Development of laminar expression of the m2 muscarinic cholinergic receptor gene in rat visual cortex and the effect of monocular visual deprivation.

The postnatal development of laminar pattern of the m2 muscarinic acetylcholine receptor subtype mRNA in the visual cortex of both normally raised and monocularly deprived rats (one eyelid sutured at the age of 11 days) was studied using in situ hybridization histochemistry and computer-assisted image analysis. In normally raised rats, on birth, the m2 transcript was found to be more concentrated in the superficial zones of the cortex. This laminar pattern alters to a more homogeneous distribution of the label throughout the cortex already detectable on day 7. From day 10 onwards a bimodal laminar pattern gradually develops with increased mRNA levels in layer IV and upper layer VI. From postnatal day 21 onwards the hybridization peak in layer VI decreases as compared to the peak level in layer IV resulting in an adult distribution with highest labeling in layer IV, low labeling in layer I to III and moderate labeling in layers V and VI. Monocular deprivation results in decreased m2 mRNA levels in visual cortical layers IV-VI in both deprived and non-deprived cortices already detectable at the age of 18 days and persisting up to the age of 21 days; but this effect disappears following further deprivation until adulthood. The data suggest that the changes in m2 receptor level from a more homogeneous distribution to a bimodal pattern during postnatal development seem to be related to synaptogenesis and final tuning of connectional pattern within the rat visual cortex.