Ontogeny of Tyrosine Hydroxylase Concentration in Locus Coeruleus of Newborn Rats: Long‐Term Effects of RU24722

: The ontogenetic variations of tyrosine hydroxylase (TH) have been studied in locus coeruleus of developing rats. During the first 2 weeks after birth, a large increase in TH content (6.04–23.99 TH units) in the noradrenergic structure was observed, followed by a period of progressive increase of the protein concentration (42 TH units in adult rats). The expression of TH was studied in the same ontogenetic period after treatment by RU24722 (20 mg/kg, i.p.). The long‐term increase in TH concentration produced by the drug was found to follow ontogenetic variations. It becomes significant around the middle of the second week after birth and gradually increases until the 24th day of postnatal development, indicating a maturation of the mechanisms involved in the inducing effect.