showed large decreases in peptide, γ-amino-butyric acid (GABA), cholinergic and dopaminergic gene expression (Fig. 3,Fig. 4 and Table 1). Examples include peptides such as NPY and somatostatin receptors, which decreased 70-80% during brain maturation across receptor subtypes (Table 1). These peptides are often found in inhibitory GABAergic interneurons. Between P38 and P88, GABAA receptor subunit mRNA and the GABA synthetic enzyme, glutamate decarboxylase, decreased 70-80%. Also, acetylcholine receptor mRNA, both muscarinic and nicotinic, and the acetylcholine synthetic enzyme ChAT decreased approximately 60-80% during adolescent brain maturation (Table 1), although acetylcholinesterase (Ache), the acetylcholine inactivating enzyme, maintained a constant level of expression during adolescent brain development (Figure 2A). A reduction in neurotransmitter mRNA expression between adolescence and young adulthood is consistent with maturation of neurocircuitry.
