steroids regulate delta9-THC effects on PENK gene expression in the hypothalamus of male and female rats remains unknown. To test this hypothesis, experiments were carried out on intact, 2-week-gonadectomized, 1-week-gonadectomized, 1-week-dihydrotestosterone (DHT) replaced male rats, and 2-week-gonadectomized, 1-week-gonadectomized, 1-week-oestradiol replaced female rats. One week after hormonal replacement, animals were treated with vehicle or delta9-THC (5 mg/kg/day, i.p. 7 days). In males, delta9-THC administration to intact animals induced PENK mRNA in the paraventricular nucleus (PVN) and ventromedial nucleus (VMN) of the hypothalamus. Orchidectomy did not affect basal PENK mRNA levels in the PVN, but reduced PENK mRNA levels in the VMN. However, delta9-THC treatment induced PENK gene expression to the same extent in both hypothalamic nuclei of intact, castrated and DHT-replaced males. In females, ovariectomy decreased PENK gene expression in PVN and VMN. delta9-THC administration increased PENK gene expression in castrated females, but had no effect in the oestradiol-replaced group. Taken together, these results suggest gender differences in the response of chronic exposure to cannabinoids on PENK gene expression in the hypothalamus. Furthermore, it appears that alterations in opioid gene expression induced by cannabinoids in female rats depend upon the presence or absence of circulating oestradiol.     

The effect of lesions of dorsal or median raphe nucleus on rat behavior

The effect of lesion of the dorsal raphe nucleus (DRN) or median raphe nucleus (MRN) on the locomotor activity, open field performance, and reactivity to noxious stimulus was tested in rats. During the first 7 days after the lesion, the rats with lesioned DRN showed higher basal and explorative locomotor activity than the rats with MRN lesions. The lesion of DRN increased the open-field performance and reactivity to pain. The results of tests carried out on the rats with MRN lesions were similar as in control rats. The results indicate that the different behavioral effect of lesions of DRN and MRN reffects differences in the functions of these two raphe nuclei.     

Medial hypothalamic nuclei mediate serotonin's inhibitory effect on feeding behavior

Previous studies have demonstrated that injection of serotonin (5-HT) into the paraventricular nucleus (PVN), specifically at the onset of the active feeding cycle, causes a strong and selective suppression of carbohydrate intake, while producing no change in fat intake and, in some cases, enhancing protein consumption. The purpose of the present investigation was to determine whether this selective inhibitory effect of 5-HT on macronutrient ingestion is localized to a specific brain region, perhaps the PVN, or whether it can also occur in other sites throughout the hypothalamus or in regions outside this structure. A total of 7 hypothalamic and 5 extrahypothalamic areas were examined in brain-cannulated, freely feeding rats maintained on pure macronutrient diets of protein, carbohydrate and fat. The effect of 5-HT, a selective suppression (-55%) of carbohydrate feeding, was replicated in the PVN with a relatively low dose of 2.5 nmoles. Tests in 11 other brain sites demonstrated that this action of 5-HT is not unique to the PVN but is anatomically localized to the medial nuclei of the hypothalamus. Sites outside the hypothalamus, namely, the amygdala, nucleus accumbens, septum, diagonal band of Broca and nucleus reuniens dorsal to the PVN, failed to exhibit any response to 5-HT injection. Within the hypothalamus, the ventromedial (VMN) and suprachiasmatic (SCN) nuclei each responded to 5-HT in a manner similar to the PVN, producing a suppression of carbohydrate intake (-50% to -70%) with little or no change in either protein, fat or total kcal intake. The dorsomedial nucleus showed a somewhat smaller response relative to these other medial hypothalamic areas.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dependence of serotonin release in the locus coeruleus on dorsal raphe neuronal activity

The serotonergic innervation of the locus coeruleus paetly derives from the dorsal raphe nucleus (DRN). Using the push-pull superfusion technique, we investigated whether and to what extent the release of serotonin and the extracellular concentration of its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the locus coeruleus are influenced by the neuronal activity of the DRN. In anaesthetized rats, a push-pull cannula was inserted into the locus coeruleus, which was continuously superfused with artificial cerebrospinal fluid (aCSF). Serotonin and 5-HIAA levels in the superfusate were determined by HPLC combined with electrochemical detection. Electrical stimulation (5 Hz, 300 μA, 1 ms) of the DRN for 5 min, or its chemical stimulation by microinjection of glutamate (3.5 nmol, 50 nl), led to an increased release of serotonin in the locus coeruleus and to a slight (2 mmHg) decrease in blood pressure. Superfusion of the locus coeruleus with tetrodotoxin (1 μM) abolished the increase in the release rate of serotonin evoked by electrical stimulation of the DRN, while the slight fall in blood pressure was not influenced. Thermic lesion (75 °C, 1 min) of the DRN elicited a pronounced decline in serotonin release rate within the locus coeruleus, the maximum decrease being 52%. The decrease in the release of serotonin was associated with a long-lasting rise in blood pressure. Microinjection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5 μg, 250 nl) into the DRN led to an initial increase in the serotonin release rate that coincided with a short-lasting fall in blood pressure. Subsequently, the release of serotonin was permanently reduced and was associated with hypertension. Microinjection of the 5-HT_1A receptor agonist (±)-8-hydroxy-dipropylaminotetralin (8-OH-DPAT; 7.5 nmol, 50 nl) into the DRN led to a long-lasting reduction of the release rate of serotonin in the locus coeruleus. Microinjection of 8-OH-DPAT into the DRN also slightly lowered blood pressure (3 mmHg). Neither stimulations nor lesion of the DRN, nor microinjection of 8-OH-DPAT into this raphe nucleus, altered the extracellular concentration of 5-HIAA. Judging from the present biochemical results it appears that the serotonergic afferents to the locus coeruleus originate to more than 50% from cell bodies located in the DRN. The neuronal serotonin release in the locus coeruleus is modulated by 5-HT_1A receptors lying within the DRN. Changes in blood pressure and release of serotonin elicited by stimulating or lesioning the DRN point to the importance of serotonergic neurons extending between this raphe nucleus and the locus coeruleus in central cardiovascular control. Received: 5 November 1998 / Accepted: 21 February 1999     

Neuronal tryptophan hydroxylase mRNA expression in the human dorsal and median raphe nuclei: major depression and suicide.

Major depressive disorder (MDD) and suicide are associated with deficient serotonergic neurotransmission. Tryptophan hydroxylase (TPH) is the rate-limiting biosynthetic enzyme for serotonin. Previously, we reported elevated levels of TPH protein in the dorsal raphe nucleus (DRN) of depressed suicides and now examine expression of neuronal TPH2 mRNA in a cohort of matched controls and depressed suicides (n = 11 pairs). DRN TPH2 mRNA was measured by densitometric analysis of autoradiograms from in situ hybridization histochemistry experiments. TPH2 mRNA is confirmed as the raphe-specific isoform of TPH in human brain, and is expressed in neurons throughout the anteroposterior extent of the DRN and median raphe nucleus (MRN). TPH2 mRNA expression correlates with TPH protein distribution in the DRN, and has a negative correlation with age. In drug-free suicides, TPH2 expression is 33% higher in the DRN and 17% higher in the MRN as compared to matched nonpsychiatric controls. Higher levels of TPH2 mRNA were found throughout the entire extent of the rostrocaudal axis of the DRN, and were not specific to any single subnucleus. Higher TPH2 mRNA expression may explain more TPH protein observed in depressed suicides and reflect a homeostatic response to deficient brain serotonergic transmission.     

Nicotine stimulation of dorsal raphe neurons: effects on laterodorsal and pedunculopontine neurons.

Previous studies showed that nicotine suppresses the ponto-geniculo-occipital (PGO) spikes of rapid eye movement (REM) sleep in cats. This effect may depend on stimulation of dorsal raphe nucleus (DRN) serotoninergic neurons that inhibit the pedunculopontine (PPT) and laterodorsal tegmental (LDT) cholinergic neurons, generators of PGO spikes. For testing this hypothesis 37 experiments were performed in rat midbrain slices. Nicotine (2 mM), administered locally into DRN, significantly increased the firing rate of 81.1% DRN neurons and serotonin release while simultaneously and significantly decreasing the firing rate of 80.8% LDT neurons and of 81.8% PPT neurons. The inhibition of LDT neurons by nicotine administered into DRN was blocked by the 5-HT1A receptor antagonist WAY-100635 (140 nM) administered into LDT. These results indicate that nicotine inhibits the activity of LDT and PPT neurons and consequently the generation of PGO spikes through stimulation of DRN serotoninergic neurons.     

Differential regulation of 5-hydroxytryptamine release by GABAA and GABAB receptors in midbrain raphe nuclei and forebrain of rats.

1. Extracellular 5-hydroxytryptamine (5-HT) was determined in dorsal raphe nucleus (DRN), median raphe nucleus (MRN) and nucleus accumbens by use of microdialysis in unanaesthetized rats. 2. Infusion of the gamma-aminobutyric acid (GABA)A receptor agonist muscimol into DRN and MRN resulted in decreased 5-HT in DRN and MRN, respectively. Muscimol infusion into nucleus accumbens had no effect on 5-HT. 3. Infusion of the GABAA receptor antagonist bicuculline into DRN resulted in increased DRN and nucleus accumbens 5-HT. Bicuculline infusion into MRN had no effect on 5-HT. This suggests that endogenous GABA had a tonic, GABAA receptor-mediated inhibitory effect on 5-HT in DRN, but not in MRN. 4. Infusion of the GABAB receptor agonist baclofen into DRN produced a decrease in DRN 5-HT. Baclofen infusion into nucleus accumbens resulted in decreased nucleus accumbens 5-HT. This suggests that GABAB receptors are present in the area of cell bodies and terminals of 5-hydroxytryptaminergic neurones. 5. Infusion of the GABAB receptor antagonists phaclofen and 2-hydroxysaclofen had no effect on midbrain raphe and forebrain 5-HT. This suggests that GABAB receptors did not contribute to tonic inhibition of 5-HT release. 6. In conclusion, 5-HT release is physiologically regulated by distinct subtypes of GABA receptors in presynaptic and postsynaptic sites.     

Medullary serotonergic neurons are insensitive to 5-meodmt and LSD

A comparison was made of the effects of 5-MeoDMT or LSD on serotonergic unit activity in the dorsal raphe nucleus (DRN) and nucleus raphe pallidus (NRP) of freely moving cats. NRP neurons were substantially less responsive than DRN neurons to both drugs. NRP neurons were unresponsive to behaviorally effective low doses of these drugs whereas the activity of DRN neurons was strongly depressed. These data are discussed in terms of autoregulatory control of serotonergic neurons.     

Influence of AMPA/kainate receptors on extracellular 5-hydroxytryptamine in rat midbrain raphe and forebrain

1. The regulation of 5-hydroxytryptamine (5-HT) release by excitatory amino acid (EAA) receptors was examined by use of microdialysis in the CNS of freely behaving rats. Extracellular 5-HT was measured in the dorsal raphe nucleus (DRN), median raphe nucleus (MRN), nucleus accumbens, hypothalamus, frontal cortex, dorsal and ventral hippocampus.
2. Local infusion of kainate produced increases in extracellular 5-HT in the DRN and MRN. Kainate infusion into forebrain sites had a less potent effect.
3. In further studies of the DRN and nucleus accumbens, kainate-induced increases in extracellular 5-HT were blocked by the EAA receptor antagonists, kynurenate and 6,7-dinitroquinoxaline-2,3-dione (DNQX).
4. The effect of infusing kainate into the DRN or nucleus accumbens was attenuated or abolished by tetrodotoxin (TTX), suggesting that the increase in extracellular 5-HT is dependent on 5-HT neuronal activity. In contrast, ibotenate-induced lesion of intrinsic neurones did not attenuate the effect of infusing kainate into the nucleus accumbens. Thus, the effect of kainate in the nucleus accumbens does not depend on intrinsic neurones.
5. Infusion of α-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate (AMPA) into the DRN and nucleus accumbens induced nonsignificant changes in extracellular 5-HT. Cyclothiazide and diazoxide, which attenuate receptor desensitization, greatly enhanced the effect of AMPA on 5-HT in the DRN, but not in the nucleus accumbens.
6. In conclusion, AMPA/kainate receptors regulate 5-HT in the raphe and in forebrain sites.

     

Differential behavioural activation following intra-raphe infusion of 5-HT1A receptor agonists.

Microinfusion of the selective 5-HT1A receptor agonist, 8-hydroxy-(di-N-propylamino)tetralin (8-OHDPAT), into the dorsal raphe nucleus (DRN) produced a marked behavioural hypoactivity and flat body posture. Injections of similar doses into the median raphe nucleus (MRN) elicited hyperactivity but no postural change. Reductions in rearing and grooming were also observed after DRN and MRN infusions of 8-OHDPAT. The behavioural profiles of other 5-HT1A selective compounds, gepirone and BMY7378 were found to be similar to 8-OHDPAT. The contrasting behavioural profiles of the 5-HT1A agents observed after DRN or MRN microinfusions are probably related to the differential innervation of forebrain structures by each raphe nucleus. Thus, the present data confirms and extends previous results illustrating the influence of 5-HT systems on motor behaviour in the rat and identifies unique behavioural profiles following activation of the DRN and MRN.     

Effect of lamotrigine and carbamazepine on corticotropin-releasing factor-associated serotonergic transmission in rat dorsal raphe nucleus

Corticotropin-releasing factor (CRF) and serotonin are important transmitters of the pathophysiology of mood disorder. To clarify the mechanisms of action of lamotrigine (LTG) and carbamazepine (CBZ), we determined their effects on serotonin release associated with CRF in rat dorsal raphe nucleus (DRN) and median prefrontal cortex (mPFC) using dual-probe microdialysis. Neither perfusion with CRF1 nor CRF2 antagonists into DRN-affected serotonin release in DRN and mPFC. Perfusion of 10 μM CRF into DRN increased serotonin release in both regions, whereas 0.1 μM CRF decreased and had no effect on serotonin release in DRN and mPFC, respectively. Pre-perfusion with CRF1 antagonist into DRN inhibited 0.1 μM CRF-induced serotonin reduction, whereas pre-perfusion with CRF2 antagonist in DRN inhibited 10 μM CRF-induced serotonin elevation, without affecting 0.1 μM CRF-induced serotonin reduction. LTG perfusion concentration dependently decreased serotonin releases in DRN and mPFC. Therapeutic and supratherapeutic concentrations of CBZ increased and decreased serotonin releases in both regions, respectively. Pre-perfusion with sub-therapeutic concentration LTG inhibited CRF1-induced serotonin reduction without affecting CRF2-induced serotonin release, whereas pre-perfusion with therapeutic concentration of LTG inhibited both CRF1- and CRF2-actions. In contrast, both therapeutic and supratherapeutic concentrations of CBZ inhibited CRF2-induced serotonin release without affecting CRF1-induced serotonin reduction. Neither LTG nor CBZ affected the CRF-induced cAMP production in cells over-expressing CRF1 and CRF2 receptors. This study demonstrated that inhibition of CRF2-receptor-mediated serotonergic transmission is a mechanism shared by LTG and CBZ, two clinically related compounds, whereas LTG but not CBZ inhibits CRF1-receptor-mediated serotonergic transmission. Therefore, these mechanisms may contribute to the clinical actions of these agents.     

Role of 5-HT(1A) and 5-HT(2) receptors of dorsal and median raphe nucleus in tolerance to morphine analgesia in rats

Several studies indicate that central serotonergic neurons have important role in morphine analgesia and tolerance. The aim of this study was to investigate possible role of 5-HT(1A) and 5-HT(2) receptors in dorsal and median raphe nucleus on development of tolerance to analgesic effect of morphine using hot plate test. Chronic injection of 5-HT(1A) receptor agonist 8-OH-DPAT (8-hydroxy-2-[di-n-propylamino]tetralin) (2, 4 and 8 mug/rat/day) to dorsal raphe nucleus (DRN) delayed tolerance to morphine analgesia, whereas injection of the same doses of 8-OH-DPAT to the median raphe nucleus (MRN) did not alter tolerance to morphine. In addition, chronic administration of ketanserin (1.5, 3 and 6 mug/rat/day), as a 5-HT(2) receptors antagonist, in DRN and MRN did not produce any significant effect. We conclude that 5-HT(1A) receptors of DRN are involved in tolerance to antinociceptive effect of morphine. However, the exact mechanism of interaction between serotonergic and opioidergic systems is not clear and remains to be elucidated.     

Aging alters in a region-specific manner serotonin transporter sites and 5-HT(1A) receptor-G protein interactions in hamster brain

Key proteins regulating serotonergic activity, specifically the serotonin transporter and 5-HT(1A) receptor, were examined in the midbrain raphe nuclei of young (3-4 months) and old (17-19 months) hamsters (N=7-10/group). An age-related decrease in the maximal density of serotonin transporter sites labelled with [(3)H]paroxetine (fmol/mg protein, Old: 396+/-13; Young: 487+/-27) was observed in the dorsal raphe nucleus (DRN) but not the median raphe nucleus (MRN), without affecting the affinity of [(3)H]paroxetine. In the DRN and MRN, the stimulation of [(35)S]GTP gamma S binding by the 5-HT(1A) receptor agonist 8-OH-DPAT, or the number of 5-HT(1A) receptor sites labeled with [(3)H] MPPF, was not different in old versus young animals. Thus in the DRN, aging decreased serotonin transporter sites without changing 5-HT(1A) receptor activation of G proteins or 5-HT(1A) receptor density. In the CA(1) region of hippocampus, 8-OH-DPAT-stimulated [(35)S]GTP gamma S binding was increased in the older animals (% above basal, Old: 141+/-21; Young: 81+/-17) without changing specific [(3)H] MPPF binding sites, suggesting that the capacity of 5-HT(1A) receptors to activate G proteins is enhanced. Aging also appears to enhance this capacity in the dentate gyrus, because this region exhibited a constant level of 8-OH-DPAT-stimulated [(35)S]GTP gamma S binding in spite of an age-related decrease in the number of [(3)H] MPPF binding sites (fmol/mg protein, Old: 203+/-21; Young: 429+/-51).     

Effects of idazoxan on dorsal raphe 5-hydroxytryptamine neuronal function.

The effects of the alpha 2-adrenoceptor antagonist idazoxan on 5-hydroxytryptamine (5-HT) neuronal firing and release have been investigated. Idazoxan, administered i.v. (10 micrograms/kg and 0.5 mg/kg) increased dorsal raphe nucleus (DRN)-5-HT neuronal firing rate in a dose-dependent fashion. At the higher dose, a voltammetric study revealed increases in extracellular 5-HT and 5-hydroxyindole acetic acid (5-HIAA) levels, there was no effect with the lower dose. Intra-raphe administration of idazoxan (1 ng) also elevated the firing rate of 5-HT neurones in the dorsal raphe, suggesting that idazoxan may produce the increase in firing by a direct effect in the DRN. However, microiontophoretic application of idazoxan did not increase the firing rate of 5-HT neurones in the DRN. Thus the increase in the firing rate of 5-HT neurones in the DRN observed with systemic and local administration of idazoxan is probably not due to a direct action of idazoxan on the 5-HT neurone. Possibly the idazoxan acted at alpha 2-adrenoceptors located on noradrenergic terminals thus stimulating noradrenaline release and consequently increased 5-HT activity. Chronic administration of idazoxan (0.8 mg/kg per h for 14 days), using osmotic mini-pumps, caused an elevation in basal firing rate and an attenuation of the inhibitory response of DRN 5-HT neurones to the 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) (10 micrograms/kg i.v.). This finding suggests that chronic infusion with idazoxan leads to desensitisation of the 5-HT1A somatodendritic autoreceptor.