Immunocytochemical localization of glutamic acid decarboxylase in neuronal somata following colchicine inhibition of axonal transport

The enzyme that synthesizes the neurotransmitter γ-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD), has been immunocytochemically localized in the somata and dendrites of certain neurons in rat cerebellum and Ammom's horn following colchicine injections into these two brain regions. In the cerebellum. GAD-positive reaction product was observed in the somata and proximal dendrites of Purkinje, Golgi II, basket and stellate neurons. Occasional staining of the proximal portions of axons was also observed in these colchicine-injected preparations. None of the somata or dendrites of these same cell types exhibited reaction product in preparations that were not pretreated with colchicine, although the axon terminals of these neurons were GAD-positive. In Ammon's horn, the somata of a few cells that are classified as probable basket and other short-axon neurons contained detectable concentrations of GAD in preparations that were not pretreated with colchicine. However, following colchicine injections into the Ammon's horn, there was approximately a five-fold increase in the number of GAD-positive somata of basket and other short-axon neurons. There was also a substantial increase in the extent of dendritic staining exhibited by these neurons. Control injections of saline and lumicolchicine produced the same results as those observed in preparations which were not pretreated with colchicine. Thus, the results from the control injections indicate that the increases in somal and dendritic staining are due to a colchicine-mediated inhibition of the somatofugal transport of GAD rather than to a non-specific effect of the drug and/or the injection procedure. The results of the present study permit the direct identification of the neuronal somata in the cerebellum and Ammon's horn whose synaptic terminals probably use GABA as their neurotransmitter. On the basis of the present findings, a reasonable explanation for the failure of earlier immunocytological studies to detect somal GAD in certain GABAergic neurons is that the axonal transport of GAD appears to occur at a sufficiently rapid rate to limit the somal concentration of GAD to low, undetectable levels.