The focus 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. As a part of ongoing studies examining alcohol effects on NMDA-type glutamate receptor function, we have also continued our studies using gene-targeted mice in which the NR2A subunit was knocked out to examine the relative role of this subunit in ethanol inihibition of NMDAR-mediated synaptic transmission. We have observed that ethanol inihibits NMDAR-dependent long-term potentiation (LTP) more effectively in hippocampal slices from NR2A knockout mice relative to those from WT mice. Ethanol inhibits NMDAR-mediated synaptic transmission to a similar extent in both mouse strains. Thus, the different in inhibition of LTP is likely due to two factors. First, LTP is less strongly expressed in hippocampus of NR2A-/- mice. Second, ethanol inhibits NMDARs in slices from both sets of mice. The combination of weaker LTP and equal ethanol inhibition likely adds up to greater loss of LTP in the NR2A-/- mice. This effect of ethanol may contribute to the inability of NR2A-/- mice to learn ethanol place preference, a task that involves both cognitive and drug-related components.? We are also examining ethanol effects on synaptic plasticity in the dorsal striatum, a brain region implicated in instrumental conditioning and habit formation. We have observed NMDA-dependent LTP in striatum, as well as a form of long-term synaptic depression (LTD) that requires activation of D2 dopamine receptors, group I metabotropic glutamate receptors and endocannabinoid activation of CB1-type cannabinoid receptors. In the dorsomedial striatum, a brain region normally implicated in learning goal-directed actions, we normally observe LTP after high frequency stimulation. However, in the presence of ethanol at 10-50 mM LTP was lost and LTD was observed. Even at a very low concentration of ethanol (2 mM) LTP was slightly decreased in magnitude. Blockade of NMDARs alone prevented LTP but did not result in LTD, but addition of ethanol with the NMDAR antagonist gave rise to LTD induction. The LTD observed in the presence of ethanol is dependent on activation of D2 and CB1 receptors, consistent with the usual characteristics of LTD in thsi brain region. Thus, in the striatum ethanol inhibits LTP but also enhances LTD. These new findings suggest a mechanisms by which alcohol could alter instrumental learning via actions on the stiratum. Specifically, prevention of LTP in the dorsomedial striatum might impair goal-directed learning and enhance the development of habitual behavior, such as that seen during addiction. This is a promising avenue of investigation that will be pursued in future research.? Our interest in the role of striatal synaptic plasticity in learning and memory mediated by this brain region also led us to an interesting collaboration with the laboratory of Dr. Yuqing Li at the University of Illinois. Dr. Li and coworkers have created mice in which the NR1 subunit of the NMDAR can be excised due to Cre-recombinase actions on a lox-P-flanked NR1 allele. They also created a transgenic mouse in which Cre expression is driven by the promoter for the RGS-9L protein, and this transgene is expressed almost exclusively in the striatum. When these two mouse lines are mated, NMDA receptor expression and function are almost completely eliminated in striatum, but not in other brain regions. These mice do not show normal improvement in their performance on a motor skill task measured using a rotating rod apppartus, but show normal learning in a shock-avoidance paradigm. Synaptic responses mediated by NMDARs are completely lost in these mice, as is LTP in the dorsomedial striatum. In contrast, LTD in the dorsolateral striatum is unimpaired, consistent with the idea that LTD does not depeend on NMDA receptor function. These findings indicate an important role for NMDARs and NMDAR-mediated synaptic plasticity in striatum-based motor skill learning. The use of this conditional knockout mouse approach should help us to gain a better understanding of a variety of striatal proteins in synaptic plasticity, learning, memory and addiction.? We have also continued our efforts to gain a better understanding of the mechanisms underlying striatal LTD. A recent study performed in collaboration with the laboratory of Dr. D. James Surmeier at Northwestern University indicated that the Cav1.3 form of the L-type calcium channel plays crucial role in postsynaptic calcium signaling needed for LTD induction. To determine if activation of this channel is sufficient to induce LTD without need for other postsynaptic receptors we combined application of an L-channel agonist with mild depolarization and examined the effect of this treatment on synaptic responses evoked by afferent stimulation in corticostriatal slices. This treatment does indeed induce a long-lasting decrease in glutamatergic transmission at this synapse. This long-lasting depression is blocked by a CB1 receptor antagonist, and thus shares common mechanisms with the striatal LTD mentioned above. Blockers of metabotropic glutamate receptors and D2 dopamine receptors do not block this L-channel activator-induced LTD. This finding indicates that L-channel activation is likely upstream of, or independent of, the L-channel-mediated rise in postsynaptic calcium needed for endocannabinoid production and LTD initiation. However, this form of LTD is not observed if afferent stimulation is not given during the period of L-channel activator application. This suggests that L-channel activation is not wholly sufficient to induce LTD, and that some process engaged during synaptic transmission, perhaps activation of the presynaptic terminal, needs to synergize with L-channel activation for LTD induction to occur.? We have also explored the role of protein synthesis in striatal LTD. Using a slice preparation in which the cortex has been removed and only striatum remains allowed us to examine LTD in the absence of the cell bodies of the presyanptic afferents that give rise to the corticostriatal projections. We could induce LTD in this preparation, indicating that LTD does not require transcription in the presynaptic neuron. Application of inhibitors of RNA translation prevented LTD expression, while transcription inhibitors did not. Loading the translation inhibitors into the postsynaptic neuron did not alter LTD, indicating that the relevant translation occurs at a site other than the postsynaptic neuron. Control experiments indicated that translation inhibitors did not alter the function of receptors of channels implicated in LTD induction. Our findings suggest that protein synthesis, RNA translation in particular, is necessary for induction of striatal LTD. If the postsynaptic neuron is not the site of the important translational events, then these must occur at some other site, perhaps even the presynaptic terminal. Past studies have indicated the possibility of presynaptic translation, and this will be an interesting possibility to examine in future studies.? These several lines of research should allow us to gain a better understanding of synaptic mechanims of learning and addiction. In addition, we are obtaining further evidence of the mechanisms through which alcohol can alter neuronal mechanisms involved in learning and memory in brain regions implicated in addiction.
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