Caudate unit activity and somal diameters in intact and nigral lesioned cats

In a search for morphofunctional relationships in the head of the caudate nucleus (CN), we recorded extracellular unit activity in intact cats and in cats that had received bilateral injections of 6-OHDA into the substantia nigra (SN) 30 days previously. Only units firing spontaneously and continuously for 2 min were studied. In dorsal regions, potentials were small and iterative at almost constant intervals; the somal diameters were relatively small. In the ventrolateral region, potentials were bigger and appeared in bursts; somal diameters were significantly larger (p less than 0.05). For the centromedial region a histogram of numbers of neurons as a function of diameters revealed a Gaussian distribution extending from small to large neurons. Most dorsal neurons increased their firing rate to radial nerve, visual, SN, and/or nucleus centralis medialis (NCM) stimulation. Ventral neurons usually responded with excitation followed by long lasting inhibition, particularly to SN and NCM stimulation. A few neurons responded to all four inputs and some showed long-lasting potentiation in response to low frequency stimulation, suggesting a more general function. Greatest convergence (65%) was found for NCM and SN inputs. In lesioned cats, there was no SN driving, NCM's inhibitory actions almost disappeared, and the excitatory action of the other stimuli was reduced.