Two systems of giant axon terminals in the cat medial geniculate body: Convergence of cortical and GABAergic inputs

The thalamus plays a critical role in processing sensory information that i nvolves interactions between extrinsic connections and intrinsic circuitry. Little is known regarding how these different systems might interact. We f ound an unexpected nuclear convergence of two types of giant axon terminals , each of which must have independent origins, in the dorsal division of th e cat medial geniculate body. The first class of giant terminal was labeled after injections of biotinylated dextran amines (BDA) in seven auditory co rtical areas. A second type was found in sections immunostained for gamma-a minobutyric acid (GABA); these endings had the same nuclear distribution, a nd they were numerous. The origin of this GABAergic terminal is unknown. Th e giant corticothalamic terminals were presumably those described in prior accounts using different tracers (Rouiller and de Ribaupierre [1990] Neuros ci. Lett. 208:29-35; Ojima [1994] Cerebral Cortex 6:646-663), but with BDA they are labeled more fully. Clusters of such endings were often linked, an d hundreds may occur in a single section. Their boutons formed a substantia l proportion of the corticothalamic population. Other types of corticogenic ulate axon terminals were also labeled, including two kinds that are much s maller and that match closely the classical descriptions of corticothalamic axons. The giant GABAergic endings were found in all dorsal division nucle i and in thalamic visual nuclei such as the lateral posterior nucleus. Like the giant cortical endings, the giant GABAergic terminals often encircled large, pale, immunonegative profiles that may be dendritic. This implies a close spatial, and perhaps a close functional, relationship between the pop ulations of giant axon terminals. Insofar as physiological studies found th at pharmacological inactivation of rat somatic sensory cortex suppresses pe ripheral information transmission through the posterior thalamus, corticofu gal input may be essential for normal processing (Diamond et al. [1992] J. Comp. Neurol. 319:66-84). Our findings suggest that the giant corticothalam ic endings could play an important role in descending control. Perhaps they are counterbalanced by a GABAergic system and affect thalamic oscillations implicated in shifts in vigilance and attention. (C) 1999 Wiley-Liss, Inc.