Ascending thalamic projections from the obex region in ranid frogs.

Amphibian forebrains display few obvious specializations and have frequently served as models of a primitive tetrapod condition from which more specialized sauropsid and theropsid forebrains have been derived. This approach to tetrapod forebrain evolution has yet to be fully exploited, as our knowledge of amphibian forebrain pathways is embryonic. Most experimental studies have focused on primary and secondary visual connections, and information concerning other systems is relatively sparse. Information on forebrain somatosensory pathways is particularly slight, and experimental data is available only for anurans. Anatomical studies using selective silver impregnation methods have failed to reveal a direct spinothalamic projection in frogs4,5, 9. Although a projection to the thalamus has been described following hemisection near the obex in Rana, it appears to be light and is restricted to a very small portion of the caudalmost thalamus 5. The apparent paucity of somatosensory projections to the forebrain seen in anatomical studies contrasts strikingly with the electrophysiological results of Vesselkin et a lJ 7, who described an extensive thalamic somatosensory representation on the basis of recordings following sciatic nerve stimulation in Rana temporaria. Since there appears to be some conflict between anatomical and physiological studies regarding the extent of thalamic somatosensory representation in anurans, and since a precise determination of this representation is essential to an understanding of tetrapod forebrain evolution, we have initiated a series of experiments on the ascending somatosensory system in ranid frogs. This preliminary report will describe our major findings to date. Nine bullfrogs, Rana catesbeiana, received single unilateral injections of 1--4 #Ci of [3H]proline or a [3H]proline-leucine mixture at a concentration of 20 /~Ci/#l. Injections were made at various levels between the caudal (lumbar) enlargement of the spinal cord and the obex. Survival times ranged from 2-9 days at temperatures of