2002, Number 3
Posible modulación de los efectos del síndrome de abstinencia a morfina por los receptores A1 de la adenosina
Cano A, Rocha L, Briones M, Villalobos R
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Language: Spanish
References: 46
Page: 47-55
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ABSTRACT
The opiate withdrawal syndrome has attracted the interest of scientists and society since realizing that in the 1880s the medically useful opiate drugs induce state of physiological and psychological dependency. Explanations for this syndrome of autonomic disturbance and psychic distress, which appears when drug intake ceases, have ranged from elaborate psychological and moral theories to increasing precise biochemical theories. New compounds, introduced partly for their lack of withdrawal effects, have included both heroin and methadone; now both are known for their withdrawal syndromes and substantial abuse. Nearly every treatment suspected to affect brain functions has been tried without significant success against the withdrawal syndrome, including electroconvulsive therapy, major and minor tranquilizers, insuline-induced hypoglycemic shock, and many others. Not a single opiate treatment so far has been convincingly proven effective, thus the problems of opiate dependency remain substantial for the individuals involved, and costly to society due to lost productivity, treatment delivery, and crime associated with drug supply and procurement.The scientific excitement about treatment for opiate withdrawal phenomena was increased by the discoveries that opiates act as receptors that are targets for numerous endogenous opiate-like peptide neurotransmitters. The research into the mechanisms of opiate action, tolerance and withdrawal suddenly appeared to offer insights into fundamental processes of central neural regulation and physiology. At the same time the increasing numbers of neurotransmitters, receptors, intracellular modulators, and interacting neuronal systems make more complex models and explanations necessary. At present, data suggest that every known neurotransmitters and neuroregulatory systems could be involved. Drug withdrawal is an integral part of most types of dependence and, to a large extent, opiate withdrawal has been considered the prototypical classical measure of opiate dependence. The opiate withdrawal syndrome is characterized by multiple behavioral and physiological signs such as behavioral activation, ptosis, diarrhea, wet dog shakes and motivational dysfunction, which may be represented in the central nervous system at multiple sites. Much recent evidence have been derived from studies of the nucleus locus coeruleus, the primary source of noradrenergic innervation of the limbic system, cerebral and cerebellar cortices, and a quantitatively smaller sources of innervation of hypothalamic and other brainstem nuclei. The locus coeruleus presents a noradrenergic inervation and is enriched with opioid receptors of the μ and κ subtypes. In addition, it receives afferents from primary pain and sensory systems and sends efferents to many areas of the brain associates with specific physiological changes described during classical morphine withdrawal syndrome. Postsynaptic receptors for these efferents are the adrenoceptors subtypes, where noradrenaline release, in addition to some specific effects, seems to modulate other neurotransmitter inputs. The locus coeruleus appears to contain presynaptic receptors on the perikarya or dendrites, and on the nerve terminals that mediate modulatory responses. Receptors for substance P, muscarinic acethylcholine, and glutamate are excitatory, and inhibitory responses are seen following application of α-2 adrenoceptors, adenosine, GABA and benzodiazepine agonists, with evidence suggesting auto- and heteroregulation, which involves the participation of efferents from the limbic system, cerebral cortex and hypothalamic innervation.
The development of dependence with repeated use and the expression of withdrawal syndrome are characteristic features of all the opiates, represented by morphine. Although many studies regarding several neurotransmitters and receptors have been conducted to define the mechanisms involved in the development of dependence and expression of opiate withdrawal, these have not been fully clarified despite the huge accumulation of experimental and clinical data at present. The known mechanism associated to the presentation and termination of morphine withdrawal is important in the cases of dependence from voluntary administration, when withdrawal results of suppression of morphine administration in the pain treatment, as well as in the cases of neonatal abstinence, which leads to prolonged hospitalization for both mother and infant.
Morphine is an opioid agonist with higher affinity by μ opioid receptors, however, many evidences indicate that other receptors, such as adenosine receptors, participate in the morphine effects. Opiate receptor-mediated adenylate cyclase signalling in the locus coeruleus is an important pathway in opiate dependence. Acutely administered opiate binds to opioid receptors and inhibits production of cyclic adenosin monophosphate (cAMP) by adenylate cyclase and decrease cAMPdependent protein kinase (PKA) activity. This results in the alteration of ion channels and the inhibitions of firing of locus coeruleus neurons. The continued presence of opiates and ongoing inhibition of adenylate cyclase activity may produce a compensatory enhancement of adenylate cyclase and PKA activity. Tolerance to opiates is associated with an increase in adenylate cyclase activity and the return of intracelular cAMP to baseline levels after two or three days of morphine treatment. Abrupt withdrawal of opiate after chronic administration produces adenylate cyclase hyperactivity associated with an acute flood of intracellular cAMP. Intravenous or intraventricular infusion of cAMP antagonizes morphine analgesia and accelerates the development of opiate tolerance and withdrawal syndrome. Complex regional alterations in adenylate cyclase and PKA activity and G protein subunities, after chronic morphine treatment, have been demonstrated in several brain regions.
Adenosine receptors, like opioid receptors, play an important role in regulating neuronal adenylate cyclase activity and intracellular cAMP levels and can therefore affect opiate dependence. Adenosine A1 receptor activation inhibits both adenylate cyclase activity and intracelular cAMPC formation, while A2a receptor activation stimulates both; these receptors ultimately affect PKA activity via cAMPC regulation.
A variety of studies have suggested that adenosine receptors are involved in central opiate actions. On the other hand, the decrease of morphine withdrawal behavioral signs after a certain period of time, suggests endogenous changes that attenuate or terminate with morphine withdrawal. In rodents, the A1 adenosine agonists blockade the morphine withdrawal signs apparition; which suggests the participation of A1 receptors in the endogenous response related with attenuation of morphine withdrawal. Morphine withdrawal has been related with neural activity increased from locus coeruleus and many studies suggest the participation of other brain regions, such as limbic system and cerebral cortex. The main purpose of the present study was to known the endogenous changes in number (Bmax) and affinity (Kd)of the adenosine A1 receptors in five rat brain structures related with activity of the locus coeruleus when the morphine withdrawal has finished. Wistar male rats received six injections of morphine (20 mg/kg, i.p.), one each twenty four hours. The frequency of presentation of five morphine withdrawal behavioral signs (anxiety, pupillary dilatation, jumping, wet dog shakes and tremor), was recorded. Animals were sacrificed when the morphine withdrawal signs were observed in less that 10% of the population, and brain regions were dissected. In receptor binding studies by saturation assays with 3H-clorocyclopentiladenosine (3H-CCPA), a selective radioligand to adenosine A1 receptors was used to obtain the corresponding Bmax and Kd values from amygdala, cortex, striatum, hippocampus and hypothalamus membranes. Forty-eighth hours after the last morphine administration, the withdrawal signs were only present in 8% of the animals. At this time there was an increase in A1 receptor number in the hippocampus (45%), the amygdala (12%), and the cerebral cortex (142%). No differences in the A1, Bmax, and Kd values were detected in the hypothalamus or the striatum in comparison with the control group. No significant changes were found in the affinity values in the hippocampus and amygdala; however, in the cortex the Kd values were higher (50%) than in the control group. Therefore, initial μ receptor activation results in adenosine A1 receptor upregulation in brain cortex and limbic structures (hippocampus and amygdala), which are associated with the decrease of morphine withdrawal signs. Simultaneously there are no changes in the hypothalamus and striatum. These results indicate that A1 receptors of only certain cerebral structures participate in the morphine withdrawal regulation. The A1 receptor upregulation found in the cortex and limbic system could be a endogenous response related with μ and A1 receptor interaction that increases the ability of endogenous adenosine to decrease neural excitability to locus coeruleus in association to natural morphine withdrawal termination. The effects of endogenous adenosine on upregulated A1 receptors in limbic and cortical afferent pathways could attenuate effects of opioid-mediated upregulation of adenylate cyclase and PKA which could decrease the neuronal firing rate in locus coeruleus in association with a decrease in the excitatory neurotransmitter release. Our results suggest that adenosine A1 receptors represent an important site of therapeutic intervention in opiate dependence and in the treatment of withdrawal resulting when drug intake ceases.
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