Response of cerebellum to stimulation of telencephalon in the catfish (Ictalurus nebulosus).

In the siluroid teleost Ictalurus nebulosus, averaged evoked potentials and unit responses were recorded from various parts of the cerebellum following electrical stimulation applied to the telencephalon. Responses are most readily evoked in the cerebellum when area dorsalis pars centralis (Dc), a group of scattered large cells in the middle of the telencephalon, is stimulated. The configuration of the evoked field potentials differs according to the rate of stimulation. At a relatively low repetition rate, 0.5/s or lower, the response takes the shape of a positive wave, Pmol2, peaking at about 58 ms in the molecular layer. In the granular layer, the response is primarily a negative wave, Ngr1, followed by a slow positive deflection, Pgr3, with peak latencies of 52 and 130 ms, respectively. Ngr1 appears to be the result of summed mossy fiber-granule cell synaptic activations, with Pmol2 its concomitant passive source. No neuronal basis can be proposed for Pgr3. At higher rates of stimulation (greater than 2/S), the response to each shock is of longer latency and more complex. The stabilized waveform (i.e., after 5-9 cycles) is dominated by a surface-negative wave, Nmol, peaking at about 80 ms. The negative wave disappears at about 300 micron below the surface (slightly above the level where Purkinje cells are located) where it is replaced by a positive and negative sequence, P-Npur, peaking at about 80 and 160 ms, respectively. Both responses are bilaterally distributed but the ipsilateral responses are usually larger in amplitude and shorter in latency. Rostrocaudally, a latency gradient with longer rostral latencies is also found in both responses. To facilitate further comparative studies, the changes of peak latency and amplitude with stimulus strength of these two patterns of response were examined. Other dynamic characteristics of these two patterns of response were also examined and compared by stimulating with pairs and short trains of electric shocks. In discussing the functional implications of these findings, this telencephalocerebellar pathway appears to be a striocerebellar pathway.