Arachidonic acid modulates Na+ currents by non-metabolic and metabolic pathways in rat cerebellar granule cells

Abstract

AA (arachidonic acid), which possesses both neurotoxic and neurotrophic activities, has been implicated as a messenger in both physiological and pathophysiological processes. In the present study, we investigated the effects of both extracellular and intracellular application of AA on the activity of Na(V) (voltage-gated Na(+) channels) in rat cerebellar GCs (granule cells). The extracellular application of AA inhibited the resultant I(Na) (Na(V) current), wherein the current-voltage curve shifted to a negative voltage direction. Because this effect could be reproduced by treating the GCs with ETYA (eicosa-5,8,11,14-tetraynoic acid) or a membrane-impermeable analogue of AA, AA-CoA (arachidonoyl coenzyme A), we inferred that AA itself exerted the observed modulatory effects on I(Na). In contrast, intracellular AA significantly augmented the elicited I(Na) peak when the same protocol that was used for extracellular AA was followed. The observed I(Na) increase that was induced by intracellular AA was mimicked by the AA cyclo-oxygenase metabolite PGE(2) (prostaglandin E(2)), but not by ETYA. Furthermore, cyclo-oxygenase inhibitors decreased I(Na) and quenched AA-induced channel activation, indicating that the effect of intracellular AA on Na(V) was possibly mediated through AA metabolites. In addition, the PGE2-induced activation of I(Na) was mimicked by cAMP and quenched by a PKA (protein kinase A) inhibitor, a G(s) inhibitor and EP (E-series of prostaglandin) receptor antagonists. The results of the present study suggest that extracellular AA modulates Na(V) channel activity in rat cerebellar GCs without metabolic conversion, whereas intracellular AA augments the I(Na) by PGE(2)-mediated activation of cAMP/PKA pathways. These observations may explain the dual character of AA in neuronal pathogenesis.

PGE2 Modulates GABAA Receptors via an EP1 Receptor-Mediated Signaling Pathway

“…Applying 1 µM PGE 2 increased GABA A currents by 42.6 ± 18.3%, while 10 µM of PGE 2 led to a current increase of only 25.0 ±15.5%. Comparing to our previous report in which 10 µM of PGE 2 applying by perfusion increased voltage-gated Na + current ( I Na ) through EP2 receptor coupled PKA signal pathway [25], it is likely that concentrations of PGE 2 greater than 1 µM might activate EP2 or other types of EP receptors CGNs simultaneously, which offsets the EP1 receptor-driven effect on GABA A currents. Very small changes in PGE 2 concentration cause different effects, implying that it may be a fine tuning mechanism by which neurons adjust their response to PGE 2 through the multiple EP receptors expressed in the same cells.…”

“…In the present of SC19220, PGE 2 only increased pPKC levels by 12.0 ± 7.8% ( n = 5, p >0.05 compared with control). To further verify that only EP1 was responsible for the intracellular effects of PGE 2 , we administered either an EP2 or EP4 receptor antagonist, AH6809 or AH23848, at concentrations that we have previously demonstrated to be effective [25,32]. Either antagonist was unable to eliminate the effects of PGE 2 on pPKC levels (Fig.…”

“…The EP1 receptor activates phospholipase C (PLC), which mediates activation of Ca 2+ /phospholipid dependent protein kinase (PKC) and elevation of cytosolic free calcium [35]. Although our previous study revealed that all the four types of EP receptors are expressed in CGNs [25], pharmacological experiments with both EP receptor-specific antagonists and agonists indicated that PGE 2 -induced increase of GABA A currents is mediated mainly by activation of the EP1 receptor. This is partially consistent with Suzuki et al's report and explain their findings of which the centrally administered PGE 2 exhibited anxiolytic-like activity through EP1 and EP4 receptors and the GABA A receptor [9].…”

“…Very small changes in PGE 2 concentration cause different effects, implying that it may be a fine tuning mechanism by which neurons adjust their response to PGE 2 through the multiple EP receptors expressed in the same cells. Due to the fact that the exogenous PGE 2 was given at concentrations higher than the physiological range in our studies [25], the affinity of EP1-4 to exogenous PGE 2 in CGNs, and the physiological or pathological relevance of our report need further exploration.…”

“…Importantly, all four types of EP receptors are transcribed in this neuronal type [25]. Therefore, the objective of this study was to determine whether PGE 2 was able to modulate GABA A receptor by specific EP receptor.…”