5-HT inhibits lateral entorhinal cortical neurons of the rat in vitro by activation of potassium channel-coupled 5-HT1A receptors

TREK-1 inhibition promotes synaptic plasticity in the prelimbic cortex

Synaptic plasticity is one of the putative mechanisms involved in the maturation of the prefrontal cortex (PFC) during postnatal development. Early life stress (ELS) affects the shaping of cortical circuitries through impairment of synaptic plasticity supporting the onset of mood disorders. Growing evidence suggests that dysfunctional postnatal maturation of the prelimbic division (PL) of the PFC might be related to the emergence of depression. The potassium channel TREK-1 has attracted particular interest among many factors that modulate plasticity, concerning synaptic modifications that could underlie mood disorders. Studies have found that ablation of TREK-1 increases the resilience to depression, while rats exposed to ELS exhibit higher TREK-1 levels in the PL. TREK-1 is regulated by multiple intracellular transduction pathways including the ones activated by metabotropic receptors. In the hippocampal neurons, TREK-1 interacts with the serotonergic system, one of the main factors involved in the action of antidepressants. To investigate possible mechanisms related to the antidepressant role of TREK-1, we used brain slice electrophysiology to evaluate the effects of TREK-1 pharmacological blockade on synaptic plasticity at PL circuitry. We extended this investigation to animals subjected to ELS. Our findings suggest that in non-stressed animals, TREK-1 activity is required for the reduction of synaptic responses mediated by the 5HT1A receptor activation. Furthermore, we demonstrate that TREK-1 blockade promotes activity-dependent long-term depression (LTD) when acting in synergy with 5HT1A receptor stimulation. On the other hand, in ELS animals, TREK-1 blockade reduces synaptic transmission and facilitates LTD expression. These results indicate that TREK-1 inhibition stimulates synaptic plasticity in the PL and this effect is more pronounced in animals subjected to ELS during postnatal development.

Serotonin 5-HT1A Receptor-Mediated Reduction of Excitatory Synaptic Transmission in Layers II/III of the Parasubiculum

Serotonin (5-HT) has important effects on cognitive function within the hippocampal region where it modulates membrane potential and excitatory and inhibitory synaptic transmission. Here, we investigated how 5-HT modulates excitatory synaptic strength in layers II/III of the parasubiculum in rat brain slices. Bath-application of 1 or 10 μM 5-HT resulted in a strong, dose-dependent, and reversible reduction in the amplitude of field excitatory postsynaptic potentials (fEPSPs) recorded in the parasubiculum. The 5-HT reuptake blocker citalopram (10 μM) also reduced fEPSP amplitudes, indicating that 5-HT released within the slice inhibits synaptic transmission. The reduction of fEPSPs induced by 5-HT was blocked by the 5-HT_1A receptor blocker NAN-190 (10 μM), but not by the 5-HT₇ receptor blocker SB-269970 (10 μM). Moreover, the 5-HT_1A agonist 8-OH-DPAT induced a reduction of fEPSP amplitude similar to that induced by 5-HT. The reduction was prevented by the 5-HT_1A receptor blocker NAN-190. The reduction in fEPSPs induced by either 5-HT or by 8-OH-DPAT was accompanied by an increase in paired-pulse ratio, suggesting that it is due mainly to reduced glutamate release. Our data suggest that the effects of serotonin on cognitive function may depend in part upon a 5-HT_1A-mediated reduction of excitatory synaptic transmission in the parasubiculum. This may also affect synaptic processing in the entorhinal cortex, which receives the major output projection of the parasubiculum.

Serotonin modulates the excitatory synaptic transmission in the dentate granule cells

Serotonergic fibers from the raphe nuclei project to the hippocampal formation, the activity of which is known to modulate the inhibitory interneurons in the dentate gyrus. On the other hand, serotonergic modulation of the excitatory synapses in the dentate gyrus is not well examined. In the present study, we examined the effects of 5-HT on the excitatory postsynaptic potentials (EPSPs) in the dentate granule cells evoked by the selective stimulation of the lateral perforant path (LPP), the medial perforant path (MPP), or the mossy cell fibers (MCF). 5-HT depressed the amplitude of unitary EPSPs (uEPSPs) evoked by the stimulation of LPP or MPP, whereas uEPSPs evoked by MCF stimulation were little affected. The effect was partly explained by the decrease of the resting membrane resistance following the activation of 5-HT1A receptors, which was confirmed by computer simulations. We also found that the probability of evoking uEPSP by LPP stimulation but not MPP or MCF stimulation was reduced by 5-HT and that the paired-pulse ratio of LPP-evoked EPSP but not that of MPP- or MCF-evoked ones was increased by 5-HT. These effects were blocked by 5-HT2 antagonist, suggesting that the transmitter release in the LPP-granule cell synapse is inhibited by the activation of 5-HT2 receptors. The present results suggest that 5-HT can modulate the EPSPs in the dentate granule cells by at least two distinct mechanisms.

From the background to the spotlight: TASK channels in pathological conditions

TWIK-related acid-sensitive potassium channels (TASK1-3) belong to the family of two-pore domain (K(2P) ) potassium channels. Emerging knowledge about an involvement of TASK channels in cancer development, inflammation, ischemia and epilepsy puts the spotlight on a leading role of TASK channels under these conditions. TASK3 has been especially linked to cancer development. The pro-oncogenic potential of TASK3 could be shown in cell lines and in various tumor entities. Pathophysiological hallmarks in solid tumors (e.g. low pH and oxygen deprivation) regulate TASK3 channels. These conditions can also be found in (autoimmune) inflammation. Inhibition of TASK1,2,3 leads to a reduction of T cell effector function. It could be demonstrated that TASK1(-/-) mice are protected from experimental autoimmune inflammation while the same animals display increased infarct volumes after cerebral ischemia. Furthermore, TASK channels have both an anti-epileptic as well as a pro-epileptic potential. The relative contribution of these opposing influences depends on their cell type-specific expression and the conditions of the cellular environment. This indicates that TASK channels are per se neither protective nor detrimental but their functional impact depends on the "pathophysiological" scenario. Based on these findings TASK channels have evolved from "mere background" channels to key modulators in pathophysiological conditions.

Hearing loss alters serotonergic modulation of intrinsic excitability in auditory cortex

Sensorineural hearing loss during early childhood alters auditory cortical evoked potentials in humans and profoundly changes auditory processing in hearing-impaired animals. Multiple mechanisms underlie the early postnatal establishment of cortical circuits, but one important set of developmental mechanisms relies on the neuromodulator serotonin (5-hydroxytryptamine [5-HT]). On the other hand, early sensory activity may also regulate the establishment of adultlike 5-HT receptor expression and function. We examined the role of 5-HT in auditory cortex by first investigating how 5-HT neurotransmission and 5-HT(2) receptors influence the intrinsic excitability of layer II/III pyramidal neurons in brain slices of primary auditory cortex (A1). A brief application of 5-HT (50 μM) transiently and reversibly decreased firing rates, input resistance, and spike rate adaptation in normal postnatal day 12 (P12) to P21 rats. Compared with sham-operated animals, cochlear ablation increased excitability at P12-P21, but all the effects of 5-HT, except for the decrease in adaptation, were eliminated in both sham-operated and cochlear-ablated rats. At P30-P35, cochlear ablation did not increase intrinsic excitability compared with shams, but it did prevent a pronounced decrease in excitability that appeared 10 min after 5-HT application. We also tested whether the effects on excitability were mediated by 5-HT(2) receptors. In the presence of the 5-HT(2)-receptor antagonist, ketanserin, 5-HT significantly decreased excitability compared with 5-HT or ketanserin alone in both sham-operated and cochlear-ablated P12-P21 rats. However, at P30-P35, ketanserin had no effect in sham-operated and only a modest effect cochlear-ablated animals. The 5-HT(2)-specific agonist 5-methoxy-N,N-dimethyltryptamine also had no effect at P12-P21. These results suggest that 5-HT likely regulates pyramidal cell excitability via multiple receptor subtypes with opposing effects. These data also show that early sensorineural hearing loss affects the ability of 5-HT receptor activation to modulate A1 pyramidal cell excitability.

Acute citalopram administration may disrupt contextual information processing in healthy males

Selective serotonin reuptake inhibitors (SSRI) are the most commonly prescribed antidepressants. It has been suggested however that SSRI administration may affect response inhibition and contextual processing but the available evidence is minimal. Therefore, the purpose of this study was to identify the effect size of acute (within 24 hours) and chronic (28 days) administration of the highly selective SSRI, Citalopram, compared to placebo on response inhibition (measured by the Degraded Symbol Continuous Performance Task [DS-CPT]) and contextual processing (assessed using a Delayed Non-Matching to Sample Task [DNMS]) in healthy males (n=20) using a randomised double-blind design. We found no effect of Citalopram on participants' performance on the DS-CPT which suggests either that SSRIs do not affect response inhibition or that this measure is insensitive to any potential disinhibition effects of SSRI. Acute, but not chronic, Citalopram administration was associated with a measurable decrement in the DNMS suggestive of a negative impact of SSRI administration on contextual processing at least during treatment initiation. These findings provide a useful guide for designing future studies in clinical populations.

Dopaminergic suppression of synaptic transmission in the lateral entorhinal cortex

Dopaminergic projections to the superficial layers of the lateral entorhinal cortex can modulate the strength of olfactory inputs to the region. We have found that low concentrations of dopamine facilitate field EPSPs in the entorhinal cortex, and that higher concentrations of dopamine suppress synaptic responses. Here, we have used whole-cell current clamp recordings from layer II neurons to determine the mechanisms of the suppression. Dopamine (10 to 50 μM) hyperpolarized membrane potential and reversibly suppressed the amplitude of EPSPs evoked by layer I stimulation. Both AMPA- and NMDA-mediated components were suppressed, and paired-pulse facilitation was also enhanced indicating that the suppression is mediated largely by reduced glutamate release. Blockade of D₂-like receptors greatly reduced the suppression of EPSPs. Dopamine also lowered input resistance, and reduced the number of action potentials evoked by depolarizing current steps. The drop in input resistance was mediated by activation of D₁-like receptors, and was prevented by blocking K⁺ channels with TEA. The dopaminergic suppression of synaptic transmission is therefore mediated by a D₂ receptor-dependent reduction in transmitter release, and a D₁ receptor-dependent increase in a K⁺ conductance. This suppression of EPSPs may dampen the strength of sensory inputs during periods of elevated mesocortical dopamine activity.

5-HT1A gene variants and psychiatric disorders: a review of current literature and selection of SNPs for future studies

5-HT1A receptors are key components of the serotonin system, acting both pre- and post- synaptically in different brain areas. There is a growing amount of evidence showing the importance of 5-HT1A in different psychiatric disorders, from mood to anxiety disorders, moving through suicidal behaviour and psychotic disorders. Findings in the literature are not consistent with any definite 5-HT1A influence in psychiatric disorders. 5-HT1A gene variants have been reported to play some role in mood disorders, anxiety disorders and psychotic disorders. Again, the literature findings are not unequivocal. Concerning response to treatment, the C(-1019)G variant seems to be of primary interest in antidepressant response: C allele carriers generally show a better response to treatment, especially in Caucasian samples. Together with the C(-1019)G (rs6295) variant, the Ile28Val (rs1799921), Arg219Leu (rs1800044) and Gly22Ser (rs1799920) variants have been investigated in possible associations with psychiatric disorders, also with no definitive results. This lack of consistency can be also due to an incomplete gene investigation. To make progress on this point, a list of validated single nucleotide polymorphisms (SNPs) covering the whole gene is proposed for further investigations.

Serotonin increases GABA release in rat entorhinal cortex by inhibiting interneuron TASK-3 K+ channels

Whereas the entorhinal cortex (EC) receives profuse serotonergic innervations from the raphe nuclei in the brain stem and is critically involved in the generation of temporal lobe epilepsy, the function of serotonin (5-hydroxytryptamine, 5-HT) in the EC and particularly its roles in temporal lobe epilepsy are still elusive. Here we explored the cellular and molecular mechanisms underlying 5-HT-mediated facilitation of GABAergic transmission and depression of epileptic activity in the superficial layers of the EC. Application of 5-HT increased sIPSC frequency and amplitude recorded from the principal neurons in the EC with no effects on mIPSCs recorded in the presence of TTX. However, 5-HT reduced the amplitude of IPSCs evoked by extracellular field stimulation and in synaptically connected interneuron and pyramidal neuron pairs. Application of 5-HT generated membrane depolarization and increased action potential firing frequency but reduced the amplitude of action potentials in presynaptic interneurons suggesting that 5-HT still increases GABA release whereas the depressant effects of 5-HT on evoked IPSCs could be explained by 5-HT-induced reduction in action potential amplitude. The depolarizing effect of 5-HT was mediated by inhibition of TASK-3 K(+) channels in interneurons and required the functions of 5-HT(2A) receptors and Galpha(q/11) but was independent of phospholipase C activity. Application of 5-HT inhibited low-Mg(2+)-induced seizure activity in slices via 5-HT(1A) and 5-HT(2A) receptors suggesting that 5-HT-mediated depression of neuronal excitability and increase in GABA release contribute to its anti-epileptic effects in the EC.

Inhibiting dopamine reuptake blocks the induction of long-term potentiation and depression in the lateral entorhinal cortex of awake rats

Synaptic plasticity in olfactory inputs to the lateral entorhinal cortex may result in lasting changes in the processing of olfactory stimuli. Changes in dopaminergic tone can have strong effects on basal evoked synaptic responses in the superficial layers of the entorhinal cortex, and the current study investigated whether dopamine may modulate the induction of long-term potentiation (LTP) and depression (LTD) in piriform cortex inputs to layer II of the lateral entorhinal cortex in awake rats. Groups of animals were pretreated with either saline or the selective dopamine reuptake inhibitor GBR12909 prior to low or high frequency stimulation to induce LTD or LTP. In saline-treated groups, synaptic responses were potentiated to 122.4+/-6.4% of baseline levels following LTP induction, and were reduced to 84.5+/-4.9% following induction of LTD. Changes in synaptic responses were maintained for up to 60min and returned to baseline levels within 24h. In contrast, induction of both LTP and LTD was blocked in rats pretreated with GBR12909. Dopaminergic suppression of synaptic plasticity in the entorhinal cortex may serve to restrain activity-dependent plasticity during reward-relevant behavioral states or during processing of novel stimuli.

Serotonin Inhibits Neuronal Excitability by Activating Two-Pore Domain K+ Channels in the Entorhinal Cortex

The entorhinal cortex (EC) is regarded as the gateway to the hippocampus; the superficial layers (layers I-III) of the EC convey the cortical input projections to the hippocampus, whereas deep layers of the EC relay hippocampal output projections back to the superficial layers of the EC or to other cortical regions. The superficial layers of the EC receive strong serotonergic projections from the raphe nuclei. However, the function of serotonin in the EC is still elusive. In the present study, we examined the molecular and cellular mechanisms underlying serotonin-mediated inhibition of the neuronal excitability in the superficial layers (layers II and III) of the EC. Application of serotonin inhibited the excitability of stellate and pyramidal neurons in the superficial layers of the EC by activating the TWIK-1 type of the two-pore domain K(+) channels. The effects of 5-HT were mediated via 5-HT(1A) receptors and required the function of Galpha(i3) subunit and protein kinase A. Serotonin-mediated inhibition of EC activity resulted in an inhibition of hippocampal function. Our study provides a cellular mechanism that might at least partially explain the roles of serotonin in many physiological functions and neurological diseases.

Effects of serotonin on the intrinsic membrane properties of layer II medial entorhinal cortex neurons

Although serotonin (5-HT) is an important neuromodulator in the superficial layers of the medial entorhinal cortex (mEC), there is some disagreement concerning its influences upon the membrane properties of neurons within this region. We performed whole cell recordings of mEC Layer II projection neurons in rat brain slices in order to characterize the intrinsic influences of 5-HT. In current clamp, 5-HT evoked a biphasic response consisting of a moderately short latency and large amplitude hyperpolarization followed by a slowly developing, long lasting, and small amplitude depolarization. Correspondingly, in voltage clamp, 5-HT evoked a robust outward followed by a smaller inward shift of holding current. The outward current evoked by 5-HT showed a consistent current/voltage (I/V) relationship across cells with inward rectification, and demonstrating a reversal potential that was systematically dependent upon the extracellular concentration of K(+), suggesting that it was predominantly carried by potassium ions. However, the inward current showed a less consistent I/V relationship across different cells, suggesting multiple independent ionic mechanisms. The outward current was mediated through activation of 5-HT(1A) receptors via a G-protein dependent mechanism while inward currents were evoked in a 5-HT(1A)-independent fashion. A significant proportion of the inward current was blocked by the I(h) inhibitor ZD7288 and appeared to be due to 5-HT modulation of I(h) as 5-HT shifted the activation curve of I(h) in a depolarizing fashion. Serotonin is thus likely to influence, in a composite fashion, the information processing of Layer II neurons in the mEC and thus, the passage of neocortical information via the perforant pathway to the hippocampus.

Dopamine has bidirectional effects on synaptic responses to cortical inputs in layer II of the lateral entorhinal cortex

Dopaminergic modulation of neuronal function has been extensively studied in the prefrontal cortex, but much less is known about its effects on glutamate-mediated synaptic transmission in the entorhinal cortex. The mesocortical dopamine system innervates the superficial layers of the lateral entorhinal cortex and may therefore modulate sensory inputs to this area. In awake rats, systemic administration of the dopamine reuptake inhibitor GBR12909 (10 mg/kg, ip) enhanced extracellular dopamine levels in the entorhinal cortex and significantly facilitated field excitatory postsynaptic potentials (fEPSPs) in layer II evoked by piriform cortex stimulation. An analysis of the receptor subtypes involved in the facilitation of evoked fEPSPs was conducted using horizontal slices of lateral entorhinal cortex in vitro. The effects of 15-min bath application of dopamine on synaptic responses were bidirectional and concentration dependent. Synaptic responses were enhanced by 10 microM dopamine and suppressed by concentrations of 50 and 100 microM. The D(1)-receptor antagonist SCH23390 (50 microM) blocked the significant facilitation of synaptic responses induced by 10 microM dopamine and the D(2)-receptor antagonist sulpiride (50 microM) prevented the suppression of fEPSPs observed with higher concentrations of dopamine. We propose here that dopamine release in the lateral entorhinal cortex, acting through D(1) receptors, can lead to an enhancement of the salience of sensory representations carried to this region from adjacent sensory cortices.

Lecozotan (SRA-333): A Selective Serotonin 1A Receptor Antagonist That Enhances the Stimulated Release of Glutamate and Acetylcholine in the Hippocampus and Possesses Cognitive-Enhancing Properties

Recent data has suggested that the 5-hydroxytryptamine (5-HT)(1A) receptor is involved in cognitive processing. A novel 5-HT(1A) receptor antagonist, 4-cyano-N-{2R-[4-(2,3-dihydrobenzo[1,4]-dioxin-5-yl)-piperazin-1-yl]-propyl}-N-pyridin-2-yl-benzamide HCl (lecozotan), which has been characterized in multiple in vitro and in vivo pharmacological assays as a drug to treat cognitive dysfunction, is reported. In vitro binding and intrinsic activity determinations demonstrated that lecozotan is a potent and selective 5-HT(1A) receptor antagonist. Using in vivo microdialysis, lecozotan (0.3 mg/kg s.c.) antagonized the decrease in hippocampal extracellular 5-HT induced by a challenge dose (0.3 mg/kg s.c.) of 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) and had no effects alone at doses 10-fold higher. Lecozotan significantly potentiated the potassium chloride-stimulated release of glutamate and acetylcholine in the dentate gyrus of the hippocampus. Chronic administration of lecozotan did not induce 5-HT(1A) receptor tolerance or desensitization in a behavioral model indicative of 5-HT(1A) receptor function. In drug discrimination studies, lecozotan (0.01-1 mg/kg i.m.) did not substitute for 8-OH-DPAT and produced a dose-related blockade of the 5-HT(1A) agonist discriminative stimulus cue. In aged rhesus monkeys, lecozotan produced a significant improvement in task performance efficiency at an optimal dose (1 mg/kg p.o.). Learning deficits induced by the glutamatergic antagonist MK-801 [(-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate] (assessed by perceptually complex and visual spatial discrimination) and by specific cholinergic lesions of the hippocampus (assessed by visual spatial discrimination) were reversed by lecozotan (2 mg/kg i.m.) in marmosets. The heterosynaptic nature of the effects of lecozotan imbues this compound with a novel mechanism of action directed at the biochemical pathologies underlying cognitive loss in Alzheimer's disease.

Tandospirone activates neuroendocrine and ERK (MAP kinase) signaling pathways specifically through 5-HT1A receptor mechanisms in vivo

Tandospirone, an azapirone, is a selective serotonin(1A) (5-HT(1A)) receptor agonist. The effects of tandospirone on plasma hormones and on mitogen-activated protein (MAP) kinase activity in the brain of male rats were studied. Tandospirone produced a time- and dose-dependent increase in plasma levels of oxytocin, adrenocorticotropin (ACTH), corticosterone, and prolactin. The minimal dose of tandospirone that led to a significant elevation of plasma oxytocin, ACTH, and prolactin levels was 1.0 mg/kg (s.c.), while the minimal dose for corticosterone release was 3.0 mg/kg (s.c.). The ED(50) of tandospirone was 1.3 mg/kg for oxytocin, 1.2 mg/kg for ACTH, 3.0 mg/kg for corticosterone, and 0.24 mg/kg for prolactin. Pretreatment with the specific 5-HT(1A) receptor antagonist WAY 100,635 (0.3 mg/kg, s.c.) completely blocked the effects of tandospirone on plasma levels of oxytocin, ACTH, and corticosterone but shifted the dose-response curve for prolactin to the right. Tandospirone injection (10 mg/kg, s.c.) stimulated the MAP kinase signaling cascade, specifically the phosphorylation of p42/44 extracellular signal-regulated kinase (ERK). Western blot analysis revealed a significant increase in phosphorylated ERK (p-ERK) levels in the hypothalamic paraventricular nucleus (PVN) as well as the dorsal raphe nucleus 5 min following tandospirone injection. These increases were blocked by pretreatment with WAY 100,635 (0.3 mg/kg). The results are the first evidence that systemic 5-HT(1A) receptor agonist administration produces a rapid increase in p-ERK levels in vivo, providing further insight into the signaling mechanisms of the 5-HT(1A) receptor.

GABA Targets for the Treatment of Cognitive Dysfunction in Schizophrenia

Cognitive deficits, including impairments in working memory that have been linked to the prefrontal cortex, are among the most debilitating and difficult to treat features of schizophrenia. Consequently, the identification of potential targets informed by the pathophysiology of the illness is needed to develop novel pharmacological approaches for ameliorating these deficits. Postmortem studies of the prefrontal cortex in schizophrenia subjects have revealed disturbances restricted to a subpopulation of inhibitory neurons that includes chandelier neurons, whose axon terminals synapse on the axon initial segment of pyramidal neurons. Chandelier neurons play an important role in synchronizing pyramidal neuron activity and appear to be a critical component of the prefrontal cortical circuitry that subserves working memory function. Therefore, in this paper we review evidence suggesting that drugs which selectively enhance chandelier neuron-mediated inhibition of prefrontal pyramidal neurons may improve working memory dysfunction in schizophrenia. Potential novel targets for such agents include GABA(A) receptors that contain the α(2) subunit. In addition, we discuss potential complementary mechanisms for enhancing inhibitory input to pyramidal cell bodies, including drugs with activity at the CB1 receptor of the endocannabinoid system. The development of pathophysiologically-based treatments that selectively remediate disturbances in specific neural circuits underlying working memory may provide an effective approach to improving cognitive deficits in schizophrenia.

Valproate: past, present, and future

Preclinical studies have been carried out during the past four decades to investigate the different mechanisms of action of valproate (VPA). The mechanisms of VPA which seem to be of clinical importance include increased GABAergic activity, reduction in excitatory neurotransmission, and modification of monoamines. These mechanisms are discussed in relation to the various clinical uses of the drug. VPA is widely used as an antiepileptic drug with a broad spectrum of activity. In patients, VPA possesses efficacy in the treatment of various epileptic seizures such as absence, myoclonic, and generalized tonic-clonic seizures. It is also effective in the treatment of partial seizures with or without secondary generalization and acutely in status epilepticus. The pharmacokinetic aspects of VPA and the frequent drug interactions between VPA and other drugs are discussed. The available methods for the determination of VPA in body fluids are briefly evaluated. At present, investigations and clinical trials are carried out and evaluated to explore the new indications for VPA in other conditions such as in psychiatric disorders, migraine and neuropathic pain. Furthermore, the toxicity of VPA, both regarding commonly occurring side effects and potential idiosyncratic reactions are described. Derivatives of VPA with improved efficacy and tolerability are in development.

Selective 5-HT1A antagonists WAY 100635 and NAD-299 attenuate the impairment of passive avoidance caused by scopolamine in the rat

Systemic administration of the muscarinic-receptor antagonists atropine and scopolamine produces cognitive deficits in humans, nonhuman primates and rodents. In humans, these deficits resemble symptoms of dementia seen in Alzheimer's disease. The passive avoidance (PA) task has been one of the most frequently used animal models for studying cholinergic mechanisms in learning and memory. The present study examined the ability of two selective 5-HT(1A) receptor antagonists WAY 100635 and NAD-299 (robalzotan) and two acetylcholinesterase (AChE) inhibitors tacrine and donepezil to attenuate the impairment of PA retention caused by the nonselective muscarinic receptor antagonist scopolamine in the rat. Although demonstrating differences in their temporal kinetics, both WAY 100635 and NAD-299 attenuated the impairment of PA caused by scopolamine (0.3 mg/kg s.c.). Donepezil did not block the PA deficit caused by the 0.3 mg/kg dose of scopolamine, but it prevented the inhibitory effects of the 0.2 mg/kg dose of scopolamine. In contrast, tacrine was effective vs both the 0.2 and 0.3 mg/kg doses of scopolamine. These results indicate that (1). a functional 5-HT(1A) receptor antagonism can attenuate the anterograde amnesia produced by muscarinic-receptor blockade, and (2). the AChE inhibitors tacrine and donepezil differ in their ability to modify muscarinic-receptor-mediated function in vivo. These results suggest that 5-HT(1A) receptor antagonists may have a potential in the treatment of cognitive symptoms in psychopathologies characterized by reduced ACh transmission such as Alzheimer's disease.

Effects of cholecystokinin tetrapeptide (CCK(4)) and anxiolytic drugs on the electrically evoked [(3)H]5-hydroxytryptamine outflow from rat cortical slices

The outflow of [(3)H]5-hydroxytryptamine ([(3)H]5-HT) from electrically stimulated rat cortical slices was measured to ascertain the modulatory role of endogenous cholecystokinin (CCK) on the amine outflow and to test the hypothesis that different anxiolytic compounds inhibit 5-HT secretion. The [(3)H]5-HT outflow evoked at 10 Hz was increased up to +30% by CCK(4) 300-1000 nM, the effect being prevented by the CCK(B) receptor antagonist GV 150013, 3 nM. The limited sensitivity to CCK(4) seemed to depend on 5-HT auto-receptor feedback because pre-treatment with 100 nM methiothepin enhanced the [(3)H]5-HT outflow and lowered the CCK(4) threshold concentration from 300 to 30 nM. In addition, pre-treatment with 1 microM bacitracin to inhibit CCK metabolism increased [(3)H]5-HT efflux. This effect was concentration-dependently counteracted by GV150013 suggesting the presence of an endogenous CCK positive modulation. GV150013 30 nM, the 5-HT(1A) partial agonist buspirone 300 nM and the GABA(A) receptor modulator diazepam 10 nM, known to have anxiolytic properties, all significantly reduced the [(3)H] amine outflow from cortical slices by about 20%. This inhibition depended on their interaction with their respective receptors, which seemed to restrain the activity of functionally interconnected glutamatergic interneurones. In fact, APV (50 microM) and NBQX (10 microM) prevented the effect of the anxiolytic compounds. Thus, anxiolytic drugs with different receptor targets can reduce 5-HT outflow by dampening the glutamatergic signal, and in turn, the secretory process of the serotonergic nerve ending.

Synergistic mu-opioid and 5-HT1A presynaptic inhibition of GABA release in rat periaqueductal gray neurons

The periaqueductal gray (PAG) plays a critical role in descending antinociception. In mechanically dissociated rat PAG neurons, pharmacologically separated spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the nystatin-perforated patch technique. Both DAMGO, a specific mu-opioid receptor agonist, and serotonin inhibited mIPSC frequency in a dose-dependent manner without affecting mIPSC amplitude, respectively, in the same PAG neurons. The presynaptic opioid effect was blocked by a specific mu-opioid receptor antagonist, CTOP. The presynaptic serotonergic effect was mimicked by a specific 5-HT(1A) receptor agonist, 8-OH-DPAT, and blocked by the specific antagonist, NAN-190. These opioidergic and serotonergic inhibitions of GABA release employed the similar intracellular mechanism of opening 4-AP-sensitive K(+) channels via GTP-binding proteins (G-proteins). Subthreshold concentrations of DAMGO (3 nM) significantly decreased mIPSC frequency with subthreshold concentrations of serotonin (3 nM) and this effect was completely blocked by pretreatment with N-ethylmaleimide (NEM), a PTX-sensitive G-protein inhibitor. In contrast, maximum doses of DAMGO (10 microM) did not further inhibit mIPSC frequency with maximum doses of serotonin (10 microM). In conclusion, activation of presynaptic mu-opioid and 5-HT(1A) receptors synergistically inhibited GABA release. These results suggest a cellular mechanism within PAG for the analgesic effectiveness of combined therapies using opioids in conjunction with classes of anti-depressants which increase synaptic serotonin levels.

Pyramidal cell axons show a local specialization for GABA and 5-HT inputs in monkey and human cerebral cortex

Various mechanisms are thought to control excitation of pyramidal cells of the cerebral cortex. With immunocytochemical methods, we found that the proximal portions of numerous pyramidal cell axons (Pyr-axons) in the human and monkey neocortex are immunoreactive for the serotonin (5-HT) receptor 5-HT-(1A). With double-labeling experiments and confocal laser microscopy, we found that most (93.4%) of the 5-HT(1A)-immunoreactive Pyr-axons present in layers II and III were innervated by parvalbumin-immunoreactive chandelier cell axon terminals. In addition, Pyr-axons were compartmentalized: 5-HT-(1A) receptors were found proximal to inputs from chandelier cells. Although we found close appositions between GABAergic chandelier cell axon terminals and Pyr-axons, suggesting synaptic connections, we did not observe 5-HT-immunoreactive fibers in close proximity to the Pyr-axons. These results suggested that Pyr-axons are under the influence of 5-HT in a paracrine manner (via 5-HT-(1A) receptors) and, more distally, are under the influence of gamma-aminobutyric acid (GABA) in a synaptic manner (through the axons of chandelier cells). The local axonal specialization might represent a powerful inhibitory mechanism by which the responses of large populations of pyramidal cells can be globally controlled by subcortical serotonin afferents, in addition to local inputs from GABAergic interneurons.

Involvement of the 5-HT1A receptors in classical fear conditioning in C57BL/6J mice

The present study examined the involvement of the 5-HT(1A) receptors in classical fear conditioning using the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propyloamino)tetralin hydrobromide (8-OH-DPAT) and the selective "silent" 5-HT(1A) receptor antagonist (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo- hexane carboxamide trihydrochloride (WAY 100635). The drugs were administered both subcutaneously and bilaterally into the dorsal hippocampus of male C57BL/6J mice. The training was performed in a single trial in which a tone was followed by a footshock. The retention of context- and tone-dependent fear was examined in separate tests conducted either 1 or 24 hr after training. Subcutaneous 8-OH-DPAT (0.1-1.0 mg/kg), when injected before but not after training, caused a dose-dependent impairment of contextual fear in both 1 and 24 hr tests, whereas tone-dependent fear was less affected. Pretraining intrahippocampal injections of 5.0 microg but not 1.0 microg 8-OH-DPAT caused a severe deficit in contextual fear when tested 24 hr after training. When injected both subcutaneously and intrahippocampally, 8-OH-DPAT induced the 5-HT syndrome, indicative of postsynaptic 5-HT(1A) receptor activation at the dose ranges that impaired fear conditioning. However, the behavioral changes induced by 8-OH-DPAT at the time of training could not account for inhibitory effects of 8-OH-DPAT on fear conditioning. Neither subcutaneous (0.03 mg/kg) nor intrahippocampal (0.5 microg per mouse) WAY 100635 altered context- or tone-dependent fear. However, subcutaneous WAY 100635 blocked both the 5-HT syndrome and the impairment of fear conditioning induced by subcutaneous or intrahippocampal 8-OH-DPAT. In contrast, intrahippocampal WAY 100635 blocked the impairment caused by intrahippocampal but not subcutaneous 8-OH-DPAT, indicating the involvement of extrahippocampal 5-HT(1A) receptors in fear conditioning. It is concluded that the deficits in fear conditioning induced by 8-OH-DPAT are a result of postsynaptic 5-HT(1A) receptor activation that interferes with learning processes operating at acquisition but not consolidation. Furthermore, the dorsohippocampal 5-HT(1A) receptors play an important but not exclusive role in the limbic circuitry subserving contextual fear conditioning.

Estrogen desensitizes 5-HT(1A) receptors and reduces levels of G(z), G(i1) and G(i3) proteins in the hypothalamus

The present study investigated whether estrogen would desensitize hypothalamic serotonin(1A) (5-HT(1A)) receptors by examining the neuroendocrine response to 8-OH-DPAT, a 5-HT(1A) agonist. Rats were ovariectomized, allowed to recover for 5 days, then given 2 daily injections of estradiol benzoate or vehicle (10 microg/day, s.c.). Twenty-four hours after the second injection, rats were challenged with a sub-maximal dose of 8-OH-DPAT (50 microg/kg, sc) or saline 15 min prior to sacrifice. 8-OH-DPAT produced a significant increase in plasma oxytocin, ACTH and corticosterone levels in ovariectomized rats. While estrogen treatment for 2 days did not alter basal hormone levels, it did significantly reduce the magnitude of oxytocin, ACTH and corticosterone responses to 8-OH-DPAT. The reduction in hormone responses was accompanied by a significant reduction in hypothalamic levels of G(z), G(i1) and G(i3) proteins (by 50%, 30% and 50%, respectively). These findings suggest that a reduction in these G proteins may contribute to the mechanisms underlying estrogen-induced desensitization of 5-HT(1A) receptors. The desensitization of 5-HT(1A) receptors has been suggested to underlie the therapeutic effects of antidepressant 5-HT uptake inhibitors (SSRIs). Thus, the present results suggest that estrogen or estrogen-like substances in combination with SSRIs may prove effective in developing novel therapeutic strategies for neuropsychiatric disorders in women.