Transient tyrosine hydroxylase-like immunoreactive neurons contain somatostatin and substance P in the developing amygdala and bed nucleus of the stria terminalis of the rat.

Tyrosine hydroxylase-like immunoreactive (TH-IR) neurons were observed from the embryonic day 17 (E17) to 6 weeks postnatally in two closely related nuclei of the limbic system, the bed nucleus of the stria terminalis (BNST) and the central nucleus of the amygdala (CNA) where they were restricted to circumscribed zones. These cells were scarce with an immature morphological aspect at E17. They progressively differentiated and increased in number until postnatal day 5 (P5), when their maximal density was reached. They were characterized as neurons by their ultrastructural appearance and the presence of both axo-somatic and axo-dendritic synaptic junctions. Moreover, TH-IR axons could be followed in the stria terminalis leaving the CNA, suggesting that part of TH-IR cells could be long projecting neurons rather than interneurons. A gradual decrease in the intensity of TH-IR and in density of labeled neurons was noted from P15 on, in both nuclei, (-50% at 4 weeks) until their total disappearance at 7 weeks. The significance of this TH-IR labeling regarding the catecholaminergic transmission remains unclear since these neurons did not contain the other catecholaminergic synthetic enzymes (DOPA-decarboxylase, dopamine-beta-hydroxylase, phenylethanolamine-N-methyl transferase) nor endogenous catecholamines. Double-labeling immunocytochemical methods, indicated that almost all the TH-IR neurons were colocalized with somatostatin 28 (SST) and with substance P (SP). Therefore these neurons expressed simultaneously 3 phenotypes, TH, SST and SP. This observation brings forth the notion of multiple neurotransmitter expression in transient neuronal populations and raises the question of neurotransmitter plasticity in the late postnatal development of the central nervous system (CNS). These neurons which were observed in two closely interconnected structures could be involved in early limbic circuits.