Muscarinic acetylcholine receptors in the normal, developing and regenerating newt retinas

Immunoreactivity for m2 and m4 muscarinic acetylcholine receptors (mAChRs) was demonstrated in the adult newt retina. The m2 mAChR was localized to somata on either side of the inner plexiform layer (IPL), especially ganglion cells, and also distributed into two bands within the IPL. The distal band at a depth of 0-15% IPL co-localized with one of two choline acetyltransferase (ChAT) immunoreactive bands, while the proximal band at 85-100% depth did not overlap with either of the ChAT-ir bands. The m4 mAChR was localized to somata closely apposed to either side of the IPL, probably amacrine cell somata, and no immunoreactivity was detectable throughout the IPL. The time course of appearance of the m2 and m4 mAChRs was examined in both developing and regenerating retinas. Like acetylcholinesterase (AChE), the m2 was first detected in somata located at the most proximal level of the retina well before ChAT-ir cholinergic neurons appeared, while the m4 was detected at the time of appearance of ChAT, in both developing and regenerating retinas. When the outer plexiform layer (OPL) began to form, somata in the horizontal cell layer became transiently immunoreactive to the m2. The discrepancy in distribution of the m2 and ChAT in the IPL suggests that mAChR may play a role other than cholinergic neurotransmission. Furthermore, the similarity in time course of appearance of the m2 and m4, as well as other cholinergic system components [4], in both developing and regenerating retinas would suggest that the mechanisms that control neuronal differentiation during retinal development and regeneration are similar.