EVIDENCE FOR A PROJECTION FROM THE PERIRETICULAR THALAMIC NUCLEUS TO THE DORSAL THALAMUS IN THE ADULT-RAT AND FERRET

During early development, the perireticular thalamic nucleus is very l arge (i.e. has many cells) and has a strong projection to the dorsal t halamus and to the cerebral neocortex. By adulthood, the nucleus has m uch reduced in size and only a few cells remain. It is not clear wheth er these perireticular cells that remain into adulthood maintain their connections with the dorsal thalamus and with the neocortex. This stu dy examines this issue by injecting neuronal tracers into various nucl ei of the dorsal thalamus (dorsal lateral geniculate nucleus, medial g eniculate complex, ventroposteromedial nucleus, lateral posterior nucl eus, posterior thalamic nucleus) and into different areas of the neoco rtex (somatosensory, visual, auditory). After injections of tracer int o the individual nuclei of the rat and ferret dorsal thalamus, retrogr adely-labelled perireticular cells are seen. In general, after each in jection, the retrogradely-labelled perireticular cells lie immediately adjacent to a group of retrogradely-labelled reticular cells. For ins tance, after injections into the medial geniculate complex, perireticu lar cells adjacent to the auditory reticular sector are retrogradely-l abelled, whilst after an injection into the dorsal lateral geniculate nucleus, retrogradely-labelled perireticular cells adjacent to the vis ual reticular sector are seen. By contrast, injections of tracer into various areas of the rat and ferret neocortex result in no retrogradel y-labelled cells in the perireticular nucleus. Thus, unlike during per inatal development when perireticular cells project to both neocortex and dorsal thalamus, perireticular cells in the adult seem to project to the dorsal thalamus only: the perireticular projection to the neoco rtex appears to be entirely transient.