| We have investigated the effects of acetylcholine (ACh) and the cholinergic agonist carbachol on cells in the developing rat hippocampus. These drugs induce a membrane depolarization and a conductance decrease in mature pyramidal cells. For most cells, there was little difference in the magnitude or pharmacology of the response to applied ACh or carbachol in immature vs. mature pyramidal tissue. The cells were responsive to cholinergic applications on the first day examined (postnatal day 2). However, unlike mature pyramidal cells, a number of cells in immature tissue exhibited decreased input resistance (R{dollar}sb{lcub}rm IN{rcub}{dollar}) during the cholinergic-induced depolarization.; The study revealed other features of immature cells which may have functional significance. Lucifer Yellow injections showed significant dye coupling among CA3 (but not CA1) pyramidal cells in immature tissue, suggesting close metabolic and/or electrostatic coupling between those cells during development. Maturing CA3 cells showed less dye coupling, but increasing anomalous rectification and lengthening time constant.; The pyramidal cell was not the only cell type sensitive to cholinergic stimulation. Two non-pyramidal cell types from animals as young as eight days demonstrated excitatory responses to application of cholinergic agonists. These excitatory responses were mostly blocked by atropine in basket cell interneurons, and were completely blocked in L-M interneurons. In both types of cells, application of cholinergic drug while synaptic transmission was blocked with TTX or low Ca/high Mg medium elicited similar excitatory effects, indicating a direct effect of drug on the post-synaptic cell. It is suggested that concomitant cholinergic effects on neighboring interneurons and pyramidal cells may be responsible for the decreased R{dollar}sb{lcub}rm IN{rcub}{dollar} seen in some immature cells.; Finally, the properties developed by putative cholinergic cells from fetal basal forebrain were examined electrophysiologically. Cells were identified morphologically via intracellular staining with the fluorescent dye Lucifer Yellow. Although demonstration of graft origin is still a difficulty, putative graft cells appeared to exhibit normal morphology and to form synaptic connections, both graft-to-host and host-to-graft. |
