POTASSIUM CURRENTS OPERATED BY THYROTROPIN-RELEASING-HORMONE IN DISSOCIATED CA1 PYRAMIDAL NEURONS OF RAT HIPPOCAMPUS

1. Membrane currents activated by thyrotrophin-releasing hormone (TRH) were investigated in the dissociated rat hippocampal CA1 pyramidal neurone using the nystatin perforated patch recording configuration. 2. Under current-clamp condition, TRH caused a transient hyperpolarization accompanied by a decrease of firing activity and a successive long-lasting depolarization. The latter induced a blockade of firing. 3. When neurones were held at a holding potential (VH) of -40 mV under voltage clamp, TRH elicited a transient outward current with an increase in the membrane conductance, which was followed by a sustained inward current with a decrease in membrane conductance. The inactive TRH metabolite, TRH free acid, did not induce any currents. 4. The reversal potential of TRH-induced outward current (E(TRH)) was close to the K+ equilibrium potential (E(K)). The change in E(TRH) for a 10-fold change in extracellular K+ concentration was 56.4 mV, indicating that the membrane behaves like a K+ electrode in the presence of TRH. On the other hand, the TRH-induced inward current was due to suppression of a slow inward current relaxation during hyperpolarizing voltage commands to -50 mV from a V(H) of -40 mV, indicating the suppression of the voltage- and time-dependent component of the K+ current (M-current). 5. The TRH-induced outward current (I(TRH)) increased in a concentration-dependent manner over the concentration range 10(-8)-10(-6) m. The half-maximum concentration was 7.4 x 10(-8) m and the Hill coefficient was 1.5. 6. The TRH-induced outward current (I(TRH)) was antagonized by K+ channel blockers such as tetraethylammonium (TEA), 4-aminopyridine (4-AP) and Ba2+ in a concentration-dependent manner. I(TRH) was insensitive to both apamin an iberiotoxin. 7. The first application of TRH to neurones perfused with Ca2+-free external solution containing 2 mm EGTA could induce I(TRH) but the TRH response diminished dramatically with successive applications. Intracellular perfusion with a Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), also diminished the TRH response. 8. The depletion of Ca2+ from the intracellular Ca2+ store by thapsigargin blocked the TRH response without affecting the caffeine response. Pretreatment with Li+ significantly enhanced I(TRH), suggesting that I(TRH) is involved in the elevation of intracellular free Ca2+ released from the inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ store site but not from the caffeine-sensitive one. 9. Staurosporine, a protein kinase C (PKC) inhibitor, suppressed I(TRH) in a concentration-dependent manner (the half-maximum inhibitory concentration (IC50), was 2.45 x 10(-8) M). The inhibition was partially restored by adding 1-oleoyl-2-acetylglycerol (OAG), an activator of PKC. 10. These findings suggest that TRH stimulates the hydrolysis of inositol phospholipids and that both the IP, and PKC pathways are involved in the operation of K+ channels in rat hippocampal CA1 neurones.