CROSSTALK BETWEEN THE 2 SIDES OF THE THALAMUS THROUGH THE RETICULAR NUCLEUS - A RETROGRADE AND ANTEROGRADE TRACING STUDY IN THE RAT

In order to investigate the possible routes linking the thalamus in th e two sides of the brain, the connections of the reticular nucleus (RT ), the major component of the ventral thalamus, with contralateral dor sal thalamic nuclei were systematically investigated in the adult rat. This study was performed with several tract-tracing techniques: singl e and double retrograde labeling with fluorescent tracers, and anterog rade tracing with biocytin. Retrograde tracing was also combined with immunocytochemistry to provide additional criteria for the identificat ion of labeled RT neurons. The data obtained with the retrograde trans port of one fluorescent tracer showed that RT neurons project to contr alateral dorsal thalamic domains. In particular, retrograde labeling f indings indicated that the anterior intralaminar nuclei, as well as th e ventromedial (VM) nucleus, are preferential targets of the contralat eral RT projections. Commissural neurons were concentrated in two port ions of RT: its rostral part, including the rostral pole, which projec ts to the contralateral central lateral (CL) and paracentral (Pc) nucl ei, and the ventro-medial sector of the middle third of RT, which proj ects to the contralateral VM and posterior part of CL and Pc. The doub le retrograde labeling study of the bilateral RT-intralaminar connecti on indicated that at least part of the commissural RT cells bifurcate bilaterally to symmetrical portions of the anterior intralaminar nucle i. The targets of the RT commissural system inferred from the retrogra de labeling data were largely confirmed by anterograde tracing. Moreov er, it was shown that RT fibers cross the midline in the intrathalamic commissure. The present data demonstrate that bilateral RT connection s with the dorsal thalamus provide a channel for interthalamic crossta lk. Through these bilateral connections with thalamic VM and intralami nar neurons, RT could influence the activity of wide territories of th e cerebral cortex and basal ganglia of both hemispheres.