Effects of Diethyldithiocarbamate (DDC), Fusaric Acid, Catecholaminergic and Serotonergic Receptor Blockers on the Ascending Pathway from the Ventrolateral Part of the Medulla Oblongata

The effect of a potent inhibitor of dopamine-β-hydroxylase, diethyldithiocarbamate (DDC) or fusaric acid, on the antidromically evoked potentials by electrical stimulation of the preoptic suprachiasmatic area (POSC) was examined. Thirty seven neurons in the ventrolateral part of medulla oblongata (VLMO) were successfully tested after DDC injection. Nineteen of them decreased in amplitude to approximately 50% of control levels at 120min after DDC injection, 20% at 180min. These 19 neurons were characterized by having a notch in the rising phase and taking a longer time to complete the initial deflection. After fusaric acid injection, ten neurons in the VLMO were successfully tested and four of them decreased in amplitude similar to DDC injection.Effects of receptor blockers of noradrenaline, dopamine and serotonin on the single unit responses of the POSC neurons to the VLMO stimulation in proestrous rats were examined in order to know which of noradrenaline, dopamine or serotonin are involved in the neural transmission from the VLMO to the POSC. After the injection of phenoxybenzamine, an α-adrenergic receptor blocker, the rates of facilitatory and inhibitory responses in the POSC neurons to the VLMO stimulation were significantly decreased. Percentages of facilitatory and inhibitory responses were not changed significantly after injection of pimozide, a dopamineric receptor blocker, or methysergide, a serotonergic receptor blocker. These findings suggest that noradrenaline might be involved in the transmission of the VLMO stimulation effect on the POSC neurons and that VLMO noradrenaline-containing neurons send their axons directly to the POSC.