Synaptic Transmission: Modulation, Plasticity And Effects Of Drugs Of Abuse

National Institute On Alcohol Abuse And Alcoholism

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Abstract

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. We have been focusing on plasticity involving the neurotransmitters glutamate, dopamine, 5-hydroxytryptamine (5-HT or serotonin), and endocannabinoids. We have continued to examine LTD induced by exogenous application of 5-HT and endogenous activation of 5-HT release in striatum. Application of 5-HT produces long-lasting depression of both glutamatergic and GABAergic synaptic transmission in striatum. Pharmacological experiments indicate the involvement of 5-HT1B receptors in this form of LTD. Previous studies had suggested that LTD at glutamtergic synapses involves a decrease in probability of glutamate release, and newer experiments with local photolysis-induced glutamate application indicate no change in postsynaptic responsiveness. Activation of release of endogenous 5-HT by afferent stimulation combined with application of the selective serotonin reuptake inhibition (SSRI) citalopram also produces 5-HT1b-mediated 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 drug (including drugs of abuse that target serotonergic synapses). Ethanol (EtOH) effects on GABAergic synaptic transmission in dorsal striatum (DS) have not been examined in much detail. Given the prominent EtOH/GABA interactions noted in other brain regions, and the DS role in addiction, it is important to examine this issue. We have begun performing brain slice experiments recording from neurons in dorsomedial and dorsolateral striatal subregions (DMS and DLS respectively) of EtOH-nave mouse brains. Our initial results indicate that EtOH inhibits GABAergic inhibitory synaptic transmission in DLS via presynaptic mechanism, while in DMS we observed potentiation of transmission. These opposing effects are surprising, and may indicate that EtOH suppresses the output of the DMS that is important for goal-directed actions, while enhancing the output of DLS which is involved in habit formation. Preliminary data using vibrodissociated neurons from DLS that include pinched-off GABAergic presynaptic terminals indicate that EtOH decreases transmission in this preparation as well. Thus, EtOH inhibition of GABA release appears to result from a direct action on the axon terminal that does not require factors found in the slice milieu. It is also important to determine the effect of chronic EtOH exposure on striatal synaptic function, given our interest in the role of this brain region in alcohol intake and addiction. We have initiated these studies using a vapor chamber inhalational chronic intermittent EtOH (CIE) exposure model in adult mice. Exposure using the CIE model for 2-4 weeks produces intoxication during exposure, tolerance to EtOH in some behavioral tests, and mild signs of withdrawal/physical dependence such as increased susceptibility to handling-induced convulsions. It is especially interesting that inhalational exposure for 3-4 weeks produces elevated EtOH consumption. We are examining striatal synaptic transmission and plasticity in brain slices from CIE-exposed mice. Preliminary results indicate that CIE exposure decreases GABAergic synaptic transmission in both DLS and DMS. The chronic EtOH effects in DMS may be homeostatic or indicate tolerance, in that GABA release is increased during acute exposure, but decreased with chronic exposure. In contrast, chronic EtOH effects on DLS appear to be non-homeostatic in that both acute and chronic EtOH decrease GABA release. Findings to date indicate no change in glutamatergic synaptic transmission. Our studies of chronic EtOH effects in mice have revealed synaptic changes that could contribute to prolonged, habitual alcohol use and abuse. The synaptic changes that we have observed could set up a situation in which depression of inhibitory synaptic transmission may enhance cortical drive that will have a greater influence on DLS output in the CIE-exposed mice. We have also examined effects of fetal/early-postnatal EtOH exposure on striatal function. This project was stimulated by reports implicating altered corticostriatal function in humans with fetal alcohol spectrum disorder (FASD). We are delivering EtOH via inhalation to pregnant mice throughout gestation (E0.5-birth) and to neonates and mothers (P0-10), allowing for stable exposure levels. The rationale for continuing exposure into the early postnatal period is that development of the striatal circuitry that takes place in utero in humans, occurs in the early postnatal period in rodents. During neonatal exposure, at vapor levels sufficient to produce the desired levels in the neonates the mothers show no measureable blood alcohol (due to the reduced EtOH metabolism of the pups relative to the mother). Thus, the mothers are unimpaired by EtOH in this model (nests are intact, pups are well fed). We are characterizing striatal physiology in the EtOH-exposed offspring starting at postnatal day 0 and continuing into adulthood using a combination of single-cell and slice electrophysiology, immunochemistry and characterization of cell types, as well as behavioral analysis. We plan to examine effects of EtOH exposure at different fetal and neonatal time points to try to identify critical or sensitive periods for disruption of striatal function. Brain slice electrophysiological studies conducted to date examining DLS MSNs in >3 month old mice have revealed that GABAergic synaptic transmission is reduced in DLS from EtOH-exposed mice. It appears that both pre- and postsynaptic function is reduced at the GABAergic DLS synapses. Thus, our studies will uncover new and interesting information about striatal dysfunction following early-life EtOH exposure. We have also found effects of fetal/neonatal EtOH exposure on striatal-based behavior. Adult male mice that were given fetal/neonatal exposure are hyperactive relative to control mice, as seen in FASD sufferers and noted previously in animal models of fetal EtOH actions. We have examined goal-directed and habitual instrumental learning using a lever-pressing task. Mice are trained on both random ratio (RR) and random interval (RI) schedules in two slightly different environments. Mice show reward devaluation in the RR training environment, indicating goal-directed learning, and show no such devaluation in the RI environment, indicating habitual stimulus-response learning. This parallel training allows the same mouse to be tested in both environments on the same day. We have now examined the performance of fetal/neonatal EtOH -exposed mice (tested as adults) in this training paradigm. Surprisingly, mice demonstrated reward devaluation even when trained on the RI schedule that normally fosters habitual responding. Non-EtOH-exposed mice run at the same time showed no devaluation in the RI environment, similar to normal mice. Thus, it appears that fetal/neonatal EtOH exposure impairs habit learning.