Classical acetylcholine receptors do not play a direct role in neuronal transmission of photic information in the suprachiasmatic nucleus in rats

Paradoxical sleep suppresses immediate early gene expression in the rodent suprachiasmatic nuclei

Light stimulates neuronal activity with subsequent expression of the protein product of the immediate early gene, c-fos, in the Suprachiasmatic Nuclei (SCN). Non-photic stimuli is also thought to modulate activity within the SCN. Here, we sought to determine the effects intrinsic stimuli, specifically, the states of sleep upon c-fos protein expression in the SCN. In 16 rats, c-fos protein expression was evaluated at a fixed time of 1600 h (subjective night), following 1 h of electroencephalographically defined sleep. During sleep, as the state of paradoxical sleep (PS) increased, c-fos protein expression decreased (r = −0.41, p < 0.033). The PS-associated reduction of c-fos positive cells occurred equally between animals asleep in the light and those asleep in the dark. We propose a model whereby PS duration might function as a homeostatic-entraining mechanism to reduce neuronal activity within the SCN, and thereby modulate circadian rhythms during sleep.

Modulation of photic resetting in rats by lesions of projections to the suprachiasmatic nuclei expressing p75 neurotrophin receptor

The suprachiasmatic nuclei of the hypothalamus (SCN) are the site of the master circadian clock in mammals. The SCN clock is mainly entrained by the light-dark cycle. Light information is conveyed from the retina to the SCN through direct, retinohypothalamic fibres. The SCN also receive other projections, like cholinergic fibres from basal forebrain. To test whether cholinergic afferents are involved in photic resetting, lesions of cholinergic projections were performed in rats with intracerebroventricular (i.c.v.) injections or intra-SCN microinjections of 192 IgG-saporin. When injected in the SCN, this immunotoxin destroys the cholinergic projections and retinohypothalamic afferents that express p75 low-affinity nerve growth factor (p75(NGF)) receptors. The extent of lesions in the basal forebrain and SCN was assessed by acetylcholinesterase histochemistry, p75(NGF) receptor, choline acetyl-transferase, calbindin-D28K and VIP immunocytochemistry. The intra-SCN treatment reduced light-induced phase advances by 30%, and induced a complete loss of forebrain and retinal afferents expressing p75(NGF) receptors within the SCN and a decrease of forebrain cholinergic neurons, most likely those projecting to the SCN. The i.c.v. treatment reduced light-induced phase advances by 40%, increased phase delays and led to extensive damage of forebrain p75(NGF)-expressing neurons, while sparing half of the fibres expressing p75(NGF) receptors (retinal afferents?) in the SCN. Because the integrity of forebrain p75(NGF)-expressing neurons appears to be critical in mediating the effects on light-induced phase advances, we therefore suggest that anterior cholinergic projections expressing p75(NGF) receptors modulate the sensitivity of the SCN clock to the phase advancing effects of light.

Muscarinic receptors mediate carbachol-induced phase shifts of circadian activity rhythms in Syrian hamsters

Carbachol, a non-specific cholinergic agonist, when administered intraventricularly or directly into the suprachiasmatic nucleus (SCN), causes phase-dependent shifts in circadian rhythms of wheel-running activity in rodents. The cholinergic receptor subtype involved in mediating these carbachol-induced phase shifts, however, remains uncertain. In order to investigate this issue we injected carbachol into the SCN through indwelling cannulas at circadian times (CT) 6, 14 and 22 in Syrian hamsters (Mesocricetus auratus) maintained in constant darkness. Carbachol elicited large phase advances at CT 6 (69.8 +/- 15.7 min; mean +/- S.E.M.) and CT 22 (83.9 +/- 24.8 min) and phase delays at CT 14 (59.7 +/- 18 min). We attempted to block the carbachol-induced phase shifts at these three phases using specific antagonists of nicotinic and muscarinic receptors. Mecamylamine, a nicotinic receptor antagonist, did not block carbachol-induced phase shifts at any of the phases tested. Atropine, a muscarinic receptor antagonist, blocked carbachol-induced phase shifts at CT 6 (-11.6 +/- 4.8 min; Mean phase shift +/- S.E.M.) and CT 22 (-20 +/- 6.6 min), suggesting that carbachol mediates its phase-shifting effects at these phases through muscarinic receptors.

Nicotine administration differentially affects gene expression in the maternal and fetal circadian clock

Exposure to nicotine by active and passive cigarette smoke is a common public health problem. Recent studies have demonstrated that human fetuses are also exposed to significant levels of nicotine and that there is a five-fold increase in the incidence of Sudden Infant Death Syndrome among infants born to smoking mothers. We examined the effect of nicotine administration and expression of the immediate early gene c-fos in the maternal and fetal rat brain by in situ hybridization. Nicotine injection (1 mg/kg s.c.) on embryonic day 20 (E20) induced detectable c-fos mRNA in the maternal habenula and hypothalamic paraventricular nucleus whereas, in the fetal brain, c-fos was induced in both these structures and also in the suprachiasmatic nucleus (SCN). Nicotine-induced c-fos expression in the fetal SCN was confirmed by Northern analysis and found to return to near basal levels by 3 h post-injection. These responses were blocked by pre-administration of mecamylamine, indicating that the effect of nicotine is mediated through the cholinergic system. Investigation of the development of this response revealed that nicotine failed to induce c-fos expression in the SCN on E16, caused minimal expression on E18, robust expression on E20 and postnatal day 0 (P0), and no expression on P2 or thereafter. These observations suggest that an alteration in the composition of the nicotinic receptors (nAChR), or the subsequent intracellular responses leading to c-fos expression, occurs in the SCN during the perinatal period. Induction of c-fos mRNA in the SCN by light has been associated with phase-shifts of the circadian system, however, the behavioral consequences of the transient sensitivity of the fetal and neonatal SCN to nicotine administration and the consequences for maternal-fetal entrainment remain to be directly determined.

Immunocytochemical localization of the alpha 7 subunit of the nicotinic acetylcholine receptor in the rat central nervous system

Previous molecular cloning studies have revealed that alpha-bungarotoxin binding proteins present in the brain are members of the neuronal nicotinic acetylcholine receptor gene family. The alpha 7 subunit is structurally related to the agonist binding subunits present in the central and peripheral nervous systems and, when expressed in Xenopus oocytes, forms functional channels blockable by alpha-bungarotoxin. In the present study, three different monoclonal antibodies raised against the alpha 7 subunit were used to map its distribution throughout the central nervous system of the rat. Immunohistochemical localization revealed that the alpha 7 subunit is expressed in most regions of the brain, being, overall, well correlated with previous "in situ" localization of alpha 7 transcripts and alpha-bungarotoxin autoradiographic binding studies. Particularly strong immunoreactivity was observed in several sensory and motor nuclei of the brainstem as well as the red nucleus. At the cellular level, alpha 7 immunostaining was usually found both in somata and dendrites, whereas axonal and terminal labeling was not observed. The widespread distribution of the alpha 7 subunit polypeptide is consistent with immunoprecipitation data demonstrating that it is a component of the predominant subtype of brain alpha-bungarotoxin-sensitive nicotinic receptors.

Circadian variations of amino acid content of suprachiasmatic nucleus in rats

The suprachiasmatic nucleus (SCN) has been known as an oscillator of circadian rhythm and recently some reports suggested that excitatory amino acids were functioning in this nucleus. The purpose of this study is to measure the physiological amino acid content of this nucleus, to estimate the circadian variation in amino acids and the effect of light on amino acid content in the SCN. The content of aspartate, asparagine, glycine and serine tended to be lower in the light phase compared to the dark, but was not significant. As to the light effect on amino acid content of the SCN, only aspartate decreased when a light pulse was applied. These results suggest that aspartate is involved in the transmission of photic information within the SCN.