Alterations in components of the pineal melatonin synthetic pathway by acute insulin stress in the rat and Syrian hamster

Acute insulin stress increased plasma catecholamine levels in both the Syrian hamster and albino rat within 3 h after an intraperitoneal injection of either 5 or 10 units of insulin. In the rat, this stress caused a concurrent increase in pineal serotonin N-acetyltransferase (NAT) activity and melatonin content with no observable change in hydroxyindole-O-methyltransferase (HIOMT) activity. In the hamster, on the other hand, acute insulin stress did not alter pineal NAT activity, but depressed both HIOMT activity and melatonin content up to 3 h after the stress. These results present further evidence that catecholamines do not control hamster pineal melatonin synthesis by the same mechanism as observed in the rat.