Daily variation of muscarinic receptors in visual cortex but not suprachiasmatic nucleus of Syrian hamsters

Intraventricular administration of carbachol can induce phase shifts in wheel-running activity in rodents, which depend on circadian phase and are mediated via muscarinic cholinergic receptors in Syrian hamsters. We studied the circadian variation in binding of ³H]-N-methylscopolamine ([³H]NMS), a hydrophilic muscarinic receptor antagonist, in micropunches obtained from the anterior hypothalamus and occipital cortex of Syrian hamsters housed in a 14:10 light:dark cycle. Binding sites were characterized on cells contained within 1 mm punches (obtained from slices 300 μm thick), using a method to selectively detect cell surface (functional) receptors. Atropine sulphate was used to determine nonspecific binding. Cortex showed a significant daily rhythm in [³H]NMS binding with a peak occurring late in the light phase and a trough at lights on, while the hypothalamus showed no detectable rhythm. Following suprachiasmatic nucleus (SCN) ablation or maintenance in constant darkness, the rhythm in the cortex was abolished. These findings suggest that photic information conveyed via the SCN is responsible for the receptor binding rhythm in the cortex. Autoradiographic studies ([³H]NMS; 2 nM, 3 weeks exposure) clearly revealed both M1 and M2 subtypes of muscarinic receptors in the region of the SCN and the visual cortex.