The current opioid epidemic is a pressing public health concern that has been difficult to address because there is no generally accepted hypothesis to explain the underlying neurophysiological mechanism(s) that lead to tolerance and withdrawal, which in turn could serve as the basis for developing therapeutic interventions. As a step to developing such a unifying mechanistic hypothesis we studied both the electrophysiological firing patterns of individual neurons and behaviors indicative of tolerance and withdrawal in rats following chronic administration of morphine. Neuronal activity recordings from the ventral tegmental area (VTA), nucleus accumbens (NAc), prefrontal cortex (PFC), thalamus, hypothalamus, hippocampus (HIPP), Amygdala (AMYG) and caudate nucleus (CN) following exposure of repetitive (chronic) morphine produced a new baseline pattern of neuronal firing rates, which we refer to as an “opioid-induced pattern” in the mesocorticolimbic neuronal activity circuit, which is thought to be involved in mediating the “reward” effects of opioid and other drugs of abuse. These changes in neuronal firing rate was paralleled by behavioral changes indicative of the development of dependence such as tolerance and withdrawal suggesting a possible cause-effect relationship between the opioid induced pattern change of baseline neuronal firing and the development of opioid tolerance and withdrawal. Briefly, (i) morphine produces a new baseline pattern of neuronal firing (i.e., an “opioid-induced pattern”), (ii) there is an intrinsic neurophysiological mechanism that seeks to maintain newly established patterns of baseline neuronal firing once established, (iii) continued morphine administration maintains the new pattern of baseline neuronal activity so that withdrawal behaviors do not occur, but (iv) eventual discontinuation of the drug leads to opioid withdrawal symptoms. Consistent with this proposed hypothesis, co-administration of the immunomodulator such as interferon, cyclosporin and cortisol attenuated both the development of an altered baseline pattern of neuronal firing and the parallel behavioral changes. This observation suggests that immunomodifiers treatment to morphine dependence subject restore the neuronal firing rate to its pre-morphine baseline and thus alleviate the withdrawal symptoms that make is so difficult for addicts to discontinue opioid use. The studies in this review describe in more detail the findings that led to our proposed hypothesis for the underlying neurophysiological basis of the development of opioid tolerance and withdrawal and the possible use of immunomodulators to decrease the development of dependence and thereby attenuate withdrawal symptoms that make it so difficult for addicts to discontinue drug use. Repetitive use of opioids results in dependence on the drug, a complex condition that is considered to be an opiate use disorder (OUD). The reduction or cessation of opioid consumption leads to severe withdrawal behaviors. The degree of opiate dependence can be assessed by the intensity of the withdrawal behavior. To prevent this devastating opiate withdrawal syndrome, the subject will continue to take the drug. Success in modifying the withdrawal behavior may shed light on the dynamics of OUD and help to curb the opiate epidemic. Classical therapeutic addiction research has focused on cellular and molecular alterations within neurons and their neuronal circuits. As such, most of the pharmacotherapies for opioid addiction are designed to target the neuronal processes known to be affected by drug intake. In addition to the pivotal role of neurons in the initiation, transition, and maintenance of opioid dependence, the glial cells within the central nervous system are also of particular importance. According to some studies, 60 to 80% of the cellular brain is composed of glial cells. Recent studies have shown that glial cells participate in synaptogenesis, neuronal excitability, and neurotransmission. Following opioid exposure, glial cells demonstrate robust changes in their morphology and physiology in key central nervous system regions known to contribute to drug dependence. They play a pivotal role in opioid-addiction like behaviors. Glial cells are also part of the immune system. This review summarizes studies demonstrating that the immune system participates in the expression of opiate withdrawal and that a single dose of immunological substances such as α-interferon, cyclosporine, or cortisol significantly attenuates the severity of the naloxone-induced withdrawal symptoms in opioid-dependent animals. These preclinical studies provide a new approach to treat opiate dependence using immunomodulators that do not belong to the opiate family. We hope that this review will encourage translational studies to use immunomodulators in combating the opioid epidemic and save lives.
Evidence that the Immune System Participates in the Expression of Opiate Withdrawal Behavior
