Stress-related psychiatric disorders such as anxiety and depression involve dysfunction of the serotonin [5-hydroxytryptamine (5-HT)]system. Previous studies have found that the stress neurohormone corticotropin-releasing factor (CRF) inhibits 5-HT neurons in the dorsal raphe nucleus (DRN) in vivo. The goals of the present study were to characterize the CRF receptor subtypes (CRF-R1 and -R2) and cellular mechanisms underlying CRF-5-HT interactions. Visualized whole-cell patch-clamp recording techniques in brain slices were used to measure spontaneous or evoked GABA synaptic activity in DRN neurons of rats and CRF effects on these measures. CRF-R1 and -R2-selective agonists were bath applied alone or in combination with receptor-selective antagonists. CRF increased presynaptic GABA release selectively onto 5-HT neurons, an effect mediated by the CRF-R1 receptor. CRF increased postsynaptic GABA receptor sensitivity selectively in 5-HT neurons, an effect to which both receptor subtypes contributed. CRF also had direct effects on DRN neurons, eliciting an inward current in 5-HT neurons mediated by the CRF-R2 receptor and in non-5-HT neurons mediated by the CRF-R1 receptor. These results indicate that CRF has direct membrane effects on 5-HT DRN neurons as well as indirect effects on GABAergic synaptic transmission that are mediated by distinct receptor subtypes. The inhibition of 5-HT DRN neurons by CRF in vivo may therefore be primarily an indirect effect via stimulation of inhibitory GABA synaptic transmission. These results regarding the cellular mechanisms underlying the complex interaction between CRF, 5-HT, and GABA systems could contribute to the development of novel treatments for stress-related psychiatric disorders.

Swim stress regulates forebrain 5-hydroxytryptamine (5-HT) release in a complex manner and its effects are initiated in the serotonergic dorsal raphe nucleus (DRN). The purpose of this study was to examine the effects of swim stress on the physiology of DRN neurons in conjunction with 5-HT immunohistochemistry. Basic membrane properties, 5-HT(1A) and 5-HT(1B) receptor-mediated responses and glutamatergic excitatory postsynaptic currents (EPSCs) were measured using whole-cell patch clamp techniques. Rats were forced to swim for 15min and 24h later DRN brain slices were prepared for electrophysiology. Swim stress altered the resting membrane potential, input resistance and action potential duration of DRN neurons in a neurochemical-specific manner. Swim stress selectively elevated glutamate EPSC frequency in 5-HT DRN neurons. Swim stress non-selectively reduced EPSC amplitude in all DRN cells. Swim stress elevated the 5-HT(1B) receptor-mediated inhibition of glutamatergic synaptic activity that selectively targeted 5-HT cells. Non-5-HT DRN neurons appeared to be particularly responsive to the effects of a milder handling stress. Handling elevated EPSC frequency, reduced EPSC decay time and enhanced a 5-HT(1B) receptor-mediated inhibition of mEPSC frequency selectively in non-5-HT DRN cells. These results indicate that swim stress has both direct, i.e., changes in membrane characteristics, and indirect effects, i.e., via glutamatergic afferents, on DRN neurons. These results also indicate that there are distinct local glutamatergic afferents to neurochemically specific populations of DRN neurons, and furthermore that these distinct afferents are differentially regulated by swim stress. These cellular changes may contribute to the complex effects of swim stress on 5-HT neurotransmission and/or the behavioral changes underlying the forced swimming test model of depression.

Rationale 5-HT1A receptor knockout (1AKO) mice have a robust anxiety phenotype. Tissue-specific “rescue” strategies and electrophysiology have implicated a critical role for postsynaptic 5-HT1A receptors, particularly in the CA1 region of the hippocampus. Objectives In this study, we evaluated differences in membrane properties and synaptic activity in CA1 hippocampal pyramidal cells between 1AKOs and wild-type (WT) controls to better understand the cellular correlates of anxiety in this mouse model. Methods Whole-cell patch-clamp recordings were conducted in CA1 pyramidal cells in hippocampal brain slices from 1AKOs and WTs that had previously been screened for anxiety with the elevated-plus maze. Spontaneous miniature inhibitory and excitatory postsynaptic currents (IPSCs and EPSCs) and stimulus-evoked eIPSCs and eEPSCs were recorded in addition to the effect of the benzodiazepine agonist diazepam or the inverse agonist FG 7142 on γ-aminobutyric acid (GABA)ergic eIPSCs. Results Evoked EPSC amplitude was greater in 1AKOs than WTs. When subjects were pooled across genotypes, anxiety measures correlated with eEPSC amplitude, indicating enhanced postsynaptic glutamate synaptic activity under conditions of synaptic activation in anxious subjects. While GABA synaptic activity and sensitivity to diazepam were not affected by genotype or correlated with anxiety, sensitivity to the anxiogenic FG 7142 was smaller in anxious subjects. Conclusions These data indicate enhanced postsynaptic glutamate receptor sensitivity and decreased GABAergic inhibition by a benzodiazepine inverse agonist in CA1 hippocampal neurons of anxious mice are produced by deletion of the 5-HT1A receptor. These data provide new information about interactions between 5-HT, GABA, and glutamate systems during the expression of chronic anxiety.

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