One of the major foci of research in the Laboratory for Integrative Neuroscience (LIN), Section on Synaptic Pharmacology, is the determination of mechanisms underlying neuromodulation and plasticity and the effects of alcohol and other drugs of abuse on these neuronal functions. In particular, we are interested in the function of the striatum, a brain region involved in action control and selection, as well as action learning. We have continued our studies of long-term synaptic depression (LTD) at synapses in striatum. This is a form of long-lasting synaptic plasticity that is thought to contribute to striatal-based learning and memory. In the past we have focused on plasticity involving the neurotransmitters glutamate, dopamine and endocannabinoids. However, the neurotransmitter 5-hydroxytryptamine (5-HT or serotonin) is present at appreciable levels in striatum. Thus, we wondered if 5-HT could induce or modulate synaptic plasticity in this brain region. We have examined effects of application of 5-HT and specific agonists for 5-HT receptors in striatal brain slices. Synaptic transmission mediated by both glutamate and gamma-aminobutyric acid (GABA) were examined. Application of 5-HT inhibited glutamatergic synaptic transmission, while results to date indicate no effect on GABAergic transmission. The inhibitory action of 5-HT was long lasting, persisting even after the neurotransmitter was removed from the solution bathing the brain slice. Selective agonists of 5-HT1B and 5-HT2 receptors mimicked the actions of 5-HT, and antagonists of these two receptor subtypes were able to block effects of 5-HT. Agonists selective for the 5-HT1A receptor did not produce synaptic depression. Analysis of responses to paired afferent stimuli and spontaneous excitatory postsynaptic currents (sIPSCs) indicate that serotonin-induced synaptic depression involves a presynaptic mechanism, most likely a decrease in probability of glutamate release. Studies to date have not revealed any involvement of postsynaptic mechanisms in this form of synaptic depression. The lack of reversal upon removal of 5-HT from the slice suggests that serotonin produces LTD. This form of long-lasting synaptic depression could be involved in striatal-based learning, as well as locomotor side effects and other actions of long-term exposure to selective serotonin reuptake inhibitors (SSRIs) and other serotonergic drugs (including drugs of abuse that target serotonergic synapses). We have also continued our studies of the 5-HT3 serotonin-activated ligand-gated ion channel, including examination of the actions of ethanol on this receptor in neurons from the brain. Ethanol has been shown to potentiate 5-HT3R function in cultured cells and heterologous gene expression systems. However, very little is known about how ethanol and other alcohols affect the function of 5-HT3 receptors in mammalian brain. We examined responses to 5-HT and selective 5-HT3 agonists in isolated neurons of the basolateral amygdala (BLA) and hippocampus. Work in the previous year established that the major effect of this receptor is to promote GABA release onto the major projection neurons in these two brain regions. This work was primarily carried out using mechanically-isolated neurons that retain intact GABAergic synapses. In this preparation 5-HT3 activation enhances GABA release onto projection neurons, with no apparent postsynaptic effect. This effect was consistently observed in neurons from both BLA and hippocampus. Thus, the most consistent action of this receptor is to boost inhibitory GABAergic transmission. Our recent findings in hippocampal neurons indicate that ethanol potentiates this presynaptic 5-HT3 receptor action . When 5-HT is used to activate the receptor, results are inconsistent. However, consistent potentiation is observed when the receptor is activated by a selective 5-HT3 agonist. Subsequent experiments have revealed that 5-HT not only activates 5-HT3 receptors that promote GABA release, but also activates 5-HT1A receptors that reduce GABA release, at least in a subset of cells. This 5-HT1A action may limit the response to 5-HT and the potentiation by ethanol. Thus, we examined ethanol effects using a combination of 5-HT and a selective 5-HT1A antagonist. Under this condition we observed consistent potentiation by ethanol. When a fairly high concentration of ethanol was used (80 mM) the potentiation of the response to 5-HT persisted even after ethanol was washed out of the preparation. This effect was concentration dependent, as potentiation was reversible upon washout when 20 mM ethanol was applied to the preparation. No potentiation was observed when voltage-gated sodium channels were blocked, indicating that potentiation by ethanol requires presynaptic depolarization. This finding indicates that the combination of ethanol and 5-HT3 receptor activation produces a long-term potentiation of GABA release at hippocampal synapses. We are currently exploring the role of presynaptic calcium transients in this potentiation. To this end, we have developed a system for simultaneous electrophysiological recording, rapid drug application and real-time calcium imaging in presynaptic terminals in the mechanically-isolated neuron preparation. Results to date indicate that we can detect both spontaneous and 5-HT-induced increases in presynaptic calcium concentration using this approach. We have observed enhancement of these calcium transients by ethanol, and are now examining the concentration-dependence and persistence of these ethanol effects. We have also begun to explore effects of early-life alcohol exposure on striatal function. Fetal and neonatal mice are exposed to low-to-moderate levels of alcohol using a vapor inhalation model. Striatal slices from these mice are then examined to determine effects of this alcohol exposure on GABAergic synaptic transmission. Mice are also examined using an instrumental conditioning paradigm to determine their ability to learn and remember using goal-direct and habitual strategies. Results to date indicate alterations in striatal GABAergic transmission in the fetal alcohol-treated mice even at the adult stage. These mice also appear to have deficits in habit learning. Fetal alcohol spectrum disorders continue to be a pervasive problem in this and other countries. Many of the effects of fetal alcohol exposure, including hyperactivity and attention problems, are likely related to dysfunction of corticostriatal circuitry. However, there has been little emphasis on this circuitry in laboratory research on this disorder. Our goal is to better understand how this circuitry malfunctions after fetal alcohol exposure, and to help develop therapeutic approaches to reduce or prevent these untoward effects.
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