An important new hypothesis for understanding and treating drug addiction is that it may be an inappropriate form of learning. This hypothesis is based on studies demonstrating that glutamate, a transmitter critical for learning and memory, is required for the development of neuroadaptations believed to underlie addiction and relapse. We have shown that the repeated administration of psychostimulants (amphetamine or cocaine) causes profound alterations in glutamate systems. These include alterations in AMPA receptor expression and responsiveness in the nucleus accumbens (Nac), a brain region critical for the rewarding and addictive properties of psychostimulants. Medium spiny GABA-containing neurons, which represent 95% of all neurons in the Nac, receive convergent inputs from midbrain dopamine (DA) neurons and glutamate neurons originating in cortex and limbic regions. Our long-term goal is to understand how psychostimulants, which initially target the DA transporter, ultimately produce adaptations in glutamate transmission in the Nac. This is an important question, because glutamate systems are likely to mediate the neuronal plasticity underlying the transition from drug experimentation to drug dependency. Unfortunately, we know little about chronic interactions between DA and glutamate receptors in the Nac. A logical starting point is to determine how acute DA receptor stimulation can influence glutamate transmission. This application will use postnatal Nac cultures to test the hypothesis that DA receptors modulate the phosphorylation of the AMPA receptor subunit GluR1. We are focusing on AMPA receptors because they are the primary mediators of excitatory transmission in medium spiny neurons and on phosphorylation as a regulatory mechanism because GluR1 phosphorylation leads to marked enhancement of AMPA receptor-mediated currents. Phosphorylation of GluR1 will be detected using phosphorylation site-specific antibodies to GluR1 developed by Dr. Richard Huganir. One selectively recognizes GluR1 phosphorylated on ser-845 [protein kinase A (PKA) site]. The other recognizes GluR1 phosphorylated on ser-831 [protein kinase C (PKC)/Ca2+ -calmodulin dependent protein kinase type II (CaMKII) site].
The first Aim i s to characterize conditions regulating basal phosphorylation of GluR1 and the kinases involved in its phosphorylation. First, control cultures will be compared with cultures incubated in the absence of Ca2+, the absence of synaptic activity (TTX, bicuculline, MK-801, CNQX), the absence of inhibitory activity (bicuculline), and the absence of excitatory activity (MK-801, CNQX). Second, we will use activators and inhibitors of PKA, PKC and CaMKII to determine which kinases phosphorylate GluR1 in our cultures.
The second Aim i s to determine if DA receptor stimulation modulates GluR1 phosphorylation. Experiments will be designed to detect both stimulatory and inhibitory effects of D1 and D2 DA receptor activation. The identity of the residue phosphorylated, combined with experiments involving selective protein kinase inhibitors, will help identify the kinases involved in DA receptor-mediated effects.
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