Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain.

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Prenatal opioid exposure reprograms the behavioural response to future alcohol reward.

As the opioid crisis has continued to grow, so has the number of infants exposed to opioids during the prenatal period. A growing concern is that prenatal exposure to opioids may induce persistent neurological changes that increase the propensity for future addictions. Although alcohol represents the most likely addictive substance that the growing population of prenatal opioid exposed will encounter as they mature, no studies to date have examined the effect of prenatal opioid exposure on future sensitivity to alcohol reward. Using a recently developed mouse model of prenatal methadone exposure (PME), we investigated the rewarding properties of alcohol and alcohol consumption in male and female adolescent PME and prenatal saline exposed (PSE) control animals. Conditioned place preference to alcohol was disrupted in PME offspring in a sex-dependent manner with PME males exhibiting resistance to the rewarding properties of alcohol. Repeated injections of alcohol revealed enhanced sensitivity to the locomotor-stimulating effects of alcohol specific to PME females. PME males consumed significantly more alcohol over 4 weeks of alcohol access relative to PSE males and exhibited increased resistance to quinine-adulterated alcohol. Further, a novel machine learning model was developed to employ measured differences in alcohol consumption and drinking microstructure to reliably predict prenatal exposure. These findings indicate that PME alters the sensitivity to alcohol reward in adolescent mice in a sex-specific manner and suggests prenatal opioid exposure may induce persistent effects on reward neurocircuitry that can reprogram offspring behavioural response to alcohol later in life.

Prenatal methadone exposure disrupts behavioral development and alters motor neuron intrinsic properties and local circuitry.

Despite the rising prevalence of methadone treatment in pregnant women with opioid use disorder, the effects of methadone on neurobehavioral development remain unclear. We developed a translational mouse model of prenatal methadone exposure (PME) that resembles the typical pattern of opioid use by pregnant women who first use oxycodone then switch to methadone maintenance pharmacotherapy, and subsequently become pregnant while maintained on methadone. We investigated the effects of PME on physical development, sensorimotor behavior, and motor neuron properties using a multidisciplinary approach of physical, biochemical, and behavioral assessments along with brain slice electrophysiology and in vivo magnetic resonance imaging. Methadone accumulated in the placenta and fetal brain, but methadone levels in offspring dropped rapidly at birth which was associated with symptoms and behaviors consistent with neonatal opioid withdrawal. PME produced substantial impairments in offspring physical growth, activity in an open field, and sensorimotor milestone acquisition. Furthermore, these behavioral alterations were associated with reduced neuronal density in the motor cortex and a disruption in motor neuron intrinsic properties and local circuit connectivity. The present study adds to the limited body of work examining PME by providing a comprehensive, translationally relevant characterization of how PME disrupts offspring physical and neurobehavioral development.

The far-reaching opioid crisis extends to babies born to mothers who take prescription or illicit opioids during pregnancy. Opioids such as oxycodone and methadone can freely cross the placenta from mother to baby. With the rising misuse of and addiction to opioids, the number of babies born physically dependent on opioids has risen sharply over the last decade. Although these infants are only passively exposed to opioids in the womb, they can still experience withdrawal symptoms at birth. This withdrawal is characterized by irritability, excessive crying, body shakes, problems with feeding, fevers and diarrhea. While considerable attention has been given to treating opioid withdrawal in newborn babies, little is known about how these children develop in their first years of life. This is, in part, because it is difficult for researchers to separate drug-related effects from other factors in a child's home environment that can also disrupt their development. In addition, the biological mechanisms underpinning opioid-related impairments to infant development also remain unclear. Animal models have been used to study the effects of opioid exposure during pregnancy (termed prenatal exposure) on infants. These models, however, could be improved to better replicate the typical pattern of opioid use among pregnant women. Recognizing this gap, Grecco et al. have developed a mouse model of prenatal methadone exposure where female mice that were previously dependent on oxycodone were treated with methadone throughout their pregnancy. Methadone is an opioid drug commonly prescribed for treating opioid use disorder in pregnant women and was found to accumulate at high levels in the fetal brain of mice, which fell quickly after birth. The offspring also experienced withdrawal symptoms. Grecco et al. then examined the physical, behavioral and brain development of mice born to opioid-treated mothers. These included assessments of the animals' motor skills, sensory reflexes and behavior in their first four weeks of life. Additional experiments tested the properties of nerve cells in the brain to examine cell-level changes. The assessments showed that methadone exposure in the womb impaired the physical growth of offspring and this persisted into 'adolescence'. Prenatal methadone exposure also delayed progress towards key developmental milestones and led to hyperactivity in three-week-old mice. Moreover, Grecco et al. found that these mice had reduced neuron density and cell-to-cell connectivity in the part of the brain which controls movement. These findings shed light on the potential consequences of prenatal methadone exposure on physical, behavioral and brain development in infants. This model could also be used to study new potential treatments or intervention strategies for offspring exposed to opioids during pregnancy.

Mu opioid receptors on vGluT2-expressing glutamatergic neurons modulate opioid reward.

The role of Mu opioid receptor (MOR)-mediated regulation of GABA transmission in opioid reward is well established. Much less is known about MOR-mediated regulation of glutamate transmission in the brain and how this relates to drug reward. We previously found that MORs inhibit glutamate transmission at synapses that express the Type 2 vesicular glutamate transporter (vGluT2). We created a transgenic mouse that lacks MORs in vGluT2-expressing neurons (MORflox-vGluT2cre) to demonstrate that MORs on the vGluT2 neurons themselves mediate this synaptic inhibition. We then explored the role of MORs in vGluT2-expressing neurons in opioid-related behaviors. In tests of conditioned place preference, MORflox-vGluT2cre mice did not acquire place preference for a low dose of the opioid, oxycodone, but displayed conditioned place aversion at a higher dose, whereas control mice displayed preference for both doses. In an oral consumption assessment, these mice consumed less oxycodone and had reduced preference for oxycodone compared with controls. MORflox-vGluT2cre mice also failed to show oxycodone-induced locomotor stimulation. These mice displayed baseline withdrawal-like responses following the development of oxycodone dependence that were not seen in littermate controls. In addition, withdrawal-like responses in these mice did not increase following treatment with the opioid antagonist, naloxone. However, other MOR-mediated behaviors were unaffected, including oxycodone-induced analgesia. These data reveal that MOR-mediated regulation of glutamate transmission is a critical component of opioid reward.

Adeno-Associated Viral Vectors in Neuroscience Research.

Adeno-associated viral vectors (AAVs) are increasingly useful preclinical tools in neuroscience research studies for interrogating cellular and neurocircuit functions and mapping brain connectivity. Clinically, AAVs are showing increasing promise as viable candidates for treating multiple neurological diseases. Here, we briefly review the utility of AAVs in mapping neurocircuits, manipulating neuronal function and gene expression, and activity labeling in preclinical research studies as well as AAV-based gene therapies for diseases of the nervous system. This review highlights the vast potential that AAVs have for transformative research and therapeutics in the neurosciences.