Behavioral and anatomical consequences of small intrastriatal injections of kainic acid in the rat.

The effects of bilateral injections of kainic acid into the anteromedial neostriatal region were examined behaviorally and anatomically in two groups of rats. Behaviorally, kainic acid injections resulted in a severe impairment of delayed alternation retention, while the ability for visual discrimination remained unaffected. Anatomically it was found that axons traversing the injected area remain able to transport horseradish peroxidase. Furthermore, histological examinations of the injected regions revealed a heavy loss of neurons and a decrease of histochemical staining for specific acetylcholinesterase. Silver impregnation showed slightly disorganized, but continuous, axons in bundles of the capsula interna. On the other hand, the axonal network throughout the neuropil of the injected area was markedly diminished. No conspicuous change was found in myelin staining or in the intensity or catecholamine fluorescence. The anatomical results suggest that kainic acid appears to affect only perikarya of the neostriatum and the axons originating from these perikarya, whereas passing axons seem to remain intact. Thus, the observed behavioral impairment must be attributed to changes in the neostriatum itself. It is concluded that the neostriatum has 'complex' or 'cognitive' functions and that some mental symptoms in Huntington's chorea may be attributed to a dysfunction of this part of the brain.