SYNAPTIC RELATIONSHIPS BETWEEN GABA-IMMUNOREACTIVE BOUTONS AND PRIMARY AFFERENT TERMINALS IN THE RAT CUNEATE NUCLEUS

The present study examined the synaptic relation between the primary a fferent terminals and intrinsic neuronal elements in the rat cuneate n ucleus. For this purpose, experimental degeneration after multiple cer vicothoracic dorsal rhizotomies or anterograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase were used to identify the primary afferent terminals, while immunogold postembedding stainin g was employed to identify the GABA-immunoreactive boutons. The combin ed procedure allowed us to demonstrate a direct synaptic relationship between the primary afferent terminals and GABA-immunoreactive boutons . At least two types of synaptic relation were observed between the pr imary afferent terminals, identified by their degenerating features or labeled by wheatgerm agglutinin conjugated to horseradish peroxidase, and the immunogold-labeled GABA-immunoreactive boutons (i) a GABA-imm unoreactive bouton making a simple presynaptic contact with the primar y afferent terminal; and (ii) a synaptic glomerular complex in which t he centrally located primary afferent terminal was postsynaptic to a G ABA-immunoreactive bouton and presynaptic to dendrites closely associa ted with it; both terminals were sometimes presynaptic to a common den drite. It is speculated from this study that the incoming impulses fro m the forelimb area are modulated by the GABA-immunoreactive boutons i n the cuneate nucleus of the rat.