Inactivation of Voltage-Activated Na Currents Contributes to Different Adaptation Properties of Paired Mechanosensory Neurons

Inactivation of Voltage-Activated Na Currents Contributes to Different Adaptation Properties of Paired Mechanosensory Neurons PÄIVI H. TORKKELI, SHIN-ICHI SEKIZAWA, AND ANDREW S. FRENCH Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada Received 27 March 2000; accepted in final form 2 January 2001 Torkkeli, Päivi H., Shin-ichi Sekizawa, and Andrew S. French. Inactivation of voltage-activated Na currents contributes to different adaptation properties of paired mechanosensory neurons. J Neurophysiol 85: 1595 1602, 2001. Voltage-activated sodium current (I Na ) is primarily responsible for the leading edge of the action potential in many neurons. While I Na generally activates rapidly when a neuron is depolarized, its inactivation properties differ significantly between different neurons and even within one neuron, where I Na often has slowly and rapidly inactivating components. I Na inactivation has been suggested to regulate action potential firing frequency in some cells, but no clear picture of this relationship has emerged. We studied I Na in both members of the paired mechanosensory neurons of a spider slit-sense organ, where one neuron adapts rapidly (type A) and the other slowly (type B) in response to a step depolarization. In both neuron types I Na activated and inactivated with single time constants of 2 3 ms and 5 10 ms, respectively, varying with the stimulus intensity. However, there was a clear difference in the steady-state inactivation properties of the two neuron types, with the half-maximal inactivation (V 50 ) being 40.1 mv in type A neurons and 58.1 mv in type B neurons. Therefore I Na inactivated closer to the resting potential in the more slowly adapting neurons. I Na also recovered from inactivation significantly faster in type B than type A neurons, and the recovery was dependent on conditioning voltage. These results suggest that while the rate of I Na inactivation is not responsible for the difference in the adaptation behavior of these two neuron types, the rate of recovery from inactivation may play a major role. Inactivation at lower potentials could therefore be crucial for more rapid recovery. INTRODUCTION Voltage-activated I Na is the main current responsible for the rising phase of the action potential in many neurons. Some neurons have two components of I Na, one inactivating in the millisecond range and the other being more persistent. The slower component has been found in neurons that adapt slowly, where it recovers from inactivation more rapidly than the fast component (e.g., Elliott and Elliott 1993), and it was suggested to have a role in allowing high-frequency firing. However, a different role for slow I Na inactivation has also been presented, where it was assumed to cause adaptation of action potential firing (Gestrelius and Grampp 1983). The lyriform slit-sense organ VS-3 in the patella of the spider, Cupiennius salei (nomenclature of Barth and Libera 1970) has seven to eight slits, which are each innervated by a pair of bipolar mechanosensory neurons. Both neurons in each pair respond to a sustained mechanical or electrical stimulus Address reprint requests to P. H. Torkkeli (E-mail: Paivi.Torkkeli@dal.ca). with a rapidly adapting burst of action potentials. However, adaptation to silence in type A neurons occurs in 20 ms, often producing only one or two action potentials, while type B neurons respond with a burst of action potentials lasting several hundred milliseconds (Seyfarth and French 1994). Part of the difference in adaptation to mechanical stimuli can be explained by differences in the dynamic properties in the transduction step from mechanical stimulus to receptor potential (Juusola and French 1998), but a similar adaptation difference is present even when the neurons are stimulated electrically, bypassing the mechanical stage. We have studied the active membrane conductances and their contributions to overall dynamic behavior of the VS-3 neurons. We began by examining the kinetics of voltage-activated I K (Sekizawa et al. 1999) but found only small differences between type A and type B neurons, which may contribute to the differences in their adaptation properties but are unlikely to be major factors. Subsequently, we found a large transient low-voltage activated (LVA) I Ca in both types of VS-3 neurons (Sekizawa et al. 2000), but this also seemed very similar in the two neuron types. We have also found that these neurons do not have Ca 2 -activated K currents [I K(Ca) ] or a classical A- type current (I K ) (Sekizawa et al. 1999), which have both been associated with spike frequency adaptation in some insect mechanosensory neurons (Torkkeli and French 1994, 1995) and other neurons (e.g., Crest and Gola 1993; Rogawski 1985). The action potentials in VS-3 neurons are normally driven by the voltage-activated I Na. Therefore any differences in the kinetics of I Na between type A and type B neurons could have a significant impact on their adaptation properties. Research into the properties of I Na in VS-3 neurons is limited by the fact that these neurons are too small for two-electrode voltage clamp, and it has not been possible to voltage clamp this propagating current, with rapid activation and inactivation kinetics, using single-electrode voltage-clamp methods. Here, we reduced the bath Na concentration to about one-half that in normal saline, which reduced the amplitude of I Na to a level where it could be well clamped. This approach allowed us to investigate the kinetic differences between type A and type B neurons to determine the role of this current in action potential frequency modulation. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. www.jn.physiology.org 0022-3077/01 $5.00 Copyright 2001 The American Physiological Society 1595

1596 P. H. TORKKELI, S. SEKIZAWA, AND A. S. FRENCH FIG. 1. Hypodermis preparation of VS-3 neurons. Intracellular recording electrodes are shown impaling both neurons in pair 4. Type A neurons usually fire only one action potential, while type B neurons fire bursts of up to several hundred milliseconds. Methods of measuring the threshold depolarization (a), action potential amplitude (b), and action potential duration (c) are shown in the recording from the type A neuron. In each case the letter indicates the distance between the 2 lines where it is placed. METHODS Preparation Experiments were carried out using autotomized legs from adult Central American Wandering Spiders, Cupiennius salei, Keys. The spiders were kept in a laboratory colony at room temperature (22 2 C, mean SD). A piece of patellar cuticle containing slit-sense organ VS-3 was dissected free in spider saline, which contained (in mm) 223 NaCl, 6.8 KCl, 8 CaCl 2, 5.1 MgCl 2, 5 sucrose, and 10 HEPES, ph 7.8 (Höger et al. 1997). The hypodermis preparation of VS-3 neurons described in detail by Sekizawa et al. (1999) was used in all experiments and is shown in Fig. 1. Briefly, the neurons were detached from the cuticle but remained embedded in an internal membrane (hypodermis), which was spread onto a small coverslip. The axons and dendrites were crushed at 100 m from the somata to improve the voltage-clamp conditions. The coverslip was placed on a preparation holder where the neurons could be superfused with different solutions during recordings. Statistical analyses were performed using Prophet 6.0 software (AbTech Corporation, Charlottesville, VA). Data were analyzed using a two-sided unpaired t-test for significantly different means or oneway unblocked ANOVA. Chemicals To record I Na, extracellular Na concentration was reduced from 223 to 100 mm, because normal Na concentration did not allow stable voltage clamp. Voltage-activated I Ca was blocked by a combination of 50 M NiCl 2 and 100 M CdCl 2, and voltage-activated I K were blocked by a combination of 25 mm tetraethylammonium chloride (TEA, Kodak) and 25 mm 4-aminopyridine (4-AP) in the bath. CsCl (3 M) in the electrode further reduced I K, but with sharp electrode only a small amount of the pipette solution can reach the cell interior, and therefore the blockade of I K was never complete at strong depolarizations as seen in Fig. 2A. I Na was completely blocked by bath Recording and stimulation Recordings were performed using the discontinuous single-electrode current- and voltage-clamp methods (Finkel and Redman 1984) with an SEC-10 l amplifier (NPI Electronic, Tamm, Germany). We have previously described the conditions for successful single-electrode voltage- and current-clamp in detail (Torkkeli and French 1994), and the same methods have been used in VS-3 neurons to study voltage-activated I K (Sekizawa et al. 1999) and I Ca (Sekizawa et al. 2000). A horizontal puller (P-2000, Sutter Instrument, Novato, CA) was used to pull microelectrodes from borosilicate glass (1 mm OD and 0.5 mm ID). Electrodes were filled with 3 M CsCl, and their resistances were 40 90 M. Electrodes were coated with petroleum jelly to reduce stray capacitance (Juusola et al. 1997), and they had time constants of 1 3 s in solution. Switching frequencies of 20 40 khz and a duty cycle of 1/4 (current passing/voltage recording) were used in all experiments. All voltage-clamp experiments were performed by averaging three current recordings with 5-s intervals. The neurons were observed under bright-field optics (Olympus, Tokyo, Japan). They were impaled with high-frequency oscillation ( buzzing ) followed by a 15-min stabilizing period before recordings. All current- and voltage-clamp experiments were controlled by an IBM compatible computer with custom written software (ASF Software, Halifax, Nova Scotia, Canada). Voltage and current stimuli were provided by the computer via a 12-bit D/A converter. The membrane potential recording was low-pass filtered at 33.3 khz and the current at 3.3 khz by the voltage-clamp amplifier. Membrane resistance was calculated from the linear part of the current-voltage curve at hyperpolarizing potentials, and this value was used for leakage subtraction. FIG. 2. I Na in VS-3 neurons. A: currents elicited from a holding potential of 100 mv to potentials from 90 to 30 mv at 10-mV intervals. Top panel shows the control recording, and the bottom panel shows identical recording after I Na was blocked with 1 M TTX. B: an example of subtraction of the current trace after TTX from a trace in a control recording to eliminate the outward currents. This recording was obtained with a voltage pulse of 20 mv.

SODIUM CURRENT IN MECHANORECEPTOR NEURONS 1597 TABLE 1. Summary of parameters for VS-3 neurons Parameter Type A Type B P Membrane potential, mv 56.9 6.4 (28) 61.6 6.0 (32) 0.0047 Threshold, mv 31.9 6.8 (25) 25.0 7.4 (31) 0.0008 Action potential amplitude, mv 34.1 6.9 (27) 35.4 7.8 (32) 0.493 Action potential duration, ms 6.1 0.8 (27) 6.2 1.1 (31) 0.8615 I Na max, na 0.53 0.27 (20) 0.51 0.34 (29) 0.8336 V 50 activation, mv 30.6 8.1 (11) 32.7 6.8 (12) 0.494 s activation, mv 5.1 1.6 (11) 4.5 1.4 (12) 0.363 V 50 inactivation, mv 40.1 7.8 (7) 58.1 13.3 (8) 0.008 s inactivation, mv 6.8 4.3 (7) 9.3 4.8 (8) 0.319 Values are means SD with the number of experiments in parentheses. Membrane potential, resting membrane potential; threshold, action potential amplitude, and action potential duration, as defined in Fig. 1; I Na max, maximum current; V 50, half-maximal current; s, slope factor. P values were obtained using 2-sided unpaired t-test. application of 1 M TTX, and the subtracted currents (Fig. 2B) could be used for kinetic analysis of I Na. Sucrose was used to adjust the osmolarity. RESULTS Voltage response Na is responsible for producing action potentials in the VS-3 neurons (Sekizawa et al. 2000; Seyfarth and French 1994). We examined the voltage response of both types of neurons to learn whether there were any differences between their action potential firing, other than the well-known difference in the number of action potentials. Figure 1 shows typical responses of both types of neurons, and Table 1 lists the electrophysiological parameters and P values for statistical tests between type A and type B neurons. While the action potential amplitudes and durations were very similar in both neuron types, there were statistically significant differences in the action potential thresholds and membrane potentials. In type A neurons the level of depolarization needed to produce an action potential was about 7 mv more positive than in the type B neurons. Voltage-activated Na current (I Na ) Figure 2A shows a typical voltage-clamp recording from a type A neuron in response to positive voltage stimuli when the extracellular solution contained blockers for I K and I Ca and had low Na concentration (see METHODS). A significant part of the outward I K remained at high depolarizing voltages. However, I Na could be completely blocked by 1 M TTX, and the residual outward currents subtracted to uncover I Na (Fig. 2B). The time courses of activation and inactivation of I Na at different test potentials were fitted by the Hodgkin-Huxley equation for an inactivating current I I 1 e t/t1 m e t/t2 (1) where I is the current level expected in the absence of inactivation, t 1 is the time constant of activation, t 2 is the time constant of inactivation, and m is an integer exponent (Hodgkin and Huxley 1952). m values of 3 or 4 gave the best fit for all of the current traces. The inset in Fig. 3A shows an example of this fit. The mean ( SD) activation and inactivation time constants of I Na for 6 type A and 11 type B neurons are shown in Fig. 3. Both time constants were voltage dependent, being slightly faster at more depolarizing potentials, but there were no statistically significant differences in either parameter between the two neuron types. I Na activated at about 50 mv (Fig. 4A) and reached its maximum amplitude at potentials close to 20 mv in both types of neurons. There were no statistically significant differences in I Na amplitudes between the two neuron types (Table FIG. 3. Activation and inactivation time constants of I Na. A, inset: an example of a current trace following a stimulus to 20 mv fitted with an exponential power equation (Eq. 1) with a time constant of 1.5 ms for activation and 16.9 ms for inactivation. Mean activation time constants ( SD) from 6 type A( ) and 11 type B ( ) neurons are plotted against test voltage. B: mean inactivation time constants ( SD) from the same neurons as in A at different test voltages.

1598 P. H. TORKKELI, S. SEKIZAWA, AND A. S. FRENCH FIG. 4. Voltage dependence of activation of I Na. A: points show the normalized mean ( SD) peak I Na from 11 type A (F) and 11 type B ( ) neurons plotted against test potentials. B: steady-state activation was determined by fitting the peak currents from each experiment with a Boltzmann distribution (Eq. 2). Data from the same neurons as in A. 1). The experimental activation data were fitted by a Boltzmann distribution of the form 1 I/I max (2) 1 e V V50 /s where I is the current at test potential V, I max is the maximum current, V 50 is the potential giving the half-maximum current, and s is the slope factor. Fitting was performed by the Levenberg-Marquardt general nonlinear fitting algorithm (Press et al. 1990). Boltzmann fits for 11 neurons of both types are shown in Fig. 4B, and the mean ( SD) values for V 50 and s from fits of the same neurons are given in the Table 1. There were no statistically significant differences in these values between the two neuron types. I Na inactivation was also voltage dependent. We determined the steady-state inactivation using 20-ms conditioning pulses followed by a test pulse to a fixed potential (Fig. 5A). Currents were then normalized by the maximum current, plotted as functions of test potential, and fitted by Eq. 2 with appropriate change in the exponential sign (Fig. 5B). The fitted parameters from seven experiments with type A neurons and eight experiments with type B neurons are given in Table 1. While the s values were not different for the two neuron types, the V 50 values were significantly more negative for type B than for type A neurons, indicating that I Na inactivation occurred at more positive potentials in type A than type B neurons. In fact, when the conditioning potential was greater than or equal to 20.0 8.2 mv (n 7) in the type A neurons and greater than or equal to 31.3 11.3 mv (n 8) in the type B neurons, I Na was completely suppressed. Maximum current could be produced when the conditioning potential was less than or equal to 78.6 19.5 mv (n 7) in type A neurons and less than or equal to 98.8 8.3 mv (n 8) in type B neurons. Therefore the ability of type B neurons to fire a burst of action potentials, in contrast to the one or two action potentials produced by type A neurons, does not arise from I Na being more easily inactivated in type A neurons. Inactivation at more depolarized potentials could allow a faster return to potentials from which the neuron can fire a new action potential. To examine the time course of I Na recovery from inactivation, we activated the current from a series of holding potentials ( 40, 60, 80, and 100 mv) to the voltage where a peak current for each cell was recorded ( 30 to 0 mv) for 10 and 20 ms and then applied a similar test pulse after varying periods at the same holding potential (Fig. 6A). Peak I Na activated by the second test pulse was normalized to the peak I Na elicited by the first test pulse, and the normalized current was plotted against time between the two test pulses, and fitted by a single exponential decay. The data revealed a difference in time course of recovery from inactivation between the two neuron types. Type B neurons recovered significantly faster than type A neurons, and the difference between the time constants of the two neuron types was significant at all holding potentials and stimulus durations that were tested (Figs. 6 and 7 and Table 2). However, the recovery from inactivation was also significantly dependent on the holding potential, being fastest at the lowest and slowest at the highest holding potential (Table 2). Interestingly, the duration of the test pulse only caused a statistically significant difference in the type B neurons being faster when the test pulse lasted 10 ms FIG. 5. Steady-state inactivation of I Na. A: example of a recording where holding potentials from 80 to 20 mv for 20 ms were used before a test pulse to a constant potential of 20 mv. B: steady-state inactivation was determined from experiments such as A by fitting the peak currents from each experiment at different voltages with a Boltzmann distribution (Eq. 2). Points show the mean ( SD) for 7 type A neurons ( ) and 8 type B neurons ( ).

SODIUM CURRENT IN MECHANORECEPTOR NEURONS 1599 FIG. 6. I Na recovery from inactivation. A: recovery from inactivation was studied by varying the duration (1 50 ms) at a constant holding potential of 40, 60, 80, and 100 mv between test pulses to fixed voltage, in this example 0 mv. Each experiment was performed using test pulse durations of 10 and 20 ms. B D: experiments with 10-ms test pulses with holding potentials 40 mv (B), 60 mv (C), and 80 mv (D). Normalized data from both types of neurons were fitted with single exponential decay time constants. The mean values and statistical comparisons are shown in Table 2. and slower with 20-ms test pulses (Table 3). These results indicate that the ability of I Na in type B neurons to inactivate at more depolarized potentials allows a more rapid recovery from inactivation and contributes to the production of more action potentials. DISCUSSION This is our third investigation into the properties of voltageactivated currents in the two types of spider VS-3 neurons. We showed previously that the difference between the adaptation properties of the two neuron types is not determined by LVA I Ca (Sekizawa et al. 2000), even though the calcium current can produce large enough depolarizations to bring the neurons to threshold, and even produce large action potentials when I Na and I K are blocked. LVA I Ca is not significantly different between the two neuron types, and its ultimate functional significance has yet to be established (Sekizawa et al. 2000). It is clear that the normal action potentials in both types of neurons are produced by an inactivating I Na and their repolarization is driven by I K (Sekizawa et al. 1999). I K activation occurs closer to the resting potential in type B neurons than in FIG. 7. I Na recovery from inactivation. Similar pulse protocols were used as described in Fig. 6A. In these experiments a test pulse duration of 20 ms was used from holding potentials of 40 mv (A), 60 mv (B), 80 mv (C), and 100 mv (D). Normalized mean values of experimental data were fitted using single exponential decay time constants, and the statistical comparison is shown in Table 2.

1600 P. H. TORKKELI, S. SEKIZAWA, AND A. S. FRENCH TABLE 2. Time constants for recovery from inactivation at different holding potentials Time Constant, ms 40 mv 60 mv 80 mv 100 mv P Type A 61.3 12.8 (5) 25.2 7.1 (14) 19.8 3.7 (9) 13.7 5.6 (6) 0.0001 Type B 29.4 19.3 (6) 14.7 7.1 (25) 10.4 4.4 (15) 6.4 3.4 (7) 0.0001 P 0.0116 0.0001 0.0001 0.0144 Values are means SD with the number of experiments in parentheses. Time constant, single exponential fit to the experimental data; 40, 60, 80, and 100 mv, holding potentials. P values were obtained using 2-sided unpaired t-test for comparison between the type A and type B neurons and 1-way analysis of variance for comparison between different holding voltages. the type A neurons (Sekizawa et al. 1999), and here we found that the same is true for the I Na inactivation. In addition, we showed here that I Na recovers faster from inactivation in type B than type A neurons. Are the differences in I Na inactivation properties responsible for the difference in the adaptation properties of these two neuron types? I Na The extracellular Na concentration was reduced to allow successful voltage-clamp experiments, so the amplitude of I Na was also decreased from its normal value at the physiological Na concentration. The maximum I Na recorded here was small (530 pa in type A neurons and 510 pa in type B neurons) when compared with the I Na amplitudes in some other neurons that use Na action potentials, where it is measured in tens or even hundreds of nanoamps (Baker and Bostock 1997; Benoit et al. 1985; Lapied et al. 1990; Purali and Rydqvist 1998). Another factor affecting I Na amplitude here was that the axons and dendrites of the VS-3 neurons were crushed, and the recordings restricted to the somata and initial axons. I Na amplitudes from dissociated neurons (Elliott and Elliott 1993; Herness and Sun 1995; Honmou et al. 1994) are closer to the values recorded here. However, the resting somatic resistance of these neurons typically exceeds 100 M (Sekizawa et al. 1999) so I Na recorded here are adequate to produce normal action potentials, as we have previously found (e.g., Höger et al. 1997; Sekizawa et al. 2000). The rates of activation and inactivation of I Na in both types of VS-3 neurons were voltage dependent (Fig. 3), but the dependence was not very strong. At 20 mv, where I Na reached its maximum amplitude, the current activated with a time constant of about 2 3 ms and inactivated with a time constant of about 5 10 ms. Significantly faster inactivation time constants have been reported for I Na in other neurons, where they are usually close to or less than 1 ms (Baker and Bostock 1997; Benoit et al. 1985; Bezanilla and Armstrong 1977; Herness and Sun 1995; Lapied et al. 1990; Purali and Rydqvist 1998). However, in many of these neurons, I Na inactivation follows two exponential decay time constants, and the TABLE 3. Time constants for recovery from inactivation at different pulse durations Time Constant, ms 10 ms 20 ms P Type A 26.1 16.6 (10) 27.5 16.9 (24) 0.8307 Type B 10.2 6.0 (18) 16.1 11.4 (35) 0.0164 P 0.015 0.0067 Values are means SD with the number of experiments in parentheses. Time constant, single exponential fit to the experimental data; 10 and 20 ms, test pulse durations. P values were obtained using 2-sided unpaired t-test. slower component varies from 3 ms (Benoit et al. 1985) to persistent (Baker and Bostock 1997; Elliott and Elliott 1993). Slowly inactivating I Na has been suggested to be part of the adaptation process in the slowly adapting lobster stretch receptor neuron (Gestrelius and Grampp 1983) and to decrease the adaptation in rat dorsal root ganglion neurons (Elliott and Elliott 1993). In VS-3 neurons we only found one type of I Na, with no indication of a more slowly inactivating component. These results suggest that I Na inactivation in the VS-3 neurons may account for their relatively rapid adaptation properties, compared with more slowly adapting neurons such as the crustacean stretch receptor neurons, but it may not be able to account for the smaller difference between type A and type B neurons. Peak I Na in both types of VS-3 neurons was produced close to 20 mv (Fig. 4A), and the steady-state activation curves followed similar courses (Fig. 4B). The V 50 values for I Na activation in type A and type B neurons were 30.6 and 32.7 mv, respectively, both close to the values reported in other invertebrate (Lapied et al. 1990) and vertebrate (Elliott and Elliott 1993; Herness and Sun 1995) neurons. These results show that I Na activates at similar potentials in both types of neurons, but when the actual threshold voltages (Resting potential Threshold in Table 1) are applied to the activation curves of Fig. 4B, they indicate that type A neurons need 67% of total I Na at threshold, in contrast to 49% in type B neurons. The major finding here was the significant difference in the steady-state inactivation properties of VS-3 neurons. Type A neurons inactivated at significantly more positive potentials than type B neurons, with the V 50 values being 40.1 mv for type A neurons and 58.1 mv for type B neurons. These values are within the range observed for other sensory neurons in vertebrates (Baker and Bostock 1997; Benoit et al. 1985; Elliott and Elliott 1993; Herness and Sun 1995) and invertebrates (Lapied et al. 1990; Purali and Rydqvist 1998), where V 50 varied between 40 and 65 mv. Interestingly, the rapidly and slowly adapting crayfish stretch receptor neurons have V 50 values of 45 and 41 mv, respectively (Purali and Rydqvist 1998), which was not a statistically significant difference. However, it was in the opposite direction to that found here for VS-3 neurons, indicating that there is no simple relationship between this parameter and the firing patterns of sensory neurons. We found that type B neurons recovered significantly faster from inactivation than type A neurons. It is well known that recovery of I Na from inactivation is strongly dependent on the amplitude of the conditioning voltage, and this is also the case with VS-3 neurons; the more negative the holding voltage, the faster was the recovery of I Na (Fig. 8 and Table 2). The time constant of I Na recovery from inactivation in the squid giant

SODIUM CURRENT IN MECHANORECEPTOR NEURONS 1601 FIG. 8. I Na recovery from inactivation at different holding potentials in type A and type B neurons. Pulse protocols were the same as described in Fig. 6. Normalized mean values of experimental data were fitted using single exponential decay time constant, and the statistical comparison is shown in Table 2. axon was 2.7 ms when a 70-mV prepulse was used and accelerated to 0.6 ms when the prepulse was 130 mv (Bezanilla and Armstrong 1977). Similar differences were seen in cockroach dorsal unpaired median neurons, where a prepulse of 60 mv produced a recovery with two time constants of 2.8 and 31.4 ms, and a 100-mV prepulse reduced these values to 0.8 and 8.4 ms (Lapied et al. 1990). Our results with both types of VS-3 neurons are similar to above-mentioned observations (Fig. 8 and Table 2), and a clear difference between the two neuron types is seen at all holding potentials. These results indicate that the faster recovery from inactivation of the type B neurons may be the factor that explains why these neurons have different adaptation properties. Comparison with the crustacean stretch receptor neuron The crustacean stretch receptor organ has been subject of a number of studies regarding the different adaptation properties of the slowly and rapidly adapting mechanosensory neurons (Swerup and Rydqvist 1992), with recent work concentrating on the action potentials and Na current in the crayfish stretch receptor neurons (Lin and Rydqvist 1999; Purali and Rydqvist 1998). In these studies the rapidly adapting neurons were shown to produce action potentials of lower amplitude and shorter duration than slowly adapting neurons and having smaller inward I Na (Purali and Rydqvist 1998). Both type A and type B spider VS-3 neurons adapt significantly more rapidly in response to a constant stimulus than the crayfish stretch receptor neurons, and they produce action potentials with smaller amplitudes and longer durations. However, we found no significant differences in the action potential amplitudes or durations between type A and type B neurons, and the amplitude of I Na was also similar in both neurons (Table 1). Purali and Rydqvist (1998) suggested that the large difference in the action potential and I Na amplitudes in the rapidly and slowly adapting stretch receptor neurons were due to differences in the densities of Na channels close to the somata (Lin and Rydqvist 1999). The VS-3 neuron Na -channel distribution has been studied using immunocytochemistry, and no differences between the two neuron types were found (Seyfarth et al. 1995). Concluding remarks The most plausible explanation for the different adaptation properties of type A and type B neurons based on our results is that the Na channels have different inactivation properties: I Na inactivated closer to the resting potential in the more rapidly adapting type A neurons, and it recovered from inactivation significantly slower in type A than type B neurons. The tendency of type A neurons to remain inactivated for a longer period than type B neurons could explain why they adapt so much more rapidly. These assumptions may be tested in the future by single-channel analysis. We thank J. Nason and A. Widmer for maintaining the animals. This work was supported by the Canadian Institutes of Health Research. REFERENCES BAKER MD AND BOSTOCK H. Low-threshold, persistent sodium current in rat large dorsal root ganglion neurons in culture. J Neurophysiol 77: 1503 1513, 1997. BARTH FG AND LIBERA W. Ein Atlas der Spaltsinnesorgane von Cupiennius salei Keys. 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