Elsevier

Brain Research

Volume 965, Issues 1–2, 7 March 2003, Pages 114-120
Brain Research

Research report
Interactions of 5-HT2 receptor agonists with acetylcholine in spinal analgesic mechanisms in rats with neuropathic pain

https://doi.org/10.1016/S0006-8993(02)04145-8Get rights and content

Abstract

Serotonin type 2 (5-HT2) receptors reportedly inhibit neuropathic pain in the spinal cord, but little is known about how spinal 5-HT2 receptors might act against such abnormal sensitivity. We examined whether the cholinergic and tachykinin systems were involved in the antiallodynic effect of intrathecally administered 5-HT2 receptor agonists in rats with nerve injury. Allodynia was produced by tight ligation of the left L5 and L6 spinal nerves, and determined by applying von Frey hairs to the left hindpaw. Effects of intrathecal pretreatment with 5-HT2 receptor antagonists (ketanserin and RS-102221), muscarinic receptor antagonists (atropine and scopolamine), a choline uptake blocker (hemicholium-3), and an NK1 receptor antagonist (L-706336) were assessed in rats subsequently given a 100-μg intrathecal dose of a 5-HT2 receptor agonist either α-methyl-5-HT or iododimethoxy aminopropane (DOI). Antiallodynic effects of 5-HT2 receptor agonists were attenuated by the 5-HT2A receptor antagonist ketanserin (30 μg), but not by the 5-HT2C receptor antagonist RS-102221 (40 μg). Muscarinic receptor antagonists (30 μg each), the choline uptake blocker (10 μg), and the NK1 receptor antagonist (30 μg) also inhibited the antiallodynic effects of 5-HT2 receptor agonists. Antiallodynic effects of intrathecally administered 5-HT2 receptor agonists may be mediated by spinal release of acetylcholine induced via 5-HT2A and NK1 receptors.

Introduction

In some patients, peripheral nerve injury results in neuropathic pain that is poorly alleviated by conventional treatments such as opioids or nonsteroidal anti-inflammatory drugs. While symptoms of neuropathic pain vary widely among these patients, hallmark manifestations include allodynia (pain evoked by nonnoxious stimuli), hyperalgesia (increased pain intensity in response to noxious stimuli), and spontaneous pain. Anomalous pain states evoked by nerve injury in humans can be mimicked by partial nerve injury in rats. For example, spinal nerve ligation of L5 and L6 just distal to the dorsal root ganglion leads to neuropathic pain [17]. There are multiple models of nerve injury, and this is one example of such.

The neurotransmitter 5-hydroxytryptamine (5-HT) is recognized to be important in spinal inhibition of nociceptive transmission. Intrathecally administered 5-HT has antinociceptive effects in acute pain models [3], [31], [39]. Autoradiographic studies have identified at least three families of 5-HT receptors (5-HT1, 5-HT2 and 5-HT3) in the spinal cord [13], [16], [20], [36]. Although some controversy persists, 5-HT1A, 5-HT1B, 5-HT2 and 5-HT3 receptor subtypes have been reported to contribute to the antinociceptive action of 5-HT in the spinal cord [10]. However, most previous investigations of spinal antinociceptive effects of 5-HT used acute pain models. Actions of 5-HT in chronic pain such as neuropathic pain induced by nerve injury have not yet been fully clarified.

Recently we reported that intrathecal administration of 5-HT2 receptor agonists, but not 5-HT1 or 5-HT3 receptor agonists, suppressed tactile allodynia in a rat model of neuropathic pain produced by spinal nerve ligation [23]. The mechanism of this antiallodynic effect mediated by spinal 5-HT2 receptor agonists is not clear. Several lines of evidence suggest that 5-HT regulates acetylcholine (ACh) release in the central nervous system [4], [11], [27]. Interactions with other neurotransmitters such as noradrenaline [29], [30], γ-aminobutyric acid (GABA) [1], or adenosine [29], [35]also have been implicated in the spinal antinociceptive effects of 5-HT. In preliminary experiments, we found that the muscarinic receptor antagonists, atropine and pirenzepine, but not various other receptor antagonists, reduced the antiallodynic effect of an intrathecally administered 5-HT2 receptor agonist [24]. Feuerstein et al. [11] reported that in rat hippocampal slices, activation of 5-HT2A receptors facilitated substance P (SP) release, which in turn acts at NK1 receptors located on cholinergic terminals to stimulate ACh release. We hypothesized that similar interactions in the spinal cord could represent a mechanism underlying antiallodynic actions of intrathecally administered 5-HT2 receptor agonists. Therefore we examined interactions between spinal 5-HT2 receptors and the cholinergic system to define the pharmacological mechanism of the antiallodynic effect of intrathecally injected 5-HT2 receptor agonists in rats with spinal nerve ligation.

Section snippets

Animal preparation

After Animal Care and Use Committee approval, male Wistar rats weighing 150–180 g underwent spinal nerve ligation under isoflurane anesthesia as previously described [17]. On the left side, spinal nerves L5 and L6 were isolated and ligated tightly with 6-0 silk sutures just distal to the dorsal root ganglion. After surgery the rats were housed individually with free access to food and water, and were allowed to recover for at least 7 days. Tactile allodynia in the left hind paw was confirmed by

Results

Intrathecal administration of 100 μg of α-m-5-HT or DOI showed an antiallodynic effect with a peak effect at 30 min after injection. A more pronounced antiallodynic effect was observed after intrathecal injection of α-m-5-HT than DOI. After injection, 100 μg of DOI produced slight restlessness that continued for approximately 30 min. This restlessness did not influence measurement of withdrawal threshold.

Discussion

In this study we clearly demonstrated that the antiallodynic effects of intrathecally administered 5-HT2 receptor agonists were attenuated by intrathecal pretreatment with muscarinic receptor antagonists and a choline uptake blocker. Our data strongly suggested that the antiallodynic effect of 5-HT2 receptor agonists is attributable to the action of ACh released in the spinal cord.

Recently we reported that intrathecal administration of a 5-HT2 receptor agonist, either α-m-5-HT or DOI, produced

References (39)

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