Regulation of glycogen content in rat pineal gland by norepinephrine

Abstract

In the rat pineal gland the glycogen stores were cytochemically localized in astrocytes and pinealocytes. Moreover, it was found that norepinephrine (NE) induced a time- and concentration-dependent reduction in pineal glycogen content and yielded lactic acid. The NE effect was prevented by blocking α 1- but not α 2 or β-adrenoceptors. Activation of α 2-adrenoceptors induced a small decrease in glycogen levels that could have pre- and postsynaptic components. Activation of β-adrenoceptors with 10 −12–10 −3 M isoproterenol (ISO) induced a bell shape concentration-response curve, presumably due to desensitization, since the response induced by 10 −4 M ISO was greater with shorter period of stimulation. On the other hand, activation of α 1-adrenoceptors with 10 −12–10 −3 M phenylephrine (PHN) induced a hyperbolic concentration-response curve with a maximum at concentrations above 10 −8 M. Moreover, treatment with ISO drastically reduced the response induced by PHN concentrations lower but not higher than 10 −6 M, favoring a concentration-dependent response between 10 −6 and 10 −4 M PHN, similar to that induced by equimolar NE concentrations. Thus, the NE-induced reduction in glycogen content of the rat pineal gland is mainly mediated by α 1-adrenoceptors and modulated by intracellular mechanisms activated by β-adrenoceptors.