Pharmacological and biophysical characterization of voltage-gated calcium currents in the endopiriform nucleus of the guinea pig

The endopiriform nucleus (EPN) is a well-defined structure that is located deeply in the piriform region at the border with the striatum and is charac terized by dense intrinsic connections and prominent projections to pirifor m and limbic cortices. The EPN has been proposed to promote synchronization of large populations of neurons in the olfactory cortices via the activati on of transient depolarizations possibly mediated by Ca2+ spikes. It is kno wn that principal cells in the EPN express both a low- and high-voltage-act ivated (HVA) Ca2+ currents. We further characterized HVA conductances possi bly related to Ca2+-spike generation in the EPN with a whole cell, patch-cl amp study on neurons acutely dissociated from the EPN of the guinea pig. To study HVA currents in isolation, experiments were performed from a holding potential of -60 mV, using Ba2+ as the permeant ion. Total Ba2+ currents ( I-Ba) evoked by depolarizing square pulses peaked at 0/+10 mV and were comp letely abolished by 200 muM Cd2+. The pharmacology of HVA I(Ba)s was analyz ed by applying saturating concentrations of specific Ca2+-channel blockers. The L-type blocker nifedipine (10 muM; n = 11), the N-type-channel blocker omega -conotoxin GVIA (0.5 muM; n = 24), and the P/Q-type blocker omega -c onotoxin MVIIC (1 muM; n = 16) abolished fractions of total I(Ba)s equal on average to 24.7 +/- 5.4%, 27.1 +/- 3.4%, and 22.2 +/- 2.4%, respectively ( mean +/- SE). The simultaneous application of the three blockers reduced I- Ba by 68.5 +/- 6.6% (n = 10). Nifedipine-sensitive currents and most N- and P/Q-type currents were slowly decaying, the average fractional persistence after 300 ms of steady depolarization being 0.77 +/- 0.02, 0.60 +/- 0.06, and 0.68 +/- 0.04, respectively. The residual, blocker-resistant (R-type) c urrents were consistently faster inactivating, with an average fractional p ersistence after 300 ms of 0.30 +/- 0.08. Fast-decaying R-type currents als o displayed a more negative threshold of activation (by about 10 mV) than n on-R-type HVA currents. These results demonstrate that EPN neurons express multiple pharmacological components of the HVA Ca2+ currents and point to t he existence of an R-type current with specific functional properties inclu ding fast inactivation kinetics and intermediate threshold of activation.