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Alcohol exposure alters the signaling of the serotoninergic system, which is involved in alcohol consumption, reward, and dependence. In particular, dysregulation of serotonin receptor type 1A (5‐HT1AR) is associated with alcohol intake and withdrawal‐induced anxiety‐like behavior in rodents. However, how ethanol regulates 5‐HT1AR activity and cell surface availability remains elusive. Using neuroblastoma 2a cells stably expressing human 5‐HT1ARs tagged with hemagglutinin at the N‐terminus, we found that prolonged ethanol exposure (18 h) reduced the basal surface levels of 5‐HT1ARs in a concentration‐dependent manner. This reduction is attributed to both enhanced receptor internalization and attenuated receptor recycling. Moreover, constitutive 5‐HT1AR internalization in ethanol naïve cells was blocked by concanavalin A (ConA) but not nystatin, suggesting clathrin‐dependent 5‐HT1AR internalization. In contrast, constitutive 5‐HT1AR internalization in ethanol‐treated cells was blocked by nystatin but not by ConA, indicating that constitutive 5‐HT1AR internalization switched from a clathrin‐ to a caveolin‐dependent pathway. Dynasore, an inhibitor of dynamin, blocked 5‐HT1AR internalization in both vehicle‐ and ethanol‐treated cells. Furthermore, ethanol exposure enhanced the activity of dynamin I via dephosphorylation and reduced myosin Va levels, which may contribute to increased internalization and reduced recycling of 5‐HT1ARs, respectively. Our findings suggest that prolonged ethanol exposure not only alters the endocytic trafficking of 5‐HT1ARs but also the mechanism by which constitutive 5‐HT1AR internalization occurs. [ABSTRACT FROM AUTHOR]

Alcoholism is a complex disorder involving, among others, the serotoninergic (5-HT) system, mainly regulated by 5-HT1A autoreceptors in the dorsal raphe nucleus. 5-HT1A autoreceptor desensitization induced by chronic 5-HT reuptake inactivation has been associated with a decrease in ethanol intake in mice. We investigated here whether, conversely, chronic ethanol intake could induce 5-HT1A autoreceptor supersensitivity, thereby contributing to the maintenance of high ethanol consumption. C57BL/6J mice were subjected to a progressive ethanol intake procedure in a free-choice paradigm (3–10% ethanol versus tap water; 21 days) and 5-HT1A autoreceptor functional state was assessed using different approaches. Acute administration of the 5-HT1A receptor agonist ipsapirone decreased the rate of tryptophan hydroxylation in striatum, and this effect was significantly larger (+75%) in mice that drank ethanol than in those drinking water. Furthermore, ethanol intake produced both an increased potency (+45%) of ipsapirone to inhibit the firing of 5-HT neurons, and a raise (+35%) in 5-HT1A autoreceptor-mediated stimulation of [35S]GTP-γ-S binding in the dorsal raphe nucleus. These data showed that chronic voluntary ethanol intake in C57BL/6J mice induced 5-HT1A autoreceptor supersensitivity, at the origin of a 5-HT neurotransmission deficit, which might be causally related to the addictive effects of ethanol intake. [ABSTRACT FROM AUTHOR]

Interaction between brain endocannabinoid (EC) and serotonin (5-HT) systems was investigated by examining 5-HT-dependent behavioral and biochemical responses in CB1 receptor knockout mice. CB1 knockout animals exhibited a significant reduction in the induction of head twitches and paw tremor by the 5-HT2A/C receptor selective agonist (±) DOI, as well as a reduced hypothermic response following administration of the 5-HT1A receptor agonist (±)-8-OH-DPAT. Additionally, exposure to the tail suspension test induced enhanced despair responses in CB1 knockout mice. However, the tricyclic antidepressant imipramine and the 5-HT selective reuptake inhibitor fluoxetine induced similar decreases in the time of immobility in the tail suspension test in CB1 receptor knockout and wild-type mice. No differences were found between both genotypes with regard to 5-HT2A receptor and 5-HT1A receptors levels, measured by autoradiography in different brain areas. However, a significant decrease in the ability of both, the 5-HT1A receptor agonist (±)-8-OH-DPAT and the 5-HT2A/C receptor agonist (−)DOI, to stimulate [35S]GTPγS binding was detected in the hippocampal CA1 area and fronto-parietal cortex of CB1 receptor knockout mice, respectively. This study provides evidence that CB1 receptors are involved in the regulation of serotonergic responses mediated by 5-HT2A/C and 5-HT1A receptors, and suggests that a reduced coupling of 5-HT1A and 5-HT2A receptors to G proteins might be involved in these effects. [ABSTRACT FROM AUTHOR]

5-HT3 (serotonin type 3) receptors are targets of antiemetics, antipsychotics, and antidepressants and are believed to play a role in cognition. Nevertheless, contrasting results have been obtained with respect to their functions in the CNS and in the control of transmitter release. We used rat hippocampal neurons in single-neuron microcultures to identify the roles of presynaptic 5-HT3 receptors at central synapses. 5-HT (10 μm) caused a transient > 10-fold increase in the frequency of miniature inhibitory postsynaptic currents without affecting amplitudes or kinetics. This effect was abolished by tropisetron (30 nm) and when Ca2+ channels were blocked by 100 μm Cd2+ it was mimicked and occluded when neurons were depolarized by 20 mm, but not 10 mm, K+. Thus, activation of presynaptic 5-HT3 receptors increased spontaneous GABA release by causing depolarization and opening of voltage-gated Ca2+ channels. In microculture neurons, 5-HT transiently reduced action potential-evoked inhibitory autaptic currents by > 50%; this effect was blocked by tropisetron and mimicked by 20 mm, but not 10 mm, K+. Miniature excitatory postsynaptic currents were not altered by 5-HT. Excitatory autaptic currents were tonically reduced, an effect attenuated by 5-HT1A antagonists. Thus, presynaptic 5-HT3 receptors control GABA, but not glutamate, release and mediate opposite effects on spontaneous and action potential-dependent release. [ABSTRACT FROM AUTHOR]

We studied the role of 5-HT1A receptors in controlling the release of glutamate (GLU) in the medial prefrontal cortex (mPFC) of conscious rats with the in vivo microdialysis technique. The effect of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin infused in the prefrontal cortex was examined under basal conditions and on the rise of extracellular GLU (+106%) induced by co-infusion of the competitive N-methyl-d-aspartate receptor antagonist 3-[(R)-2-carboxypiperazin-4yl]-propyl-1-phosphonic acid (CPP). 8-OH-DPAT (0.3 and 3 μm) had no effect on basal extracellular GLU, but the higher concentration completely abolished the rise of extracellular GLU induced by CPP. CPP also increased extracellular serotonin (5-HT) in the mPFC (+50%) and this effect was antagonized by 3 μm 8-OH-DPAT which, by itself, had no effect on basal 5-HT release. The effects of 8-OH-DPAT on extracellular GLU and 5-HT were reversed by the 5-HT1A receptor antagonist WAY100 635 (100 μm), indicating a selective involvement of 5-HT1A receptors. WAY100 635 had no effect by itself. These results show that the stimulation of cortical 5-HT1A receptors prevents the CPP-evoked rise of extracellular GLU and 5-HT and suggest that these effects may contribute to the ability of intracortical 8-OH-DPAT to counteract cognitive deficits caused by the blockade of NMDA receptors. [ABSTRACT FROM AUTHOR]

Stimulation of hippocampal 5-HT1A receptors impairs memory retention. The highly selective 5-HT1A antagonist, WAY-100635, prevents the cognitive deficits induced not only by 5-HT1A stimulation but also by cholinergic or NMDA receptor blockade. On this basis, the effects of WAY-100635 on molecular events associated with memory storage were explored. In rat hippocampus, WAY-100635 produced a rapid increase in phosphorylated Ca2+/calmodulin-dependent protein kinase II (CaMKII) and in Ca2+-independent CaMKII and protein kinase A (PKA) enzyme activity. This increase was followed a few hours later by an enhanced membrane expression of AMPA receptor subunits, especially of the GluR1 subunit phosphorylated at the CaMKII site, pGluR1(Ser831). The same qualitative effects were found with the weaker 5-HT1A antagonist NAN-190. The effects of both antagonists were no longer apparent in rats with a previous 5-HT depletion induced by the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA), suggesting that 5-HT1A receptor blockade removes the tonic inhibition of 5-HT through 5-HT1A receptor stimulation on excitatory hippocampal neurons, with the consequent increase in PKA activity. In addition, administration of WAY-100635 potentiated the learning-specific increase in the hippocampus of phospho-CaMKII, Ca2+-independent CaMKII activity, as well as the phosphorylation of either the CaMKII or the PKA site on the AMPA receptor GluR1 subunit. This study suggests that blockade of hippocampal 5-HT1A receptors favours molecular events critically involved in memory formation, and provides an in vivo molecular basis for the proposed utility of 5-HT1A receptor antagonists in the treatment of cognitive disorders. [ABSTRACT FROM AUTHOR]