| The well-characterized anatomy, pharmacology, and electrophysiology of the cerebellar Purkinje cell make it ideal for assessing relationships between neural activity, gene expression and enzyme activity. The two Purkinje cell afferent inputs, the climbing fiber and mossy fiber systems, generate distinct Purkinje cell spike characteristics, the complex spike (CS) and simple spike (SS), respectively. Purkinje cell firing, mediated through gamma-aminobutyric acid (GABA) synthesized by glutamic acid decarboxylase (GAD), in turn inhibits target neurons in the lateral vestibular (LVN) and deep cerebellar nuclei (DCN). In the current study drugs producing unique electrophysiological profiles were used to produce specific patterns of neuronal stimulation, and the effects on GAD gene expression and enzymatic activity in Purkinje cells determined.; The neurotoxin 3-acetylpyridine (3-AP) selectively destroys climbing fibers, abolishing CS activity and increasing SS and overall Purkinje cell firing. In the current studies this was associated with changes in Purkinje cell GAD mRNA expression, and GAD activity at projection sites. The earliest increase in GAD activity was in the medial DCN 3 days post-lesion (+20%). Maximal GAD increases in the DCN were found at 14 days. Enzymatic activity then declined over the next two weeks, but remained significantly elevated. In contrast, GAD activity in the LVN reached maximal levels at 7 days, and was not significantly elevated at either 14 or 30 days post-lesion. Significant increases in Purkinje cell GAD mRNA levels were seen at 1 day post-lesion (+38%), and remained elevated for one week. The levels were slightly, but non-significantly, elevated over the subsequent 3 weeks.; The drug harmaline, which increases CS activity and suppresses SS and overall cell activity, decreased GAD activity in the medial DCN ({dollar}-{dollar}18%) but did not affect GAD mRNA levels. Diazepam, which suppresses SS activity and overall cell firing without altering CS activity, had no effect on GAD activity or mRNA levels. These studies suggest that GAD mRNA expression and GAD activity in Purkinje cells is inversely related to the level of CS activity, implicating the climbing fiber-Purkinje cell synapse in the regulation of GAD activity and gene expression. |
