Actions of baclofen on components of the Ca‐current in rat and mouse DRG neurones in culture

Ca currents in rat and mouse sensory dorsal root ganglion (DRG) neurones were inhibited by concentrations of (-)-baclofen as low as 1 micron. The proportion of neurones responding to baclofen was low (less than 20%), except in young cultures of neonate rat DRG neurones (3 days in culture), where 86% of the neurones were responsive. 2. Three types of unitary Ca currents were observed in the rat DRG neurones, corresponding to the T-, N- and L-type currents of chick DRG neurones. 3. Baclofen produced two types of response on whole-cell currents of DRG neurones from both species. The first was on an early inactivating component of the Ca current. This early current was partially inactivated at a holding potential of -40 mV. It was also reduced during the second of a pair of depolarizing command pulses. The results suggest that this action of baclofen is due to an action on an N-type component of the current. The second response to baclofen persisted throughout the command step and was not reduced during the second of a pair of command pulses, indicating that this effect is due to an action on the L-type current. 4. Unitary or ensemble Ca currents recorded in cell-attached patches, on neurones previously shown to respond to baclofen in whole-cell clamp mode, were generally not inhibited by baclofen application external to the patch electrode. This indicates that a readily diffusible internal second messenger substance is probably not involved in coupling the GABAB receptor to the ion channels.