Biphasic changes in locomotor behavior and in expression of mRNA for NGFI-A and NGFI-B in rat striatum following acute caffeine administration.

The time course of expression of mRNA for NGFI-A and NGFI-B after a single intraperitoneal injection of saline or caffeine was examined using in situ hybridization. Administration of a high dose of caffeine (100 mg/kg) decreased locomotor behavior and increased NGFI-A and NGFI-B mRNA in the entire striatum. A lower dose of caffeine (50 mg/kg) caused a weak enhancement of both messages, which was confined to the lateral part of caudate-putamen. This dose increased horizontal, but not vertical, movement. In rats treated with the lowest dose of caffeine (25 mg/kg), the expression of both investigated genes tended to be lower than for saline-treated rats and both horizontal and vertical locomotor activity increased markedly. The reduction in the number of labeled neurons seemed to occur predominantly in enkephalin-containing neurons, which coexpress adenosine A2A receptors and dopamine D2 receptors. The decrease of mRNA for NGFI-A, NGFI-B, and jun B caused by caffeine (25 mg/kg) could be mimicked by the D2 agonist quinpirole (1 and 3 mg/kg). Moreover, caffeine could significantly decrease the expression seen following treatment with the D2 antagonist raclopride (2 mg/kg). In addition, in the parietal cortex, 25 mg/kg of caffeine caused a significant elevation of both examined immediate early genes. Thus, biphasic changes in locomotion induced by caffeine are paralleled by biphasic changes in mRNA for NGFI-A, NGFI-B, and jun B. The results also provide additional support for a functionally important interaction between adenosine and dopamine D2 receptors.