Postnatal sequential development of dopaminergic and enkephalinergic perineuronal formations in the lateral septal nucleus of the rat correlated with local neuronal maturation.

Tyrosine-hydroxylase (TH-IR) and methionine-enkephalin like immunoreactivity (MetE-IR) were analyzed in the lateral septal nucleus (LSN) of the rat from birth (PO) to adulthood. TH-IR labeled specifically the dopaminergic (DA) pericellular arrangements of the LSN, as checked by negative dopamine-beta-hydroxylase and phenylethanolamine-N-methyl transferase-IR. TH-IR and Met-IR processes were present at birth in the medial LSN and extended lateralwards and caudalwards from P0 to P6 to constitute two main DA terminal fields (medial and lateral) surrounding a MetE one. Within these fields, the development of perineuronal baskets followed a similar medial to lateral sequence: DA axons first surrounded a few neuronal cell bodies at P3 in the medial part of the intermediate LSN; at P6, Met-IR axons encircled more laterally located perikarya, and only at P9, some neurons located along the ventricle in the lateral DA field became surrounded. The initial aspect of TH-IR baskets consisting of few axons surrounding the cell body rapidly evolved in a positive network encapsulating the perikaryon and long segments of the proximal dendrites, whereas MetE-IR varicosities remained restricted around the perikaryon and the initial dendritic segments. Ultrastructural study at P14 revealed numerous TH-IR and MetE-IR axosomatic and axodendritic profiles. TH-IR axosomatic varicosities exhibited asymmetrical synapses, whereas MetE-IR ones displayed rare symmetrical contacts. The medio-lateral gradient of development of the perineuronal baskets was parallel to the postnatal neuronal development of the LSN as evaluated by cytological criteria: neuronal density, cell size and Nissl staining. Therefore, the formation of DA and MetE perineuronal arrangements in the LSN does not seem to be subordinate to the nature of the neurotransmitter they contain but related to the level of differentiation of their target neurons. A similar sequential set-up in the development of afferences paralleling the neuronal differentiation is discussed.