Changes of the cholinergic input to the superior colliculus following enucleation in neonatal and adult rats

The effects of neonatal and adult enucleation on the adult pattern of choli nergic inputs to the rat superior colliculus (SC) was analysed. In the supe rficial layers immunohistochemical labelling revealed that choline acetyltr anferase (ChAT) was predominantly confined to single boutons which were alm ost continuously distributed throughout the rostrocaudal and lateromedial a xes. In these layers a higher density of boutons was observed in the stratu m zonale (SZ) and lower stratum griseum superficiale (SGS(1)) than in the u pper stratum griseum superficiale (SGS(u)) and stratum opticum (SO). In int ermediate collicular layers ChAT-immunostaining was mainly found in axonal profiles which were arranged in a patchy fashion. Neonatal enucleation caus ed a drastic increase in bouton density in the SZ, SGS(u) and SGS(1). The d ensity of boutons was particularly high in the SGS(u), giving the appearanc e of an almost homogeneous distribution of boutons from the collicular surf ace down to the upper limit of SO. Visual deafferentiation at the adult sta gs was followed by an increase in the bouton density exclusively in the SZ. Neonatal enucleation produced a dorsoventral enlargement of the region con taining patches of ChAT staining which was slightly greater following adult deafferentiation. The results described here show that after visual deaffe rentiation an increase in ChAT innervation to superficial and intermediate collicular layers occurs, providing new information regarding plasticity in the visual system. In view of previous data on cholinergic function in the central nervous system, such an increase could compensate for the loss of retinal excitatory input by facilitating neuronal responses in the SC. (C) 2001 Published by Elsevier Science B.V.