Effect of continuous darkness on diurnal rhythms in small vesicles in sympathetic nerve endings of the mouse pineal-quantitative electron microscopic observations.

Quantitative electron microscopic studies on the effect of continuous darkness on the diurnal rhythm of small vesicles in sympathetic nerve endings of the mouse pineal demonstrate that the numerical change of type 1 granulated vesicles, predominant small granulated vesicles, and non-granulated elliptical vesicles seen during the first half of the dark period under diurnal lighting conditions may persist for at least 14 days under continuous darkness. Furthermore, it is found that the alteration in the number of type 1 granulated vesicles and the total vesicle density observed during the light period under diurnal lighting conditions may be lost in mice kept in continuous darkness for 7 days. This result indicates that the type 1 granulated vesicles may be composed of two different populations of vesicles, i.e., granulated vesicles of population 1 and 2, and that the diurnal rhythm in the number of the granulated vesicles of population 1 may be abolished, while that of the granulated vesicles of population 2 may be maintained, under continuous darkness. The diurnal variation in the number of the type 2 granulated vesicles, which constitute a small population of small granulated vesicles, may persist in darkness. Thus, the diurnal rhythm in the number of small granulated vesicles in sympathetic nerve endings of the mouse pineal may be controlled by two different mechanisms, exogenous and endogenous.