Electrophysiological evidence for retinal projections to the hypothalamic supraoptic nucleus and its perinuclear zone

Secretion of vasopressin (VP) and oxytocin (OT) displays a daily rhythm. Using electrophysiological methods, we investigated the projections from the optic nerve to the supraoptic nucleus (SON) and its perinuclear zone (PNZ) which might underlie the rhythm. Extracellular recordings were made from magnocellular cells in the SON and its PNZ in 22 urethane-anaesthetized female Wistar rats while stimulating the optic nerve. The responses of magnocellular and PNZ cells were classified as orthodromic excitatory (OD+) or inhibitory (OD-) after creating peri-stimulus time histograms (PSTHs). Twenty-six of 73 (35.6%) VP and OT cells and 16 of 42 (38.1%) PNZ cells were excited by optic nerve stimulation. PNZ cells displayed both short (for 7 cells 30 ms or less) and long (> 60 ms) latency responses. Most (6/7) short latency responses had a short duration but longer latency responses were longer. No magnocellular cells showed responses with both short latency and short duration. Short latency responses with a short duration probably reflect direct monosynaptic inputs whereas longer latency responses with longer duration may reflect complex inputs. Thus the retina projects to the PNZ and to the SON but the PNZ receives a stronger direct input. Such projections might provide a light-related input to SON cells and suggest a role for the PNZ in this input.