A COMPARATIVE-ANALYSIS OF COORDINATED NEURONAL-ACTIVITY IN THE THALAMIC VENTROBASAL COMPLEX OF RATS AND CATS

There are substantial differences in the incidence of inhibitory neuro ns in the ventrobasal complex of rat and cat thalamus. This marked dis similarity in neuronal composition suggests that there should be corre sponding differences in the orchestration of neural activity in these regions during cutaneous stimulation. To explore this possibility, we conducted a cross-correlation analysis of neuronal activity in the ven troposterolateral (VPL) nucleus of anesthetized rats and cats. Pairs o f neurons representing hairy skin were recorded simultaneously with on e or two electrodes during air jet stimulation of multiple sites throu ghout the receptive fields. Cross-correlation histograms indicated tha t correlated activity among adjacent neurons occurred in three distinc t patterns. In one pattern, classified as narrow-unimodal, the dischar ge of one neuron preceded a discharge in the partner neuron over a nar row interval of time (< 5 ms). Narrow-bimodal patterns were characteri zed by responses in which the temporal order of discharges from the tw o neurons was variable, but the interspike intervals were always < 5 m s. In wide-unimodal patterns, the discharge of one neuron was correlat ed with subsequent discharges in the partner neuron over a wide interv al of time (> 5 ms). In rat VPL, two-thirds of the 58 neuron pairs sho wing correlated responses were characterized by narrow-unimodal respon ses and nearly one-third of the neuron pairs displayed narrow-bimodal patterns. Only one pair of rat VPL neurons were characterized by a wid e-unimodal pattern of coordination. By comparison, half of the 61 adja cent neuron pairs with coordinated responses in cat VPL were character ized by narrow-unimodal patterns. Slightly more than one-third of the correlated neuron pairs had narrow-bimodal patterns, while the remaind er (13%) were classified as wide-unimodal responses. Pairs of neurons separated by 340-405 mu m discharged synchronously in a pattern that w as similar to the temporal relationship expressed in the narrow-bimoda l patterns found among adjacent neurons. In both species, the wide-uni modal patterns had the strongest coordinated responses as measured by the correlation coefficient. Although inhibitory relationships did not appear in correlation histograms that had been corrected for stimulus coordination, cross-correlation analysis of the raw spike trains reve aled brief (10-40 ms) periods of inhibition that were associated with cat VPL neurons exhibiting wide-unimodal coordination patterns. In rat VPL, most inhibition involved longer (30-60 ms) periods of inhibitory oscillations appearing amidst a much larger rhythmic pattern. These r esults suggest that correlation patterns transpiring over narrow (< 5 ms) time intervals represent the coordination of activity among neighb oring thalamocortical relay neurons. By contrast, wide-unimodal patter ns appear to represent coordinated activity between a thalamocortical relay cell and an intrinsic inhibitory neuron.