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The distribution of immunoreactive neurones and fibres was studied in rat brain using an antiserum to rat prolactin. Neurones containing the immunoreactive material were localized in the arcuate, ventromedial, premamillary, supraoptic and paraventricular nuclei of the hypothalamus. Immunoreactive nerve fibres were widely distributed within the brain. No differences were observed in labelling between male and female rats, or as a consequence of hypophysectomy.

By means of a specific anti-rat renin antiserum, immunohistochemical staining was observed restricted to Leydig cells of rat testis. Specificity of the staining was ascertained by the absence of reaction with nonimmune serum or with the antiserum preincubated with rat renin. Specific staining of Leydig cells was absent in newborn rats; it developed with the onset of puberty. Staining was suppressed or abolished by hypophysectomy and estrogen treatment and was reduced by gonadotropin stimulation. Vasectomy destroyed the seminiferous epithelium but did not impair renin-like immunoreactivity of the interstitial tissue. It is concluded that Leydig cells contain a pituitary-dependent renin-like substance.

By an immunoperoxidase (PAP) technique, ACTH- and prolactin-like substances were detected in neurons of several hypothalamic nuclei in young and adult rats. Neurons in the arcuate and ventromedial nuclei were simultaneously both ACTH-like and PRL-like positive. Labelled fibres were followed to their fields of projections. Intensity of labelling was not influenced by experimental procedures known to change PRL and ACTH content of the pituitary. Colchicine enhanced labelling of perikarya while lowering that of fibres. The results support neuronal synthesis of hormonal ACTH and PRL related molecules and their involvement in the control of autonomic nerve functions.

The cerebellar molecular layer in adult rats has been studied with the electron microscope at several early and consecutive survival times following 3-acetylpyridine intoxication. Climbing fiber (CF) terminals underwent a fast process of electron-dense degeneration which became apparent from 16 hours onwards. A small proportion of degenerating terminals were depleted of vesicles and filled with a dark flocculent and granular homogeneous matrix. Microtubular changes in degenerating CF tendrils were observed. CF terminals were found in relation with every Purkinje cell in normal animals and completely disappeared within 72 hours after the treatment. CF synapses were found on Purkinje dendritic and somatic thorns, sometimes also on the dendritic shafts or even on the Purkinje soma. Convincing evidencd of synaptic contacts of CF varicosities on either basket or stellate cells could not be obtained. CF synapses with Golgi II cell dendrites in the molecular layer were described. Decrease in the number of post-synaptic dendritic thorns normally assigned for CF synapses was observed consequential to CF anterograde degeneration. The observations are consistent with previous conclusions drawn from light microscopic studies that the clearing up of CF debris in the molecular layer is completed within the short time of three days, and that the inferior olive seems to be the only source of CFs.

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