Neuroendocrine rhythms

Concentrations and distribution of vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI) and peptide histidine valine (PHV) in the cerebral cortex and the suprachiasmatic nucleus of the mouse

Prepro-vasoactive intestinal peptide (prepro-VIP) is processed to at least three biologically active peptides: VIP, peptide histidine isoleucine (PHI) and an extended PHI, peptide histidine valine (PHV). The aim of the present investigation was by chromatography combined with RIA and immunocytochemistry to determine which of these peptides were present in the cerebral cortex and the hypothalamic suprachiasmatic nucleus (SCN) of the mouse. These regions were chosen since they are known to contain a high concentration of VIP but the relative concentration of PHI and PHV is not known. Tissue was extracted and subjected to gel chromatography and high-pressure liquid chromatography (HPLC). VIP and PHI immunoreactivities co-eluted with synthetic rat VIP and PHI. A minor peak of PHI and prepro-VIP(111-122) immunoreactivities eluted at the position of synthetic PHV. Surprisingly, a major peak of prepro-VIP(111-122) immunoreactivity eluted in a position not related to any other immunoreactivity indicating the presence of prepro-VIP(111-122). Measurements of these immunoreactivities in cortical and suprachiasmatic extracts revealed that VIP was found in the highest concentration whereas PHV was found in the lowest. Immunoreactivity for PHI and prepro-VIP(111-122) was found in moderate concentrations. Except for prepro-VIP(111-122) which was found to be approximately 3 x higher concentrated in the SCN than in the cerebral cortex, the other immunoreactivities were found in almost similar relative concentrations in the two tissues. Using immunocytochemistry, elongated neurons mostly of the bipolar type with prominent processes observed in the cerebral cortex reacted with all antisera tested. More PHI/PHV/prepro-VIP(111-122)- than VIP-immunoreactive (ir) nerve fibers were found in the cerebral cortex. In the SCN, the density of immunoreactivity was the same whatever antiserum used. VIP-, PHI- and prepro-VIP(111-122)/PHV-ir neurons were observed in the ventral part of the nucleus with numerous axons coursing caudodorsally into the subparaventricular area. A substantial number of terminals was detected caudal to the paraventricular nucleus. Minor projections spread to the medial part of the anterior nucleus and to the medial preoptic area hypothalamic. These data show that VIP and PHI are the major active peptides derived from prepro-VIP in the mouse cerebral cortex and SCN whereas PHV was found in minor concentrations. Prepro-VIP(111-122), which so far has been found to have no functional significance, is, therefore, most likely a vaste fragment of processing of PHI in central neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of the efferent projections of the lateral geniculate nucleus with special reference to the innervation of the subcommissural organ and related areas

In order to define central neurons projecting to the subcommissural organ (SCO) and to related areas in the postero-medial diencephalon, Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected into the lateral geniculate nucleus of the rat. PHA-L-labelled neurons send axonal processes medially through the posterior thalamic nuclei and the posterior commissure to the other hemisphere. Branches of fibres originating from this projection form a plexus of nerve terminals in the underlying precommissural nucleus and in the nucleus of the posterior commissure. A small number of PHA-L-immunoreactive nerve fibres penetrate from the precommissural nucleus into the lateral part of the SCO. A few labelled fibres penetrate directly from the posterior commissure into the medial part of the caudal SCO. Most of the PHA-L-immunoreactive fibres occur in the hypendymal layer, although a few terminate near the ependymal cells of the organ. Many labelled fibres are found in the ventricular ependyma adjacent to the SCO, some fibres lying close to the ventricular lumen. These results were obtained only if the tracer was delivered into the intergeniculate leaflet of the lateral geniculate nucleus (IGL). The IGL innervates both the suprachiasmatic nucleus and the pineal organ; the connections between the IGL and the midline structures, including the SCO, suggest that these areas are influenced by the circadian system.

Circadian rhythms

Changes in behavior which occur on a daily or circadian basis represent one of the most ubiquitous strategies by which most living organisms have adapted to their environment. Underlying the daily changes in behavior are a multitude of endocrine and metabolic rhythms which provide adaptively significant temporal organization within the organism. In mammals there appears to be a central circadian clock in the SCN which is responsible for generating and coordinating the entire 24-hour temporal organization of the animal. The circadian clock regulates the timing, duration, and characteristics of sleep, and together the circadian clock and sleep interact to control the timing of endocrine secretions. While the impact of disturbed endocrine circadian rhythms for the survival of the species has received very little attention, the almost universal presence of circadian rhythmicity within the endocrine system argues in support of the hypothesis that a disruption of the normal circadian organization within the endocrine system can have serious consequences for the health and well-being of the organism. It is particularly noteworthy that in advanced age, various alterations in circadian endocrine rhythms have been observed and these alterations may impair the ability of the animal to adapt normally to the environment. Relatively speaking, the study of circadian rhythms is a new field of biology, and as a result, much remains to be discovered about the physiological mechanisms that underlie rhythmicity, as well as the functional significance of 24-hour temporal organization for the survival of the species.

Circadian variations in the amplitude of corticotropin-releasing hormone 41 (CRH41) episodic release measured in vivo in male rats: correlations with diurnal fluctuations in hypothalamic and median eminence CRH41 contents

The possible correlation between the circadian and episodic release of corticotropin-releasing hormone 41 (CRH41) in male rats was explored in a comparative study, including the measurement at 0700 hr and 1700 hr of (1) the quantitative parameters of the episodic release pattern of CRH41 into the push-pull-cannulated median eminence (ME); (2) CRH41 content measured by radioimmunoassay in the hypothalamus, and immunocytochemically in the ME; and (3) plasma adrenocorticotropic hormone (ACTH). The data showed that in early evening, the 3.4-fold rise in plasma ACTH coincided with a doubling of CRH41 content in the hypothalamus and in the ME, and of the CRH41 release from the perfused ME. The immunocytochemical data further indicated that the ME area labeled with CRH41 immunoreactivity, rather than the labeling intensity of CRH41-stained neurons, increased in the evening, which may point to an evening recruitment of additional CRH41-producing neurons as the origin of the evening increment in CRH41 and ACTH releases. Finally, the computerized analysis of the CRH41-releasing pattern with three different algorithms (Pulsar, Ultra, and the Santen and Bardin algorithm) showed for the first time that the evening rise in CRH41 output was associated with correlative increases of three parameters of the episodic pattern--peak amplitude (+55% to +80%), peak duration (+20%), and mean absolute peak values (+73%)--while the pulse frequency remained at the baseline level of 3 cycles.hr-1. The data suggest the occurrence of a connection between the circadian pacemaker and the machinery generating the episodic release of CRH41.

The effect of photoperiod on plasma levels of melanin-concentrating hormone in the trout

The neurohypophysial melanin-concentrating hormone, MCH, plays a role in adaptive colour change in teleost fishes inducing pallor when the fish is placed in pale-coloured surroundings. The present study shows that its plasma concentration, measured in groups of white-adapted fish, is not uniformly high throughout the day but follows a clear diurnal pattern. Over a 24 h cycle, plasma concentrations rise gradually during the morning to reach peak values around the middle of the photophase, after which they decline significantly before night. Lowest concentrations are observed during the dark period. This pattern was observed under a long photoperiod in summer and a short photoperiod in winter. The peak was shifted within a week of changing the onset of either light or dark. When dawn was delayed by 6 h for fish held under short photoperiod conditions, then peak concentrations were attained 6 h precociously. Fish from a long photoperiod placed in constant light showed a pattern of MCH release which approximated to the normal over the first 24 h period but plasma values then became raised and periodicity was no longer discernible. Plasma hormone concentrations were much reduced in trout kept in black coloured tanks in which nocturnal and daytime values differed, but significant differences during the photophase were not demonstrable. The results suggest that an illuminated white background can initiate the early morning release of MCH, and that an endogenous pacemaker underlies the pattern of MCH secretion.

Clinical aspects of the melatonin action: impact of development, aging, and puberty, involvement of melatonin in psychiatric disease and importance of neuroimmunoendocrine interactions

During the last decade we have learned much on physiological changes in the secretion of the pineal hormone melatonin (MLT) in man. Reportedly, there is little or no MLT secreted before age 3 months. Then MLT production commences, becomes circadian, and reaches highest nocturnal levels at the age of 1-3 years. During all of childhood nocturnal peak levels drop progressively by 80% until adult levels are reached. This alteration appears to be the consequence of increasing body size in face of constant MLT production during childhood. The biological significance of this MLT alteration is presently unknown. Because of conceptual considerations, major depressive syndrome (MDS) and seasonal affective disorder (SAD) have been in the focus of pineal research for several years. Although in these disorders alterations in MLT levels could not be substantiated, light therapy, a consequence of this research, was discovered as an effective treatment for SAD and perhaps for MDS. In addition, there is some recent evidence for low MLT levels in schizophrenia. Finally, the potential effect of MLT in neuroimmunoendocrine interactions is presently explored. Reportedly, in vitro studies and animal experiments give evidence for a modulatory role of MLT in the immune response. However, the exact way of this possible action of MLT remains to be clarified. Clinical studies are too scant for a meaningful estimation of MLT's involvement in human neuroimmunoendocrine interactions.

The suprachiasmatic nucleus of the mink (Mustela vison): apparent absence of vasopressin-immunoreactive neurons

The hypothalamic suprachiasmatic nucleus is centrally involved in generation of several circadian rhythms. Neurons of the mammalian suprachiasmatic nucleus express a number of neuropeptides including vasopressin. The suprachiasmatic nucleus of the mink (Mustela vison) is easily distinguished from neighbouring hypothalamic areas and the underlying optic chiasm as a small nucleus containing densely packed parvocellular neurons. A dorsal and ventral subdivision were clearly recognized within the midportion and caudal part of the nucleus. Using immunohistochemistry, we have identified vasopressin-, neurophysin-, and vasoactive intestinal peptide-immunoreactive neuronal elements in the hypothalamus of the mink. Vasoactive intestinal peptide-immunoreactive neurons can be observed in the ventral aspect of the suprachiasmatic nucleus, but to our surprise, no vasopressin immunoreactive perikarya are found within the suprachiasmatic nucleus, this absence being independent of the experienced annual cycle. The hypothalamic paraventricular and supraoptic nuclei contain large numbers of vasopressin-, neurophysin- and vasoactive intestinal peptide-immunoreactive magnocellular neurons with extensive projections towards the infundibulum and neurohypophysis. A comparative analysis of the distribution of vasopressin-immunoreactive elements in a number of conventional laboratory animals has demonstrated that, in contrast to the rat, golden hamster and Mongolian gerbil, neither vasopressin-containing perikarya in the suprachiasmatic nucleus nor fine calibered immunoreactive fibres entering the adjacent subparaventricular zone are present in the mink. The mink is a photodependent seasonal breeder, and thus vasopressin-immunoreactive neurons in the suprachiasmatic nuclei may not be essential for the photoperiodic regulation of reproduction and seasonal events experienced by this species.

Photoperiodic and pineal influences on estrogen-stimulated behaviors in female Syrian hamsters

Three experiments investigated the effects of short photoperiod exposure on the estrogenic facilitation of locomotor activity and lordosis. In Experiment 1, ovariectomized female hamsters were administered exogenous estrogen to stimulate locomotor activity in running wheels. Estrogen was effective in the long photoperiod group but did not stimulate running-wheel activity in the short photoperiod group. In Experiment 2, the role of the pineal gland in mediating photoperiodic influences on female hamster behavior was examined. Both estrogen-induced locomotor activity and estrogen+progesterone-stimulated lordosis behavior were significantly reduced in short photoperiod females. Both these photoperiodic effects were absent in pinealectomized hamsters. Sham-pinealectomized, short photoperiod females expressed behavioral deficits; pinealectomized hamsters in the short photoperiod did not. Experiment 3 investigated lordosis only and used hormone injections rather than silastic implants to administer estrogen. The photoperiodic and pineal effects observed in Experiment 2 were replicated in Experiment 3. Additionally, the suppression of lordosis responsiveness by short photoperiod exposure was estrogen dose dependent. Photoperiodic effects were present when 2 micrograms estradiol cypionate was used but absent when higher estrogen doses were used. These findings are discussed in the context of other results that suggested photoperiodic effects on hamster lordosis were pineal independent.

Rhythms in the immune system

This report reviews the evidence that cells within the immune system are subject to rhythmic influences that affect numbers of circulating cells and their function both in vitro and in vivo. It is concluded that, although periodicity has clearly been demonstrated for numbers of immunocompetent cells in the circulation, significant functional changes have not been consistently observed. A number of neuroendocrine hormones, which modulate immune responsiveness in vitro and which are released in a rhythmic manner, are considered as mediators of the observed effects on the immune system and this is related to changes in expression and activity of immune-mediated diseases such as rheumatoid arthritis.

Central sites mediating reproductive responses to melatonin in juvenile male Siberian hamsters

Juvenile male Siberian hamsters received infusions of varying doses of melatonin (MEL), or saline vehicle, via microdialysis probes implanted in brain regions which have previously been shown to contain MEL receptors. Daily infusions were 10 h in length and occurred during exposure to constant light on days 22-34 of age. All animals were sacrificed on day 35 and paired testis weights recorded prior to preparation of the brain tissue for histological evaluation of the infusion site. Some animals were also blood-sampled prior to sacrifice for determination of circulating levels of prolactin (PRL). Saline infusions did not have a significant effect upon gonadal maturation, regardless of the infusion site, when compared with unoperated control animals reared under similar photoperiod conditions. In contrast, animals which received infusions of 75 pg MEL into the suprachiasmatic nucleus (SCN), paraventricular nucleus of the thalamus, or nucleus reuniens regions, showed a marked inhibition of gonadal growth. Infusions of this dose of MEL into various other neural regions (e.g. lateral hypothalamus, ventromedial nucleus of the hypothalamus, paraventricular nucleus of the hypothalamus) did not result in decreased testis weights at the time of sacrifice. Daily administration of 20 pg MEL inhibited gonadal maturation and resulted in decreased circulating PRL levels only when infused into the SCN region. For animals receiving the 7.5 pg dose, infusions into the midline thalamic nuclei were not successful in inhibiting testis growth, and infusions in the SCN region had only a marginal effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Melatonin regulates the synthesis and secretion of several proteins by pars tuberalis cells of the ovine pituitary

The pars tuberalis (PT) of the pituitary may be an important target for melatonin action, but the secretory output of the melatonin-responsive cells is unknown. Using [(35) S]methionine, protein synthesis and secretion have been studied in primary cultures of ovine PT cells, and analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Only 4% of the labelled proteins appeared in the medium with the majority retained in the cells. Stimulation of the cells with 10μM forskolin increased the accumulation of several labelled proteins in the medium without corresponding changes in the cell (72, 62, 44, 39, 29, 24, 23, 18 and 14 kd). Two-dimensional gel electrophoresis showed the proteins to have mildly acidic isoelectric points. Melatonin (1 μM) counteracted the stimulatory effect of forskolin on all but one (23 kd) of these secreted proteins. Immunoprecipitation showed this to be prolactin. Furthermore, melatonin alone appeared to have an inhibitory effect on the synthesis and release of proteins into the medium. The synthesis and secretion of the melatonin-responsive proteins was not inhibited by actinomycin D (1 μg/ml), indicating control at the translational level. This contrasts with the regulation of prolactin which is actinomycin D-sensitive. Pulse-chase experiments demonstrated that it requires 30 min for the secretory proteins to appear in the medium, consistent with intracellular processing and packaging prior to secretion. The secretory proteins labelled in the ovine PT, and responsive to melatonin, did not appear to be specific to the PT, as a similar profile of labelled secretory proteins was produced in primary cultures of pars distalis cells. However, melatonin had no effect on the synthesis and secretion of proteins by the pars distalis. These results demonstrate that in the ovine PT melatonin regulates the synthesis and export of several secretory proteins. These are possibly packaging proteins of secretory granules, similar to the granin family of proteins. Thus, the results confirm that melatonin-responsive cells are secretory cells and further imply that the PT-specific product is not a protein.

Non-photic phase shifting of the circadian activity rhythm of Syrian hamsters: the relative potency of arousal and melatonin

This study investigated the relative potency of melatonin and arousal as Zeitgebers in the non-photic phase shifting of circadian rhythmicity in the adult Syrian hamster. Animals held under dim red light (DD) exhibited robust free-running rhythms of wheel-running activity. Melatonin (1 mg/kg) or ethanolic saline vehicle, delivered manually by subcutaneous injection after removing the animal from its cage, resulted in phase advances of the activity rhythm. This effect was phase dependent, injections at CT 8 and 10 being effective (CT 12 = anticipated activity onset), whereas injection at CT 2, 6, 14 and 20 did not cause a shift. There was no significant difference between the magnitude or timing of phase shifts in response to injections of saline or melatonin. To determine whether the observed shifts were related to arousal of the animals induced by handling, a second group held under DD were fitted with chronic s.c. cannulae so that melatonin solution or vehicle could be delivered remotely at projected CT 10. Neither solution had any effect upon the free-running rhythm. However, when these animals received manual s.c. injection of saline or melatonin solution, they exhibited phase advances similar to those observed in Expt. 1. These results fail to support the hypothesis that melatonin can exert a chemically specific, acute phase-shifting action in the adult Syrian hamster. They do, however, demonstrate the potent effect of arousing stimuli upon the circadian clock in this species.