- Email: [email protected]
Neuronal adaptability
1~2
TINS. ~
19"/8
Neuronal adaptability John E. Treherne ,.
|l
|
i i
Recent eleclrophyf~ologtcal research hat J~own thai tome htpertdwate m eelle are able to maintain excitability in theface o f naz~ve changes in the osmotic and ionic concenlrations o f the bathing medium. ?'hit eslablhdwt the important pthtetple that tmarotw~ need IJot necetlarily be at the mercy of their lmmedlme fluid ~ i r a n n ~ t , but can adapt, relatively rapidly, to changes in the chemical composition of their body fluids. In lklt article John Treherne explains the mechanisms by which an um~tal esluartne worm maintains ne'aan~l excitability.
Nerve ceils are I~merally supposed to require a fluid environment of stable chemical composition. This. supposition results from the fact that our knowledge of the ionic basis of neuronal excitability has been largely derived from studies of the giant nerve ceils of marine invertebrates •E (in which ionic and osmotic homeostasis is provided by environmental stability, or by reSulation of the Mood composition), or from the nerves of vertebrate animals and insects (in which the neuronal microenvironment is regulated by well-developed Mood-brain barrier systems)t. sdsptsl~ worm The nerve cells of some invertebrates, however, have been showli recently to he
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Fit. 2. Continuous recording of tie axowl rtttint and action potentials, mttuwtd with aR iatracflhdar microel¢ctrode, during prolreulve dlluttml of the basking mediumt. Despite tie tJtbMalttlatdilution of external ~llum (from an i~tlal com'eatration of ,182J down to 36,2raM) ~ tie reduced osmotic concentratioR (from 1024 to 77 mOtnwl), actiolt potentials coMInue to be tlleited. 7lie hypo.otmotlc adaptatiolt it associated witlt 4 pro~ntiw Diocese ill retting potential (It.vlm'polarlgatlon).
able to function adequately, despite large clum~gesin the ionic and osmotic concentration of the body fluids. A most spectacular example of this occurs in a small estuarine worm, Mercierella enigmutica
F'~. !. Tie polyckaete ~orm. Merciere~ enigrustics,/: the emxstextremeo.¢moco~ormerImmm. TAisspecies can witlntmvdvarimiottsin the osmotic com'entratiolt of its 6ody flwtds from 84 to 2J04 mOsmoP.
© F.i~/Nott~,~HolluOBiomufiud~
1978
(Fig. I), whose nerve cells ca~ adapt, relatively rapidly, to massive ,Jilution of the Mood resulting from clumges in environmental salinity6. This remarkable ability can he demonstrated in ~itro by progressive dilution of the physiological saline bathing the isolated preparation of the so-called giant u o n (Fig. 2p. The Mercierella u o n is, nevertheless, similar to those of other animals (for example, the giant u o n of the squid) in that action potentials appear to he mediated by an inward movement of sodium ices and are blocked, as in other axons, by the poison tetrodotoxin which specifically Mocks the sodium channel in conventional axon membranes*, it is, therefore, surprising that the amplitudes of the action potentials can in some cases actually increase, and their rise times become faster, following
substantial dilution of external sodium and
other iota (Fig 3). S m m . ~ mlq~t The axons of Mercierella use a number of intriguing strategies which enable them to function in such an inconstant environmerit as is fc~md in estuaHt~ surroundings. The axon membrane is protected from the effects of increased internal hydrostatic pressure, following abrupt dilution of the bathing medium, by unustut ultrastructural specializations: hemidesmosomelike structures associated with the axonal membrane which are connected to a network of neurofilaments within the axoplasm (Fig. 4). The provision of regular, closely-spaced, supports for the membrane appears to be a hishly effective way of reducing the tension to which it is subjected during hypo-osmotic stress. With such an arrangement a small membrane extension allows a very large dernm~ in tension. It is calculated, for example, that for a notional excess of
I.',3
T I N S - D e e e m b ~ 1978
÷20
-20
--40
°i
-60 -80
R I ms
Fig. 3. Imrace!lularly recorded action potentials measured initiai l.v in anax on in I00 % saline (left) and in the same axon after proltrestive dilution o f tile bathing medium to one.fifth o f the original concentration (right). Thit correH~ond~ to a dilution o f external sodium from 482.3 to 96.$ mM which, paradoxica(ly, i~ associated with an Mcrea~e in the amplitude and the rate o f rise o f the ~odium. mediated action potentials.
internal osmotic concentration of 100 mOsmol a 10% membrane extension would reduce the tension to less than 1% of that of an unextended axon {i.e. to about the tension which th~ membrane of a human red cell can withstand before haemolysis occurs)L
Another specialization of the Mercierella axon results from the unusually high potassium concentration ( > 3 0 raM) of the blood o f sea water-adapted individuals. The significance of this is that, although potassium is the principal ion responsible for maintaining the resting potential of most nerve cells, low concentrations o f other ions (especially sodium) make sight. fic~nt contributions, so that the curve relating the resting potential to potassium concentration becomes progressively less steep as the internal ionic concentration is lowered. In most animals the blood potassium concentration lies in the region of I0 mM or less, where the resting potential (and hence the general level of excitability) of the neurones is relatively insensitive to changes in the external concentration. In Mercierella, on the other hand, the normal blood potassium concentration of 30 mM lies on a much steeper part of the resting potent; d/concentration curvea Deleterious c~Leets resulting from dilution of the blood can thus he partly counteracted, since the concomitant fall in potassium concentration can cause a larger increase in the resting potential (hyperpolarization) of the nerve cell membrar~:s. This accounts for the appreciable hyperpolar/zatiott observed during adaptation to hypo-osmo:ic dilution (Fig. 2), for, although the intracellular potassium is lost
from the axon, the dilution of the axoplasmic potassium is proportionately less than that in the external medium z. The reduced level of intracellular potassium during hylm-osmotic dilution represents a balance between the necessity to contribute to osmotic equilibration and to maintain a potassium gradient across the axon membrane sufficient to produce an appreciable increase in the resting potential during dilution of the external medium. The increase in axonat resting potential observed during hypo-osmotic adaptation has two important consequences. First. it helps to maintain the amplitude of the action potential by compensating for the fall in the peak of the action potential which results from the dilution of the external sodium ions that carry the inward current of the action potential. Secondly, this increase in resting potential reduces the inactivation of the sodium channels (the built-in shut-off device which becomes increasingly effective as the membrane potential is reduced)~. This reduction in inactivation increases the rapid rise of the action potential (Fig. 3) by increasing the transitory entry, of sodium ions through the axon membrane. Thus, unlike the squid axon, in which dilution of the external medium causes a marked reduction in both amplitude and rise time of the action potential, the action potentials in the Merclerel/a axon are maintained at a #
.
relatively large amplitude, with rapid rat,:s of rise. during dilution of the blood. Sodium balance Dqutioa of the external medium also causes a reduction in the concentration of sodium ions within the axon. This reduction is abolished by the sodium transport inhibitor, ouabain, and could result from an increa~ in activeextrusion of this cation from within the axon, as appears to occur in the giant axon of the squid during exposure to sodium-deficient saline ~. Whatever the mechanism invol-ed, the import. ant conclusion is that the reduction in intrac¢llular sodium in dilute media tends to maintain the inward electrochemical gradient of sodium ions across the axon membrane during ionic dilution. The existence of these adaptive strategies provides an explanation o~" an apparent physiological paradox: an ,,xon in which the amplitude and ri~e times of the sodiummediated action potentials are maintained. or even increased, during extreme dilution of external sodium and other ions. These observations also establish that the elet:trical properties of nerve cells are not necessarily dependent on their ionic milieu. for the Merciere'la axon, at least, can adapt, relati~el} rapidb, to large changes in the chemical composition of ~ts en~ Jronment. The adaptability of the .tler('ierella axons is ob~iously an extreme condition, but the possibilit) exists that neurones in the n e r v o u s systems of higher orgamsms could adapt to more subtle changes in their immediate fluid en~irorm¢nt. Q
Rculdlag I ~ I. Abbott. N. J. an,: Trchern¢. J, l:. ( l ~ 7 , j In. B. L. Gupta. R. B..Morcton. J L O~hman and B. J. Wall (¢J.l, Tran.~po,r ,# I,m~ .tz,t Water in .4nimuh. A~adcnls¢ Pres~, London
and N¢, York. 2. Iknson. J. A. an.~ l"rehern¢. J E. (197~ J £.[p. B~oL Iin press). 3. Carlson. A. D and ]'rch¢rn¢, J |: (19"", J
E.~p. Biol. 67. 205 ZIS. 4, Hodgkzn. A, L, dad Huxlo. A F t|95-~) J Physiol. I Io. 49"- 506. 5. Hodlgkln. A. L. and Keyn¢,, k D. (1955) J Physiol. 12S. 28.60. 6, Skaer, H, i¢ B. t1974j J. Etp. B~ol. 60..~21 3~, 7. $kaer, H. I¢ B., rrcherne. J. F., Bcnson, ) A. and Morcton, R. D. (1978) J. I=ap. Biol. (m prg~s :. 8. Trehcrne, J. E.. Bgnson, J. A. and Skaer, H. I¢ B. ~.1977) Nature {London) 269, 430-431,
Fig. 4, Electronmicrograph o f a giant axon ~GA) o f Mercicr¢lla sha,.ing an unusual structural
spt.ciali:mion: a system o f m'uro]ilamems a,hich insert inla ~pccialt:ed mcmbram, ar,,as at, th ; a.ton membrane (small arroa's). The aaon sue/me has incomplete glial coverage (g) and iJ directly actessibl¢ from tlw blood (large arrow).
J E. Treherne is Unil'ersity Reader il~ inr¢~fcb~ut," Physiology at Ca,nbridge. Fellow ol Downing College. and Director o f the A.R.C. t't~t: o/ Inwrtebrate Chemistry and Phys'.olog) at tlu" Deparlmrn! o f Z,,olog). L'niver~ity o f Camhr,t.¢c. Do~,winp Street. Camb:-idg¢ CB2 3El. U.K.
TINS. ~
19"/8
Neuronal adaptability John E. Treherne ,.
|l
|
i i
Recent eleclrophyf~ologtcal research hat J~own thai tome htpertdwate m eelle are able to maintain excitability in theface o f naz~ve changes in the osmotic and ionic concenlrations o f the bathing medium. ?'hit eslablhdwt the important pthtetple that tmarotw~ need IJot necetlarily be at the mercy of their lmmedlme fluid ~ i r a n n ~ t , but can adapt, relatively rapidly, to changes in the chemical composition of their body fluids. In lklt article John Treherne explains the mechanisms by which an um~tal esluartne worm maintains ne'aan~l excitability.
Nerve ceils are I~merally supposed to require a fluid environment of stable chemical composition. This. supposition results from the fact that our knowledge of the ionic basis of neuronal excitability has been largely derived from studies of the giant nerve ceils of marine invertebrates •E (in which ionic and osmotic homeostasis is provided by environmental stability, or by reSulation of the Mood composition), or from the nerves of vertebrate animals and insects (in which the neuronal microenvironment is regulated by well-developed Mood-brain barrier systems)t. sdsptsl~ worm The nerve cells of some invertebrates, however, have been showli recently to he
1O07O60.5O40353O
25
IS
20
7.5 Dilution (%)
I0
,40 ,20
o -20
Action potentials
--4O -6O
.
-8O L 0
i I
i 2
.
.
.
.i J
•
I 4 Hours
i $
,
i 6
i 7
! 8
Au
Fit. 2. Continuous recording of tie axowl rtttint and action potentials, mttuwtd with aR iatracflhdar microel¢ctrode, during prolreulve dlluttml of the basking mediumt. Despite tie tJtbMalttlatdilution of external ~llum (from an i~tlal com'eatration of ,182J down to 36,2raM) ~ tie reduced osmotic concentratioR (from 1024 to 77 mOtnwl), actiolt potentials coMInue to be tlleited. 7lie hypo.otmotlc adaptatiolt it associated witlt 4 pro~ntiw Diocese ill retting potential (It.vlm'polarlgatlon).
able to function adequately, despite large clum~gesin the ionic and osmotic concentration of the body fluids. A most spectacular example of this occurs in a small estuarine worm, Mercierella enigmutica
F'~. !. Tie polyckaete ~orm. Merciere~ enigrustics,/: the emxstextremeo.¢moco~ormerImmm. TAisspecies can witlntmvdvarimiottsin the osmotic com'entratiolt of its 6ody flwtds from 84 to 2J04 mOsmoP.
© F.i~/Nott~,~HolluOBiomufiud~
1978
(Fig. I), whose nerve cells ca~ adapt, relatively rapidly, to massive ,Jilution of the Mood resulting from clumges in environmental salinity6. This remarkable ability can he demonstrated in ~itro by progressive dilution of the physiological saline bathing the isolated preparation of the so-called giant u o n (Fig. 2p. The Mercierella u o n is, nevertheless, similar to those of other animals (for example, the giant u o n of the squid) in that action potentials appear to he mediated by an inward movement of sodium ices and are blocked, as in other axons, by the poison tetrodotoxin which specifically Mocks the sodium channel in conventional axon membranes*, it is, therefore, surprising that the amplitudes of the action potentials can in some cases actually increase, and their rise times become faster, following
substantial dilution of external sodium and
other iota (Fig 3). S m m . ~ mlq~t The axons of Mercierella use a number of intriguing strategies which enable them to function in such an inconstant environmerit as is fc~md in estuaHt~ surroundings. The axon membrane is protected from the effects of increased internal hydrostatic pressure, following abrupt dilution of the bathing medium, by unustut ultrastructural specializations: hemidesmosomelike structures associated with the axonal membrane which are connected to a network of neurofilaments within the axoplasm (Fig. 4). The provision of regular, closely-spaced, supports for the membrane appears to be a hishly effective way of reducing the tension to which it is subjected during hypo-osmotic stress. With such an arrangement a small membrane extension allows a very large dernm~ in tension. It is calculated, for example, that for a notional excess of
I.',3
T I N S - D e e e m b ~ 1978
÷20
-20
--40
°i
-60 -80
R I ms
Fig. 3. Imrace!lularly recorded action potentials measured initiai l.v in anax on in I00 % saline (left) and in the same axon after proltrestive dilution o f tile bathing medium to one.fifth o f the original concentration (right). Thit correH~ond~ to a dilution o f external sodium from 482.3 to 96.$ mM which, paradoxica(ly, i~ associated with an Mcrea~e in the amplitude and the rate o f rise o f the ~odium. mediated action potentials.
internal osmotic concentration of 100 mOsmol a 10% membrane extension would reduce the tension to less than 1% of that of an unextended axon {i.e. to about the tension which th~ membrane of a human red cell can withstand before haemolysis occurs)L
Another specialization of the Mercierella axon results from the unusually high potassium concentration ( > 3 0 raM) of the blood o f sea water-adapted individuals. The significance of this is that, although potassium is the principal ion responsible for maintaining the resting potential of most nerve cells, low concentrations o f other ions (especially sodium) make sight. fic~nt contributions, so that the curve relating the resting potential to potassium concentration becomes progressively less steep as the internal ionic concentration is lowered. In most animals the blood potassium concentration lies in the region of I0 mM or less, where the resting potential (and hence the general level of excitability) of the neurones is relatively insensitive to changes in the external concentration. In Mercierella, on the other hand, the normal blood potassium concentration of 30 mM lies on a much steeper part of the resting potent; d/concentration curvea Deleterious c~Leets resulting from dilution of the blood can thus he partly counteracted, since the concomitant fall in potassium concentration can cause a larger increase in the resting potential (hyperpolarization) of the nerve cell membrar~:s. This accounts for the appreciable hyperpolar/zatiott observed during adaptation to hypo-osmo:ic dilution (Fig. 2), for, although the intracellular potassium is lost
from the axon, the dilution of the axoplasmic potassium is proportionately less than that in the external medium z. The reduced level of intracellular potassium during hylm-osmotic dilution represents a balance between the necessity to contribute to osmotic equilibration and to maintain a potassium gradient across the axon membrane sufficient to produce an appreciable increase in the resting potential during dilution of the external medium. The increase in axonat resting potential observed during hypo-osmotic adaptation has two important consequences. First. it helps to maintain the amplitude of the action potential by compensating for the fall in the peak of the action potential which results from the dilution of the external sodium ions that carry the inward current of the action potential. Secondly, this increase in resting potential reduces the inactivation of the sodium channels (the built-in shut-off device which becomes increasingly effective as the membrane potential is reduced)~. This reduction in inactivation increases the rapid rise of the action potential (Fig. 3) by increasing the transitory entry, of sodium ions through the axon membrane. Thus, unlike the squid axon, in which dilution of the external medium causes a marked reduction in both amplitude and rise time of the action potential, the action potentials in the Merclerel/a axon are maintained at a #
.
relatively large amplitude, with rapid rat,:s of rise. during dilution of the blood. Sodium balance Dqutioa of the external medium also causes a reduction in the concentration of sodium ions within the axon. This reduction is abolished by the sodium transport inhibitor, ouabain, and could result from an increa~ in activeextrusion of this cation from within the axon, as appears to occur in the giant axon of the squid during exposure to sodium-deficient saline ~. Whatever the mechanism invol-ed, the import. ant conclusion is that the reduction in intrac¢llular sodium in dilute media tends to maintain the inward electrochemical gradient of sodium ions across the axon membrane during ionic dilution. The existence of these adaptive strategies provides an explanation o~" an apparent physiological paradox: an ,,xon in which the amplitude and ri~e times of the sodiummediated action potentials are maintained. or even increased, during extreme dilution of external sodium and other ions. These observations also establish that the elet:trical properties of nerve cells are not necessarily dependent on their ionic milieu. for the Merciere'la axon, at least, can adapt, relati~el} rapidb, to large changes in the chemical composition of ~ts en~ Jronment. The adaptability of the .tler('ierella axons is ob~iously an extreme condition, but the possibilit) exists that neurones in the n e r v o u s systems of higher orgamsms could adapt to more subtle changes in their immediate fluid en~irorm¢nt. Q
Rculdlag I ~ I. Abbott. N. J. an,: Trchern¢. J, l:. ( l ~ 7 , j In. B. L. Gupta. R. B..Morcton. J L O~hman and B. J. Wall (¢J.l, Tran.~po,r ,# I,m~ .tz,t Water in .4nimuh. A~adcnls¢ Pres~, London
and N¢, York. 2. Iknson. J. A. an.~ l"rehern¢. J E. (197~ J £.[p. B~oL Iin press). 3. Carlson. A. D and ]'rch¢rn¢, J |: (19"", J
E.~p. Biol. 67. 205 ZIS. 4, Hodgkzn. A, L, dad Huxlo. A F t|95-~) J Physiol. I Io. 49"- 506. 5. Hodlgkln. A. L. and Keyn¢,, k D. (1955) J Physiol. 12S. 28.60. 6, Skaer, H, i¢ B. t1974j J. Etp. B~ol. 60..~21 3~, 7. $kaer, H. I¢ B., rrcherne. J. F., Bcnson, ) A. and Morcton, R. D. (1978) J. I=ap. Biol. (m prg~s :. 8. Trehcrne, J. E.. Bgnson, J. A. and Skaer, H. I¢ B. ~.1977) Nature {London) 269, 430-431,
Fig. 4, Electronmicrograph o f a giant axon ~GA) o f Mercicr¢lla sha,.ing an unusual structural
spt.ciali:mion: a system o f m'uro]ilamems a,hich insert inla ~pccialt:ed mcmbram, ar,,as at, th ; a.ton membrane (small arroa's). The aaon sue/me has incomplete glial coverage (g) and iJ directly actessibl¢ from tlw blood (large arrow).
J E. Treherne is Unil'ersity Reader il~ inr¢~fcb~ut," Physiology at Ca,nbridge. Fellow ol Downing College. and Director o f the A.R.C. t't~t: o/ Inwrtebrate Chemistry and Phys'.olog) at tlu" Deparlmrn! o f Z,,olog). L'niver~ity o f Camhr,t.¢c. Do~,winp Street. Camb:-idg¢ CB2 3El. U.K.