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 effects of 4-aminopyridine (4-AP), a specific blocker of outward K current, on voltage-activated transient outward K current 1(I) and inward Na current (I) were investigated on cultured rat cerebellar granule cells using the whole cell voltage-clamp K(A) Na 1technique. At the concentration of 1–5 mM, 4-AP inhibited both I and I. It reduced the amplitude of peak Na current without K(A) Na significant alteration of the steady-state activation and inactivation properties. The inhibitory effect was not enhanced by repeated 1depolarizing pulses (0.5 or 0.1 Hz), suggesting that the binding affinity of 4-AP on Na channels is state-independent. In contrast, the 1 effect of 4-AP on Na channels appeared to be voltage-dependent, the weaker inhibition occurred at more depolarization. Moreover, 4-AP 1 slowed both the activation and inactivation kinetics of Na current. These effects were similar to those induced by a-scorpion toxin and 1 sea anemone toxins on Na channels in other cell model. Our data demonstrate for the first time that 4-AP is able to block not only 11 1 A-type K channels, but also Na channels in rat cerebellar granule cells. It is concluded that the inhibition exerted by 4-AP on Na current likely differs from that provoked by local anesthetics. The possibility that the binding site of neurotoxin receptor 3 may be involved is discussed.

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 effects of adenosine on the voltage-sensitive delayed-rectifier Kq I. currents and hyperpolarization-activated cationic inward K current Ih. were studied in cultured frog melanotrophs using the whole-cell configuration of the patch-clamp technique. The A1 receptor agonist R-N6-phenylisopropyl-adenosine R-PIA; 50 mM. reversibly increased I. Perfusion of dibutyryl-cAMP 1 mM. in the external K solution did not modify the R-PIA-induced enhancement of IK. Pretreatment of melanotrophs with pertussis toxin 1 mgrml; 12 h.totally abolished the R-PIA-evoked response. Application of hyperpolarizing voltage pulses from y60 to y120 mV to melanotrophs induced a two-component inward current corresponding to an I-like conductance. This conductance was characterized by a high Kq hselectivity and a low Naq permeability and was resistant to etrodotoxin 1 mM.. R-PIA had no effect on I. The present study hdemonstrates that in frog melanotrophs adenosine inhibits the electrical activity by activating IK through an A1 receptor subtype coupled to a pertussis toxin-sensitive pathway independent of the cAMPrPKA system. This study also demonstrates the existence of a Ih conductance in frog melanotrophs which is not modulated by A1 receptors. q1998 Elsevier Science B.V. All rights reserved.

1. Transient outward current was recorded in cultured frog melanotrophs with the whole-cell configuration of the patch-clamp technique. The ionic dependence, kinetics and pharmacological properties of the current were studied. The effects of the A1 adenosine receptor agonist R-N6 enylisopropyl-adenosine (R-PIA) on this current were also investigated. 2. In tetrodotoxin-and cobalt-containing solution, depolarization from -120 mV elicited both transient and delayed outward currents. Pulses from -60 mV activated only a sustained late current. 3. 4-Aminopyridine (4 mM) reduced the transient outward current much more than the delayed outward current. In contrast, tetraethylammonium (10-20 mM) selectively reduced the delayed current. 4. Tail current measurements showed a positive shift in the reversal potential when external K+ concentration was increased, indicating that K+ was the predominant charge carrier. 5. Steady-state inactivation was complete at potentials positive to -10 mV and removed by hyperpolarization. 6. Inactivation of the transient current was slowed and accelerated in oxidizing and reducing conditions, respectively, confirming the involvement of an inactivating 'ball and chain'peptide. 7. R-PIA increased the transient current. The steady-state inactivation curve was shifted towards more positive potentials without changing the activation kinetics. Pretreatment with pertussis toxin (1 jug ml-') blocked the response to R-PIA. 8. It is concluded that frog melanotrophs possess an A-type current that is likely to play an important role in excitability. This current, which is directly modulated by A, adenosine receptors through a Gi/Go protein, appears to be responsible for the inhibitory effects of adenosine on electrical activity.