The effect of alphaMSH on beta-adrenergic receptor mechanisms in the rat pineal

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Stimulation of a nicotinic ACh receptor causes depolarization and activation of L-type Ca2+ channels in rat pinealocytes

1. Membrane voltage (Vm) recordings were obtained from isolated rat pinealocytes using the patch-clamp technique. In parallel to the electrophysiological experiments, intracellular Ca2+ measurements were performed using fura-2. 2. The resting Vm averaged -43 mV and replacement of extracellular NaCl by KCl completely depolarized the cells. This indicates that the resting Vm is dominated by a K+ conductance. Single-channel recordings revealed the presence of a large conductance Ca(2+)-activated charybdotoxin-sensitive K+ channel. 3. Application of ACh (100 microM) depolarized the pinealocytes on average by 16 mV. The depolarizing effect of ACh was mimicked by nicotine (50 microM) and was prevented by tubocurarine (100 microM). 4. The ACh-induced depolarization was largely abolished in the absence of extracellular Na+, but was not significantly affected by extracellular Ca2+ removal. 5. Application of ACh (100 microM) caused an increase in [Ca2+]i. This increase was completely dependent on the presence of extracellular Ca2+ and was largely reduced after extracellular Na+ removal. Nifedipine (1 microM) reduced the ACh-induced increase in [Ca2+]i by about 50%. 6. Our findings indicate that in rat pinealocytes stimulation of a nicotinic ACh receptor (nAChR) induces depolarization mainly by Na+ influx via the nAChR. The depolarization then activates L-type Ca2+ channels, which are responsible for the nifedipine-sensitive portion of the intracellular Ca2+ increase. Ca2+ influx via the nAChR probably also contributes to the observed rise in [Ca2+]i.

MIF-induced augmentation of melatonin functions: possible relevance to mechanisms of action of MIF-1 in movement disorders

MIF-1, a synthetic tripeptide with MSH-release inhibitory properties, has been reported to improve symptoms of Parkinson's disease, attenuate levodopa-related dyskinesias and diminish the dyskinetic movements of Tardive dyskinesia. More recently, MIF-1 has been reported partially to protect against the nigro-striatal dopamine depleting effects of MPTP in mice, raising the possibility that it may exert protective effects against the development of Parkinson's disease. There is evidence to suggest that MIF-1 increases nigro-striatal dopaminergic activity, but its ability to improve symptoms in patients with Parkinson's disease, levodopa-related dyskinesias and Tardive dyskinesia cannot be explained solely on the basis of the drug's effect on striatal dopaminergic neurons. MIF-1 has been reported to potentiate the melanocyte-lightening effect of melatonin in rats and its effects in patients with Parkinson's disease and Tardive dyskinesia are associated with marked mood elevation. It is, therefore, possible that the effects of MIF-1 in movement disorders are associated with increased melatonin secretion. Thus, hypothalamic MIF may modulate nigro-striatal dopaminergic functions in part via pineal melatonin. Such an interaction represents a novel mechanism by which hypothalamic peptides act to modulate the expression of movement disorders.

Immunohistochemical evidence for the presence of peptides derived from proenkephalin, prodynorphin and proopiomelanocortin in the guinea pig pineal gland

By using a plethora of region-specific antisera, this light microscopic immunohistochemical study revealed that derivatives from the three opioid precursors, i.e. proenkephalin, prodynorphin and proopiomelanocortin are differentially distributed in the pineal gland of guinea pig. Various molecular forms of immunoreactive opioid peptides derived from proenkephalin or prodynorphin were present in a minority of pinealocytes as well as in nerves. In contrast to this dual distribution pattern of opioid-active peptides, the opioid-inactive derivative from proopiomelanocortin, alpha-melanocyte stimulating hormone, was exclusively present in a large proportion of pinealocytes. A multiple and differential origin and function of opioidergic pineal innervation involving sympathetic, parasympathetic and sensory components is suggested. alpha-MSH is proposed as a pineal hormone which may act in concert with melatonin to regulate pineal rhythms or may function like MSH of pituitary origin.

Hormonal modulation of pineal-mediated seasonal events: effects of thyroid or gonadal manipulations

The ability of various manipulations of the endocrine system to affect pineal-mediated events was examined in the present studies. Male Syrian hamsters were analyzed for pineal-induced gonadal regression and depressions in serum thyroxine and testosterone levels after treatments which altered thyroid or gonadal function. Hamsters were thyroidectomized, received thyroxine implants (5 mg), or were thyroidectomized plus implanted with thyroxine. The animals were exposed to short photoperiods (10L:14D) for 9 wk, and plasma hormone levels and gonadal status were determined at the end of the experimental period. Likewise, hamsters were castrated, received testosterone implants (5 mg), or were castrated plus implanted with testosterone, and subsequently were exposed to short photoperiods for 9 wk. These animals' responses to photoperiod exposure were compared to animals which received identical hormonal treatments but remained in long photoperiod (14L:10D). All of the hamsters responded to the photoperiodic treatments equally, regardless of the hormonal treatment. The results of these studies indicate that experimentally induced alterations in plasma thyroxine or testosterone levels are unable to prevent or attenuate the ability of the pineal to elicit gonadal regression in response to short photoperiod exposure.

Injections of alpha-melanocyte stimulating hormone affect pineal serotonin, melatonin and N-acetyltransferase activity

To determine if exogenously administered alpha-melanocyte stimulating hormone (alpha-MSH) affects nighttime pineal N-acetyltransferase activity, pineal levels of 5-hydroxytryptophan, serotonin and melatonin, and plasma prolactin levels, adult male hamsters were injected at 1900 hr (lights out 2000-0600 hr) with two doses of the peptide and killed at 0300 hr. The low dose of alpha-MSH (200 ng) produced a significant fall in pineal serotonin, pineal NAT activity and plasma prolactin values. The high dose of the peptide (20 micrograms) increased circulating prolactin titers and pineal serotonin levels and caused a concomitant decrease in pineal melatonin levels.

Dual neuroendocrine glands: sensitivity of the pineal gland to hormonal feedback

Numerous studies have tried to determine whether alterations in the hormonal content of the plasma would modify pineal function. Recent evidence has indicated that the pineal is relatively insensitive to alterations in plasma hormone concentration, as contrasted with the neuroendocrine hypothalamus which is very responsive to hormonal feedback. Therefore, two types of neuroendocrine glands appear to exist based on their ability to respond to hormonal feedback. The first type of neuroendocrine gland, exemplified by the neuroendocrine portion of the hypothalamus, is extremely sensitive to hormonal modulation and is concerned with maintaining the endogenous balance of plasma hormones. The second type of neuroendocrine gland, exemplified by the pineal gland, is principally responsible for converting its neural input, usually a environmental stimulus, into a hormonal messenger, irrespective of plasma hormone levels. These two types of neuroendocrine glands would act in synchrony to maintain homeostasis throughout a variety of external conditions.

Demonstration of an endogenous circadian rhythm of alpha-melanocyte stimulating hormone in the rat pineal gland

alpha-Melanocyte stimulating hormone (alpha-MSH) has been identified and characterized in the rat pineal gland by a combination of immunochemical and high pressure liquid chromatographic techniques. The immunoreactivity in pineal extracts was separated into two chromatographic components. The major component had a retention time identical to that of alpha-MSH while the minor component eluted just slightly before standard alpha-MSH. Male rats maintained in a 12 h photoperiod demonstrated a marked circadian rhythm in pineal alpha-MSH concentration. Concentrations that peaked at 07.00 h, 1 h after the lights were turned on, were greater than 5 times the nadir which occurred at 01.000 h. Animals house in chronic dark for 7 days maintained the diurnal variation of alpha-MSH concentrations. However, in chronically dark housed rats, the peak shifted to 05.000 h and was greater than 10 times the nidir of this rhythm and approximately 4 time the peak at 07.00 in alternating light/dark conditions. Rats exposed to chronic light for 7 days maintained a pineal alpha-MSH rhythm although the amplitude of the peak was significantly decreased compared to the rhythm in animals housed in alternating light/dark conditions. Neither hypophysectomy nor superior cervical ganglionectomy had any effect on the alpha-MSH rhythm. Lesion of the arcuate nucleus, the major source of alpha-MSH-containing nerves in the brain, did not significantly affect pineal alpha-MSH concentrations. These data demonstrate a circadian alpha-MSH rhythm in the rat pineal and suggest an alpha-MSH involvement in the rhythmic processes of the pineal gland.