A Diurnal Rhythm of Immunoreactive <i>α</i>-Melanocyte-Stimulating Hormone in Discrete Regions of the Rat Brain

Abstract

A significant diurnal variation in α-melanocyte stimulating hormone (α-MSH) concentrations was observed in discrete regions of the rat brain. During the 12 h light: 12 h dark cycle, α-MSH concentrations in each case were highest during the light period and lowest during the dark period. At 09.00 h, 3 h after lights were turned on, the peak α-MSH concentration occurred in the median eminence, the arcuate and dorsomedial hypothalamic nuclei and the periventricular thalamic nucleus. The paraventricular and anterior hypothalamic nuclei had highest α-MSH concentrations at 13.00 h. In the medial preoptic nucleus, the peak α-MSH concentrations appeared at 17.00 h. These changes in α-MSH content may reflect an α-MSH role in circadian variations in behavioral and neuroendocrine processes.

Changes in Alpha-Melanocyte-Stimulating Hormone Content in Discrete Hypothalamic Areas of the Male Rat during a Twenty-Four-Hour Period

“…In contrast, O'Donohue et al [26] found a peak of a-MSH content in the hypothalamus during the light period; they related the diur nal variations with circadian changes in pi tuitary and blood a-MSH content. This could indicate that the values of releasable a-MSH in the hypothalamus is not necessar ily identical with its a-MSH content.…”

Diurnal Variation in the Release of Alpha-MSH from Rat Hypothalamus and Pituitary

“…Only at 10.00 h when we observed a measurable spontaneous a-MSH release, could enhanced release be elicited by high potassium. This corresponds to the time at which O'Dono hue et al [18] observed a peak in a-MSH concentration in the hypothalamus. In our study, this time corresponds to a low level of a-MSH in blood.…”

“…In our study, this time corresponds to a low level of a-MSH in blood. Furthermore, the hypothala mus presents a single peak of a-MSH concentration [18], whereas two such peaks of a-MSH exist in blood [27, pre sent study]. Neither one of these peaks coincides with the pe riod of maximal a-MSH concentration in the hypothala mus, when the peptide can be released from this structure by high potassium.…”

“…Both spontaneous and potassium-induced a-MSH release were related to diurnal variation. Marked release from the slices was observed at 10.10 h, corresponding to a peak in the a-MSH concen tration in the hypothalamus [18] and to a low level of a-MSH in the blood. Contrarily, no significant release from the hypo thalamus was obtained at 17.00 h when hypothalamic a-MSH content was low, but blood levels exhibited a peak.…”

“…Numerous authors have localized the peptide by immunological methods in different regions of the brain [I, 6-8, 12, 13, 17, 19, 22, 28, 30]. Immunopositive nerve terminals are present in the hypothalamus, and cell bodies in the arcuate nucleus are immunoreactive for a-MSH [18]. In addition, due to the presence of a-MSH in hypothalamic synaptosomes [8] and of Ca-dependent a-MSH release [I, 16, 23], a neuromodulator or neurotrans mitter role for a-MSH in the hypothalamus and perhaps in the central nervous system was postulated.…”