Postnatal alterations of GABA receptor profiles in the rat superior colliculus

Midbrain sections taken from Sprague-Dawley rats of varying ages within the first four postnatal weeks were used to determine, immunocytochemically, p utative changes of GABA(A) receptor beta2/3 subunits, GABA(B) receptor (R1a and R1b splice variants), and GABA(C) receptor rho1 subunit expression and distribution in the superficial, visual layers of the superior colliculus. Immunoreactivity for the GABA(A) receptor beta2/3 subunits was found in th e superficial grey layer from birth. The labelling changed with age, with a n overall continuous reduction in the number of cells labelled and a signif icant increase in the labelling intensity distribution (neuropil vs soma). Further analysis revealed an initial increase in the labelling intensity be tween postnatal days 0 and 7 in parallel with an overall reduction of label led neurones. This was followed by a significant decrease in labelling inte nsity distribution between postnatal days 7 and 16, and a subsequent increa se in intensity between postnatal days 16 and 28. The labelling profiles fo r GABA(B) receptors (R1a and R1b splice variants) and GABA(C) receptors (rh o1 subunit) showed similar patterns. Both receptors could be found in the s uperficial layers of the superior colliculus from birth, and the intensity and distribution of labelling remained constant during the first postnatal month. However, the cell body count showed a significant decrease between p ostnatal days 7 and 16. These changes may be related to the time-point of e ye opening, which occurred approximately two weeks after birth. For all thr ee receptor types, the cell body count remained constant after postnatal da y 16. By four weeks of age, there was no significant difference between the cell numbers obtained for the different receptors. Both GABA itself and ne urofilament labelling were also obtained in the superficial superior collic ulus at birth. Neurofilament, although found at birth, showed very little o rdered arrangement until 16 days after birth. When slices were double label led for GABA(C) receptors and neurofilament, some overlap was observed. Dou ble labelling for the presynaptic protein synaptophysin and GABA(C) recepto rs showed proximity in some places, indicative of a partly synaptic locatio n of GABA(C) receptors, When GABA(C) and GABA(A) receptors were labelled si multaneously, some but not all neurones showed immunoreactivity for both re ceptor types. In conclusion, all three GABA receptor types were found to be present in th e superior colliculus from birth, and all show some form of postnatal modif ication, with GABA(A) receptors demonstrating the most dramatic changes. Ho wever, GABA(B) and GABA(C) receptors are modified significantly around the onset of input-specific activity. Together, this points towards a contribut ion of the GABAergic system to processes of postnatal maturation in the sup erficial superior colliculus. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.