Serotonin neuron transplants: electrophysiological unit activity of intrahippocampal raphe grafts in freely moving cats.

Fetal serotonin (5HT)-containing neurons from the cat midbrain raphe were transplanted to the hippocampus of adult host cat brain. These neurons displayed spontaneous electrophysiological activity in freely moving cats, and showed the pacemaker-like discharge pattern characteristic of these cells. However, these neurons showed no significant change in activity across the sleep-waking cycle, nor were they responsive to sensory stimulation in the auditory and visual modalities. Grafted neurons were inhibited by 5HT agonists, but required higher doses than those in intact cats. These data are the first electrophysiological recordings of grafted 5HT neurons in freely moving animals, and question whether such tissue grafts can restore functional activity in the central 5HT system.