Effect of the neurotoxin AF64A on intrinsic and extrinsic neuronal systems of rat neostriatum measured by in vivo microdialysis

Abstract

In the present in vivo microdialysis study the aziridinium ion of ethylcholine mustard, AF64A and the excitotoxin ibotenic acid were comapred for their effects on extracellular striatal acetylcholine, choline, gamma-aminobutyric acid (GABA), dopamine and its metabolites, glutamate and aspartate, measured in the same perfusate sample, under basal and high KCl conditions. Ten days following unilateral striatal injections of AF64A (2×0.08 to 2×8 mM) there was a dose-dependent decrease in the extracellular striatal levels of acetylcholine and GABA, the two major intrinsic striatal neurotransmitter systems. No significant effects were on any monitored neurotransmitter systems following the lowest (2×0.08 mM) dose of AF64A, while at the intermediate (2×0.8 mM) dose, AF64A produced a unilateral > 50% and > 70% decrease in basal extracellular striatal acetylcholine and GABA levels respectively. The effects of K +-depolarization on extracellular acetylcholine and GABA levels were diminished by ≈ 50%. At the highest dose (2×8 mM), extracellular striatal acetylcholine levels were non-detectable under basal conditions, while the GABA levels were decreased by > 50%, when compared with the contralateral intact side. However, at this dose, GABA levels were bilaterally decreased compared to levels observed in control animals. Basal extracellular striatal dopamine and glutamate levels, representing the two major extrinsic neurotransmitter systems innervating the neostriatum were only affected by the highest dose of AF64A. The excitotoxin ibotenic acid (2×28.4 mM) produced a strong unilateral decrease in extracellular striatal acetylcholine (> 80%) and GABA (> 90%) levels, without significantly affecting basal dopamine and glutamate levels. Intrastriatal injections of the vehicle did not affect any of the monitored substances, with the exception of aspartate, which wasdecreased when compared to the contralateral striatum. We conclude that neurotoxin AF64A is a potent for intrinsic neuronal systems in the neostriatum. However, AF64A affects local neurons releasing acetylcholine and GABA, as well as projection neurons.