Pineal melatonin synthesis is enhanced by the activation of adrenergic and opioid receptors. The precise mechanism of how signal transduction is affected by the activation of these receptors, leading to an increase in melatonin synthesis, is not clear. An attempt has been made to investigate the effects of the activation of these two types of receptors that might lead to the induction of immediate early genes (IEGs) and nitric oxide synthase (NOS) expressions. The present study shows that the stimulation of cultured pinealocytes by 1 microM epinephrine (an alpha- and beta-adrenergic agonist) for 2 h increased the number of c-fos immunoreactive (IR) cells, and that this stimulatory effect was abolished by adding 10 microM prazosin (an alpha-adrenergic antagonist) to the culture medium. No significant change of c-fos-IR cells was found when cells were treated with either 1 microM isoproterenol (a beta-adrenergic agonist) or 100 microM morphine (an opioid agonist). Neither epinephrine (1 microM), isoproterenol (1 microM) nor morphine (100 microM) altered jun-B-IR cells in the culture. With regard to NOS, the number of IR cells was slightly increased after 2 h of incubation by 10 microM isoproterenol, whereas morphine (100 microM) and epinephrine (10 microM) did not change the number of IR cells. The results of this experiment have demonstrated that c-fos expression is induced by alpha-adrenergic but not beta-adrenergic agonists, nor by opioid agonists. Neither alpha-, beta-adrenergic nor opioid agonists had any significant effect on jun-B, whereas NOS was slightly increased by isoproterenol.