Molecular basis of opioid pharmacology in pain control

Javier Garzón

Neuropharmacology, Cajal Institute, CSIC. Dr Arce 37, 28002 Madrid, Spain.

Opioids such as morphine efficiently control nociceptive sensations and they are the analgesics of choice in the treatment of high-intensity chronic pain. Unfortunately, these positive attributes are shadowed by the loss of potency that accompanies their repeated administration. This is a common characteristic of opioids that act via Mu receptors and it is a serious drawback for their long-term clinical use in treating severe pain. In experimental paradigms, repeated administration of opioids also brings about a progressive decrease in potency. Desensitization of Mu receptors can be observed after three consecutive administrations of the same analgesic dose or even after a single dose of the opioid if this is sufficiently large. This phenomenon of acute tolerance appears within hours of agonist administration and lasts for about 2-3 days. Recently, mechanisms responsible for acute tolerance have been proposed that parallel those which underlie chronic morphine tolerance.

Opioid receptors belong to a superfamily of seven transmembrane domain-receptors and they regulate the activity of Gi/o/z/q/11 transducer proteins. Through the regulation of different effectors, the activation of these opioid receptors reduces neural excitability and diminishes synaptic activity. Indeed, opioid receptors can inhibit adenylyl cyclase and voltage-gated Ca²⁺ channels, and they may stimulate inwardly rectifying K⁺ channels. In cell lines carrying opioid receptors, opioid agonists promote the rapid inactivation of these receptors through the phosphorylation of cytosolic sequences and the ensuing internalization of the molecule. However, desensitization of opioid receptors in nervous tissue is promoted by a series of molecular events that precede their inactivating phosphorylation and it is not associated with the loss of surface receptors. The mechanisms underlying the development of tolerance are multifaceted and they are only partially understood. Functional and gene expression studies have highlighted a growing number of target proteins that may be involved in the in vivo adaptation to chronic opioid treatment. Furthermore, these proteins and their impact on opioid tolerance have been studied by different approaches, including inhibition with pharmacological antagonists, knockdown of expression with antisense oligonucleotides and genomic deletion in mice. As a result, other opioid receptors and endogenous ligands, particularly those of the Delta-opioid receptor system, have been seen to be intimately involved in the responses to acute and chronic morphine administration. Deletion of the genes encoding the Delta-opioid receptor and preproenkephalin, the precursor of endogenous peptide agonists, inhibits the development of morphine tolerance in antinociceptive tests. In addition, an ever-growing number of neurotransmitters and/or their receptors can also modify the development of tolerance to the antinociceptive effects of morphine. These studies implicate well-known transmitters such as neurokinins or cholecystokinin in these processes. Moreover, phospholipase C-mediated activation of N-methyl-d-aspartate (NMDA) receptors and of nitric oxide synthetase (NOS) also impairs the effects of opioids. Indeed, it has been demonstrated that NMDA receptor antagonists or NOS inhibitors enhance opioid analgesia and decrease the development of tolerance. This indicates the close functional connection between opioid receptors, NMDA receptors and the NO system. Another relevant mechanism of tolerance that is employed to reduce opioid function involves phosphorylation of the opioid receptor proteins, their coupled G proteins and of several related effector proteins. The enzymes producing these changes include second messenger-dependent protein kinases, protein kinase C, cyclic AMP-dependent protein kinase (PKA), Ca²⁺/camodulin-dependent protein kinase II (CaMKII)), G protein-coupled receptor kinases (GRKs) and mitogen-activated protein kinases (MAPKs). Thus, these kinases play important roles in the regulation of opioid signal transduction.

Our knowledge of the particular elements involved in the processing of extracellular signals in neuronal cells has increased greatly in recent years. In these cells, a series of signaling proteins have been identified that participate in desensitization of G-protein coupled receptors and whose expression is virtually restricted to nervous tissue, e.g. Gz proteins, their regulators (Regulators of G protein Signalling) RGSZ1 and RGSZ2, the subfamily of RGS-R7 proteins and the phosducin like proteins. These RGS proteins control the intensity and the duration of the effects produced by Mu receptor agonists and thus, they influence the tolerance that develops in response to specific doses of opioids. The RGS-Rz proteins reduce the amplitude of opioid analgesia and also coordinate with RGS-R7 proteins to bind and sequester Mu receptor activated G-proteins. Thus, they drain away the pool of receptor-regulated G proteins and hence, they buffer the effects of agonists. The desensitization observed following morphine administration correlates with the transfer of Gα subunits from Mu receptors to these RGS proteins. The subsequent stabilization of this association by means of phosphorylation and binding to third partner proteins contributes further to this effect. This regulatory machinery is virtually constrained to the cell membrane, and it is finely controlled by phosphorylation/dephosphorylation processes that are triggered by NMDA receptors, activation of PKC/CaMKII/GRK and heterodimerization of Mu and Delta opioid receptors.

These molecular interactions provide a framework in which pharmacological treatments that are effective in reverting or delaying Mu receptor desensitization can be understood. Furthermore, they should provide a substrate to identify novel therapeutical targets for the better treatment of severe pain.

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