Postnatal development of GABA-immunoreactive neurons and terminals in rat periaqueductal gray matter: a light and electron microscopic study.

The development of intrinsic gamma-aminobutyric acid (GABA)-ergic neurons was studied in the first month of postnatal life in the rat periaqueductal gray matter (PAG) by light and electron microscopy using an anti-GABA serum. At birth (postnatal day 0: P0) GABA-immunopositive (GABA(IP)) neurons were detected only on the outer edge of dorsolateral PAG (PAG-DL) and were rare in the other PAG subdivisions. Their distribution did not change from P0 to P5, while they increased progressively from P5 to P10 in PAG-DL and began to be detected in ventrolateral PAG (PAG-VL). At the end of the second postnatal week the immunostaining pattern was nearly adult-like, and between P20 and P30 the adult pattern of GABA immunoreactivity was established. Quantitative light microscopic examination indicated that in the first postnatal month the cross-sectional area of GABA(IP) neurons gradually increased from 67.63 and 78.69 microm(2) at P0 to 122.15 and 119.16 microm(2) at P30 in PAG-DL and PAG-VL, respectively. Electron microscopic observations disclosed GABA labeling from P0 in cell bodies, dendrites, growth cones, and axon terminals. GABA(IP) terminals were few in neonatal rats and became more numerous and morphologically mature around the second week. Synapse development and maturation were examined by quantitative ultrastructural analysis. Synaptic vesicle number and size of GABA(IP) axon terminals progressively grew in the first postnatal month. In conclusion, the number and size of GABA(IP) cells progressively increase in postnatal PAG, with two populations of intrinsic neurons expressing their GABAergic nature in two different periods.