R. D. Sanders • M. Maze (✉)
Academic Anaesthetics, Imperial College, Chelsea and Westminster Hospital, 369 Fulham
Road, London SW10 9NH, UK
Abstract Adrenergic and cholinergic signalling contributes signiﬁcantly to the endogenous antinociceptive system. Exogenous α2 adrenergic agonists have a well-established analgesic proﬁle; however, recent investigations suggest that this class of agents is underused, and herein we highlight the potential for both current application and future development of these agents. Nicotinic and muscarinic cholinergic ligands represent a novel class of agents with much promise for the management of problematic pain. In this chapter we review
advances in both preclinical and clinical arenas and highlight potential avenues for further research.
Keywords Pain · Antinociception · Adrenergic · Clonidine · Dexmedetomidine · Cholinergic · Nicotinic · Muscarinic
Pain management remains a real and current problem in clinical medicine; in the United States 70%–80% of surgical patients experience moderate to se- vere post-operative pain (Owen et al. 1990; Svensson et al. 2000; Thomas et al.
1998). This does not merely reﬂect inadequate pain management strategies at a local level but also poor efﬁcacy and poor tolerability of the analgesic agents.
For example, opioid administration is commonly limited by side-effects from respiratory and gastrointestinal symptoms. To enable more effective therapy multi-modal strategies are now employed; however, new agents with improved
efﬁcacy are required to help combat problematic pain management. In ad- dition, chronic and neuropathic pain syndromes remain resistant to current approaches, with only a minority of patients responding mostly at the expense
of signiﬁcant side-effects (Arner and Meyerson 1988).
With administration via systemic or regional approaches for acute, chronic and neuropathic pain, α2 adrenergic agonists remain a potent but relatively underused class of analgesic agents. Below we review supporting evidence for an expanding role of this class of agents and explore their mechanisms of action.
At present, cholinergic compounds, both nicotinic and muscarinic, are being developed as novel analgesics and herein we review their progress. Further- more, there is substantial overlap in the mechanisms of action of both of these classes of agents; further scientiﬁc exploration is required to inform us about their potential adjuvant administration.
Adrenergic signalling is one of the primary components of the endogenous antinociceptive system that modulates pain responses. Acting at spinal and supraspinal sites, norepinephrine release is involved in the control of a wide
range of pain responses via activation of α2 and α1 adrenoceptors. Descending inhibitory neurons (DINs) are an important component of the antinociceptive system. Activated from supraspinal sites such as the periaqueductal grey and
dorsal raphe nucleus (Jones and Gebhart 1984; Tjolsen et al. 1990) as well as other brainstem nuclei such as the A5 and A7, they inhibit the nociceptive responses in the dorsal horn of the spinal cord via release of norepinephrine,
serotonin and acetylcholine (Li and Zhuo 2001). In the dorsal horn, nore- pinephrine depresses wide-dynamic-range neuron responses after Aδ and C nociceptive ﬁbre activation by stimulation of α2 adrenoceptors (Jones and Gebhart 1984). This effect of norepinephrine is mimicked by exogenous α2 adrenoceptor agonists (Millar et al. 1993) and is thought to be dependent on
stimulation of spontaneously active neurons in the deep layer of the spinal
cord which release acetylcholine and enkephalins. Further indirect evidence is provided by the observation that acute pain increases norepinephrine and acetylcholine levels in the cerebrospinal ﬂuid (CSF), and the α2 adrenoceptor agonist clonidine increases acetylcholine in the CSF (Eisenach et al. 1996; De- tweiler et al. 1993). This also indicates an interdependent antinociceptive effect
exerted between the cholinergic and adrenergic systems.
α2 Adrenergic Receptors and Substrates
When stimulated, α2 adrenoceptors inhibit adenyl cyclase via pertussis-sen- sitive G proteins. These receptors are coupled, via the subunits of the G protein, to ligand-gated ion channels including the N-type calcium channel (inhibition; Adamson et al. 1989), the P/Q-type calcium channel (inhibition; Ishibashi et al.
1995), the IA potassium channel (activation; North et al. 1987), the calcium activated potassium channel (activation; Ryan et al. 1998), the ATP-sensitive potassium channel (activation; Galeotti et al. 1999), the voltage-dependent potassium channels (activation; Galeotti et al. 1999) and the Na+/H+ anti- porter (activation; Ryan et al. 1998). Furthermore, recent work has highlighted
the association of α2 adrenoceptors with G protein-coupled inwardly rectifying potassium (GIRK) channels (Blednov et al. 2003; Mitrovic et al. 2003).
Using D79N mice which express dysfunctional α2A adrenoceptors, Lakhlani and colleagues showed that adrenoceptor agonist antinociception (assessed by the hot plate test) and sedation were dependent on this receptor sub- type (Lakhlani et al. 1997). In the absence of functional α2A adrenoceptors, the agents could not suppress voltage-gated calcium or activate potassium currents. This mutation did not affect morphine analgesia. It is notewor-
thy, though, that pharmacogenetic analysis of different inbred mouse strains showed signiﬁcant correlation between strain dependence of morphine and clonidine analgesia (in hot plate and formalin tests; Wilson et al. 2003). Fur- thermore, as the interaction between clonidine and morphine is synergistic (Wilcox et al. 1987) and there are overlapping pharmacogenomic substrates, investigation of downstream effectors beyond the receptor may lead to the development of novel agents which separate analgesic and sedative effects of
α2 adrenoceptor agonists.
Site of Action
Supraspinal and spinal targets contribute to the potent antinociceptive efﬁcacy of α2 adrenoceptor agonists. This is of importance because drugs which rely
on DINs such as nitrous oxide are ineffective in the young, as DINs are imma- ture in early development (Ohashi et al. 2002). The α2A adrenoceptor agonist dexmedetomidine (Dex) is effective in the immature phenotype as it targets both supraspinal and spinal sites, circumventing DINs (Sanders et al. 2005).
The locus coeruleus (LC) is an adrenergic centre in the brainstem that tonically inhibits the A5 and A7, which are then coupled to DINs. Activation of α2 adrenoceptors in the LC inhibits neuronal ﬁring in this region (Guo et al. 1996). Inhibition of the LC by discrete administration of α2 adrenoceptor agonists leads to ‘disinhibition’ (i.e. activation) of the A5 and A7 and therefore DINs.
Comparison between the analgesic effectiveness of clonidine after systemic or neuraxial (spinal or epidural) administration revealed that the spinal cord was an important site of action for α2 adrenoceptor agonist-induced analgesia (Bernard et al. 1995; Eisenach et al. 1998). Furthermore, in human volun-
teers intrathecal (IT) clonidine was superior to intravenous clonidine against capsaicin and thermal pain (Eisenach et al. 1998).
In the dorsal horn of the spinal cord, activation of pre-synaptic α2 adreno- ceptors reduces glutamate (Li and Eisenach 2001), substance P and calcitonin
gene-related peptide (CGRP) release (Takano and Yaksh 1993). Post-synaptic effects are related to activation of voltage-dependent potassium channels (Ga- leotti et al. 1999) and GIRK channels (Blednov et al. 2003; Mitrovic et al. 2003).
In two separate studies the action of clonidine was examined in GIRK-2-null mutant mice using the hot plate test and tail ﬂick latency (Blednov et al. 2003; Mitrovic et al. 2003); the mutation reduced clonidine antinociception almost to baseline, indicating primarily a post-synaptic action of clonidine. We have pre-
viously found electrophysiological evidence to support this genetic evidence of a post-synaptic action for α2 adrenoceptor agonists as Dex reduces ventral root potentials (induced by substance P) in an ex vivo isolated neonatal rat spinal cord preparation. In addition, α2 adrenoceptor agonists inhibit adenyl cyclase, which is a pivotal enzymatic step in the development of hyperalgesia at post-synaptic sites (Hoeger-Bement and Sluka 2003; Sanders et al. 2005).
Systemic administration of α2 adrenoceptor agonists induces antinocicep- tion in several animal models with effects both at supraspinal and spinal sites. Both clonidine and dexmedetomidine increase latency of tail ﬂick during the hot plate test in a dose-related manner (Sabbe et al. 1994). Formalin and cap-
saicin induce inﬂammatory pain with a typical biphasic pain response (acute pain and secondary hyperalgesia). α2 Adrenoceptor agonists inhibit this pain
response (Wilson et al. 2003; Sanders et al. 2005) likely via both pre-synaptic (reduction of glutamate, substance P and CGRP release) and post-synaptic (ac- tivation of GIRK channels and inhibition of adenyl cyclase) mechanisms. This
underlies the known efﬁcacy of α2 adrenoceptor agonists in hyperalgesia and neuropathic pain-associated allodynia; α2 adrenoceptor agonists may even show increased efﬁcacy against neuropathic pain (Puke and Wiesenfeld-Hallin
1993). Furthermore, α2 adrenoceptor agonists reduce allodynia after nerve le- sioning models of neuropathic pain (Poree et al. 1998), which may be related to peripheral adrenoceptor activation. This potent anti-neuropathic pain effect is also reduced by acetylcholine depletion in the spinal cord (Paqueron et al.
2001) and muscarinic cholinergic antagonists (Pan et al. 1999) likely via M4 receptors (Kang and Eisenach 2003). The α2 adrenoceptor agonists also show efﬁcacy against visceral pain (Harada et al. 1995; Iwasaki et al. 1991) and at peripheral—such as intra-articular injection—sites (potentially mediated by
local enkephalin release; Nakamura and Ferreira 1988).
Likewise, α2 adrenoceptor agonists have long been administered for regional anaesthesia; after spinal administration in sheep they interact synergistically
with opioid and cholinergic agonists and cholinesterase inhibitors such as neostigmine (Detweiler et al. 1993). This synergistic interaction has yet to be observed in humans, but opioid analgesia is enhanced in the presence of
clonidine (at doses of up to 75 μg IT; Grace et al. 1994). Similarly, combination of spinal neostigmine and clonidine prolongs post-operative analgesia (Pan et al. 1998). Therefore, whether administered systemically or neuraxially, α2
adrenoceptor agonists exert their primary analgesic effect at the level of the spinal cord.