Temperature compensation of the threshold potential for excitation of the snail bursting neuron

Camp. Biochem. Ph~~siol.Vol. 76A, No. 1, pp. 173 to 176, 1983 Printed in Great Britain

0300.9629/83$3.00+0.00 a; 1983 Pergamon Press Ltd.

TEMPERATURE COMPENSATION OF THE THRESHOLD POTENTIAL FOR EXCITATION OF THE SNAIL BURSTING NEURON DEJAN ZE~EVI~:

Institute

for Biological

Research,

University

and

of Belgrade,

(Rewired

MIRA

PAW

29 Novembar

6 January

142, 11000 Beograd,

Yugoslavia

1983)

Abstract-l. The efTects of short- and long-term temperature transition on the threshold potential for excitation (0,) and on temporal characteristics of the action potential were examined in the identified bursting neuron of the snail Hel;.~ poma/iu. 2. Abrupt cooling (from 25 to -0.5 C) induced a consistent and reversible increase of the threshold voltage in the Br neurons from warm acclimated animals. 3. Acclimation of intact animals to low (5 C) temperature resulted in a complete compensation in 0,. That is, threshold potential of the Br neurons from cold acclimated animals recorded at -0.5 C was not different from the value recorded at 25 C in warm acclimated neurons. 4. The duration of evoked action potentials was markedly prolonged by cooling but showed no compensatory changes with acclimation.

MATERIALS

INTRODUCTlON

The significance of the adaptive changes induced in the nervous system of poikilotherms by thermal acclimation is well established (Prosser and Nelson, 1981) although there is much to be learned about compensatory processes at the level of single nerve cells. We have previously shown that the isolated ganglion complex of land snails (Helix) with its identifiable nerve cells is a favorable system for the investigation of the neuronal correlates of thermal acclimation. It was found that thermal acclimation of intact snails to low (5 C) temperature resulted in a perfect compensation in the frequency of endogenous firing of the single, identified nerve cell, and in a partial compensation in the frequency of repetitive firing evoked by depolarization in the same neuron (ZeEevii and Levitan, 1980; Zecevik and Pa%, 198 I). Also, we recently established that acclimationinduced effects in excitability are accompanied by compensatory changes in some characteristics of membrane inward currents elicited by depolarization. Contrary to the effects of abrupt cooling, adaptation to low temperature induced an increase in the maximum peak inward current of identified neuron, as well as the shortening of the process of recovery of inward current from inactivation (ZeEeviC and PaSit, 1981; ZeEeviC, Levitan and Pa%, in preparation). For further experiments concerning the characteristics of voltage-dependent conductances which could be modified by prolonged exposure of the animals to low temperature, we examined here the effects of short- and long-term temperature transition on the threshold potential for excitation (Q,) and on temporal characteristics of the action potential (AP) of the identified (Br) neuron in Helix pomafia. We found that cold acclimation is accompanied by a change in 0, that compensates completely for the increase initially induced by abrupt cooling. We also found that thermal acclimation had no obvious effect on the duration of AP. 173

AND METHODS

Land snails Helix pomutia (collected locally) were divided into two groups and acclimated for at least 3 weeks to either 25 or 5 ‘C. The identified giant Br neuron in the right parietal ganglion was used in all experiments. This cell is probably the same bursting neuron described by Kerkut and Meech (1967) in Helix aspersa and by Sakharov and Sal6nki (1969) in H&r pomatiu. The acclimation procedure, dissection, temperature control, and composition of the normal physiological solution were as previously described (ZeEevii and Levitan. 1980). Standard techniques were used for recording membrane potential and for passing transmembrane polarizing currents by means of two intracellular microelectrodes filled with 3 M KCI. RESULTS

As previously described (ZeEevik and Levitan, 1980), the Br neurons of the warm acclimated dormant snails were, at 25 ‘C, characterized by spontaneous bioelectrical activity. Abrupt cooling from 25 to -0.5 ‘C resulted in an inhibition of their spontaneous firing, with little or no change in the resting membrane potential. Also, at low temperature the Br neurons could not sustain repetitive discharge upon depolarization (Fig. I). These results are similar to ones reported previously after cooling the Br neuron to somewhat higher temperature (ZeEevii- and Levitan, 1980). However, since at -O.YC depolarization always initiated a single action potential, it was possible to determine the effect of abrupt cooling on the threshold, H,, and on the duration of AP. To determine the threshold potential under different thermal conditions the Br neuron, which is at high temperature spontaneously active and has no real resting membrane potential, was tested from a potential level of -60 mV under current clamp condition. When hyperpolarized to - 60 mV the cell was silent and the membrane resistance remained stable.

DEJAN ZEC?EVI~. and MIKA PA&~

174

Temperature

:

-0.5”C

Fig. 1. Responses of warm (A) and cold (B) acclimated Br neuron amplitude. Upper trace: zero potential: middle trace: membrsne stimulus current.

For 0, determination the neuron was depolarized toward threshold by transmembrane current pulses of varying amplitude and at least 5 times longer in duration than the membrane time~~~nstant. Successive pulses were separated by at least 20 sec. The threshold membrane potential was determined by increasing depolarizing current until the first AP was generated. The membrane potential reached during the largest local response was regarded as the threshold, 0, (Fig. 2). When tested at 25°C the threshold membrane potential of Br neurons from warm acclimated snails was 0, = -49 + 0.6 mV (SEM, n = 16). Abrupt cooling of these neurons from 25 to -0.5 C markedly and reversibly increased 0, to - 37 + 1.1 mV (SEM. n = 16) (Fig. 2). The duration of evoked AP (as determined at OmV) increased markedly with cooling from 5.6& 0.3msec at 25%, to 71 i_ 3Smsec at -0.5 C (SEM, n = 7). Cold ucclimatinn

The threshold membrane

potential

at -0S’C

of

at -0.5’ C to depolarization of varying potential; lower trace: t~lnsmembrane

Br neurons from animals acclimated to 5 C for at least 3 weeks was -49 + 0.9 mV (SEM. n = 5). This value is significantly (P c; 0.01, f-test) lower than in neurons from warm acclimated animals at -0.5 ‘C, but not different (P > 0.4, t-test) from the threshold potential recorded at 25 C in warm acclimated neurons (Fig. 3). The duration of action potentials evoked in cold adapted Br neurons was 63.5 t: 6.1 (SEM, II = 5) at -0.5 C. This value was not signi~cantly different (P > 0.2, t-test) from that found in neurons from warm acclimated snails at the same temperature (Fig. 3). DISCUSSION

The present results show that there is a consistent and reversible increase in threshold voltage with abrupt decrease in temperature in Br neurons from warm acclimated snails, and that acclimation of intact animals to low temperature results in a complete compensation in (I, value of the identified neuron.

Fig. 2. The depolarization required to initiate an impulse at different temperatures from a fixed membrane potential of -60 mV in Br neurons from warm (A) and (B) and cold (C) acclimated snails. In each trace subthreshold sweeps are superimposed with one which was just suprathreshold. Upper trace: zero potential; middle trace: membrane potential: lower trace: transmembranc current.

---___ 7 Temperature

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Fig. 3. The eflect of temperature (full line) and cold acclimation (dashed line) on threshold membrane potential (open circles) and on action potential duration (filled circles).

The duration of evoked APs, although markedly temperature sensitive, shows no compensatory changes with acclimation. Cooling The threshold voltage of excitable tissue from different animals is increased by lowering temperature including Aplysia (Murray, 1966) chick embryos (Sperelakis, 1970), land snail Otala h-tea (Barker and Gainer, 1975), and locust (Heitler et al., 1977). Particularly clear effect of abrupt cooling was demonstrated in an identified motoneurone of the warm (31’C) acclimated locust (Heitler et al., 1977) where temperature reduction clearly increased the absolute threshold voltage for action potentials initiated by short and long depolarizing current pulses, as well as by naturally occurring EPSPs. This increase in absolute voltage threshold was found to be the primary cause of reduced excitability of the locust motoneurone at low environmental temperature. These results are consistent with changes in threshold potential with temperature that we recorded in Br neurons of warm acclimated snails, Helix pomatiu. In squid giant axon (Guttman, 1966) and Aplysiu nerve cells (Carpenter, 1967) however, the threshold potential is either independent of temperature or decreases slightly after cooling. These findings are in apparent contrast to our results. but such differences are not surprising since it has been shown that thermal characteristics of threshold for excitation of identified neurons may be different even between individuals of the same species (Heitler et al., 1977). Unfortunately, precise interpretation of the possible bases for the inter- and intra-species differences is hampered by the lack of data concerning thermal characteristics of different membrane parameters (passive membrane characteristics, voltage-dependent conductance changes, see FitzHugh, 1966) that determine threshold phenomena. Our finding that abrupt cooling markedly increased the duration of the action potential of Br neuron confirms the results reported by others (cf Tasaki and Fujita, 1948; Hodgkin and Katz, 1949; Kerkut and Ridge, 1962; Zeeevik and Levitan, 1980) and this effect is certainly based on the influence of temperature on the kinetics of voltage-dependent

in snail neuron

175

membrane conductance changes (cf Hodgkin and Huxley, 1952; Frankenhauser and Moore, 1963: Partridge and Connor, 1978). Cold ucclimution

The marked increase in threshold potential with abrupt cooling from 25 to -0.5 C was transitory. After a period of cold acclimation the threshold potential of the Br neuron decreased and, at -0.5 C, was about the same as in the neurons from warm acclimated animals at 25 C. Such complete physiological compensation associated with a period of acclimation is undoubtedly important in achieving reactivation of the nervous system in an animal that must function over a wide range of body temperatures. The adaptive influence of long-term temperature transitions on the threshold potential for excitation has been previously recorded by Dierolf and McDonald (1969) in the giant axons of the earthworm. Contrary to our results. they recorded a decrease in threshold potential with abrupt cooling, and a compensatory increase resulted from adaptation to low (5 C) temperature. These results are, however, not readily comparable to ours, since 0, in the earthworm was determined relative to the resting membrane potential, which itself varied markedly with temperature and with thermal acclimation. Compensatory changes in the threshold potential with acclimation described in the present work should be attributed to underlying membrane phenomena. Therefore, further work should be directed toward examination of voltage-dependent ionic conductances under different thermal conditions. Our preliminary results show that cold acclimation influences, in compensatory manner. maximum peak inward current and the kinetics of the recovery of inward current from inactivation (ZeEevik and Pa%, 1981; ZeEevi& Levitan and Pa%, in preparation). In more general terms, one may envisage the adaptive changes in excitability as a process based upon several well documented long-term effects of temperature on membrane lipid composition and fluidity (Sinensky, 1971: Cossins. 1977: Cossins and Prosser. 1978: Willis, 1979). Fluidity of the lipids in the membrane seems to have an effect on the kinetics of asymmetric charge movement (functionally connected to the opening and closing of ionic channels) as proposed previously (Hammel and Zimmermann, 1970; Kimura and Meves, 1979; Schwartz, 1979). The lack of effect of temperature acclimation on AP duration confirmed previous results of ours obtained over a narrower temperature range (ZeceviC and Levitan, 1980). This is in contrast to the pronounced compensatory effects of acclimation on the duration of APs described in the giant axons of the earthworm (Dierolf and McDonald, 1969). The discrepancy might be based upon unequal capability for temperature adaptation in different species.

REFERENCES

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176

DEJAN ZE~EVI~. and MIKA PASI~‘

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