Pineal influence on annual nuclear volume changes in ventromedial hypothalamic nucleus (VMH) neurons of the male Wistar rat

The ventromedial hypothalamic nucleus (VMH) regulates various autonomic, en docrine, and behavioral activities. These activities show annual changes, a nd the pineal gland is involved in their adjustment to environmental cues. Therefore, this study investigated whether the VMH belongs to the effector structures of the pineal gland. To abolish the rhythmic melatonin release, male Wistar rats were subjected to pinealectomy (PX) or ganglionectomy (sym pathetic denervation of the pineal gland, GX) regularly at the beginning of any of the four seasons. Brains from animals of PX-, GX-, and sham-operate d control groups were prepared 3 months later for measurement of the nuclea r volume, which changes according to the general gene activity. At each of the four seasons, 2000 nuclei of VMH neurons stemming from is animals per g roup were measured to obtain both seasonal daily mean values and annual mea n values, respectively, as well as to calculate annual curves of the nuclea r volume using empirical regression and locally adjusted polynomial approxi mation. The major findings are the following. First, inactivation of the pi neal function influences the nuclear activity of VMH neurons. (2) PX and GX mainly depress the nuclear activity, indicating that the pineal influence on the VMH may predominantly be a stimulatory one. Third, size and directio n of the changes caused by PX and GX vary in a seasonally dependent manner. Fourth, the annual rhythm of the nuclear activity of the VMH is modified b y PX and GX. To explain how the pinealeffects on the VMH may be mediated, a possible inhibitory influence of the suprachiasmatic nucleus (SCN), which has been activated in the same animals following both PX and CX, is discuss ed. In conclusion the results confirm that the nuclear activity of VMH neur ons underlies pineal influences. This also indicates an involvement of the pineal gland in many VMH-regulated functions.