There is a large variability in the kinetic properties o f voltage-gated outward currents in different DRG neurones. The delayed rectifier type potassium currents can be crudely separated into two groups, inactivating and non-inactivating, based on their inactivation properties. Total potassium currents were evoked from -80mV with a step potential to +10mV and the non inactivating potassium currents were evoked after a prepulse to -5OmV from a test potential of+10m V . The inactivating potassium current could be separated by subtraction. The voltage protocol and a typical trace is shown in figure 3.20a. Figure 3.20b and c shows that there was no significant difference in the normalised amplitude o f the non-inactivating and inactivating delayed rectifier type potassium current.
lOpA 10ms n=22 0 .4 0 - n=21 - 0 .3 5 - < 0 .3 0 - 0 .2 5 - 0.2 0- 0 .1 5 - 0.1 0- 0 .0 5 - 0.00 control WtHSV i r n=15 Z 0.75 -o 0.50 o 0.25 control W tH S V 17
Figure 3.20. The effect o f HSV infection o f DRG neurones on inactivating and non inactivating potassium currents, a Inactivating and non-inactivating delayed rectifier type potassium currents from an uninfected DRG neurone (leak subtracted). The total potassium currents were evoked from -80mV with a step potential to +IOmV and the non-inactivating potassium currents were evoked after a prepulse to -50mV w ith a test potential o f + 10mV. The inactivating potassium current could be separated by subtraction, b The norm alised mean non inactivating, potassium current amplitude ± s.e.m is shown for control and wt HSV 17"^ infected DRG neurones. There was no significant difference between the norm alised potassium current amplitude o f control or infected DRG neurones (Student t test, P>0.3). c The normalised inactivating type potassium current amplitudes o f infected and control DRG neurones. There was no significant difference between the normalised current am plitude o f control or infected DRG neurones (Student t test, P>0.2).
A family o f outward potassium currents in DRG neurones was evoked from a holding potential o f -8OmV by a series o f 15ms depolarising voltage steps between -90mV and +40mV. The membrane was then stepped back to -40mV. The tail currents upon stepping back to -40mV are displayed in figure 3.21a. On stepping to -40mV the current falls to an instantaneous value which was proportional to the number o f channels open immediately prior to the step. The tail currents were measured and normalised with respect to maximum tail current, and then plotted against the voltage prior to the -40mV step, giving the activation curve as shown in figure 3.21c.
To analyse the steady-state inactivation properties o f the outward potassium currents, a family o f currents was evoked at +45mV after a Is prepulse to membrane potentials ranging from -120mV to OmV (figure 3.21b). Activation o f outward currents can be seen for the last few prepulse potentials.
The activation and steady-state inactivation curves o f the delayed rectifier type potassium currents are shown in figure 3.21c. There was no apparent change in the voltage dependence o f activation or inactivation o f wt HSV 17^ infected or control DRG neurones. There was no significant difference in the slope factors and V5 0, voltage at half maximal conductance, as shown in table 3.8.
0 .5 n A
0.6- ■2? 'ob 0 . 4 - 0.2- 0.0 -1 2 0 -1 0 0 -8 0 -6 0 -4 0 -20 0 20 4 0 potential (mv)
Figure 3.21. The activation and steady-state inactivation o f control or wt H SV I?"*" infected DRG neurones, a A family o f outward potassium currents in DRG neurones. N eurones were held at -80mV. The activation protocol consisted o f a series o f 15ms depolarising voltage steps between -90mV and +40mV. Note the outward tail currents that occur when the membrane is stepped back to -40mV. b The steady-state
inactivation o f outward potassium currents were evoked at +45mV after a Is prepulse at m embrane potentials ranging from -120mV to OmV. Activation o f outward currents can be seen for the last few prepulse potentials. Leak currents were subtracted, c The activation and steady-state inactivation curves o f control or wt HSV IT’*' infected DRG neurones. The graph shows the normalised mean conductance ± s.e.m plotted against the command potential. The activation and steady-state inactivation curves o f control ( • ) n=25, and wt HSV 17+ infected (■ ) n=l 1 DRG neurones, are shown. The curves are obtained by fitting the data points with a Boltzmann function.
Activation Steady-State inactivation
V 5 0 slope factor V 5 0 slope factor number
(mV) (mV) (mV) (mV) o f neurones control -26.8 ± 1.9 12.8 ± 1.3 -64.7 ± 1.2 17.5 ± 0 .9 25 w tH S V \ T -23.7 ± 1.7 17.3 ± 2 .7 -63.8 ± 1.5 1 5 .7 ± 1 .2 11
Table 3.8. Eletrophysiological characteristics of the delayed rectifier type potassium conductances of control or wt HSV 17"^ infected DRG neurones. V5 0 refers to the
potential ± s.e.m at which conductance (g) is half o f the maximum value (gmax). The slope factor is a function of the voltage dependence of the conductances. The V5 0 and
slope factor values were calculated from Boltzmann curves fitted between the data points from each cell. There was no significant difference between the V5 0 and slope factor
values of the activation curves (Student t test, P>0.07). There was also no significant difference between the V5 0 and slope factor values of the steady-state inactivation curves
of infected and uninfected DRG neurones (Student t test, P>0.1).