[125I]Vasoactive intestinal peptide binding in rodent suprachiasmatic nucleus: developmental and circadian studies

Temporal profiles of vasoactive intestinal polypeptide precursor mRNA and its receptor mRNA in the rat suprachiasmatic nucleus

Many vasoactive intestinal polypeptide (VIP) neurons are distributed in the suprachiasmatic nucleus (SCN) and most of them arborize throughout the nuclei. We determined temporal patterns of VIP receptor (VPAC2 receptor) mRNA in the rat SCN in constant darkness and light-dark cycles, by means of in situ hybridization histochemistry. Amounts of VIP precursor mRNA in the SCN were also quantified in adjacent sections to compare its temporal profile with that of VPAC2 receptor mRNA. In constant darkness no circadian rhythms in VIP precursor mRNA or VPAC2 receptor mRNA in the SCN were detected. In light-dark cycles VIP precursor mRNA and VPAC2 receptor mRNA in the SCN showed robust but different daily rhythms. VPAC2 receptor mRNA decreased over the course of the light period and then gradually recovered during the dark period. VIP precursor mRNA was also reduced in response to light but the reduction was transient. This difference in the nature of light responsiveness between VIP mRNA and VPAC2 receptor mRNA suggests that pre- and postsynaptic events of the VIP transduction system represent different stages of light information processing in the SCN.

An immunotoxin, anti-VIP antibody-ricin A chain conjugate eliminates neurons in the hypothalamic suprachiasmatic nucleus selectively and abolishes the circadian rhythm of water intake

In mammals, a master circadian oscillator is known to be located in the suprachiasmatic nucleus (SCN) of the hypothalamus. We examined the function of SCN neurons involved in the mechanism of circadian rhythm of water intake by lesioning them with an immunotoxin, anti-vasoactive intestinal polypeptide (VIP) antibody-ricin A conjugate. We found that the immunotoxin had a specific lethal effect on cultured PC12h cells when VIP was added to the medium. When the conjugate was infused into the third cerebral ventricle of rats above the SCN, two specific types of selective lesions of neurons were observed in the SCN: selective lesions of neurons containing arginine vasopressin (AVP) (AVP-neurons), and selective lesions of neurons containing VIP (VIP neurons). The former lesions caused disappearance of the circadian rhythm of drinking behavior, whereas the latter lesions did not affect the rhythm of water intake under constant dim lighting. Lesions that did not selectively affect one of these neurochemically identified SCN cell populations were also observed after the infusion of the conjugate or normal rabbit serum immunoglobulin G-ricin A chain conjugate. If these nonspecific lesions included entire region of the SCN, the circadian rhythm of water intake was abolished. These findings suggest that SCN neurons bearing VIP receptors such as AVP neurons, but not VIP neurons, may be involved in the mechanism of the circadian rhythm of water intake.

The distribution of vasoactive intestinal peptide2 receptor messenger RNA in the rat brain and pituitary gland as assessed by in situ hybridization

The distribution of rat vasoactive intestinal peptide2 (VIP2) receptor messenger RNA in the brain and the pituitary gland was examined by in situ hybridization and by ribonuclease protection assay. labelled cells were found chiefly in the suprachiasmatic nucleus, the central nucleus of the amygdala and the thalamus (the lateral geniculate nucleus, and the paraventricular, mediodorsal and ventral nuclei of the thalamus). The distribution of the VIP2 receptor overlaps only in part with that of the VIP1 receptor, for example in the hippocampus, where VIP2 receptor messenger RNA was found in the pyramidal cells of the CA1-CA3 subfields and in the granule cells of the dentate gyrus. Small numbers of neurons containing high concentrations of VIP2 receptor messenger RNA were present in the brainstem in the principal sensory trigeminal nucleus and in the substantia gelatinosa of the spinal cord, suggesting a role for the VIP2 receptor in the processing of sensory information. The presence of the VIP2 receptor in the suprachiasmatic nucleus suggests that it is this receptor subtype which is involved in the control of circadian rhythms.

Gene expression in suprachiasmatic nucleus and circadian rhythms

The suprachiasmatic nuclei (SCN) contain a circadian system consisting of circadian oscillator (clock) that is normally synchronized by the light/dark cycle (input) and drives circadian rhythms (output) that are intrinsic to the SCN. Gene expression of immediate-early genes, such as c-fos and jun-B, in the ventrolateral SCN is associated with circadian synchronization by light pulses and subjected to circadian control. Vasopressin and somatostatin gene expression shown distinct circadian rhythms intrinsic to the dorsomedial SCN with higher peptide levels occurring during the day. In addition, embryonic SCN grafted into the brain of an SCN-lesioned arrhythmic host define the period of the restored circadian locomotor rhythm. Taken together, these and other findings support the notion that the expression of genes underlying circadian synchronization, oscillation and output takes place within individual SCN neurons. However, no information regarding the nature and number of those neurons as well as the molecular mechanisms of the single cell-circadian oscillator and output is currently available. Therefore, we propose a simple two-neuron model as a framework for critically discussing the molecular genetic strategies to analyze the circadian system in SCN.

Neurochemical organization of circadian rhythm in the suprachiasmatic nucleus

The circadian rhythm in mammals is under control of the pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. This tiny nucleus contains a number of neurochemicals, including peptides, amines and amino acids. Heterogeneous distribution of these neurochemicals defines the substructures of the SCN. In the present review, functional significance of such neurochemical heterogeneity in the SCN is discussed in the light of circadian patterns of the concentrations of these neurochemicals in the SCN and their effects on SCN neurons in in vitro slice preparation. In particular, the hypothesis that the dorsomedial SCN is involved in maintaining the circadian rhythm, while the ventrolateral SCN is involved in adjusting the phase of the rhythm, is critically discussed. These considerations suggest that distinct sub-components of the SCN as marked by neurochemicals, interact with each other and this organizational architecture could be the basis of the proper operation of the circadian time keeping system in this nucleus.

Development of vasoactive intestinal polypeptide neurons in the human suprachiasmatic nucleus in relation to birth and sex

The development of vasoactive intestinal polypeptide (VIP) neurons was determined in the human suprachiasmatic nucleus (SCN) of 43 subjects ranging from 27 weeks of gestation to 30 years of age using immunocytochemistry and morphometry. VIP neurons were first observed at 31 weeks of gestation in the ventrolateral part of the SCN. From 3 months postnatally onwards, VIP positive neurons were observed in some subjects in the centromedial part of the SCN. The centromedial type of VIP staining became a constant finding only at 19 years of age, at term the SCN was still very immature. Only in a few subjects some VIP neurons stained in the ventrolateral SCN and their number and nuclear diameter was small. Postnatally the number of VIP neurons increased gradually until around 3 years of age adult values were reached. After the age of 10 a clear sex difference in the number of VIP neurons was found: males having on average twice as many VIP neurons in the SCN as females. The adult VIP cell numbers in the SCN amounted only 35% of those found earlier for vasopressin. The present data do not support a particular role for VIP neurons in those rhythms that are already present in early development, e.g., of the temperature rhythm in prematures of around 30 weeks gestational age. Our observations in this and earlier papers as well as animal studies do suggest though a possible role for VIP neurons in the SCN in sexual dimorphic functions such as reproduction and sexual behavior.

NMDA receptor binding in rodent suprachiasmatic nucleus

NMDA receptors are thought to mediate effects of light on circadian rhythms and on immediate-early gene expression in the suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals. The present study characterized NMDA receptors in autoradiographs of SCN incubated with the NMDA antagonist [3H]MK-801. In both rat and hamster, [3H]MK-801 binding did not delineate the SCN and was fairly uniformly distributed across the SCN region. Binding levels were unaffected by circadian time, light vs. dark conditions, or enucleation. Scatchard analyses revealed species differences in both receptor number and affinity in the SCN. The [3H]MK-801 binding sites characterized in this study could mediate the NMDA antagonist-sensitive effects of light on the SCN and circadian rhythms.

VIP binding sites in adult rat hypothalamus: nuclear distribution and daily variations

Vasoactive intestinal peptide (VIP) neurons are present in the suprachiasmatic nucleus (SCN) and project to various hypothalamic nuclei. Several results suggest that hypothalamic VIP is involved in the mediation of synchronizing information. [125I]Vasoactive intestinal peptide binding has been reported to be especially high in the hypothalamus. We show here by radioautography by dipping that [125I]VIP binding is high and homogeneously distributed in the SCN. The correspondence between VIP binding and terminals densities supports the hypothesis that VIP acts through synapses in the SCN. Moderate and fairly homogeneous VIP binding is also found in the paraventricular nucleus (PaVN), whereas VIP terminals are reported to be restricted to the sub-paraventricular area. This suggests that VIP could diffuse and thus function as a paracrine neuromodulator in the PaVN. [125I]Vasoactive intestinal peptide binding is stable along the nycthemer in the suprachiasmatic, peri-, and paraventricular nuclei as well as in all other areas investigated.