Fornix and fifth-nerve interaction on thalamic units in awake, sitting squirrel monkeys.

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Abstract
Possible differential action of the hippocampus on somatosensory functions was studied as it is relevant to neuromechanisms of pain. The effects of fornix volleys and hippocampal afterdischarges on thalamic unit responses evoked by a potentially noxious shock to the 5th-nerve have been tested in chronically prepared, awake squirrel monkeys sitting in a special chair. The 5th-nerve shock is regarded as a potentially noxious stimulus (PNS) because it produces signs of discomfort when given repetitively. Of 1343 thalamic and tegmental units, 169 responded to the PNS. Twenty-eight were in nucleus N. ventralis posteromedialis (VPM). The fornix volleys and/or hippocampal afterdischarges had no effect on responses of VPM units to the PNS, but a large percentage of non-VPM (31%) showed signs of inhibition. The effects of hippocampal afterdischarges were often profound, lasting almost 2 minutes. The affected units were located in caudal intralaminar, tegmental and hypothalamic areas. Hippocampal afterdischarges may alter responses to noxious stimuli, but not those dependent on taction and proprioception.