Astrocytes regulate the developmental appearance of GABAergic and glutamatergic postsynaptic currents in cultured embryonic rat spinal neurons

The effects of astrocytes on the emergence of synaptic transients and excit able membrane properties in cultured, embryonic, rat ventral spinal neurons were studied with electrical and optical recording techniques. Neurons on astrocytes had significantly longer neurites and an accelerated rate of gro wth in surface membrane during the initial 24 h in culture compared to neur ons on poly-D-lysine (PDL). GABAergic (GABA, gamma-aminobutyric acid) and g lutamatergic transients appeared spontaneously in co-cultured neurons by 24 h. GABAergic quanta did not appear in neurons on PDL until 4 days in cultu re, and glutamatergic transients did not emerge until 7 days in culture. As trocyte-conditioned medium (ACM) partially mimicked the effects of direct a strocytic contact. GABAergic transients appeared by 2 days, and glutamaterg ic signals by 4 days in neurons on PDL exposed to ACM. All of the spontaneo us, synaptic-like transients were eliminated by tetrodotoxin or Ca-o(2+)-fr ee saline, implicating voltage-dependent cation channels in their generatio n. Astrocytes immediately and significantly increased the density of voltag e-dependent Na+ currents compared to neurons on PDL, but by the end of 24 h , Na+ current densities were identical. Electrophysiological and optical re cording revealed comparable densities of high-voltage-activated (HVA) Ca2currents on both co-cultured neurons and neurons on PDL throughout the firs t week. However, neurons on astrocytes had significantly greater contributi ons of P/Q-type currents and lesser contributions of L-type currents beginn ing at 24 h and continuing for 7 days. The contribution of N-type current w as significantly more in co-cultured neurons only at 24 h. Thus, in vitro, astrocytes help to differentiate specific excitable membrane properties in spinal neurons, along with GABAergic and glutamatergic forms of synaptic tr ansmission.