Immunoelectron microscopic observations of hypothalamic TRH-containing neurons in rats

Immunoreactive TRH-containing neurons and their synaptic associations were studied electron microscopically in the paraventricular nucleus (PVN) and dorsomedial nucleus (DMH) of the rat hypothalamus. In propylthiouracil (PTU)-treated rats, the immunoreactive cell bodies in the PVN appeared to be activated, showing a hypertrophic perikaryon, well developed Golgi bodies and numerous secretory granules. No such alterations were evident in the TRH neurons in the DMH. These findings suggest that the PVN-TRH neurons are involved in the hypothalamic-hypophysial-thyroid axis. Further, it was shown that unlabeled nerve terminals containing small and large clear vesicles make synaptic contacts with the TRH perikarya in the PVN. Thus it is likely that PVN-TRH neurons are regulated both by thyroid hormones and by other neuronal signals. In the DMH, unlabeled nerve terminals containing small and large clear vesicles, and immunoreactive terminals form synapses with TRH neurons. Thus the DMH-TRH neurons may be under dual neuronal control. It was further noted that in the DMH and PVN, TRH nerve terminals make synaptic contacts with other unlabeled neurons. It is evident that TRH acts as a neurotransmitter or neuromodulator, although the origin of TRH terminals should be elucidated.