Where is the spawning reflex receptor for neurohypophysial peptides in the killifish, Fundulus heteroclitus?

Arginine vasotocin, injected into the third vehicle of 10 brain-cannulated, unanesthetized male Fundulus heteroclitus at estimated doses of 0.4 and 40 mU/g wt., failed to elicit a spawning reflex in five recipients; each had been both pre- and post-tested for responsiveness to intraperitoneal injections. Five others responded at the 40 mU/g dose and one also at the 0.4 mU/g dose: the first response occurred after a long delay, similar to that after intraperitoneal injection. Escape into the peripheral circulation is implicated. Since the N. preopticus is known to participate in the spawning reflex response, an indirect pathway from the peripheral AVT-receptor to the hypothalamic nucleus is suspected. Substances that might have behavioral effects, or function as neuro-transmitters, were tested with negative results: ACTH, epinephrine, norepinephrine, GABA, dopamine, L-dopa, cAMP, cGMP and the prostaglandins PGA2, PGE2 and PGE2 alpha. Serotonin failed to elicit a spawning reflex but caused rapidly developing symptoms of hypoxia. An immediate single spawning reflex was observed on three occasions after intracerebral injection: once after each of the otherwise ineffective hypothalamic releasing hormones, LRF and TRF, and once after AVP. Direct stimulation of the N. preopticus is suspected.