Activation of potassium(K+)cu rrents plays a critical role in the control of prog rammed cell death Because pituitary adenylate cyclase-activating polypeptide(PACAP)has been shown to inhibit the apoptotic cascade in the cerebellar cortex du ring development, we hav einvestigated the effect of PACAP on K’cu rrents in cultu red cerebellar g ranule cells using the patch-clamp technique in the whole-cell configu ration Two types of outward K_cu rrents, a transient K+cu rrent(k)and a delayed rectifier K’cu rrent(/K)were characterized using two different voltage protocols and specific inhibitors of K’channels Application of PACAP induced a reversible reduction of the /K amplitude, but did not affect/A, while the PACAP-related peptide vasoactive intestinal polypeptide had no effect on either types of K+ cu rrents Repeated applications of PACAP induced g radual attenuation of the electrophysiological response In the presence of guanosine 5'-rthio1rlphosphate(GTPh, S), PACAP provoked a marked and i rreversible/K depression, whereas cell dialysis with guanosine 5'-bthiodiphosphate GDPl3S totally abolished the effect of PACAR P re-treatment of the cells with pertussis toxin did not modify the effect of PACAP on/K In contrast, cholera toxin suppressed the PACAP-induced inhibition of/K Exposu re of g ranule cells to dibutyryl cyclic adenosine monophosphate(dbcAMP)mimicked the inhibitory effect of PACAP on/K Addition of the specific protein kinaseAinhibitorH89inthepatch pipettesolution preventedthe reductionof/Kinducedbyboth PACAPand dbcAMR PACAPprovoked a sustainedincreaseofthe restingmembranepotentialin cerebellarg ranule cells cultu red eitherin high orlowKCI-containingmedium, and this long-ferm depolarizing effect of PACAP was mimicked by the/K specific blocker tetraethylammonium chloride fTEA)In addition, pre-incubation of g ranule cellswithTEA suppressedthe effect of PACAP on resting membrane potential TEAmimickedthe neu roprotective effect of PACAP against ethanol-induced apoptotic cell death, and the increase of caspase-3 activity observed after exposu re of g ranule cells to ethanol was also significantly inhibited by TEA Taken together, the present results demonstrate that, in rat cerebellar g ranule cells, PACAP reduces the delayed outward rectifier K’cu rrent by activating a type 1 PACAP(PACl)receptorcoupledtothe adenylyl cyclase/protein kinaseApathwayth rougha choleratoxin-sensitiveGsprotein Our dataalso showthat PACAP and TEA induce long-term depolarization of the resting membrane potential promote cell su rvival and inhibit caspase-3 activity, suggesting that PACAP-evoked inhibition of/K contributes to the anti-apoptotic effect of the peptide on cerebellar granule cells

We have investigated the effects of sigma ligands [1,3-di(2-tolyl)guanidine (DTG) and (1)-pentazocine] on the electrical activity of cultured frog pituitary melanotrope cells by using the patch-clamp technique. DTG and (1)-pentazocine (10 mM each) induced a reversible depolarization associated with an increase in membrane resistance and action potential firing. In voltage-clamp experiments, DTG and (1)-pentazocine elicited inward currents whose intensity augmented with membrane depolarization. The currents vanished or reversed between -90 and -100 mV, at values close to the K1 equilibrium potential(EK15 -102 mV). DTG (2–500 mM) and (1)-pentazocine (0.2-200 mM) reduced the outward delayed rectifier K1 current [IK(V)] in a dose-dependent manner with EC50 of 64 and 37 mM,respectively. In contrast, naloxone (50 mM) and pirenzepine (10mM) did not affect the sigma ligand-induced inhibition of IK (V).Addition of guanosine-59-O-(3-thiophosphate) in the pipettesolution irreversibly sustained the DTG-induced current whereas guanosine-59-O-(2-thiodiphosphate) virtually suppressed the response. Cholera toxin-pretreatment (1 mg/ml; 18hr) abolished the inward current and the inhibition of IK (V)induced by sigma ligands. In contrast, pretreatment with pertussis toxin (1 mg/ml; 18 hr) had no effect. Taken together, these data indicate that DTG and (1)-pentazocine activate the electrical activity of cultured frog melanotrope cells by reducing both a tonic K1 current and a voltage-dependent [IK (V)] K1conductance through the activation of a cholera toxin-sensitive G-protein.

The molecular mechanisms regulating GABA, receptor activity in cultured frog rnelanotrophs were studied using the patch-clamp technique. In the whole-cell configuration, application of GABA evoked a dose-related increase of inward chloride currents. The ED, value, estimated from the sigmoidal dose-response curve was 2 x M and the Hill coefficient was 1.55. The amplitude of the GABA-induced current decayed with time, Kinetics analysis of the desensitization revealed that the time-course of the current decrement was fitted by one exponential. Graded doses of GABA or association of GABA with the benzodiazepine receptor agonist flunitrazepam accelerated the desensitization process. In contrast, the time-course of the current did not significantly vary at different holding potentials. In the outside-out configuration, GABA was found to activate channels which displayed three unitary conductance levels (8, 15 and 30 pS). The channel openings of the more frequent conductance level (30 pS) exhibited short and long lasting open states (1.2 and 28.3 ms at -60mV). Altogether these data reveal that frog melanotrophs possess a single population of GABA, receptors which interconvert into a higher affinity state in the presence of benzodiazepine receptor agonists. Two GABA molecules must bind to the receptor to trigger long lasting channel openings. In addition, the activity of the GABA,receptor appears to be independent of the accumulation of intracellular chloride ions.

The dendritic arbor of pyramidal neurons is not a monolithic structure.Weshow here that the excitability of terminal apical dendrites differs from that of the apical trunk. In response to ?uorescence-guided focal photolysis of caged glutamate, individual terminal apical dendrites generated cadmium-sensitive all-or-none responses that were subthreshold for somatic action potentials. Calcium transients produced by all-or-none responses were not restricted to the sites of photolysis, but occurred throughout individual distal dendritic compartments, indicating that electrogenesis is mediated primarily by voltagegated calcium channels. Compartmentalized and binary behavior of parallelconnected terminal dendrites can greatly expand the computational power of a single neuron.

The functions of the D 4 receptor, a newly cloned D2-1ike receptor, as well as the identity of cells expressing it, are still poorly defined. Using quantitative polymerase chain reaction we detected the messenger RNA of the D4, but not other D2-1ike receptor, in cultured granule cells from neonatal rat cerebellum. In these neurons, dopamine reduced high-voltage-activated calcium current, with a pharmacology corresponding to that of the D 4 receptor. The response declined from one to three days, when calcium currents were mostly sensitive to nifedipine, to 15 days, when nifedipine-insensitive calcium currents were also present and D 4 receptor messenger RNA had declined. The dopamine response was abolished after pretreatment of the cells by pertussis toxin, was potentiated and made irreversible by infusion of guanosine 5'-O-(3-thiotriphosphate) but persisted in the presence of cyclic AMP and isobutylmethylxanthine.