Long-term glutamate desensitization in locus coeruleus neurons and its role in opiate withdrawal

During opiate withdrawal, there is an elevated and prolonged efflux of glutamate and aspartate in the locus coeruleus (LC). The enhanced excitatory amino acid (EAA) release is thought to contribute to the withdrawal-induced activation of LC neurons and to the expression of the physical withdrawal syndrome. In this study, prolonged bath applications of glutamate to LC neurons in brain slices resulted in a slowly developing long-term glutamate desensitization (LTGD). LTGD was observed during extracellular recordings or in neurons voltage-clamped to -60mV, in both cases reaching a maximum of about a 50% reduction in the glutamate response. Responses in the desensitized cells gradually recovered within 3 h. Cyclothiazide, an inhibitor of rapid glutamate receptor desensitization did not prevent LTGD. LTGD could not be induced by prolonged applications of EAA agonists other than glutamate, either alone or in various combinations. However, after induction by glutamate, there was cross-desensitization to quisqualate but not to AMPA or NMDA. LTGD was blocked by either lowering extracellular Ca2+ concentrations or by treatment with the protein kinase C inhibitor chelerythrine but not by inhibitors of calcium/calmodulin-dependent kinase or nitric oxide synthase. Applications of the protein kinase C activator phorbol diacetate did not cause a decrease in glutamate responses indicating that an activation of protein kinase C may not be sufficient for desensitization to occur. A decrement of the glutamate response resembling LTGD occurred after treatment by the protein phosphatase inhibitors okadaic acid or calyculin A. LC neurons in brain slices prepared from opiate-withdrawn rats exhibited glutamate responses that were initially desensitized and recovered within 3 h after withdrawal. These results suggest that LTGD in LC neurons may occur during opiate withdrawal and could contribute to the time course of LC hyperactivity and the associated behavioral withdrawal syndrome.