Glycine- and GABA-immunoreactive nerve terminals apposed to alpha-motoneurons during postnatal development in kittens: a quantitative study using thedisector

The distribution of gamma-aminobutyric acid (GABA) and/or glycine-immunorea ctive (IR) terminallike structures apposed to somatic and dendritic membran e of lumbar a-motoneurons in column 2 was examined in 1- and 3-week-old kit tens and in the adult cat. This quantitative study was carried out using a postembedding technique on semithin sections and a stereological method, th e disector. Analysis of immunoreactive terminals showed that the percentage s of GABA-IR and glycine-IR terminals (these populations include terminals containing both GABA and glycine) apposed to somatic and proximal dendritic compartments of alpha-motoneurons are almost the same in kittens, while in the adult glycinergic innervation becomes predominant. This change results from: (1) the decrease in numbers of GABA-IR terminals contacting the soma tic compartment between 3 weeks and adult stage, while the numbers of glyci ne-IR terminals show no significant changes after birth and the numbers of terminals containing both neurotransmitters (GABA-IR+glycine-IR) present tr ansient changes and (2) the postnatal increase in the dendritic compartment , in numbers of GABA-IR, glycine-IR and GABA-IR+glycine-IR terminals; the i ncrease being larger for glycine-IR terminals. Furthermore, using EI postem bedding immunogold technique, observations by electron microscopy showed th at GABA-IR P boutons apposed to M boutons can already be identified at 1 we ek after birth.