POSTNATAL-DEVELOPMENT OF LOW [MG2+] OSCILLATIONS IN NEOCORTEX

One form of rhythmic activity intrinsic to neocortex can be induced in slices of adult somatosensory cortex by lowering [Mg2+](o) to unblock N-methyl-D-aspartate (NMDA) receptors. It has been suggested that a p opulation of intrinsically burst-firing (IB) neurons that are unique t o cortical layer 5 may play a role in the rhythmic activity seen under these conditions. Whole cell patch-clamp and field-potential recordin gs in slices of somatosensory cortex from neonatal rats were used to s tudy the development of IB cells and the development of 0 [Mg2+] oscil lations. IB cells were not encountered before postnatal day 12 (P12) i n layer 5, but from P13 to P19 an increasing proportion of cells had I B properties. Recordings from cells at P7, P17, and P19 in 0 [Mg2+] in dicate that dramatic changes occur postnatally in 0 [Mg2+]-induced act ivity. At P7, cells largely showed trains of single action potentials. In contrast, at P19, cells showed organized bursts of rhythmic activi ty lasting 0.5-5 s separated by periods of relative quiescence. Cells recorded at P17 were found to have less organized rhythmic activity th an cells from P19 cortex. Field-potential recordings in 0 [Mg2+] made at P7 showed infrequent and slowly occurring field depolarizations, wh ereas field-potential recordings at P19 consisted of spontaneous burst s of 4-12 Hz oscillations identical to those observed in the adult. Ap plication of NE, which inhibits burst-firing of layer 5 IB cells, sign ificantly altered the discharge pattern of 0 [Mg2+] oscillations at P1 9. These data suggest that the maturation of one type of rhythmic netw ork activity intrinsic to neocortex is influenced by the development o f the membrane properties of a single cell type.