Photoperiod effects on bombesin- and cholecystokinin-like immunoreactivity in the suprachiasmatic nuclei of the Djungarian hamster (Phodopus sungorus)

Cholecystokinin (CCK)-expressing neurons in the suprachiasmatic nucleus: innervation, light responsiveness and entrainment in CCK-deficient mice

The suprachiasmatic nucleus (SCN) is the principal pacemaker driving circadian rhythms of physiology and behaviour. Neurons within the SCN express both classical and neuropeptide transmitters which regulate clock functions. Cholecyctokinin (CCK) is a potent neurotransmitter expressed in neurons of the mammalian SCN, but its role in circadian timing is not known. In the present study, CCK was demonstrated in a distinct population of neurons located in the shell region of the SCN and in a few cells in the core region. The CCK neurons did not express vasopressin or vasoactive intestinal peptide. However, CCK-containing processes make synaptic contacts with both groups of neurons and some CCK cell bodies were innervated by VIPergic neurons. The CCK neurons received no direct input from the three major pathways to the SCN, and the CCK neurons were not light-responsive as evaluated by induction of cFOS, and did not express the core clock protein PER1. Accordingly, CCK-deficient mice showed normal entrainment and had similar τ, light-induced phase shift and negative masking behaviour as wild-type animals. In conclusion, CCK signalling seems not to be involved directly in light-induced resetting of the clock or in regulating core clock function. The expression of CCK in a subpopulation of neurons, which do not belonging to either the VIP or AVP cells but which have synaptic contacts to both cell types and reverse innervation of CCK neurons from VIP neurons, suggests that the CCK neurons may act in non-photic regulation within the clock and/or, via CCK projections, mediate clock information to hypothalamic nuclei.

Neurochemistry of identified motoneurons of the tensor tympani muscle in rat middle ear

The objective of the present study was to identify efferent and afferent transmitters of motoneurons of the tensor tympani muscle (MoTTM) to gain more insight into the neuronal regulation of the muscle. To identify MoTTM, we injected the fluorescent neuronal tracer Fluoro-Gold (FG) into the muscle after preparation of the middle ear in adult rats. Upon terminal uptake and retrograde neuronal transport, we observed FG in neurons located lateral and ventrolateral to the motor trigeminal nucleus ipsilateral to the injection site. Immunohistochemical studies of these motoneurons showed that apparently all contained choline acetyltransferase, demonstrating their motoneuronal character. Different portions of these cell bodies were immunoreactive to bombesin (33%), cholecystokinin (37%), endorphin (100%), leu-enkephalin (25%) or neuronal nitric oxide synthase (32%). MoTTM containing calcitonin gene-related peptide, tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were not found. While calcitonin gene-related peptide was not detected in the region under study, nerve fibers immunoreactive to tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were observed in close spatial relationship to MoTTM, suggesting that these neurons are under aminergic and neuropeptidergic influence. Our results demonstrating the neurochemistry of motoneuron input and output of the rat tensor tympany muscle may prove useful also for the general understanding of motoneuron function and regulation.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Encoding le quattro stagioni within the mammalian brain: photoperiodic orchestration through the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) is a pacemaker that not only drives circadian rhythmicity but also directs the circadian organization of photoperiodic (seasonal) timekeeping. Recent evidence using electrophysiological, molecular, and genetic tools now strongly supports this conclusion. Important questions remain regarding the SCN's precise role(s) in the brain's photoperiodic circuits, especially among different species, and the cellular and molecular mechanisms for its photoperiodic "memory." New data suggesting that SCN "clock" genes may also function as "calendar" genes are a first step toward understanding how a photoperiodic clock is built from cycling molecules.

Nycthemeral variations of cholecystokinin action on intestinal motility in rats: effects of melatonin and S 20928, a melatonin receptor antagonist

The aim of the present work was to investigate the impact of the light-dark cycle on CCK intestinal motor effect and to evaluate the consequence of the melatonin treatment and the melatonin receptors blockade on CCK action. The peripheral administration of CCK-8s (5 microg/kg iv) during the fasted state induces an irregular spiking activity corresponding to an excitation of intestinal motility on the duodenum, the jejunum and the ileum. The duration of this excitomotor effect is shorter in the dark phase only on the duodenum (-60%) and the jejunum (-40) compared to the light phase. During the light phase, melatonin (1 mg/kg iv) administered, 10 min prior to CCK-8s, reduces the duration of CCK-8s excitomotor effect only on the duodenum (-42%) and the jejunum (-52%). On the opposite, during the dark phase, the blockade of melatonin receptors by S 20928 (1 mg/kg sc), 60 min prior CCK-8s, restores the excitomotor effect of CCK-8s to its diurnal values. In conclusion, the action of the neurohormone CCK on intestinal motility follows a biological rhythm related to the light-dark cycle. Melatonin, released by the pineal gland at night, seems to be physiologically involved in this modulation.

Regulation of appetite and body weight in seasonal mammals

As models of physiological regulation of body weight, adiposity and appetite, seasonal mammals offer unique opportunities for manipulating fundamental regulatory processes that may not be available in the more frequently-studied laboratory rodents. Seasonal weight and intake cycles are anticipatory rather than reactive in nature, being manifest despite the availability of adlibitum supplies of food. They are exhibited despite all other environmental variables being held constant, and are reversible. Appropriate body weight appears to be a sliding set point in many seasonal mammals, which can move in either direction, largely independently of age. While few data are available other than from rats and mice, there appears to be a strong commonality of central neuroendocrine and peripheral signalling systems between seasonal and non-seasonal mammals, although the conditions under which endogenous regulatory pathways are activated may differ significantly between species. Peripheral and central signalling systems implicated in the regulation of appetite and body weight may be modulated during seasonal transitions. Discussion will concentrate on hypothalamic neuropeptides, gastrointestinal satiety peptides, the recently-described peptide, leptin, that is secreted by adipose tissue, and the interactions between these regulatory components.

Nitric oxide-synthesizing neurons in the hamster suprachiasmatic nucleus: a combined NOS- and NADPH- staining and retinohypothalamic tract tracing study

Neuronal nitric oxide (NO), thought to be a neuroactive substance of high potency, is produced by the enzyme nitric oxide synthase (NOS) which has been demonstrated to additionally exhibit a so-called NADPH-diaphorase (NADPH-d) activity. Since physiological results pointed to the involvement of NO in circadian regulation, and morphological descriptions are not available, we sought to study the distribution of NO-producing cells in the hypothalamic suprachiasmatic nucleus (SCN) in Djungarian hamsters (Phodopus sungorus) by means of histochemistry and immunohistochemistry (IHC). In the SCN, NADPH-d stained perikarya of varying intensity and number were found predominantly in the ventrolateral subdivision. Diaphorase staining combined with the IHC demonstration of NOS revealed a complete overlapping of both. The combination of NADPH-d staining with the demonstration of the retinohypothalamic tract using the anterograde neuronal transport of cholera toxin B (CTB) following intraocular injection showed CTB terminals accumulating at NADPH-d cell bodies mainly in the ventrolateral region of the SCN. These data provide morphological evidence for the involvement of nitric oxide in the mediation of photic stimulation of the circadian oscillator located in the SCN.

Melatonin is involved in cholecystokinin-induced changes of ileal motility in rats

The aim of this study was to determine, in the rat, the interaction between melatonin and cholecystokinin in the regulation of the ileal interdigestive motility. This was analyzed by the chronic electromyography technique. Ileal motility was defined by the presence of intermittent spike bursts corresponding to the contractile activity of the organ. In control rats, these spike bursts were organized in cyclic myoelectrical complexes. Each complex is characterized by two successive spiking activity phases: the irregular phase (ISA) followed by the regular phase (RSA). Pinealectomy suppressed the RSA phase so ileal motility was constituted only by the ISA phase. When melatonin (1 mg/kg i.v.) was injected into pinealectomized rats, RSA phases were immediately and definitively restored. RSA phases were also re-established when the "alimentary" type of cholecystokinin receptors (CCKA) were blocked by selective antagonists such as L364,718 or SR27897 (1 mg/kg i.v.). The latter had better brain accessibility than L364,718. Unlike the effects of melatonin, the effect of these antagonists was neither immediate (the latency is longer for L364,718 than for SR27897) nor definitive. In control rats, cholecystokinin (5 micrograms/kg i.v.) induced a characteristic long-lasting (29 +/- 2 min) excitomotor effect on the ileum. This effect was suppressed in pinealectomized rats and was restored after melatonin treatment. These results suggest that, via the central nervous system, melatonin is involved in the modulation of cholecystokinin action on ileal motility.

Anterograde neuronal tracing of retinohypothalamic projections in the hamster--possible innervation of substance P-containing neurons in the suprachiasmatic nucleus

The retinohypothalamic tract (RHT) in the Djungarian hamster Phodopus sungorus was studied using anterograde neuronal transport of cholera toxin subunit B (CTB) following unilateral intraocular injection. A major projection terminates bilaterally in the hypothalamic suprachiasmatic nuclei (SCN). In the anterior ventral SCN, a light ipsilateral predominance was evident. In the medial SCN, labelling was concentrated laterally where it was seen over the dorso-ventral extension of the nuclei, pronounced contralaterally to the site of CTB injection, which was even more characteristic in the posterior aspects of the nuclei. Labelled fibers and terminals were observed in the supraoptic nuclei, but not in lateral and paraventricular hypothalamic regions. Additional experiments utilizing double immunofluorescence of CTB and of substance P (SP) in the SCN showed that SP-containing perikarya were particularly observed in a central portion of the nucleus, where CTB-stained terminals were accumulated in the vicinity of immunoreactive cell bodies, fibers and terminals. Our data provide preliminary morphological evidence for the regulation of SCN function by retinal afferents and may explain the circadian and photoperiodic fluctuations in the amount of SP in the SCN.

Substance P-like immunoreactivity in the hypothalamic suprachiasmatic nucleus of Phodopus sungorus--relation to daytime, photoperiod, sex and age

The immunohistochemical distribution of substance P (SP) in the hypothalamic suprachiasmatic nucleus (SCN) was studied in adult male and female Djungarian hamsters (Phodopus sungorus) held under either long or short photoperiods. Intact animals were killed by perfusion with a fixative at the middle of the light or dark periods, respectively. The tissue was processed by routine immunohistochemical methods. Perikarya exhibiting SP-like immunoreactivity (LI) were found in the SCN of animals of all groups. These cell bodies predominantly were restricted to a distinct portion of the nucleus extending less than 150 microns rostrocaudally and were often concentrated in its lateral aspect. SP-LI fibers were rarely observed in the SCN, however, other hypothalamic parts, e.g. anterior and paraventricular regions, exhibit strong SP-LI innervation patterns. Sex-related differences were not observed. Long-term exposure to short days decreased the number of neurons exhibiting SP-LI by approximately 60% when compared to long-day animals at both day- and nighttime. At night, SP-LI neurons were augmented in number by 34% (long-day group) and 56% (short-day group). Further, the numbers of SP-LI perikarya in the SCN of aged hamsters at day- and nighttime were augmented 3- to 4-fold when compared to adult animals. These results suggest that substance P in the SCN is involved in the regulation of circadian and seasonal mechanisms in this highly photoperiodic rodent species.

[Work of the inner clock. Neuroanatomy of circadian systems of mammals]

Many aspects of mammalian life exhibit distinct alterations throughout the 24-h cycle. Morphological, physiological, and biochemical parameters display circadian rhythms which are thought to be generated by an endogenous pacemaker and regulated by environmental factors. The morphological substrates of the endogenous circadian system have been studied extensively during the last two decades. Although knowledge is far from complete, there is general agreement that the pathways involved consist mainly of retina, hypothalamus, spinal cord, sympathetic trunk, and pineal gland. This review characterizes the anatomical structures and tracts responsible for generation and maintenance of circadian rhythmicity and discusses functional implications of neurotransmitter involvement and the selectivity of connections.

Efferent projections of the suprachiasmatic nucleus in the golden hamster (Mesocricetus auratus)

The efferent projections of the suprachiasmatic nucleus (SCN) in the golden hamster have been examined by using the anterograde tracer Phaseolus vulgaris leucoagglutinin (Pha-L). SCN projections were further localized through a combination of restricted SCN-lesions and immunocytochemistry for three well-known peptidergic transmitters contained in SCN neurons, viz. vasopressin (VP), vasoactive intestinal peptide (VIP), and gastrin-releasing peptide (GRP). Thus, major terminal fields of SCN-derived VP were detected in the medial preoptic nucleus, the anterior part of the paraventricular nucleus of the thalamus (PVA), the medial parvicellular part of the paraventricular nucleus of the hypothalamus (PVN), and the medial part of the dorsomedial nucleus of the hypothalamus (DMH). VIP-containing projections from the SCN were discovered in the PVA, anterior and dorsal parvicellular divisions of the PVN, subparaventricular area, and medial DMH. Efferent fibers from the SCN containing GRP were restricted to the subparaventricular area, medial DMH, and supraoptic nucleus. In addition, Pha-L tracing indicated the existence of SCN projections which could not be ascribed to one of the presently investigated peptides. Furthermore, a pronounced innervation of the contralateral SCN was observed, of which the neurotransmitter remains to be established. The results of the present study indicate that the different neuronal populations in the SCN, as characterized by their transmitter content, also show a clear diversity in their preferential target areas.

Retinal input to the suprachiasmatic nucleus before and after puberty in Djungarian hamsters

Retinal projections to the suprachiasmatic nucleus (SCN) mediate the effect of photoperiod to entrain circadian rhythms and to control reproductive maturation in the Djungarian hamster. To determine whether the retinal innervation of the SCN had fully developed by the onset of puberty in this hamster species, prepubertal and postpubertal hamsters received an intraocular unilateral injection of horseradish peroxidase (HRP), and after 24 h, the anterograde transport of HRP to the SCN was studied. In prepubertal hamsters, the retinohypothalamic tract (RHT) was found to project to the medial and caudal SCN, principally the ventrolateral regions and, to an extent, the dorsomedial portion of the nucleus. RHT innervation was asymmetric; the SCN contralateral to the monocular injection received the dominant projection. A similar pattern of retinal projections was found postpubertally; however, the ipsilateral SCN was less extensively labelled with HRP and smaller as determined by Nissl counterstain compared to that in prepubertal hamsters. These findings indicate that modifications in the retinal innervation of the SCN occur as late as puberty, and may be part of a developmental change in the mechanism which processes photoperiodic information during sexual maturation.