Abstract
Through double staining and electron microscope investigation, the Innervation of neurons containing thyrotropin-releasing hormone (TRH) by neurons containing gamma-aminobutyric acid (GABA) in the rat hypothalamic paraventricular nucleus (PVN) was studied. To visualize the location of GABA and TRH, Vibratome sections of the hypothalamic region were stained using a preembedding double immunostaining technique using the antisera of GABA and TRH. Both the silver-gold intensified (SGI) peroxidase-antiperoxidase complex (PAP) -3, 3'-diaminobenzidine (DAB) method, and the PAP-DAB method were employed. The presence of DAB reaction products and silver-intensified gold-substituted DAB reaction products demonstrated immunoreactivities for TRH and GABA, respectively. Many TRH-like immunoreactive (TRH-LI) cell bodies and processes, containing numerous TRH-LI granular vesicles, were detected in the medial and periventricular and in the dorsal parvocellular parts of the PVN. The parvocellular and magnocellular parts of the PVN also contained numerous GABA-like immunoreactive (GABA-LI) fibers, but no GABALI cell body was detected in the PVN. Small, clear, GABA-like immunonegative vesicles and some large, densely-cored, GABA-LI vesicles were observed in GABA-LI axon terminals. On observation of double-immunostained sections of the rat hypothalamic PVN, TRH-LI neurons came into contact with both GABA-LI and GABA-like immunonegative axon terminals, and GABA-LI axon terminals were found to occasionally form symmetrical synapses on TRH-LI nerve cell bodies and processes. Our findings suggest that GABA-containing neurons may innervate TRH neurons, thereby regulating their secretory activity and/or other functions, via synapses in the rat hypothalamic PVN.
