Functional effects of chronic electroconvulsive shock on serotonergic 5-HT(1A) and 5-HT(1B) receptor activity in rat hippocampus and hypothalamus

The aims of this work were to determine the influence of chronic electroconvulsive shock (ECS) on presynaptic 5-HT(1A) receptor function, postsynaptic 5-HT(1A) receptor function in hippocampus and hypothalamus, and presynaptic 5-HT(1B) receptor function in hippocampus and hypothalamus. This represents part of an on-going study of the effects of ECS on serotonergic receptor activity in selected brain areas which may be relevant to the effects of electroconvulsive therapy (ECT) in humans. Chronic ECS reduced the ability of the 5-HT(1A) receptor agonist 8-hydroxy-2(di-n-propylamino)tetraline (8-OH-DPAT) (0.2 mg/kg s.c.) to decrease 5-HT levels in hypothalamus as shown by in vivo microdialysis, indicative of a reduction in sensitivity of presynaptic 5-HT(1A) autoreceptors. The ability of the 5-HT(1B) receptor antagonist GR 127935 (5 mg/kg s.c.) to increase 5-HT levels in both hippocampus and hypothalamus was unaffected by chronic ECS. 8-OH-DPAT (0.2 mg/kg s.c.) increased cyclic AMP levels in hippocampus measured by in vivo microdialysis approximately 2-fold. The degree of stimulation of cyclic AMP formation was not altered by chronic ECS. However the cyclic AMP response to forskolin (50 micro M) administered via the microdialysis probe, which was approximately 4-fold of basal in sham-treated rats, was almost completely abolished in ECS-treated rats. Since this indicates that either adenylate cyclase catalytic unit activity or Gs protein activity is reduced in the hippocampus after chronic ECS, the lack of change in 8-OH-DPAT-induced cyclic AMP formation may be taken as possible evidence of an increase in sensitivity of postsynaptic 5-HT(1A) receptors in the hippocampus by chronic ECS. Chronic ECS increased basal plasma levels of corticosterone, ACTH and oxytocin. The ACTH response to s.c. injections of 0.2 mg/kg or 0.5 mg/kg 8-OH-DPAT was reduced by chronic ECS. Postsynaptic 5-HT(1A) receptor activity in the hypothalamus, in contrast to the hippocampus, thus appears to be desensitized after chronic ECS. We conclude that chronic ECS has regionally specific effects on both pre- and post-synaptic 5-HT(1A) receptors, but, in contrast to some antidepressant drugs, does not affect presynaptic 5-HT(1B) receptor activity.     

Methamphetamine-induced sensitization includes a functional upregulation of ventral pallidal 5-HT2A/2C receptors

Methamphetamine (METH) causes the release of serotonin (5-HT), but little is known about how repeated exposure to METH modifies serotonergic receptor function, especially in the ventral pallidum (VP), a brain region highly innervated by serotonin inputs. The current study was designed to ascertain if ventral pallidal neurons are functionally upregulated 3 days after a behaviorally sensitizing treatment regimen of METH, and whether these effects could be revealed by activating the 5-HT2A/2C receptors. Rats treated with METH (2.5 kg/mg/day) for 5 days, responded with enhanced stereotypic behaviors. Electrophysiological evaluations of the VP conducted in anesthetized rats 3 days following this sensitizing treatment regimen of METH revealed that spiking rate increases induced by intravenous METH were augmented above that seen in rats with a history of saline treatments. The efficacy of the 5-HT2A/2C agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) to augment ventral pallidal cell firing also was enhanced in METH-sensitized rats. These data reveal that METH-induced behavioral sensitization renders the VP more responsive to METH and is associated with a functional upregulation of 5-HT2 receptors.     

Involvement of the 5-HT(1A) and 5-HT(1B) serotonergic receptor subtypes in sexual arousal in male mice

The presence of a sexually receptive female behind a partition that prevents physical contact, but not seeing or smelling, increases blood testosterone level and induces the specific behavior in CBA male mice so that they more frequently approach the partition and spend more time near it in an attempt to make their way to the female. Treatment with the selective 5-HT(1A) serotonin receptor agonist 8-OH-DPAT (0.1, 0.25, 0.5 and 2.0 mg/kg) induced a dose-dependent decrease in the amount of time spent by the males near the partition, or "partition time", which is considered the main pattern of sexual motivation. The activating effect of female exposure on the male's pituitary-testicular system was totally blocked, as no increase in plasma testosterone level was observed. The 5-HT(1A) antagonist p-MPPI (0.1, 0.2 and 0.4 mg/kg) itself did not affect behavior or alter plasma testosterone, but attenuated the inhibiting effect of 8-OH-DPAT on behavior and totally antagonised the effect of the 5-HT(1A) agonist on testosterone response. The 5-HT(1B) agonist CGS-12066A (1.0 and 2.0 mg/kg) has no influence on the plasma testosterone increase exhibited by the male in response to female exposure. At the same time, either dose of CGS-12066A significantly reduced the partition time. The conclusion was made that the 5-HT(1A) subtype is involved in controlling both behavioral and hormonal indices of sexual arousal in male mice, while the 5-HT(1B) receptors antagonise sexual motivation, but do not modify the hypothalamic-pituitary-testicular response.     

Sequential activation of the 5-HT1(A) serotonin receptor and TrkB induces the serotonergic neuronal phenotype

Serotonin (5-HT) is an important factor controlling survival, differentiation, and plasticity of neurons in serotonergic target regions of the brain and has been implicated in major psychiatric and autonomic disorders. Relatively little is known, however, of factors controlling differentiation and plasticity of developing and adult 5-HT neurons. We show now that 5-HT, the 5-HT1(A) receptor, brain-derived neurotrophic factor (BDNF), and its receptor, trkB, form an auto/paracrine loop for the regulation of the serotonergic phenotype. Serotonin applied to cultures from E14 rat raphe increased numbers of neurons expressing serotonergic markers in a dose-dependent manner. Agonists of the 5-HT1(A) receptor, BP-554 and 8-OH-DPAT, but not agonists of the 5-HT1(B) and 5-HT1(D) receptors, mimicked this effect, while the specific 5-HT1(A) antagonist, WAY-100635, inhibited it. Serotonin also increased BDNF mRNA and protein in embryonic raphe cultures. Induction of serotonergic markers by serotonin was suppressed by a trkB-IgG fusion protein but not by trkC-IgG. Taken together, our data indicate that serotonin acts on 5-HT1(A) autoreceptors, causing up-regulation of BDNF, which activates trkB to promote serotonergic phenotype-specific markers.     

Effects of triiodothyronine on 5-HT(1A) and 5-HT(1B) autoreceptor activity, and postsynaptic 5-HT(1A) receptor activity, in rat hypothalamus: lack of interaction with imipramine

Triiodothyronine (T3) is effective in both augmenting and accelerating the therapeutic response to antidepressant drugs, especially tricyclics, and there is evidence from both human and animal studies that it acts on serotonergic neurotransmission. In this work we examined the effects of T3 alone and together with imipramine on 5-HT levels in the hypothalamus and on 5-HT(1A) and 5-HT(1B) autoreceptor sensitivity, using in vivo microdialysis in the rat. The effects of T3 on postsynaptic 5-HT(1A) receptor activity in the hypothalamus were also determined using a neuroendocrine challenge procedure. T3 administered daily at 20 microg/kg s.c. for 2 weeks reduced the sensitivity of 5-HT(1A) autoreceptors which control 5-HT release, as measured by the effect of 8-OH-DPAT to decrease 5-HT in the hypothalamus, and also the sensitivity of hypothalamic 5-HT(1B) receptors as measured by the effect of the 5-HT(1B) receptor agonist CP 93129 to decrease 5-HT release. Imipramine at 10 mg/kg daily for 4 weeks by osmotic minipump reduced 5-HT(1A) autoreceptor activity, as measured by the effect of 8-OH-DPAT in the hypothalamus, but the combination of T3 and imipramine given for 2 weeks did not affect either 5-HT(1A) or 5-HT(1B) autoreceptor activity. T3 at 20 microg/kg s.c. given daily for 1 week also reduced the sensitivity of postsynaptic 5-HT(1A) receptors in the hypothalamus, as measured by injection of 8-OH-DPAT and determination of the plasma ACTH and corticosterone responses. Animals which received T3 for 7 days showed a dose-dependent reduction in plasma free T4 levels but no change in total T3 levels. We conclude that while T3 alone affects both presynaptic and postsynaptic components of the serotonergic system, these effects may not be responsible for the therapeutic acceleration action seen with a combination of a tricyclic drug and T3.     

Role of presynaptic serotonergic receptors on the mechanism of action of 5-HT1A and 5-HT1B agonists on masculine sexual behaviour: physiological and pharmacological implications

Summary In order to establish whether the 5-HT1A or the 5HT1B agonists, 8-OH-DPAT or TFMPP, produce their facilitatory or inhibitory actions on masculine sexual behaviour via a mechanism involving: (a) the serotonin synthesis or release; (b) the stimulation of presynaptic receptors, or (c) the stimulation of somatodendritic receptors, three series of experiments were performed. The administration of the serotonin synthesis inhibitor, p-chlorophenylalanine (p-CPA, 300mg/kg×3 days), facilitated sexual behaviour but does not interfere neither with the inhibitory nor with the facilitatory effects of TFMPP (0.5mg/kg) or 8-OH-DPAT (0.5 mg/kg), respectively. The icv or the intraraphé administration of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), slightly stimulated masculine sexual behaviour and produced a decrease in serotonin and its metabolite levels. In lesioned animals TFMPP (0.5 mg/kg) resulted in an inhibitory effect reflected as a prolongation of the ejaculation latency. The inhibitory effect of this drug on mounting behaviour was not observed in 5,7-DHT treated rats. In lesioned animals 8-OH-DPAT (0.5 mg/kg) produced the same facilitatory effect. Present data indicate that serotonergic postsynaptic receptors mediate both the inhibitory and the facilitatory actions of TFMPP or 8-OH-DPAT in copulation. All data further support the idea that endogenous serotonin acts via the stimulation of 5-HT1B receptors to induce its inhibitory effects on masculine sexual behaviour.     

Changes in exploratory activity following stimulation of hippocampal 5-HT1A and 5-HT1B receptors in the rat

The object exploration task allows the measure of changes in locomotor and exploratory activities, habituation, and reaction to a spatial change and to novelty. The effects of intrahippocampal (dorsal CA1 field) microinjections of serotonin 1 receptor (5-HT1) agonists on these behavioral components were evaluated in the rat. 8-Hydroxy-2-(din-propylamino)-tetralin (8-OH-DPAT,5 μg/μl) was used as a 5-HT1A agonist, 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo[3,2-b] pyrid-5-one (CP 93,129, 16 μg/μl) as a 5-HT1B agonist, and scopolamine (10 μg/μl) as a muscarinic cholinergic antagonist. Scopolamine induced a long-lasting increase in locomotor activity and a lack of reaction to spatial change; both these results are in agreement with the known crucial influence of the septo-hippocampal cholinergic system in hippocampal functioning. Stimulation of 5-HT1A and 5-HT1B receptors induced a decrease in object exploration and habituation without affecting the retrieval of spatial information. But stimulation of hippocampal 5-HT1B receptors induced a selective change in the animal's emotional state, i.e., an initial decrease in locomotor activity and a neophobic reaction in response to a new object; such effects did not occur following stimulation of 5HT1A receptors. These results have to be considered in the light of the anxiogenic propety of 5-HT1B agonists. On the whole, they support the hypothesis of the involvement of the serotonergic system, via 5HT1A and 5-HT1B receptors, in the modulation of hippocampal functions. © 1995 Wiley-Liss, Inc.     

Effects of ethanol and 5-HT1A agonists on astroglial S100B

Previous studies from this and another laboratory demonstrated that in utero ethanol exposure reduces 5-HT neurons and S100B-immunopositive glia that are proximal to these neurons. Our laboratory also found that these effects are prevented by maternal treatment with a 5-HT(1A) agonist. Because of S100B's important role in the development of 5-HT neurons, the present study used both in vivo and in vitro models to investigate the potential involvement of S100B with the damaging effects of ethanol and with the protective effects of 5-HT(1A) agonists. We used in situ hybridization to address whether a 5-HT(1A) agonist could potentially affect S100B mRNA in vivo. Maternal treatment with buspirone between gestation days 13 and 20 significantly increased S100B mRNA in neuroepithelium of G20 offspring of control (40%) and ethanol-fed dams (20%). However, S100B mRNA was not altered in neuroepithelium from ethanol-exposed offspring. In astroglial cultures, we examined whether ethanol reduces the release of S100B and whether a 5-HT(1A) agonist could stimulate the release of this protein. We also evaluated the effects of ethanol and ipsapirone on astroglial content of S100B. Neither the concentration of S100B in astroglial media nor astroglial content of S100B were affected by ethanol. However, treatment with 100 nM ipsapirone, a 5-HT(1A) agonist, between the 6th and 7th day in vitro, increased astroglial release of S100B 2- to 3-fold. Thus, the protective effects of a 5-HT(1A) agonist on ethanol-treated 5-HT neurons might be associated with the ability of these drugs to release the neurotrophic factor S100B from astrocytes.     

Effects of dorsal rhizotomy and selective lesion of serotonergic and noradrenergic systems on 5-HT1a , 5-HT1b and 5-HT3 receptors in the rat spinal cord

Summary Autoradiographic studies were performed in combination with dorsal rhizotomy or selective lesion of descending serotonergic or noradrenergic systems in an attempt to identify the neuronal cell types endowed with the serotonin 5-HT_1a , 5-HT_1b and 5-HT₃ receptors in the rat spinal cord. Unilateral sectioning of seven dorsal roots (C4–T2) at the cervical level produced a marked decrease (–75%, 10 days after the surgery) in the binding of [¹²⁵I]iodozacopride to 5-HT₃ receptors in the superficial layers of the ipsilateral dorsal horn, further confirming the preferential location of these receptors on primary afferent fibres. In addition, a significant decrease (20%) in the binding of [³H]8-OH-DPAT to 5-HT_1A receptors and of [¹²⁵I]GTI to 5-HT_1B receptors was also observed in the same spinal area in rhizotomized rats, suggesting that a small proportion of these receptors are also located on primary afferent fibres. The labelling of 5-HT_1B receptors was significantly decreased (–12%) in the dorsal horn at the cervical (but not the lumbar) level, and that of 5-HT₃ receptors was unchanged in the whole spinal cord in rats whose descending serotonergic projections had been destroyed by 5,7-dihydroxytryptamine. Conversely, the labelling of 5-HT_1A receptors was significantly increased in the cervical (+13%) and lumbar (+42%) dorsal horn in 5,7-dihydroxytryptamine-lesioned rats. Similarly, [³H]8-OH-DPAT binding to 5-HT_1A receptors significantly increased (+26%) in the lumbar (but not the cervical) dorsal horn in rats whose noradrenergic systems had been lesioned by DSP-4. The labelling of 5-HT_1B receptors was also increased (+31% at the cervical level; +17% at the lumbar level), whereas that of 5-HT₃ receptors remained unchanged in these animals. These data indicate that complex adaptive changes in the expression of 5-HT_1A and 5-HT_1B receptors occurred in the rat spinal cord following the lesion of descending monoaminergic systems.Abbreviations CP 93129 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo [3,2-b]pyrid-5-one - 5,7-DHT 5,7-dihydroxytryptamine - DSP-4 N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine - GTI serotonin-O-carboxymethylglycyl-iodotyrosinamide - HEPES 4-(2-hydroxyethyl)-l-piperazine ethane sulfonic acid - 5-HT 5-hydroxytryptamine - i.c. intracisternally - NA noradrenaline - 8-OH-DPAT 8-hydroxy-2-(di-n-propylamino)tetralin     

Opioid and GABA modulation of accumbens-evoked ventral pallidal activity

Summary The principle output of the nucleus accumbens innervates the ventral pallidum and rostral substantia innominata. GABA and opioid peptides are among the neurotransmitter candidates for this projection. The goal of the present experiments was to delineate further the physiology and pharmacology of the accumbens projection to the ventral pallidum. The trans-synaptic responsiveness of ventral pallidal and rostral substantia innominata neurons to electrical stimulation of the nucleus accumbens was examined concurrently with the ability of microiontophoretically applied morphine (an opioid agonist), naloxone (an opioid antagonist) and bicuculline (a GABA antagonist) to modulate evoked responses. Accumbens stimulation altered the firing rate in 60% of the 132 neurons tested. Fifty-two percent of responding neurons exhibited simple excitations or inhibitions in response to accumbens stimulation, while 48% exhibited complex response sequences with two or more evoked components. Predominant responses consisted of a short latency (<10 ms) and short duration (10 ms) excitation (51 % of responding neurons) and an inhibition with a variable, onset latency and, duration (52% of responding neurons). Evoked responses often occurred within limited areas within the ventral pallidum suggesting that activation of descending afferents can influence discrete targets within the region. A large majority (>80%) of neurons evoked by accumbens stimulation also exhibited a current-dependent and naloxone-sensitive increase in spontaneous firing to microiontophoretically applied morphine. Morphine shortened the duration of the accumbens-evoked, short latency excitation and attenuated the magnitude of the long-latency inhibition. Evoked responses in the presence of morphine were opposite to those observed with naloxone, but similar to bicuculline. Thus, opioid receptor activation may be functionally antagonistic to GABAergic neurotransmission in the ventral pallidum. The prominence of accumbens-evoked and morphine-sensitive neurons within the ventral pallidum corroborates the density of accumbens and opioid input to this brain region, and demonstrates that opioids serve as an important influence on neuronal activity and information processing in the ventral-striatopallidal pathway.     

Antiplatelet and antithrombotic activity of SL65.0472, a mixed 5-HT1B/5-HT2A receptor antagonist

The antiplatelet and antithrombotic activity of SL65.0472 (7-fluoro-2-oxo-4-[2-[4-(thieno [3,2-c]pyrin-4-yl) piperazin-1-yl]ethyl]-1,2-di-hydroquinoline-acetamide), a mixed 5-HT1B/5-HT2A receptor antagonist was investigated on 5HT-induced human platelet activation in vitro, and in rat, rabbit and canine platelet dependent thrombosis models. SL65.0472 inhibited 5-HT-induced platelet shape change in the presence of EDTA (IC50 values = 35, 69 and 225 nM in rabbit, rat and human platelet rich plasma (PRP)), and also inhibited aggregation induced in human PRP by 3-5 microM 5-HT + threshold concentrations of ADP (0.5-1 microM) or collagen (0.3 microg/ml) with mean IC50 values of 49 +/- 13 and 48 +/- 6 nM respectively. SL65.0472 inhibited thrombus formation when given both intravenously 5 min and orally 2 h prior to assembly of an arterio-venous (A-V) shunt in rats as from 0.1 and 0.3 mg/kg respectively. It was active in a rabbit A-V shunt model with significant decreases in thrombus weight as from 0.1 mg/kg i. v. and at 10 mg/kg p.o. The delay to occlusion in an electric current-induced rabbit femoral artery thrombosis model was increased by 251% (p <0.05) after 20 mg/kg p.o. SL65.0472 (30 microg/kg i.v.) virtually abolished coronary cyclic flow variations (7.2 +/- 1.0/h to 0.6 +/- 0.6/h, p <0.05) and increased minimum coronary blood flow (1.2 +/- 0.8 ml/min to 31.8 +/- 8.4 ml/min, p <0.05) in a coronary artery thrombosis model in the anaesthetised dog. Finally, SL65.0472 significantly increased the amount of blood lost after rat tail transection at 3 mg/kg p.o. Thus the anti-5-HT2A component of SL65.0472 is reflected by its ability to inhibit 5-HT-induced platelet activation, and platelet-rich thrombus formation.     

Physiological effects of selective 5-HT1a and 5-HT1b ligands in rat hippocampus: comparison to 5-HT

The responses of CA1 neurons to topical application of serotonin (5-HT) and selective 5-HT1a and 5-HT1b agonists were examined with intracellular recording in the hippocampal slice. 5-HT produced a uniform hyperpolarizing response associated with an increase in K conductance as previously reported. In addition a marked reduction was recorded in slow afterhyperpolarization (AHP) which follows a burst discharge. 8-OH-DPAT, ipsapirone and LY165,163 partially mimicked the hyperpolarizing response to 5-HT when first applied to the slice. However, these 5-HT1a ligands antagonized responses to subsequent applications of 5-HT. Topical application of the 5-HT1b ligand TFMPP on the slice did not produce the direct or antagonistic action seen with the 5-HT1a ligands. It is suggested that the physiological response to 5-HT in the rat hippocampus is mediated by a 5-HT1a receptor. The currently available 5-HT1a ligands show a low agonist potential and a high antagonist action towards the responses of hippocampal neurons to 5-HT. Definite classification of the hyperpolarizing response to 5-HT awaits development of more specific ligands having a pure agonistic activity.     

Effects of D1 and D2 antagonists on apomorphine-induced responses of ventral pallidal neurons

The ventral pallidum with the adjacent substantia innominata (VP) has been described as a dopaminoceptive brain region. Repeated systemic injections of the dopamine agonist, apomorphine, induce dose-dependent alterations in VP neuronal activity. The present studies evaluated the contribution of D1 and D2 receptor subtypes to apomorphine-induced alterations in extracellularly recorded VP neuronal activity. Both sulpiride (D2 antagonist) and SCH23390 (D1 antagonist) attenuated many of these responses; however, pretreatment with either antagonist did not alter the number of responding neurons, or the maximal effect induced by apomorphine. Thus, activation of either receptor subtype by apomorphine is sufficient to initiate the observed responses, and both may be involved in dopaminergic modulation of VP neurons.     

Involvement of 5-HT1B receptors within the ventral tegmental area in regulation of mesolimbic dopaminergic neuronal activity via GABA mechanisms: a study with dual-probe microdialysis

This study was designed to assess the involvement of 5-HT1B receptors within the ventral tegmental area (VTA) in the regulation of mesolimbic dopaminergic transmission. Dual-probe microdialysis was performed in freely moving adult Sprague-Dawley rats with one probe within the VTA and the other within the ipsilateral nucleus accumbens (NACC). Drugs were administered into the VTA via retrograde dialysis. Dialysates from both the VTA and the NAC were collected for determination of dopamine (DA) and gamma-aminobutyric acid (GABA) by high-performance liquid chromatography with electrochemical detection. Intra-tegmental infusion of CP 93129 (20, 40, and 80 microM), a 5-HT1B receptor agonist, increased extracellular DA concentrations in a concentration-dependent manner not only in the NACC but also in the VTA, indicating increased mesolimbic DA neuron activity. Administration of CP 93129 at 80 microM into the VTA also significantly decreased extracellular GABA concentrations in this region. Co-infusion of the 5-HT1B receptor antagonist SB 216641 (10 microM), but not the 5-HT1A receptor antagonist WAY 100635 (10 microM) or the 5-HT1D/1A receptor antagonist BRL 15572 (10 microM), antagonized not only the effects of intra-tegmental CP 93129 (80 microM) on VTA DA and NAC DA but also on VTA GABA. The results suggest that activation of VTA 5-HT1B receptors increases mesolimbic DA neuron activities. The increased DA neuron activity may be associated, at least in part, with the 5-HT1B receptor-mediated inhibition of VTA GABA release.     

Stimulation of 5-HT1A, 5-HT1B, 5-HT2A/2C, 5-HT3 and 5-HT4 receptors or 5-HT uptake inhibition: short- and long-term memory

In order to determine whether short- (STM) and long-term memory (LTM) function in serial or parallel manner, serotonin (5-hydroxtryptamine, 5-HT) receptor agonists were tested in autoshaping task. Results show that control-vehicle animals were modestly but significantly mastering the autoshaping task as illustrated by memory scores between STM and LTM. Thus, post-training administration of 8-OHDPAT (agonist for 5-HT(1A/7) receptors) only at 0.250 and 0.500 mg/kg impaired both STM and LTM. CGS12066 (agonist for 5-HT(1B)) produced biphasic affects, at 5.0 mg/kg impaired STM but at 1.0 and 10.0 mg/kg, respectively, improved or impaired LTM. DOI (agonist for 5-HT(2A/2C) receptors) dose-dependently impaired STM and, at 10.0 mg/kg only impaired LTM. Both, STM and LTM were impaired by either mCPP (mainly agonist for 5-HT(2C) receptors) or mesulergine (mainly antagonist for 5-HT(2C) receptors) lower dose. The 5-HT(3) agonist mCPBG at 1.0 impaired STM and its higher dose impaired both STM and LTM. RS67333 (partial agonist for 5-HT(4) receptors), at 5.0 and 10.0 mg/kg facilitated both STM and LTM. The higher dose of fluoxetine (a 5-HT uptake inhibitor) improved both STM and LTM. Using as head-pokes during CS as an indirect measure of food-intake showed that of 30 memory changes, 21 of these were unrelated to the former. While some STM or LTM impairments can be attributed to decrements in food-intake, but not memory changes (either increase or decreases) produced by 8-OHDPAT, CGS12066, RS67333 or fluoxetine. Except for animals treated with DOI, mCPBG or fluoxetine, other groups treated with 5-HT agonists 6 h following autoshaping training showed similar LTM and unmodified CS-head-pokes scores.     

Electrophysiological responses of serotoninergic dorsal raphe neurons to 5-HT1A and 5-HT1B agonists

A direct comparison was made of the effects of serotonin 5-HT1A and 5-HT1B selective compounds on the spontaneous firing rate of dorsal raphe serotoninergic neurons in chloral-hydrate-anesthetized rats. Following intravenous administration, the 5-HT1A selective compounds ipsapirone (TVX Q 7821) and LY 165163 potently inhibited single-unit activity in a dose-dependent manner whereas the 5-HT1B selective compounds, m-chlorophenylpiperazine (mCPP) and trifluoromethylphenylpiperazine (TFMPP), displayed only weak or irregular actions. Low microiontophoretic currents of ipsapirone and LY 165163 were also effective in suppressing spontaneous firing; dose-response relationships for the 5-HT1A compounds were indistinguishable from that of 5-HT itself. In contrast, dorsal raphe neurons were only weakly responsive to microiontophoretic application of mCPP and TFMPP; dose-response relationships for the 5-HT1B compounds were significantly displaced from that of 5-HT. In intracellular studies, ipsapirone and LY 165163, when added to the media bathing brain slices, mimicked the actions of 5-HT in hyperpolarizing dorsal raphe cell membranes and decreasing input resistance; however, the maximal effects of the 5-HT1A compounds on these membrane properties exceeded those of 5-HT. In summary, dorsal raphe 5-HT neurons appear highly responsive to 5-HT1A, but not to 5-HT1B compounds; these findings are discussed with regard to the 5-HT receptor subtypes as candidates for the somatodendritic autoreceptor of dorsal raphe neurons.     

Altered expression and functions of serotonin 5-HT1A and 5-HT1B receptors in knock-out mice lacking the 5-HT transporter

By taking up serotonin (5-hydroxytryptamine, 5-HT) released in the extracellular space, the 5-HT transporter (5-HTT) regulates central 5-HT neurotransmission. Possible adaptive changes in 5-HT neurotransmission in knock-out mice that do not express the 5-HT transporter were investigated with special focus on 5-HT1A and 5-HT1B receptors. Specific labelling with radioligands and antibodies, and competitive RT-PCR, showed that 5-HT1A receptor protein and mRNA levels were significantly decreased in the dorsal raphe nucleus (DRN), increased in the hippocampus and unchanged in other forebrain areas of 5-HTT-/- vs. 5-HTT+/+ mice. Such regional differences also concerned 5-HT1B receptors because a decrease in their density was found in the substantia nigra (-30%) but not the globus pallidus of mutant mice. Intermediate changes were noted in 5-HTT+/- mice compared with 5-HTT+/+ and 5-HTT-/- animals. Quantification of [35S]GTP-gamma-S binding evoked by potent 5-HT1 receptor agonists confirmed such changes as a decrease in this parameter was noted in the DRN (-66%) and the substantia nigra (-30%) but not other brain areas in 5-HTT-/- vs. 5-HTT+/+ mice. As expected from actions mediated by functional 5-HT1A and 5-HT1B autoreceptors, a decrease in brain 5-HT turnover rate after i.p. administration of ipsapirone (a 5-HT1A agonist), and an increased 5-HT outflow in the substantia nigra upon local application of GR 127935 (a 5-HT1B/1D antagonist) were observed in 5-HTT+/+ mice. Such effects were not detected in 5-HTT-/- mice, further confirming the occurrence of marked alterations of 5-HT1A and 5-HT1B autoreceptors in these animals.     

Increased binding at 5-HT(1A), 5-HT(1B), and 5-HT(2A) receptors and 5-HT transporters in diet-induced obese rats

5-Hydroxytryptamine (5-HT, serotonin), synthesized in midbrain raphe nuclei and released in various hypothalamic sites, decreases food intake but the specific 5-HT receptor subtypes involved are controversial. Here, we have studied changes in the regional density of binding to 5-HT receptors and transporters and the levels of tryptophan hydroxylase, in rats with obesity induced by feeding a palatable high-energy diet for 7 weeks. We mapped binding at 5-HT receptor subtypes and transporters using quantitative autoradiography and determined tryptophan hydroxylase protein levels by Western blotting. In diet-induced obese (DiO) rats, specific binding to 5-HT(1A) receptors ([3H]8-OH-DPAT) was significantly increased in the dorsal and median raphe by 90% (P<0.01) and 132% (P<0.05), respectively, compared with chow-fed controls. 5-HT(1B) receptor binding sites ([125I]cyanopindolol) were significantly increased in the hypothalamic arcuate nucleus (ARC) of DiO rats (58%; P<0.05), as were 5-HT(2A) receptor binding sites ([3H]ketanserin) in both the ARC (44%; P<0.05) and lateral hypothalamic area (LHA) (121%; P<0.05). However, binding to 5-HT(2C) receptors ([3H]mesulgergine) in DiO rats was not significantly different from that in controls in any hypothalamic region. Binding to 5-HT transporters ([3H]paroxetine) was significantly increased (P<0.05) in both dorsal and median raphe, paraventricular nuclei (PVN), ventromedial nuclei (VMH), anterior hypothalamic area (AHA) and LHA of DiO rats, by 47%-165%. Tryptophan hydroxylase protein levels in the raphe nuclei were not significantly different between controls and DiO rats. In conclusion, we have demonstrated regionally specific changes in binding to certain 5-HT receptor subtypes in obesity induced by voluntary overeating of a palatable diet. Overall, these changes are consistent with reduced 5-HT release and decreased activity of the 5-HT neurons. Reduction in the hypophagic action of 5-HT, possibly acting at 5-HT(1A), 5-HT(1B) and 5-HT(2A) receptors, may contribute to increased appetite in rats presented with highly palatable diet.     

Synaptically released 5-HT modulates the activity of tonically discharging neuronal populations in the rostral ventral medulla (RVM)

There is substantial evidence for an important modulating role of monoamines (catecholamines and serotonin, 5-HT) in the rostral ventral medulla (RVM), a region which plays an important role in cardiovascular and nociceptive functions. We investigated in slices the role of endogenous monoamines in the synaptic control of the activity of rat RVM neuronal populations using intracellular recordings in the lateral RVM plus lateral aspect of nucleus paragigantocellularis lateralis. A triple-labelling protocol allowed us to identify the location of impaled neurons and their eventual monoaminergic phenotype within the serotonergic and catecholaminergic populations of the RVM. Focal electrical stimulation revealed the existence of a functional monoaminergic input onto RVM neurons which was mediated by endogenous 5-HT acting at inhibitory 5-HT1A receptors but did not involve noradrenergic neurotransmission. The slow 5-HT-mediated inhibitory postsynaptic potential (IPSP) was only observed in the regularly discharging neurons, which were found to be neither catecholaminergic nor serotonergic. The synaptic release of 5-HT was, itself, under an inhibitory control involving GABAA (gamma-aminobutyric acid) receptors. Moreover, we characterized the effect of the 5-HT-releasing agent fenfluramine on this functional 5-HT-mediated synaptic transmission. Our results show that the effect of fenfluramine is biphasic consisting of an initial prolongation of the serotonergic IPSP followed by a decrease in amplitude. Our data provide a basis for the previously reported inhibitory effects of exogenously applied serotonin agonists/antagonists on the autonomic functions controlled by the RVM. This 5-HT pathway, which functionally links the serotonergic and catecholaminergic regions, might play an important role in cardiovascular and nociceptive functions.