Developmental changes in electrophysiological properties of pyramidal neurones correlated with the developmental decline in gap junction-dependent dye coupling were investigated in coronal slices of rat prefrontal and sensorimotor cortex. Effects of intracellular acidification induced by application of weak organic acids on neuronal dye coupling, electrotonic parameters as well as synaptic potentials were examined using the patch clamp technique. Optical monitoring of intracellular pH revealed an acidic shift of 0.4-0.5 pH units following sodium propionate application. 2. Dye coupling between layer II-III neurones was prominent during the first two postnatal weeks. During this period, pre-incubation of slices with 30 mM of the sodium salts of weak organic acids reduced the number of cells coupled to the injected neurones by 64%. 3. Between postnatal days 1 and 18, the mean neuronal input resistance decreased significantly (by 81.0%). Both the membrane time constant (tau 0) and the first equalizing time constant (tau 1) also showed a significant developmental decline of 25.8 and 65.8%, respectively. Electrotonic length decreased by 34.9%. The electrophysiological properties of neurones displayed a pronounced intercellular variability which decreased with on-going development. 4. During the first two postnatal weeks, intracellular acidification led to a mean increase in neuronal input resistance of 55.9% and a mean decreae in electrotonic length of 22.2%. The membrane time constant was reduced by approximately 25% in the majority of neurones tested. Significant electrophysiological effects induced by intracellular acidification were not detected in uncoupled neurones from 18-day-old rats. 5. EPSP width at half-maximal amplitude showed a substantial reduction of approximately 50%, while rise times of the non-NMDA receptor-mediated EPSP components displayed no significant change during development. Both weak organic acids, as well as the gap junction blocker 1-octanol, reduced excitatory synaptic transmission independent of developmental age. 6. We conclude that gap junction permeability is regulated by intracellular pH in developing layer II-III pyramidal cells in the rat neocortex. The prominent correlation between pH-induced reduction in dye coupling and changes in electrophysiological cell properties suggests a significant influence of gap junctions on synaptic integration and information transfer in the immature neocortex.
