PARAVENTRICULAR—HYPOPHYSIAL NEUROSECRETORY PATHWAYS IN THE GUINEA-PIG

SUMMARY The neurosecretory pathways from the paraventricular (PV) nuclei have been identified in the guinea-pig and plotted according to stereotaxic coordinates for the guinea-pig brain. The magnocellular neurosecretory neurones are concentrated in the lateral tip of the PV nucleus approximately half-way between the third ventricle and the fornix. Some neurosecretory fibres pass between the cranio-ventral extension of the PV nucleus and the supraoptic (SO) nucleus. Neurosecretory fibres leave the rostral region of the main part of the PV nucleus and pass ventral to the fornix, those from the middle region pass both ventral and dorsal to the fornix and those from the caudal region pass mainly dorsal to the fornix. After passing around the fornix, these fibres merge to form a broad band, the paraventricular—hypophysial tract, which curves ventrolaterally and then ventromedially in a gentle arc around the lateral edge of the anterior hypothalamic area and the hypothalamic ventromedial nucleus before turning caudomedially to run over the dorsal surface of the optic chiasma towards the median eminence. Neurosecretory fibres were not observed in the anterior hypothalamic area or in the ventromedial nucleus. An accessory PV nucleus was found dorsolateral to the fornix. Neurosecretory fibres pass over the fornix in both directions between the PV and the accessory PV nuclei, also fibres leave the accessory PV nucleus and pass ventrally to merge with those of the main paraventricular—hypophysial tract.