Signal transduction mechanisms in cultured CNS neurons and clonal pituitary cells

The experimental accessibility of monolayer culture has been used to study signal transduction mechanisms in primary CNS neurons and clonal pituitary cells. Here we review results on two signals representative of the emerging diversity of mechanisms discovered in all species studied thus far. One is mediated by micromolar concentrations of the amino acid GABA at postsynaptic membranes throughout the mammalian CNS and involves transient activation of Cl- ion channels whose distribution of conducting periods accounts for the millisecond time course of the signal. This signal serves to depress the probability that the target cell will trigger an action potential. The signal intensifies as the postsynaptic membrane is depolarized and can be modulated by clinically important drugs, primarily through changes in channel kinetics. The other signal involves nanomolar concentrations of the peptide TRH, which stimulates secretion of prolactin from clonal "GH3" pituitary cells. Intracellular recordings of GH3B6 cells show that TRH triggers a complex electrical response lasting several minutes. The response consists of Ca2+-activated K+ conductance followed by Ca2+-action potential activity. Whole-cell patch recordings, which rapidly dialyze the cell, can eliminate the TRH-induced changes in membrane excitability. Inclusion of aqueous lysates of the GH3B6 clone or the soluble second messenger factors inositol trisphosphate (IP3) or protein kinase (PKC) can restore various aspects of the change in membrane excitability. Thus, TRH alters ion conductance mechanisms through a second messenger cascade likely to involve IP3-mediated mobilization of Ca2+ from the endoplasmic reticulum and transient translocation of PKC from cytoplasm to plasma membrane. These synaptic and extrasynaptic signals reflect some of the diversity of transduction mechanisms involved in intercellular communication.