Pre- and postnatal development of GABA receptors in Macaca monkey visual cortex

Abstract

GABA is a putative inhibitory neurotransmitter in adult mammalian visual cortex but also has been implicated as playing a crucial role in cortical information processing during development. In order to understand better the role of GABA during primate visual cortex development, we have examined the time course of GABA(A) and GABA(B) receptor ontogenesis in 18 Macaca nemestrina monkeys ranging from fetal day 61 (F61d) to adulthood. The GABA and benzodiazepine binding sites of the GABA(A) receptor were detected by 3H-muscimol (3H-MS) and 3H-flunitrazepam (3H-FZ), respectively. GABA(B) receptors were detected by 3H-baclofen (3H-BA). All ligands were visualized by in vitro autoradiography. Quantitative analysis of film density was done to compare laminar changes during pre- and postnatal development. Saturation binding experiments were done for MS and FZ binding sites to determine receptor number (B(max)) and affinity (K(d)) at selected pre- and postnatal ages. Both MS and FZ binding sites were present at F61d-72d throughout the cortical plate and marginal zone. FZ binding sites were more dense than MS binding sites over the cortical plate at young ages and were especially dense over the marginal zone. FZ binding sites also were present in lesser amounts over the subplate and intermediate zone, but not over the subventricular zone. By F119d-126d, layer 4 could be distinguished by its higher density for both ligands. The basic adult laminar pattern was established for both MS and BZ binding sites by birth (birth = F165d-170d). After birth, MS density increases dramatically in all layers, but layer 4C remains most dense to adulthood. FZ labeling is heavy in both layers 4 and 3 at birth but after 4 weeks after birth (P4 wk) it declines somewhat in the supragranular layers so that layer 4C now predominates. Labeling in layers 5/6 virtually disappears after birth. BA binding sites were present at F126d, at which time layer 4 was slightly lighter than the remainder of striate cortex; this laminar pattern remained basically the same throughout our series to adulthood. Competitive binding of agonist and antagonists for the GABA(A) receptor showed that MS binding characteristics were similar at F126d and P8.5 years (yr). MS binding site B(max) was about 8% of adult values at F72d, 24% by F126d, and 56% at F152d. B(max) then rose rapidly after birth to peak at P18wk at 169% of adult values, and then declined to P1yr. A second peak of 143% was found around P3.5yr, with adult values reached by P8.5yr. K(d) values for MS at F126d showed evidence for two binding sites (34 nM and 4 nM) that existed up to P1d, and only one site (26-65 nM) after this age. The B(max) for FZ was 27% of adult values at F126d, peaked at 113% adult value at P18wk, and declined slowly to adult levels by P8.5yr. K(d) values for FZ found one site at all ages. Binding for FZ is proportionately higher in early development with a ratio of 1:2-3 FZ:MS, but in the postnatal cortex MS binding dominates with a ratio of 1:4-6 FZ:MS. The present study demonstrates the early prenatal occurrence in monkey visual cortex of both the MS and FZ binding sites of the GABA(A) receptor and also of the GABA(B) receptor. The GABA(A) receptor first appears near the age when GABAergic neurons can be detected immunocytochemically (Hendrickson et al., 1988; Meinicke and Rakic, 1989), but before synapses are found in significant numbers (Zielinski and Hendrickson, 1990). Although GABA(A) receptors are normally associated with inhibitory synaptic circuits, these data suggest that the initial expression of GABA receptors is not dependent on synaptic contact and that GABA(A) and its receptors may play some neurotrophic role in early cortical development. Postnatal changes in receptor distribution and number are more closely correlated with synaptic density changes.