Abstracts

h-channels (hyperpolarization-activated cation channels; [italic]I[/italic][sub]h[/sub]; HCN) have been hypothesized to play a role in epileptogenesis due to recent evidence that they are downregulated by seizures, are upregulated by several antiepileptic drugs, and produce epilepsy when genetically deleted in an animal model. Little is known about possible intracellular modulators of h-channel function apart from the well-described role of cAMP. Previously we found that inhibition of p38 mitogen-activated protein kinase (MAPK) downregulated [italic]I[/italic][sub]h[/sub] by causing a hyperpolarizing shift in voltage-dependent activation. We sought to characterize the role of p38 MAPK in the actions of a known upregulator of [italic]I[/italic][sub]h[/sub], the anti-epileptic drug (AED) lamotrigine (LTG)., We tested the effects of p38 MAPK activation using cell-attached and whole-cell recordings of [italic]I[/italic][sub]h[/sub] in pyramidal-like principal (PLP) neurons in the CA1 layer of the hippocampus in rat brain slices. PLPs contain a high somatic density of [italic]I[/italic][sub]h[/sub] with biophysical properties similar to [italic]I[/italic][sub]h[/sub] in CA1 pyramidal neuron dendrites. Drugs were applied through the pipette and for at least 45 min via bath application before recording., Application of an activator of p38 MAPK and c-Jun NH2 terminal kinase (JNK), anisomycin (20 [micro]M), produced an [sim]11 mV depolarizing shift in [italic]I[/italic][sub]h[/sub] voltage-dependent activation (V[sub]1/2[/sub] in control = -89 [plusmn] 2.4 mV [mean [plusmn] SEM], n=11; V[sub]1/2[/sub] in anisomycin = -78 [plusmn] 2.0 mV, n=7, p[lt].001) while a specific inhibitor of JNK, SP600125 (10 [micro]M), caused no change in voltage-dependent activation (V[sub]1/2[/sub] = -87 [plusmn] 3.6 mV, n=4, p[gt].05). Thus, the effect of anisomycin is due to the activation of p38 MAPK. In whole cell experiments, pipette-applied activated p38[alpha] MAPK (5 [micro]g/ml) led to a decrease in input resistance (IR) 15 min after the start of recording (p38[alpha] = -36.7 [plusmn] 7.7%, n=4; in control = -13.9 [plusmn] 3.7%, n=6, p[lt].05). Perfusion of p38[alpha] MAPK decreased temporal summation (TS) elicited by a train of 20 Hz alpha-function current injections after 15 min pipette application (-23.6 [plusmn] 5.6%, n=4; in control = -8.6 [plusmn] 3.1%, n=6, p[lt].05). We then tested whether p38 MAPK is necessary for the action of LTG. A specific p38 MAPK inhibitor, SB203580 (SB, 10 [micro]M), produced an [sim]22mV hyperpolarizing shift in voltage-dependent activation (V[sub]1/2[/sub] in SB = -111 [plusmn]3.6 mV, n=6, p[lt].0001). When SB was applied with LTG (100 [micro]M), the depolarizing effect of LTG was blocked by the hyperpolarizing effect of SB (V[sub]1/2[/sub] in LTG = -83.6 [plusmn] 1.6 mV, n=5; V[sub]1/2[/sub] in SB/LTG = -104.2 [plusmn]5.6 mV, n=5, p[lt].05)., These results show that p38 MAPK plays an important role in the regulation of [italic]I[/italic][sub]h[/sub]. Activation of p38 MAPK activity produced a significant upregulation of [italic]I[/italic][sub]h[/sub] in hippocampal PLPs which decreased the excitability of PLPs by diminishing IR and TS. These results also suggest that p38 MAPK is necessary for LTG action on [italic]I[/italic][sub]h[/sub]. The effects of p38 MAPK on [italic]I[/italic][sub]h[/sub] imply that the p38 MAPK pathway is a promising target for novel AEDs., (Supported by NIH Grant NS050229, GlaxoSmithKline.)