Effects of cerebellar and brain stem stimulation on hippocampal formation neurons in the monkey

Micro- and macroelectrode recording techniques were used to detect the presence of cerebellar influences on electrical activity in the hippocampal formation of the rhesus monkey. Only 2% of the cells studied responded to cerebellar stimulation. These responsive cells showed a decreased firing rate. No evoked potentials were detected after cerebellar stimulation in any of the monkeys. The effects of brain stem stimulation were tested in two monkeys. One electrode placement lateral to the central gray did not evoke any neuronal responses. The other, situated in the raphe nucleus near the border of the dorsal raphe and nucleus centralis superior, evoked inhibitory responses in 17% of the cells tested. These results (i) fail to provide support for the existence of an important pathway from the cerebellum to the hippocampal formation in monkeys, and (ii) preliminarily extend to a primate species previous results in rat and cat showing an inhibitory effect of raphe stimulation on hippocampal neurons.