Transplants of the embryonal rat somatosensory neocortex in the barrel field of the adult rat: responses of the grafted neurons to sensory stimulation.

The degree of participation of grafted neurons in sensory analysis was investigated in embryonal rat somatosensory neocortex transplanted into the cavity at the place of the barrel field in the neocortex (SI) of adult rats. The neurons were investigated extracellularly 3 to 6 months after grafting. In the majority of grafts the neurons had normal levels and patterns of spontaneous activity. Many of them (65%) responded to displacement of the whiskers with latencies insignificantly different (18 +/- 0.8 ms) from those for reactions in the intact barrel field (16 +/- 0.5 ms). The receptive fields of the grafted neurons were very large. None of the neurons responded to stimulation of a single vibrissa, as in intact cortex. As a rule, the same neuron responded to isolated deflections of several (up to 10-20) vibrissae. Many of them were responsive to stimulation of the small anterior vibrissae and tactile stimulation of nose, limbs and body surface. Nevertheless, there was some spatial gradient in the effectiveness of stimulation of the body surface at various distances from the vibrissal pad; among effective vibrissae, usually several adjacent ones (2-4) produced larger responses with shorter latencies than the other ones. All units responded to painful stimuli irrespective of their location. The data show that the grafted neurons receive and may transmit sensory signals. The grafts which were proved histologically to be isolated from the host's brain did not respond to sensory stimulation and were characterized by the presence of aperiodic hypersynchronous bursts in their background activity. Electrophysiological criteria may be used for intravital diagnosis on the degree of the graft morphofunctional integration.