Neuronal activity of the substantia nigra (pars compacta) after injection of kainic acid into the caudate nucleus

In rats the caudate nucleus (Cd) of one side was injected with 2.5 μg kainic acid (KA) in 1 μl saline and that of the other side with saline of the same volume. Three to sixty days later, neuronal activities of the substantia nigra were examined by stimulating the caudate nucleus on each side. Two major types of activity were distinguished: pure inhibition and antidromic excitation followed by inhibition. With 65 substantia nigra units sampled from the side of saline injection, the frequency of incidence for the pure inhibition was 58.5% and that for the antidromic excitation followed by inhibition was 21.5%. In five units (7.7%), caudate stimulation remained ineffective. A total of 97 substantia nigra units was recorded from the side of KA injection and characterized as follows: (i) 42.3% had no effect of caudate stimulation, this being much higher than the control. Pure inhibition was encountered at a reduced frequency (29.9%) and its duration was markedly shortened. These findings are interpreted to mean that GABAergic strionigral inhibitory neurons were killed or damaged by locally injected KA. (ii) Antidromic excitation was seen as frequently as in the control side (19.6%). Inhibition following it was not reduced in duration. The antidromic inhibition of substantia nigra neurons from the caudate nuclei was suggested to be due to dopaminergic self-inhibition as proposed by Groves and his co-workers.