Fig. 3.
TRPC1 and TRPC4 mediate the plateau potentials underlying mGluR agonist-induced epileptiform burst firing in the dorsolateral septum. A, action potentials were evoked by a brief depolarizing current step (0.8 nA, 20 ms) applied every 10 s. Superfusion of SKF96365 (20 μM) resulted in a complete block of the 1S,3R-ACPD-induced subthreshold and plateau depolarizing responses (n = 3) (red trace). The SKF96365 block was fully reversible upon washout of the drug (blue trace). B, a brief depolarizing current step (0.8 nA, 20 ms) was applied every 10 s to evoke action potentials, and 20 consecutive traces during the wash-in of mGluR agonists were shown. Note the transition from normal firing to subthreshold responses and the full plateaus as the agonist concentration increases during the wash-in. Superfusion of 1S,3R-ACPD (30 μM) in TRPC1/4 double-knockout (DKO) mice (n = 10) failed to induce the subthreshold or plateau depolarizing responses seen in age-matched wild-type controls (i.e., 100% of the dorsolateral septal neurons in TRPC1/4 DKO mice were nonresponding cells). Consistent with the complete absence of mGluR agonist-induced plateau responses, the dorsolateral septal neurons in TRPC1/4 DKO mice failed to exhibit the spontaneous bursting seen in age-matched wild-type control cells after depolarizing the cells during 1S,3R-ACPD superfusion by injecting a constant holding current. The input resistances of the dorsolateral septal neurons in the TRPC1/4 DKO mice were not significantly different from that of wild-type neurons (78.5 ± 4.2 and 68.5 ± 5.2 MΩ, respectively) (p > 0.05, unpaired t test). Scale bars for insets, 20 mV, 500 ms. C, the percentage of cells exhibiting mGluR agonist-induced full plateau potential in wild-type, TRPC1 knockout, and TRPC1/4 DKO mice. D, neurons in the dorsolateral septum were recorded in whole-cell patch-clamp configuration (Vh, −65 mV). The voltage-gated sodium and calcium channels were blocked by tetrodotoxin (1 μM) and Cd2+ (30 μM). The I-V relationship was then determined by a slow voltage ramp (−100 to +40 mV; 2 mV/s). The current induced by 30 μM 1S,3R-ACPD (I-ACPD) or by 30 μM 1S,3R-ACPD and 20 μM SKF96365 (I-ACPD,SKF) was determined by subtracting the control current from the current in the presence of 1S,3R-ACPD or 30 μM 1S,3R-ACPD and 20 μM SKF96365. Note the significant reduction of responses to 1S,3R-ACPD by SKF96365 in wild-type mice and the minimal response to 1S,3R-ACPD in TRPC1/4 DKO mice.