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Characteristics of the slow inhibitory postsynaptic potential of bullfrog symphathetic ganglion cells
Vol. 6, pp . 1827-1836, 1967 . Life Sciences Printed in Great Britain .
Pergamon Press Ltd.
CHARACTERISTICS OF THE SL04f INHIBITORY POSTSYNAPTIC POTENTIAL OF BULLFROG SYMPATHIsTIC GAAGLION CELLB' S . áoketsu and S. Nishi Neurophysiology Laboratory Departméat of Pharmacology sad Therapeutics Stritch School of Medicine, Loyola University Hines, Illinois (Received 13 April 1967 ; in final form 5 June 1967) PREGANGLIONIC volleys evoke in the curarized superior cervical ganglion of turtles and rabbits (1,2) a positive wave (P potential) which is followed by a late occurring negative wave (LN potential) .
Tetanic stimulation enhances the P potential
as well as the LN potential, whereas atropine coapletely abolishes both of them (2,3,4) .
Inasmuch as dibeaaaine de-
presses the P potential preferentially over the LN potential (4), and reserpine selectively depresses the P potential (5), it has been suggested that the P potential is produced by adrenaline which is released from the chromaffia cells is the sympathetic ganglion (4,5,6) . A long lasting hyperpolarizatioa, which corresponds to the P potential, has been recorded iatracellularly from the postsyaaptic neurons of sympathetic ganglia of rabbits, frogs and bullfrogs (x,8,9) "
The P potential can be considered
to be an inhibitory postsynaptic potential (IPSP) by reason of its tress-synaptic occurrence sad its polarity (cf . 4), although there is no experimental evidence that it actually inhi bits ganglioaic transmission .
Moreover, there is a report
' This work xas supported by the Institute of Neurological DiseaseR and Blindness, NIH Grant NB066Q2-01 and National Science Foundation Grant GB-5269x " 1827
Vol . 6, No . 17
POSTSYNAPTIC POTENTIAL
1828
contrary to the idea that catecholamines act as the mediator of the P potential (10) .
Aeverthelese, it was considered worth-
while to study the mechanism of the P potential, regardless of the àind of transmitter involved, is comparison with the IPSP of other cells in general . The present communication deals with the inhibition of postgaaglioaic discharges caused by the P potential, and with the nature of the P potential which differs in characteristics frog that of other cells so far studied . Methods Isolated sympathetic chains of bullfrogs (xaaa catesbiana ) were used throughout .
The recording of postgaaglionic dis-
charges was aade by an AC amplifier with a pair of platinum electrodes ; one was placed oa the 9th or 10th ganglion, and the other on its postgaaglionic nerve branch .
Electrical
activity of the 9th or 10th ganglionic neurons was recorded eztracellularly by meahs of the sucrose-gap technique (Fig . 1 anä cf . 11) .
Aaodal and cathodal currents xere applied to
the ganglion through a bridge circuit, as shown in Fig. 1 . ~ctrinsic currents were applied to the aicotinized ganglia more than 30 ~" after application of nicotine, by which time the depolarizing action of nicotine (cf . 12) and the accompanying rednctioa of the meavbraae resistance would have completely subsided (unpublished observation) ; thus, the membrane potential changes by the extrinsic currents would be comparable to those before the drug was applied .
Pregaaglionic stimulation was
applied to the sympathetic chain near the 9th or 10th ganglion through a pair of platiaua electrodes .
The method of intra-
POSTSYNAPTIC POTENTIAL
Vol . 6, No . 17
1829
B
FIG . 1 A : Schematic draxings of the ezperinental arrangement for the sucrose-gap aethod (lj and the bridge circuit (2) used for application of conditioning hyperpolarizing and depolarizing +currents to the bullfrog sympathetic poatganglionic neurons, respectively . P, T, 8 and 8 represent the paraffin pool, test solution, sucrose solution, and Finger's solution pool, respectively . E and E represent the calomel electrodes, ands r~presea~s the stimulator . B : Schematic drawing illustrating arrangeaent of the preparation. G% and SN represent the 10th sympathetic ganglion and spinal nerve, respectively . cellular recording was not employed in the present experiment, since steady intracellular P potentials were difficult to obtain .
The composition of Singer's solution was as folloxs :
NaCl, 112 mM ; SCl, 2 mM ; CaC1 2 , 1 .8 mM ; NaHC03, 2 mM .
1830
POSTSYNAPTIC POTENTIAL
Vol . 6, No . 17
aesults 1)
Inhibition of oostaaaalionic discharge b~ P potential . Yhea the orthodroQic response in bullfrog sympathetic
ganglia to a single pregaaglionic stimulation xas completely blocked by nicotine sulfate (1 - 5 z 10-5 g/cc), a teteaic pregaaglionic stimulation (pulse duration, 1 cosec . ; frequency,
10 - 50 sec . ; train duration, 10 - 20 sec.) produced a poxerful and long lasting after-discharge (AD), xhich consisted of txo distinctly different components, the early after~ischarge (SAD) and the late'after~lischarge (LAD)(9) .
By applying
short tetanic stimulation (1 - 5 sec.) during the production of the AD, a marked inhibition of the discharges could be observed dnriag and immediately after stimulation (see record A of Fig. 2) .
Inasmuch as the P potential of ganglion cells xas
elicited during and immediately after tetaaic pregaaglionic atiaulation (see record B-2 of Fig . 2), the P potential would be responsible for the observed inhibition of the AD . The P potential as xell as the concoaittaat inhibition of the LD xere observed xhen the presynaptic B fibers alone were stimulated .
The sire of the P potential as xell as the degree
of inhibition shored a marked increase xhen the presyaaptic B and C fibers (13) were stimulated .
Jldditioa of atropine
sulfate (1 z 10-5 g/cc) completely abolished the P potential as xell as the inhibitory effect of presynaptic stimulation . This observation also proves that the inhibition of the AD is brought about by the P potential . 2)
Aaal~sis of the nature of SIPBP . Since the P potential not only inhibits the post-
ganglionic discharges, but also exhibits a slow time course,
Vol . 6, No . 17
POSTSYNAPTIC POTENTIAL
1831
FIG . 2 A : AD (EAD plus LAli) of a aicotinized bullfrog sympathetic ganglion produced by repetitive preganglioaic stimulation (10 sec . for 20 sec .) . Two successive repetitive pregaaglioaic stimulations (10/sec . for approzimately 4 sec .) mere given during the production of the AD . Note the inhibition of AD during and immediately after the two test stimulations . Period during which stimulation was applied is shown by horizontal lines . Calibration : 40 WY, 20 sec . B and C : P otentials (B-1 to B-4) and orthodromic responses p(C-1 to C-4) induced during conditioning depolarization (B-1 and C-1) and hyperpolarization (B-3,4 and C-3,4) of the same ganglion . Orthodromic responses xere elicited by a single mazimum stimulation applied to the pregaaglionic B fibers before application of nicotine . P potentials were produced by a train of tetaaic stimulation (10/sec . for 4 sec .) given to both the preganglionic B and C ~ibers 30 ~" after application of nicotine (5 z 10 - g/cc) . The-strength of the applied cathodal current was 3 .6 z 10 anp . for B-1 and C-1:6aß that of the applied aaodal currant was 3 "0 z 10 amp . for B-3 and C-3~ and 7 " 3 z 10 - amp . for B-4 sad C-4.
183 2
Vol . 6, No . 17
POSTSYNAPTIC POT~iTIAL
mrtrineic currents xere not applied in B-2 and C-2 . Bote the decrease and increase of the P potential during conditioning depolarization (B-1) and ~yperpolarization (B-2), respectively . Calibration : 2 mY, 4 sec . it can be called the slox inhibitory postsynaptic potential (ßIPBP) . The SIPBP exhibited an unusual characteristic rhea the ganglion cells xere either depolarised or hyperpolarized by extrinsic currents applied through the E1 and T2 electrodes . The amplitude of the P potential gradually decreased and eventually disappeared as the conditioning depolarization of ganglion cells xas increased stepxise (cf. record B-1), xhereas it increased almost in proportion to the conditioning l~yperpolarizatioa xhich xas increased up to a certain level (cf . record B-3) .
Yhea the conditioning 2~yperpolarization
exceeded this level, the increment of the P potential gradually decreased .
A strong hyperpolarization, larger than that xhich
nullified the after-hyperpolarisation of the orthodromic action potential before application of nicotine, usually could not abolish nor reverse the P potential .
The P potential evoked
during such a strong hyperpolarization shored an amplitude coaparable to or slightly smaller than that of the control (cf. record B-4), xith the exception of a fex cases is xhich the P potential xas nullified, and turned into a small negative potential (eee discussion) . Ia Finger's solution coat ai~ 10 to 15 mM potassium (NaCl of Ring er'8 solution xas replaced by equiaolar aaouats of gCl), the size of the P potential xas reduced to less than 1/3 of that in aoraal Ginger's eolutloa .
Hoxever, it xas
increased by conditioning hyperpolarization of the ganglion
Vol . 6, No . 17 cells .
POSTSYNAPTIC POTENTIAL
1833
The conditioning hyperpolarizatioa, which was stronger
than that which nullified the after-hyperpolarization of the orthodromic action potential in 10 mM potassium Ringer's solution, still augmented the P potential.
A more intense
hyperpolarization reeu~ted in the decrease of the P potential . In some cases, such a strong hyperpolarizatioa abolished or reversed the P potential (see discussionj .
The P potential
simply decreased sad eventually disappeared as the conditioning depolarization was gradually increased . When the potassium concentration of Ringer'a solution was reduced to 0 .2 mM, the amplitude of the P potential decreased to approximately 2/3 of that in aoreal Ringer's solution, instead of increasing .
The effects of hyperpolariaation and
depolarization of ganglion cells in lox potassium median were essentially the sage as those in normal Ringer's solution . No changes were observed in the vise or in the nature of the P potential when the sodium chloride oY Ringer's solution was totally replaced with equimolar amounts of sodium glutamate . When the calcium concentration of Ringer's solution was raised to 20 mM (the Ca concentration of the test solution xas increased by replacing NaCl with equimolar amounts of CaC12, and the Na concentration of the control solution xas adjusted by replacing NaCl with sucrose), the amplitude of the P potential was slightly increased .
The most significant effect
of high calcium was that there was no augmentation of the P potential during moderate hyperpolarization .
The size of
the P potential simply decreased in accordance xith the stepwise increase in the strength of conditioning hyperpolarization. The effect of conditioning depolarization in this medium was
1834
POSTSYNAPTIC POTF~iTIAL
Vol . 6, No . 17
essentially the seas as that in norael Banger's solution, although a slightly larger current xas required to abolish the P potential . Discussion In general, the aeabrane hyperpolarization of the IPSP is ezplaiaed to occur as a result of the increased peraeability of the subsynaptic aaabrane to potassiua sad/or chloride ions (cf . 14) .
According to the present ezperiment, however,
the SIPSP of ganglion cells can be considered to be independent of C1 ions, on the assumption that glutaaate ions are inert and are not able to pass through the meabraae .
Assuming then
that the SIPBP is solely dependent upon the potassium peraeability of the subsyaaptic meabrane, the size of the P potential should be decreased during conditioning hyperpolarization sad increased during conditioning depolarization . Furtheraore, the P potential should be nullified or even reversed when a strong i~yperpolarization, approzimatiag or ezceeding the potassiua equilibrium potential (approziaately 30 mV sore negative xith respect to the resting meabraae potential ; unpublished observation), is applied .
There is,
hoxever, no such ezperimeatal evidence, xith the ezception that there is a decrease sad occasional reversal of the P potential xhen the applied hyperpolarization is much pore intense than that xhich nullifies the after-hyperpolarizatioa of the orthodromic action potential of the ganglion .
It should
be mentioned here that tetanic pregaaglioaic stimulation simultaneously induces the SIPSP and the slox ezcitatory postsynaptic potential (S&PSP)(9,15), and that the size of
Vol . 6, No . 17
POSTSYNAPTIC POTENTIAL
1835
the STPS,P increases in a similar wanner to that of the TPSP during hyperpolarization (unpublished observation) .
Thus,
the reduction in size and the reversal of polarity of the P potential could be simply due to the augaeatatioa of the SEP$P during a strong conditioning hyperpolarization .
tloreover,
if there were s soall fraction of the SPBP remaining after nicotinisatioa of the ganglia, a strong hyperpolarizatioa would enhance the EPSP sufficiently to contribute to the reduction as well as the reversal of the P potential .
altogether,
the present results and the aforeaentioned consideration imply that neither the potassium permeability nor the chloride permeability of the subsyaaptic membrane is involved in the production of the SIPSP .
The nature of the P potential, oa
the whole, appears to closely reseable that of the slow post-tetaaic hyperpolarization (PTH) of crayfish stretch receptor which is reported by Nakajima and Takahashi (16) to be due to the electrogenic sodium-puap .
They have suggested,
with regard to the increase of the slow PTH by hyperpolarization, that hyperpolarization influences the activity of the pumps in such a way as to increase the net outward current, concluding that there exists, for the active sodiua-puap, a feedback system by which more energy is aobilized in the preaence of a greater load .
The present experiments shows that
the augmentation of the P potential by hyperpolarizatioa is abolished by raising the external Ca concentration.
Assuaing
that the P potential is associated with as electrogenic sodiumpump and that a similar feedback system exists is the sympathetic ganglion cells, as excess of eztracellular Ca ions may interfere, by some unknown mechanism, with the relationship
1836
POSTSYNAPTIC POTENTIAL
between load and energy mobilization .
Vol. 6, No . 17 The slight increase of
the P potential in a high Ca medium might simply be due to an increase in membrane resistance .
The changes in the membrane
conductance during the P potential are now under investigation .
References 1.
Y. LAPORTE and R . LORENTE DE N0, J. cell . comp . Physiol . Suppl . 2, 61 (1950) "
2.
3.
R. M. ECCLES, J. Physiol . 1~, 196 (1952) . R. M. ECCLÈB, J. Pi~ysiol . l~, 181 (1952) "
4.
R. M. ECCLES and B . LIBET, J. Phssiol . ~, 484 (1961) .
5.
B. LIBET, Fed . Proc . 21, 345 (1962) .
6.
B. LIBET, Proc . Z%III Iat . Physiol . ConRr. Yol . II , p . 809 (1962) . T. TOSAKA and B . LIBET, Abstr . YYIII Int . Physiol. Coa~r . 905 (1965) .
8.
B. LIBET and T . TOSAKA, Fed . Proc . ~, 455 (1966) .
9.
S. NISHI and 'K . KOKETSU, Life Sciences , ~, 1991 (1966j .
lo .
R. L. vOLLE, Pharmacol . Rev. 18, 839 (1966) .
11 .
S. KOKETSU and S. NISHI, Life Sciences , is press .
12 .
B. L. GINSB08G and S . GUII3RES0, J . Physiol .
13 .
S . NISHI, H. SOEDA and K. KOKETSU, J . cell . comp . Physiol .
Z,
189 (196+1 .
66, 19 (1965) . 14 .
J. C . ECCLES, The Pl~ysioloas of Synapses . p . 173, 3pringer-verlag, Berlin (1964) .
Z,
15 .
B. LIBET, J. Physiol .
1 (1964) .
16 .
S. NAgAJIMA and K . TAK,AHASHI, J. Pl~ysiol . 1~, 105 (1966) .
Pergamon Press Ltd.
CHARACTERISTICS OF THE SL04f INHIBITORY POSTSYNAPTIC POTENTIAL OF BULLFROG SYMPATHIsTIC GAAGLION CELLB' S . áoketsu and S. Nishi Neurophysiology Laboratory Departméat of Pharmacology sad Therapeutics Stritch School of Medicine, Loyola University Hines, Illinois (Received 13 April 1967 ; in final form 5 June 1967) PREGANGLIONIC volleys evoke in the curarized superior cervical ganglion of turtles and rabbits (1,2) a positive wave (P potential) which is followed by a late occurring negative wave (LN potential) .
Tetanic stimulation enhances the P potential
as well as the LN potential, whereas atropine coapletely abolishes both of them (2,3,4) .
Inasmuch as dibeaaaine de-
presses the P potential preferentially over the LN potential (4), and reserpine selectively depresses the P potential (5), it has been suggested that the P potential is produced by adrenaline which is released from the chromaffia cells is the sympathetic ganglion (4,5,6) . A long lasting hyperpolarizatioa, which corresponds to the P potential, has been recorded iatracellularly from the postsyaaptic neurons of sympathetic ganglia of rabbits, frogs and bullfrogs (x,8,9) "
The P potential can be considered
to be an inhibitory postsynaptic potential (IPSP) by reason of its tress-synaptic occurrence sad its polarity (cf . 4), although there is no experimental evidence that it actually inhi bits ganglioaic transmission .
Moreover, there is a report
' This work xas supported by the Institute of Neurological DiseaseR and Blindness, NIH Grant NB066Q2-01 and National Science Foundation Grant GB-5269x " 1827
Vol . 6, No . 17
POSTSYNAPTIC POTENTIAL
1828
contrary to the idea that catecholamines act as the mediator of the P potential (10) .
Aeverthelese, it was considered worth-
while to study the mechanism of the P potential, regardless of the àind of transmitter involved, is comparison with the IPSP of other cells in general . The present communication deals with the inhibition of postgaaglioaic discharges caused by the P potential, and with the nature of the P potential which differs in characteristics frog that of other cells so far studied . Methods Isolated sympathetic chains of bullfrogs (xaaa catesbiana ) were used throughout .
The recording of postgaaglionic dis-
charges was aade by an AC amplifier with a pair of platinum electrodes ; one was placed oa the 9th or 10th ganglion, and the other on its postgaaglionic nerve branch .
Electrical
activity of the 9th or 10th ganglionic neurons was recorded eztracellularly by meahs of the sucrose-gap technique (Fig . 1 anä cf . 11) .
Aaodal and cathodal currents xere applied to
the ganglion through a bridge circuit, as shown in Fig. 1 . ~ctrinsic currents were applied to the aicotinized ganglia more than 30 ~" after application of nicotine, by which time the depolarizing action of nicotine (cf . 12) and the accompanying rednctioa of the meavbraae resistance would have completely subsided (unpublished observation) ; thus, the membrane potential changes by the extrinsic currents would be comparable to those before the drug was applied .
Pregaaglionic stimulation was
applied to the sympathetic chain near the 9th or 10th ganglion through a pair of platiaua electrodes .
The method of intra-
POSTSYNAPTIC POTENTIAL
Vol . 6, No . 17
1829
B
FIG . 1 A : Schematic draxings of the ezperinental arrangement for the sucrose-gap aethod (lj and the bridge circuit (2) used for application of conditioning hyperpolarizing and depolarizing +currents to the bullfrog sympathetic poatganglionic neurons, respectively . P, T, 8 and 8 represent the paraffin pool, test solution, sucrose solution, and Finger's solution pool, respectively . E and E represent the calomel electrodes, ands r~presea~s the stimulator . B : Schematic drawing illustrating arrangeaent of the preparation. G% and SN represent the 10th sympathetic ganglion and spinal nerve, respectively . cellular recording was not employed in the present experiment, since steady intracellular P potentials were difficult to obtain .
The composition of Singer's solution was as folloxs :
NaCl, 112 mM ; SCl, 2 mM ; CaC1 2 , 1 .8 mM ; NaHC03, 2 mM .
1830
POSTSYNAPTIC POTENTIAL
Vol . 6, No . 17
aesults 1)
Inhibition of oostaaaalionic discharge b~ P potential . Yhea the orthodroQic response in bullfrog sympathetic
ganglia to a single pregaaglionic stimulation xas completely blocked by nicotine sulfate (1 - 5 z 10-5 g/cc), a teteaic pregaaglionic stimulation (pulse duration, 1 cosec . ; frequency,
10 - 50 sec . ; train duration, 10 - 20 sec.) produced a poxerful and long lasting after-discharge (AD), xhich consisted of txo distinctly different components, the early after~ischarge (SAD) and the late'after~lischarge (LAD)(9) .
By applying
short tetanic stimulation (1 - 5 sec.) during the production of the AD, a marked inhibition of the discharges could be observed dnriag and immediately after stimulation (see record A of Fig. 2) .
Inasmuch as the P potential of ganglion cells xas
elicited during and immediately after tetaaic pregaaglionic atiaulation (see record B-2 of Fig . 2), the P potential would be responsible for the observed inhibition of the AD . The P potential as xell as the concoaittaat inhibition of the LD xere observed xhen the presynaptic B fibers alone were stimulated .
The sire of the P potential as xell as the degree
of inhibition shored a marked increase xhen the presyaaptic B and C fibers (13) were stimulated .
Jldditioa of atropine
sulfate (1 z 10-5 g/cc) completely abolished the P potential as xell as the inhibitory effect of presynaptic stimulation . This observation also proves that the inhibition of the AD is brought about by the P potential . 2)
Aaal~sis of the nature of SIPBP . Since the P potential not only inhibits the post-
ganglionic discharges, but also exhibits a slow time course,
Vol . 6, No . 17
POSTSYNAPTIC POTENTIAL
1831
FIG . 2 A : AD (EAD plus LAli) of a aicotinized bullfrog sympathetic ganglion produced by repetitive preganglioaic stimulation (10 sec . for 20 sec .) . Two successive repetitive pregaaglioaic stimulations (10/sec . for approzimately 4 sec .) mere given during the production of the AD . Note the inhibition of AD during and immediately after the two test stimulations . Period during which stimulation was applied is shown by horizontal lines . Calibration : 40 WY, 20 sec . B and C : P otentials (B-1 to B-4) and orthodromic responses p(C-1 to C-4) induced during conditioning depolarization (B-1 and C-1) and hyperpolarization (B-3,4 and C-3,4) of the same ganglion . Orthodromic responses xere elicited by a single mazimum stimulation applied to the pregaaglionic B fibers before application of nicotine . P potentials were produced by a train of tetaaic stimulation (10/sec . for 4 sec .) given to both the preganglionic B and C ~ibers 30 ~" after application of nicotine (5 z 10 - g/cc) . The-strength of the applied cathodal current was 3 .6 z 10 anp . for B-1 and C-1:6aß that of the applied aaodal currant was 3 "0 z 10 amp . for B-3 and C-3~ and 7 " 3 z 10 - amp . for B-4 sad C-4.
183 2
Vol . 6, No . 17
POSTSYNAPTIC POT~iTIAL
mrtrineic currents xere not applied in B-2 and C-2 . Bote the decrease and increase of the P potential during conditioning depolarization (B-1) and ~yperpolarization (B-2), respectively . Calibration : 2 mY, 4 sec . it can be called the slox inhibitory postsynaptic potential (ßIPBP) . The SIPBP exhibited an unusual characteristic rhea the ganglion cells xere either depolarised or hyperpolarized by extrinsic currents applied through the E1 and T2 electrodes . The amplitude of the P potential gradually decreased and eventually disappeared as the conditioning depolarization of ganglion cells xas increased stepxise (cf. record B-1), xhereas it increased almost in proportion to the conditioning l~yperpolarizatioa xhich xas increased up to a certain level (cf . record B-3) .
Yhea the conditioning 2~yperpolarization
exceeded this level, the increment of the P potential gradually decreased .
A strong hyperpolarization, larger than that xhich
nullified the after-hyperpolarisation of the orthodromic action potential before application of nicotine, usually could not abolish nor reverse the P potential .
The P potential evoked
during such a strong hyperpolarization shored an amplitude coaparable to or slightly smaller than that of the control (cf. record B-4), xith the exception of a fex cases is xhich the P potential xas nullified, and turned into a small negative potential (eee discussion) . Ia Finger's solution coat ai~ 10 to 15 mM potassium (NaCl of Ring er'8 solution xas replaced by equiaolar aaouats of gCl), the size of the P potential xas reduced to less than 1/3 of that in aoraal Ginger's eolutloa .
Hoxever, it xas
increased by conditioning hyperpolarization of the ganglion
Vol . 6, No . 17 cells .
POSTSYNAPTIC POTENTIAL
1833
The conditioning hyperpolarizatioa, which was stronger
than that which nullified the after-hyperpolarization of the orthodromic action potential in 10 mM potassium Ringer's solution, still augmented the P potential.
A more intense
hyperpolarization reeu~ted in the decrease of the P potential . In some cases, such a strong hyperpolarizatioa abolished or reversed the P potential (see discussionj .
The P potential
simply decreased sad eventually disappeared as the conditioning depolarization was gradually increased . When the potassium concentration of Ringer'a solution was reduced to 0 .2 mM, the amplitude of the P potential decreased to approximately 2/3 of that in aoreal Ringer's solution, instead of increasing .
The effects of hyperpolariaation and
depolarization of ganglion cells in lox potassium median were essentially the sage as those in normal Ringer's solution . No changes were observed in the vise or in the nature of the P potential when the sodium chloride oY Ringer's solution was totally replaced with equimolar amounts of sodium glutamate . When the calcium concentration of Ringer's solution was raised to 20 mM (the Ca concentration of the test solution xas increased by replacing NaCl with equimolar amounts of CaC12, and the Na concentration of the control solution xas adjusted by replacing NaCl with sucrose), the amplitude of the P potential was slightly increased .
The most significant effect
of high calcium was that there was no augmentation of the P potential during moderate hyperpolarization .
The size of
the P potential simply decreased in accordance xith the stepwise increase in the strength of conditioning hyperpolarization. The effect of conditioning depolarization in this medium was
1834
POSTSYNAPTIC POTF~iTIAL
Vol . 6, No . 17
essentially the seas as that in norael Banger's solution, although a slightly larger current xas required to abolish the P potential . Discussion In general, the aeabrane hyperpolarization of the IPSP is ezplaiaed to occur as a result of the increased peraeability of the subsynaptic aaabrane to potassiua sad/or chloride ions (cf . 14) .
According to the present ezperiment, however,
the SIPSP of ganglion cells can be considered to be independent of C1 ions, on the assumption that glutaaate ions are inert and are not able to pass through the meabraae .
Assuming then
that the SIPBP is solely dependent upon the potassium peraeability of the subsyaaptic meabrane, the size of the P potential should be decreased during conditioning hyperpolarization sad increased during conditioning depolarization . Furtheraore, the P potential should be nullified or even reversed when a strong i~yperpolarization, approzimatiag or ezceeding the potassiua equilibrium potential (approziaately 30 mV sore negative xith respect to the resting meabraae potential ; unpublished observation), is applied .
There is,
hoxever, no such ezperimeatal evidence, xith the ezception that there is a decrease sad occasional reversal of the P potential xhen the applied hyperpolarization is much pore intense than that xhich nullifies the after-hyperpolarizatioa of the orthodromic action potential of the ganglion .
It should
be mentioned here that tetanic pregaaglioaic stimulation simultaneously induces the SIPSP and the slox ezcitatory postsynaptic potential (S&PSP)(9,15), and that the size of
Vol . 6, No . 17
POSTSYNAPTIC POTENTIAL
1835
the STPS,P increases in a similar wanner to that of the TPSP during hyperpolarization (unpublished observation) .
Thus,
the reduction in size and the reversal of polarity of the P potential could be simply due to the augaeatatioa of the SEP$P during a strong conditioning hyperpolarization .
tloreover,
if there were s soall fraction of the SPBP remaining after nicotinisatioa of the ganglia, a strong hyperpolarizatioa would enhance the EPSP sufficiently to contribute to the reduction as well as the reversal of the P potential .
altogether,
the present results and the aforeaentioned consideration imply that neither the potassium permeability nor the chloride permeability of the subsyaaptic membrane is involved in the production of the SIPSP .
The nature of the P potential, oa
the whole, appears to closely reseable that of the slow post-tetaaic hyperpolarization (PTH) of crayfish stretch receptor which is reported by Nakajima and Takahashi (16) to be due to the electrogenic sodium-puap .
They have suggested,
with regard to the increase of the slow PTH by hyperpolarization, that hyperpolarization influences the activity of the pumps in such a way as to increase the net outward current, concluding that there exists, for the active sodiua-puap, a feedback system by which more energy is aobilized in the preaence of a greater load .
The present experiments shows that
the augmentation of the P potential by hyperpolarizatioa is abolished by raising the external Ca concentration.
Assuaing
that the P potential is associated with as electrogenic sodiumpump and that a similar feedback system exists is the sympathetic ganglion cells, as excess of eztracellular Ca ions may interfere, by some unknown mechanism, with the relationship
1836
POSTSYNAPTIC POTENTIAL
between load and energy mobilization .
Vol. 6, No . 17 The slight increase of
the P potential in a high Ca medium might simply be due to an increase in membrane resistance .
The changes in the membrane
conductance during the P potential are now under investigation .
References 1.
Y. LAPORTE and R . LORENTE DE N0, J. cell . comp . Physiol . Suppl . 2, 61 (1950) "
2.
3.
R. M. ECCLES, J. Physiol . 1~, 196 (1952) . R. M. ECCLÈB, J. Pi~ysiol . l~, 181 (1952) "
4.
R. M. ECCLES and B . LIBET, J. Phssiol . ~, 484 (1961) .
5.
B. LIBET, Fed . Proc . 21, 345 (1962) .
6.
B. LIBET, Proc . Z%III Iat . Physiol . ConRr. Yol . II , p . 809 (1962) . T. TOSAKA and B . LIBET, Abstr . YYIII Int . Physiol. Coa~r . 905 (1965) .
8.
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