| Since Ca2+ influx via voltage-gated Ca2+ channels has important consequences in terms of neuronal function, this dissertation addresses the general question: Why do pyramidal cells express all 6 HVA channel types? In order to do so we used the whole-cell patch clamp technique to record from acutely-dissociated neocortical pyramidal cells determined by projection targets. We used voltage steps and pharmacology to study the distribution of the Ca2+ channel types among three pyramidal cell types. The combination of voltage steps, pharmacology, and RT-PCR was used to characterize the modulation of Ca2+ currents by muscarine. Action potential waveforms were used to study Ca2+ currents in terms of the timing of current entry, the proportions of the Ca2+ channel types contributing to the whole cell current, and the degree to which the currents are modulated and inactivate.; First, we found that Ca2+ channel types are differentially distributed between particular types of pyramidal cells (defined by their projection targets: Chapter 3). Second, we found that activation of acetylcholine-muscarinic receptors led to modulation of HVA currents through two distinct pathways (Chapter 4). Finally, we used action potential waveforms to perform a detailed examination of factors influencing Ca2+ entry during spikes (Chapter 5). In that chapter we discovered that some of our hypotheses concerning the roles of the various Ca2+ subtypes, which were based upon studies using voltage steps were relevant to more physiological conditions, whereas other hypotheses were not (Chapter 5). |
