Alpha-1A adrenoceptors modulate potentiation of spinal nociceptive pathways in the rat spinal cord in vitro

Abstract

The rat hemisected spinal cord preparation was used to assess the role of different adrenoceptor subtypes on the modulation of nociceptive reflexes. These were elicited by trains of high intensity electrical stimuli delivered to a lumbar dorsal root. Responses were recorded from the corresponding ventral root in AC- and DC-amplification modes simultaneously.

Superfusion of noradrenaline produced a potentiation of action potential firing (AC channel) as well as a depression of the cumulative depolarisation (DC channel) in responses to repetitive afferent stimulation.

Noradrenaline-induced potentiation of firing was mimicked by the α_1A-adrenoceptor agonist A 61603 and the α₁-adrenoceptor agonist methoxamine in a reversible and concentration-dependent manner. The order of potency of these agonists was A61603>>noradrenaline>methoxamine. The α_1A-adrenoceptor antagonist 5-methyl-urapidil and the α₁-adrenoceptor antagonist corynanthyne blocked the excitatory effects of noradrenaline. In contrast, the α_1B/D-adrenoceptor antagonists chloroethylclonidine and BMY 7378 failed to block this effect.

Noradrenaline-induced depression of cumulative depolarisation was mimicked by the α₂-adrenoceptor agonist UK 14,304. In addition, this compound produced inhibition of firing in responses to afferent stimulation.

These results show that noradrenaline has bi-directional modulatory effects on nociceptive reflexes and indicate that selective activation of α_1A- but not α_1B/D-adrenoceptors mediate potentiation of spinal nociceptive reflexes.

Introduction

Descending noradrenergic pathways originating in pontine A5, A6 (locus coeruleus) and A7 (subcoeruleus) nuclei innervate wide regions of the spinal cord. Electrical or chemical activation of neurones in these and other related nuclei generally produces antinociception in lightly anaesthetised animals which is mediated by α₂-adrenoceptors (for a review see Proudfit, 1992). Consistently, specific pharmacological activation of α₂-adrenoceptors preferentially depresses the responses of deep and superficial dorsal horn neurones to nociceptive stimulation in in vivo and in vitro experiments (Headley et al., 1978, North and Yoshimura, 1984, Willis, 1988, Grudt et al., 1995; for a review see Millan, 1997). A fundamental role in mediating antinociception is attributed to the α_2A-adrenoceptor subtype (Hunter et al., 1997, Stone et al., 1997) but the α_2C receptor subtype may also contribute (Fairbanks and Wilcox, 1999, Graham et al., 2000).

Less well understood is the role of α₁-adrenoceptor subtypes in the control of spinal nociceptive pathways. Three subtypes of α₁-adrenoceptors are generally recognised which are termed α_1A, α_1B and α_1D (see Bylund et al., 1994, Hieble et al., 1995). According to recent reports the α₁-adrenoceptor subtype populations of ventral and dorsal horns are similar. The α_1A and α_1B subtypes are more common (70 and 30% of the total respectively) and the α_1D subtype undetectable (Wada et al., 1996). Stimulation of the locus coeruleus and related structures can excite a small percentage of spinal dorsal horn neurones (Mokha et al., 1983) via α1-adrenoceptors (North and Yoshimura, 1984, Howe and Zieglgänsberger, 1987, Grudt et al., 1995). These excited neurones are generally regarded as inhibitory interneurones and therefore their activation would contribute towards analgesic-like effects (see Millan, 1997). In the ventral horn, selective activation of α₁-adrenoceptors causes depolarisation and increased excitability of motoneurones (Elliott and Wallis, 1992, Larkman and Kelly, 1992, Parkis et al., 1995). Recently Holden et al. (1999) reported that activation of subcoeruleus neurones by direct application of morphine causes a decrease in foot withdrawal time from a thermal noxious stimulus which is blocked by intrathecal administration of α₁-adrenoceptor antagonists. These experiments clearly suggest that noradrenergic control of nociceptive pathways can be bi-directional and suggest that selective activation of α₁-adrenoceptors can lead to pro-nociceptive effects.

The aim of the present investigation was to study further the pharmacology of spinal adrenoceptors involved in the modulation of nociceptive reflexes using the hemisected spinal cord preparation in vitro. The experiments reported were designed mainly to clarify the role of α₁-adrenoceptor subtypes. Simultaneous DC–AC recordings from ventral roots were used to enable the analysis of sub- and supra-threshold events originating at motoneurones in responses to repetitive dorsal root stimulation at intensities sufficient to activate C-fibres. The involvement of different adrenoceptor subtypes in modulating nociceptive reflexes was examined by direct superfusion of selective agents at known concentrations. Preliminary results have been published in abstract form (Hedo and Lopez-Garcia, 2000).