Effects of 4-Aminopyridine on Potassium Currents in a Molluscan Neuron

The effects of4-aminopyridine (4-AP) on the delayed K + current and on the CaZ+-activated K + current of the Aplysia pacemaker neurons R-15 and L-6 were studied. The delayed outward K + current was measured in Ca 2+free artificial seawater (ASW) containing tetrodotoxin (TTX), using brief depolarizing clamp pulses. External (and internal) 4-AP blocks the delayed K + current in a dose-dependent manner but does not block the leakage current. Our results show that one 4-AP molecule combines with a single receptor site and that the block is voltage dependent with an apparent dissociation constant (K4-Ap) of ~0.8 m M at 0 mV. K4-Ap increases e-fold for a 32-mV change in potential, which is consistent with the block occurring -0 .8 of the distance through the membrane electrical field. The 4-AP block appears to depend uPon stimulus frequency as well as upon voltage. The greater speed of onset of the block produced by internal 4-AP relative to when it is used externally suggests that 4-AP acts from inside the cell. The Ca2+-activated K + current was measured in Ca2+-free ASW containing T T X , using internal Ca2+-ion injection to directly activate the K § conductance. Low external 4-AP concentrations (<2 mM) have no effect on the Ca2+-activated K + current, but concentrations of 5 m M or greater increase the K + current. Internal 4-AP has the same effect. The opposing effects of 4-AP on the two components of the K + current can be seen in measurements of the total outward K § current at different membrane potentials in normal ASW and during the repolarizing phase of the action potential. T h e a m i n o p y r i d i n e s b lock v o l t a g e d e p e n d e n t po ta s s ium cur ren ts o f ne rve axon (Pe lha te a n d Pichon, 1974; Meves a n d Pichon, 1975 a n d 1977; S c h a u f et al., 1976; U l b r i ch t a n d W a g n e r , 1976; Yeh et al., 1976 a a n d 1976 b), s o m a ( T h o m p s o n , 1977), a n d a x o n t e r m i n a l (Llin~is, et al., 1976) m e m b r a n e s a n d o f musc le m e m b r a n e (Gillespie a n d H u t t e r , 1975; F ink a n d Wet twer , 1978; Molgo , 1978). Moreove r , they b lock the vo l t age -dependen t K § cu r ren t in lower c o n c e n t r a t i o n a n d in a different m a n n e r t h a n the b lock p r o d u c e d by t e t r a e t h y l a m m o n i u m . In mol luscan neurons the K + cu r ren t o f the s o m a m e m b r a n e can be d iv ided into a c o m p o n e n t ac t iva t ed by changes in m e m b r a n e vo l tage a n d a c o m p o n e n t ac t iva t ed by the en t r ance o f C a 2+ ions into the c e l l t h e Ca2+-ac t iva ted K + cur ren t (Meech a n d S tanden , 1975; H e y e r J. GEN. PHYSIOL. 9 The Rockefeller University Press 9 0022-1295/81/07/0063/24 $1.00 63 Volume 78 July 1981 63-86 on D ecem er 9, 2017 jgp.rress.org D ow nladed fom 6 4 t H E J O U R N A L O F G E N E R A L P H Y S I O L O G Y * V O L U M E 78 9 1981 a n d Lux, 1976 b). T h e vo l t age -dependen t K + cur ren t o f the mol luscan neu ron s o m a m e m b r a n e has been fu r the r d iv ided ope ra t iona l ly into a fast o u t w a r d K + cur ren t (Connor a n d Stevens, 1971; Neher , 1971) a n d the de layed o u t w a r d K + cur ren t (or the "de l ayed o u t w a r d rect if ier") , which was first descr ibed in squid axon by H o d g k i n a n d H u x l e y (1952 a). I t has been repor ted tha t ex terna l 4 a m i n o p y r i d i n e (4-AP) selectively blocks the fast K + cur ren t o f the mol luscan neu ron s o m a ( T h o m p s o n , 1977). T h e expe r imen t s r epor ted in this p a p e r were car r ied out to c o m p a r e the effects o f ex t race l lu la r a n d in t race l lu la r 4-AP on different c o m p o n e n t s of the K § cur ren t o f a mol luscan p a c e m a k e r neuron . T h e y show tha t 4-AP also blocks the de layed o u t w a r d K + cur ren t but tha t , at h igh concent ra t ions , it increases the Ca2+-ac t iva ted K + current . T h e results suggest tha t the a m i n o p y r i d i n e s can be used to p rov ide a p h a r m a c o logical s epa ra t ion o f the vol tagea n d Ca2+-dependen t c o m p o n e n t s o f the K + current . A p r e l i m i n a r y accoun t o f some aspects o f this work has a p p e a r e d e lsewhere ( H e r m a n n a n d G o r m a n , 1978).