Inhibition of a Hyperpolarization-Activated Current by Clonidine in Rat Dorsal Root Ganglion Neurons

Abstract

Whole cell voltage- and current-clamp recordings were carried out to investigate the effects of clonidine, an alpha 2-adrenoceptor agonist, in L4 and L5 dorsal root ganglion (DRG) neurons of the rat. In voltage-clamp mode, application of 20 microM clonidine reversibly reduced the inward current evoked by hyperpolarizing voltage steps. The "clonidine-sensitive current" was obtained by subtracting the current during clonidine application from the control current, and its properties were as follows. 1) It was a slowly activating inward current evoked by hyperpolarization. 2) The reversal potential in the standard extracellular solution ([K+]o = 5 mM, [Na+]o = 151 mM) was -38.3 mV, and reduction of [Na+]o shifted it to a more negative potential, whereas an increase of [K+]o shifted it to a more positive potential, indicating that the current was carried by Na+ and K+ (PNa/PK = 0.22). 3) The relationship between the chord conductance underlying the clonidine-sensitive current and voltage could be fitted by a Boltzmann equation. These results indicate that the clonidine-sensitive current corresponds to a hyperpolarization-activated current (Ih), i.e., clonidine inhibits Ih in rat DRG neurons. DRG neurons were classified as small (15.9-32.9 microns diam), medium-sized (33-42.9 microns), and large (43-63.6 microns), and 7 of 19, 24 of 25, and 22 of 22 of these types exhibited Ih with mean +/- SE clonidine-induced inhibition values of 36.1 +/- 3.5% (n = 7), 43.1 +/- 3.7% (n = 24), and 35.1 +/- 2.7% (n = 22), respectively. Clonidine application to L4 and L5 DRG neurons excised from rats the sciatic nerves of which had been transected 14-35 days previously (transected DRG neurons) also reduced Ih. In current-clamp mode, 9 of 13 intact and 4 of 6 transected medium-sized DRG neurons that exhibited Ih responded to clonidine with hyperpolarization (> 2 mV). Some medium-sized DRG neurons exhibited repetitive action potentials in response to a depolarizing current pulse, and clonidine reduced the firing discharge frequencies in 8 of 11 intact and 3 of 4 transected neurons tested. Injection of a hyperpolarizing current pulse produced time-dependent rectification in DRG neurons that exhibited Ih, and clonidine blocked this rectification in all intact and transected neurons tested. These results suggest that inhibition of Ih due to alpha 2-adrenoceptor activation contributes to modulation of DRG neuronal activity in rats. On the basis of our findings, we discuss the possible mechanisms whereby sympathetically released norepinephrine modulates the abnormal activity of DRG neuronal cell bodies after nerve injury.

Direct Inhibition of Ih by Analgesic Loperamide in Rat DRG Neurons

“…On the functional level, I h kinetics and pharmacology were close to those reported previously (Cardenas et al 1999;Scroggs et al 1994;Yagi and Sumino 1998). The threshold for I h activation ranged between -55 and -60 mV; voltage for half-maximal activation and the slope coefficient were V 1/2 ϭ Ϫ73.2 Ϯ 0.8 mV and k ϭ 8.5 Ϯ 0.7 mV, respectively, which was similar to values reported by Cardenas et al (1999) (V 1/2 ϭ Ϫ73.3 mV, k ϭ 7.0 mV).…”

Presence of functional hyperpolarisation-activated cyclic nucleotide-gated channels in clonal alpha cell lines and rat islet alpha cells

“…To construct the steadystate activation curve of these channels, the tail currents measured at −140 mV immediately after the conditioning voltages (indicated by an arrow in Fig. 2b) were normalised and plotted as a function of the conditioning membrane voltages, then the data were fitted using the Boltzmann equation [28]. As shown (Fig.…”

Noradrenergic modulation of the hyperpolarization-activated cation current (Ih) in dopamine neurons of the ventral tegmental area

“…Norepinephrine is known to modulate the I h of thalamic relay neurons (McCormick and Pape, 1990), neurons from the medial nucleus of the trapezoid body (MNTB) (Banks et al, 1993) and primary sensory neurons of the dorsal root ganglia (Yagi and Sumino, 1998). In the thalamus, NE activates beta-adrenergic receptors elevating intracellular cAMP levels and shifting I h voltage dependence to more depolarized potentials.…”

“…The I h is a depolarizing current that affects cell excitability in three interrelated ways: it helps to determine the resting potential and firing frequency; reduces the amplitude and duration of hyperpolarizing inputs; and generates pacemaker activity when coupled to other voltage-dependent conductances (e.g., I T , a voltage dependent calcium current) (Luthi and McCormick, 1998;Pape, 1996). In the lateral geniculate nucleus of the thalamus (McCormick and Pape, 1990), in primary sensory neurons of the dorsal root ganglia (Yagi and Sumino, 1998), and in neurons of the medial nucleus of the trapezoid body (Banks et al, 1993), norepinephrine (NE) affects the state of excitability by modulation of the voltage sensitivity of I h . The possibility of noradrenergic modulation of I h in VTA DA cells has not been studied.…”