Ultrastructural characterization of prolactin-like immunoreactivity in rat medial basal hypothalamus

Prolactin-like immunoreactivity has been reported in the medial basal hypothalamus at the light microscopic level, in hypophysectomized rats. Here, with preembedding immunocytochemistry at the electron microscopic level, we have observed prolactin-immunoreactive neurons and synapses in the hypothalamus. Reaction product was discovered in medial basal hypothalamic neurons, which had typical large nucleoli and received axosomatic synapses. In the cytoplasm, reaction product was distinctly granular. Immunoreactive neurons were usually surrounded by nonreactive cells. Reaction product was also seen in dendrites, some of which had spines. Some axons in the hypothalamus contained reaction product, usually surrounded by nonreactive axons, and immunopositive synapses were detected both in the hypothalamus and in the midbrain. In a small number of cases immunoreactive axons could be seen synapsing on immunoreactive dendrites.