Neurochemical aspects of the ontogenesis of cholinergic neurons in the rat brain.

Ontogenic development of central cholinergic neurons in rat brain was examined by measuring the activity of choline acetyltransferase (CAT), concentration of acetylcholine (ACh) after focused microwave irradiation, the activity of the high affinity uptake process for choline and the apparent muscarinic receptor as quantified by specific binding of [3H]3-quinuclidinyl benzilate (QNB). For whole brain, the specific activity of CAT increases from 1 to 8% of adult between 15 days gestation and 7 days postpartum and then increases linearly to 83% by 4 weeks postpartum. The concentration of ACh is 22% of adult at 15 days gestation, rises to 29% by birth and attains adult levels by 4 weeks postpartum. The developmental rise in specific binding of [3H]QNB is intermediate between CAT and ACh with 10% of adult concentration of receptor at birth and a linear increase to 90% by 4 weeks postpartum. The development of the uptake of [3H]choline parallels that of CAT. In all regions of the neonatal rat brain, the relative level (% adult) of ACh is higher than [3H]QNB binding, which is higher than CAT. The neonatal medulla-pons has higher levels of [3H]QNB binding and activity of CAT (% adult) and develops more rapidly than the parietal cortex and corpus striatum; the hypothalamus and midbrain-thalamus exhibit intermediate rates of development.