Medullary synaptic inputs to thyrotropin-releasing hormone (TRH)-containing neurons in the hypothalamus: an ultrastructural study combining WGA-HRP anterograde tracing with TRH immunocytochemistry.

Ascending projections from the A1/C1 cell group and from the A2 cell group in the medulla oblongata was studied in the light microscope by anterograde tracing of Phaseolus vulgaris leucoagglutinin and in the electron microscope by anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) combined with thyrotropin-releasing hormone (TRH) immunocytochemistry in the hypothalamic paraventricular nucleus (PVN). WGA-HRP-labeled axon terminals originating from neurons in the A1/C1 or the A2 cell group were found to make synaptic contacts with TRH-containing cell bodies and dendrites in the medial parvocellular part of the PVN, usually forming axo-dendritic synapses. Of all the afferent synapses on TRH neurons in the PVN, 9.8-20.9% of the presynaptic axon terminals were WGA-HRP-positive. This indicates that each brain stem catecholaminergic cell group that contribute to innervation of the PVN is in a position to modulate the activity of TRH neurons.