Dynamic changes in orbitofrontal neuronal activity in rats during opiate administration and withdrawal

Role of deep brain stimulation (DBS) in addiction disorders

Background

Addiction disorders pose significant challenges to public health, necessitating innovative treatments. This assesses deep brain stimulation (DBS) as a potential intervention for addiction disorders.

Methods

A literature review was carried out with a focus on the role of DBS in addiction disorders and its future implications in neurosurgical research.

Results

The online literature shows that DBS precisely modulates certain brain regions to restore addiction-related neural circuits and promote behavioral control.

Conclusion

Preclinical evidence demonstrates DBS's potential to rebalance neural circuits associated with addiction, and early clinical trials provide encouraging outcomes in enhancing addiction-related outcomes. Ethical considerations, long-term safety, and personalized patient selection require further investigation.

Deep brain stimulation of the orbitofrontal cortex prevents the development and reinstatement of morphine place preference

The orbitofrontal cortex (OFC) is involved in compulsive drug seeking and drug relapse. Its involvement in cue-, context-, and stress-induced reinstatement of drug seeking has also been confirmed in animal models. Deep brain stimulation (DBS) was proposed to be an effective intervention for patients with treatment-refractory addiction. Therefore, in the present study, we investigated the potential efficacy of DBS in the OFC for controlling addictive-like behaviors in rats. Rats were bilaterally implanted with electrodes in the OFC and trained to the morphine conditioned place preference (CPP; 3, 5, and 7 mg/kg). High-frequency (HF; 130 Hz) or low-frequency (LF; 13 Hz) DBS-like stimulation was applied during the conditioning (40 minutes, once daily, 3 days) or extinction (20 minutes, once daily, 6-10 days) trials. Following the extinction, morphine preference was reinstated by a priming dose of morphine (2 mg/kg). When applied during the conditioning phase, HF-DBS significantly decreased preference for the morphine-associated context. HF-DBS during the extinction phase of morphine CPP reduced the number of days to full extinction of morphine preference and prevented morphine priming-induced recurrence of morphine preference. LF-DBS did not change any of these addictive behaviors. HF-DBS had no significant effect on novel object recognition memory. In conclusion, HF-DBS of the OFC prevented morphine preference, facilitated extinction of morphine preference, and blocked drug priming-induced reinstatement of morphine seeking. These findings may indicate a potential applicability of DBS in the treatment of relapse to drug use. Further studies will be necessary to assess the translatability of these findings to the clinic.

Manganese-Enhanced Magnetic Resonance Imaging: Application in Central Nervous System Diseases

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on the strong paramagnetism of Mn2+. Mn2+ is a calcium ion analog and can enter excitable cells through voltage-gated calcium channels. Mn2+ can be transported along the axons of neurons via microtubule-based fast axonal transport. Based on these properties, MEMRI is used to describe neuroanatomical structures, monitor neural activity, and evaluate axonal transport rates. The application of MEMRI in preclinical animal models of central nervous system (CNS) diseases can provide more information for the study of disease mechanisms. In this article, we provide a brief review of MEMRI use in CNS diseases ranging from neurodegenerative diseases to brain injury and spinal cord injury.

Estimating Mental Health Conditions of Patients with Opioid Use Disorder

Objectives

Noninvasive estimation of cortical activity aberrance may be a challenge but gives valuable clues of mental health in patients. The goal of the present study was to characterize specificity of electroencephalogram (EEG) electrodes used to assess spectral powers associated with mental health conditions of patients with opioid use disorder.

Methods

This retrospective study included 16 patients who had been diagnosed with opioid use disorder in comparison with 16 sex- and age-matched healthy controls. EEG electrodes were placed in the frontal (FP1, FP2, F3, F4, F7, F8, and Fz), central (C3, C4, and Cz), temporal (T3, T4, T5, and T6), parietal (P3, P4, and Pz), and occipital scalp (O1 and O2). Spectral powers of δ, θ, α, β, and γ oscillations were determined, and their distribution was topographically mapped with those electrodes on the scalp.

Results

Compared to healthy controls, the spectral powers at low frequencies (<8 Hz; δ and θ) were increased in most electrodes across the scalp, while powers at the high frequencies (>12 Hz; β and γ) were selectively increased only at electrodes located in the frontal and central scalp. Among 19 electrodes, F3, F4, Fz, and Cz were highly specific in detecting increases in δ, θ, β, and γ powers of patients with opioid use disorders.

Conclusion

Results of the present study demonstrate that spectral powers are topographically distributed across the scalp, which can be quantitatively characterized. Electrodes located at F3, F4, Fz, and Cz could be specifically utilized to assess mental health in patients with opioid use disorders. Mechanisms responsible for neuroplasticity involving cortical pyramidal neurons and μ-opioid receptor regulations are discussed within the context of changes in EEG microstates.

Activity-induced MEMRI cannot detect functional brain anomalies in the APPxPS1-Ki mouse model of Alzheimer's disease

Alzheimer’s disease (AD) is the most common cause of dementia. Aside neuropathological lesions, abnormal neuronal activity and brain metabolism are part of the core symptoms of the disease. Activity-induced Manganese-Enhanced Magnetic Resonance Imaging (MEMRI) has been proposed as a powerful approach to visualize evoked brain activity in rodents. Here, we evaluated the relevance of MEMRI in measuring neuronal (dys-)function in the APPxPS1 knocked-in (KI) mouse model of AD. Brain anomalies were firstly demonstrated in APPxPS1-Ki mice using cognitive testing (memory impairment) and histological mapping of immediate early gene products (decreased density of fos-positive neurons). Paradoxically, MEMRI analyses were not able to confirm the occurrence of neuronal hypoactivities in vivo. We then performed a neuropathological analysis that highlighted an abnormal increased permeability of the blood-brain barrier (BBB) in APPxPS1-Ki mice. We hypothesized that diffuse weakening of the BBB results in an uncontrolled diffusion of the MR contrast agent and a lack of correlation between manganese accumulation and neuronal activity. These results bring to light a limitation of the activity-induced MEMRI approach when applied to the APPxPS1-Ki mouse model as well as other mouse models harboring a compromised BBB.

Cocaine differentially affects synaptic activity in memory and midbrain areas of female and male rats: an in vivo MEMRI study

Manganese enhanced magnetic resonance imaging (MEMRI) has been previously used to determine the effect of acute cocaine on calcium-dependent synaptic activity in male rats. However, there have been no MEMRI studies examining sex differences in the functional neural circuits affected by repeated cocaine. In the present study, we used MEMRI to investigate the effects of repeated cocaine on brain activation in female and male rats. Adult female and male rats were scanned at 4.7 Tesla three days after final treatment with saline, a single cocaine injection (15 mg kg^-1, i.p. × 1 day) or repeated cocaine injections (15 mg kg^-1, i.p. × 10 days). A day before imaging rats were provided with an i.p. injection of manganese chloride (70 mg kg^-1). Cocaine produced effects on MEMRI activity that were dependent on sex. In females, we observed that a single cocaine injection reduced MEMRI activity in hippocampal CA3, ventral tegmental area (VTA), and median Raphé, whereas repeated cocaine increased MEMRI activity in dentate gyrus and interpeduncular nucleus. In males, repeated cocaine reduced MEMRI activity in VTA. Overall, it appeared that female rats showed a general trend towards increase MEMRI activity with single cocaine and reduced activity with repeated exposure, while male rats showed a trend towards opposite effects. Our results provide evidence for sex differences in the in vivo neural response to cocaine, which involves primarily hippocampal, amygdala and midbrain areas.

Identifying non-toxic doses of manganese for manganese-enhanced magnetic resonance imaging to map brain areas activated by operant behavior in trained rats

Manganese-enhanced magnetic resonance imaging (MEMRI) offers unique advantages such as studying brain activation in freely moving rats, but its usefulness has not been previously evaluated during operant behavior training. Manganese in a form of MnCl₂, at a dose of 20mg/kg, was intraperitoneally infused. The administration was repeated and separated by 24h to reach the dose of 40mg/kg or 60mg/kg, respectively. Hepatotoxicity of the MnCl₂ was evaluated by determining serum aspartate aminotransferase, alanine aminotransferase, total bilirubin, albumin and protein levels. Neurological examination was also carried out. The animals were tested in visual cue discriminated operant task. Imaging was performed using a 3T clinical MR scanner. T1 values were determined before and after MnCl₂ administrations. Manganese-enhanced images of each animal were subtracted from their baseline images to calculate decrease in the T1 value (ΔT1) voxel by voxel. The subtracted T1 maps of trained animals performing visual cue discriminated operant task, and those of naive rats were compared. The dose of 60mg/kg MnCl₂ showed hepatotoxic effect, but even these animals did not exhibit neurological symptoms. The dose of 20 and 40mg/kg MnCl₂ increased the number of omissions and did not affect the accuracy of performing the visual cue discriminated operant task. Using the accumulated dose of 40mg/kg, voxels with a significant enhanced ΔT1 value were detected in the following brain areas of the visual cue discriminated operant behavior performed animals compared to those in the controls: the visual, somatosensory, motor and premotor cortices, the insula, cingulate, ectorhinal, entorhinal, perirhinal and piriform cortices, hippocampus, amygdala with amygdalohippocampal areas, dorsal striatum, nucleus accumbens core, substantia nigra, and retrorubral field. In conclusion, the MEMRI proved to be a reliable method to accomplish brain activity mapping in correlation with the operant behavior of freely moving rodents.

Low gamma wave oscillations in the striatum of mice following morphine administration

Functional role of the striatum in motor control has been widely studied. In addition, its involvement in reward function as a brain area in the dopamine system has also been mentioned. However, neural signaling in the striatum in response to consumption of emotional enhancing substances remained to be explored. This study aimed to investigate local field potential (LFP) of the striatum following morphine administration. Male Swiss albino mice implanted with electrode into the striatum were given an intraperitoneal injection of either saline or morphine (5 or 15 mg/kg). LFP and locomotor activity of individual animals were simultaneously recorded in the recording chamber following the administration. The inspection of LFP tracings revealed the increase in fast wave induced by morphine particularly at a high dose. Statistical analyses were performed using a one way ANOVA followed by Tukey post hoc test. Frequency analysis using Fast Fourier transform also confirmed a significant elevation of low gamma (30-44.9 Hz) activity. When analyzed in time domain, significant increase in low gamma power was observed from the 15th to 65th min following 15 mg/kg morphine treatment. Moreover, morphine treatment also exhibited a stimulating effect on locomotor speed. However, regression analyses revealed no significant correlation between low gamma power and locomotor speed. In summary, this study demonstrated the increase in low gamma oscillation in the striatum and this effect was not associated with locomotor activity of animals. Thus, it is possible that low gamma oscillation induced by morphine treatment is related with the reward function.

Mouse model of OPRM1 (A118G) polymorphism has altered hippocampal function

A single nucleotide polymorphism (SNP) in the human μ-opioid receptor gene (OPRM1 A118G) has been widely studied for its association in a variety of drug addiction and pain sensitivity phenotypes; however, the extent of these adaptations and the mechanisms underlying these associations remain elusive. To clarify the functional mechanisms linking the OPRM1 A118G SNP to altered phenotypes, we used a mouse model possessing the equivalent nucleotide/amino acid substitution in the Oprm1 gene. In order to investigate the impact of this SNP on circuit function, we used voltage-sensitive dye imaging in hippocampal slices and in vivo electroencephalogram recordings of the hippocampus following MOPR activation. As the hippocampus contains excitatory pyramidal cells whose activity is highly regulated by a dense network of inhibitory neurons, it serves as an ideal structure to evaluate how putative receptor function abnormalities may influence circuit activity. We found that MOPR activation increased excitatory responses in wild-type animals, an effect that was significantly reduced in animals possessing the Oprm1 SNP. Furthermore, in order to assess the in vivo effects of this SNP during MOPR activation, EEG recordings of hippocampal activity following morphine administration corroborated a loss-of-function phenotype. In conclusion, as these mice have been shown to have similar MOPR expression in the hippocampus between genotypes, these data suggest that the MOPR A118G SNP results in a loss of receptor function.

Opiate dependence induces network state shifts in the limbic system

Among current theories of addiction, hedonic homeostasis dysregulation predicts that the brain reward systems, particularly the mesolimbic dopamine system, switch from a physiological state to a new "set point." In opiate addiction, evidence show that the dopamine system principal targets, prefrontal cortex (PFC), nucleus accumbens (NAC) and basolateral amygdala complex (BLA) also adapt to repeated drug stimulation. Here we investigated the impact of chronic morphine on the dynamics of the network of these three interconnected structures. For that purpose we performed simultaneous electrophysiological recordings in freely-moving rats subcutaneously implanted with continuous-release morphine pellets. Chronic morphine produced a shift in the network state underpinned by changes in Delta and Gamma oscillations in the LFP of PFC, NAC and BLA, in correlation to behavioral changes. However despite continuous stimulation by the drug, an apparent normalization of the network activity and state occurred after 2 days indicating large scale adaptations. Blockade of μ opioid receptors was nonetheless sufficient to disrupt this acquired new stability in morphine-dependent animals. In line with the homeostatic dysregulation theory of addiction, our study provides original direct evidence that the PFC-NAC-BLA network of the dependent brain is characterized by a de novo balance for which the drug of abuse becomes the main contributor.

Differential effects of aging on EEG after baclofen administration

Baclofen is a selective gamma-aminobutyric acid (GABA) type B agonist that may have important medicinal uses, such as in analgesics and drug addiction treatment. In addition, evidence is accumulating that suggests GABAergic-mediated neurotransmission is altered during aging. This study investigated whether baclofen administration (5 mg kg(-1)) induces differential effects on cortical electrical activity with age. Electroencephalograms (EEGs) were recorded from young (3-4 months) and aged (15-17 months) rats, and both the absolute and relative powers in five frequency bands (delta: 2-4 Hz; theta: 4-8 Hz; alpha: 8-12 Hz; beta: 12-20 Hz; gamma: 20-100 Hz) were analyzed. Before administration of baclofen, we found that the EEG relative power in the beta band was higher in the aged than that in the young rats. After administration of baclofen, there was a slower increase in the relative power in the delta band in the aged than that in the young rats. Moreover, there was no significant difference between the two age groups in absolute power in any frequency band. These findings indicate that baclofen treatment appears to differentially modify cortical EEG activity as a function of age. Our data further elucidate the relationship between GABA(B) receptor-mediated neurotransmission and aging.

Evidence of key tinnitus-related brain regions documented by a unique combination of manganese-enhanced MRI and acoustic startle reflex testing

Animal models continue to improve our understanding of tinnitus pathogenesis and aid in development of new treatments. However, there are no diagnostic biomarkers for tinnitus-related pathophysiology for use in awake, freely moving animals. To address this disparity, two complementary methods were combined to examine reliable tinnitus models (rats repeatedly administered salicylate or exposed to a single noise event): inhibition of acoustic startle and manganese-enhanced MRI. Salicylate-induced tinnitus resulted in wide spread supernormal manganese uptake compared to noise-induced tinnitus. Neither model demonstrated significant differences in the auditory cortex. Only in the dorsal cortex of the inferior colliculus (DCIC) did both models exhibit supernormal uptake. Therefore, abnormal membrane depolarization in the DCIC appears to be important in tinnitus-mediated activity. Our results provide the foundation for future studies correlating the severity and longevity of tinnitus with hearing loss and neuronal activity in specific brain regions and tools for evaluating treatment efficacy across paradigms.

Learning-associated gamma-band phase-locking of action-outcome selective neurons in orbitofrontal cortex

Gamma oscillations (30-100 Hz) correlate to a variety of neural functions, including sensory processing, attention, and action selection. However, they have barely been studied in relation to emotional processing and valuation of sensory signals and actions. We conducted multineuron and local field potential recordings in the orbitofrontal cortex (OFC) of rats performing a task in which they made go or no-go decisions based on two olfactory stimuli predicting appetitive or aversive outcomes. Gamma power was strongest during the late phase of odor sampling, just before go/no-go movement, and increased with behavioral learning. Learning speed was correlated to the slope of the gamma power increment. Spikes of OFC neurons were consistently timed to the gamma rhythm during odor sampling, regardless of the associated outcome. However, only a specific subgroup of cells showed consistent phase timing. These cells showed action-outcome selective activity, not during stimulus sampling but during subsequent movement responses. During sampling, this subgroup displayed a suppression in firing rate but a concurrent increment in the consistency of spike timing relative to gamma oscillations. In addition to gamma rhythm, OFC field potentials were characterized by theta oscillations during odor sampling. Neurons phase-locked to either theta or gamma rhythms but not to both, suggesting that they become associated with separate rhythmic networks involving OFC. Altogether, these results suggest that OFC gamma-band synchronization reflects inhibitory control over a subpopulation of neurons that express information about the emotional valence of actions after a motor decision, which suggests a novel mechanism for response inhibition.

Manganese-enhanced MRI of layer-specific activity in the visual cortex from awake and free-moving rats

Cortical responses to visual stimulation have been studied extensively in the rodent, but often require post-stimulation ex vivo examination of the tissue. Here, we test the hypothesis that visual stimulus-dependent cortical activity from awake and free-moving rats can be encoded following systemically administered MnCl(2), and activity subsequently readout using manganese-enhanced MRI (MEMRI), a technique that can be performed without sacrificing the animal. Unanesthetized Sprague-Dawley rats, with or without systemic injection of MnCl(2), were maintained for 8 h in either a visually stimulating environment or darkness. To identify vision-dependent changes in cortical activity, animals were anesthetized and cortices were examined by 3D RARE MEMRI. Mean signal intensities in sub-cortical regions (e.g., superior colliculus and the lateral geniculate), and cortical regions (primary and accessory visual cortices) were compared. Cortex linearization was performed to aid in layer-specific signal intensity comparisons. Manganese administration alone globally increased signal intensity in the brain (P<0.0001). In visually stimulated and unstimulated rats, layer-specific analysis revealed that stimulated rats had on average significantly (P<0.05) higher signal intensities in layers IV and V of the primary visual cortex, as well as in deeper portions of the superficial superior colliculus, relative to dark adapted rats. Such differences went undetected without layer-specific analysis. We demonstrate, for the first time, the feasibility of layer-specific stimulus-dependant non-invasive MEMRI readout after encoding activity in awake and free moving rats. Future MEMRI studies are envisioned that measure the effects on cortical activity of sensory stimulation, as well as normal development, disease, plasticity, and therapy in longitudinal studies.

Neuronal dysfunction of a long projecting multisynaptic pathway in response to methamphetamine using manganese-enhanced MRI

RATIONALE

Manganese (Mn2+)-enhanced magnetic resonance imaging (MEMRI) is an emerging in vivo MR approach for pharmacological research. One new application of MEMRI in this area is to characterize functional changes of a specific neural circuit that is essential to the central effects of a drug challenge.

OBJECTIVES

To develop and validate such use of MEMRI in neuropharmacology, the current study applied MEMRI to visualize functional changes within a multisynaptic pathway originating from fasciculus retroflexus (FR) that is central to a commonly abused psychostimulant, methamphetamine (MA).

METHODS

Twelve rats were injected intraperitoneally with MA (10 mg/kg) or saline every 2 h for a total of four injections. After 6 days, Mn2+ was injected into the habenular nucleus (FR origin) of all animals, and MEMRI was repeatedly performed at certain points in time over 48 h. The evolution of Mn2+-induced signal enhancement was assessed across the FR tract, the ventral tegmental area (VTA), the striatum, the nucleus accumbens, and the prefrontal cortex (PFC), in both MA-injected animals and controls.

RESULTS

MA treatment was found to affect the complexity and efficiency of Mn2+ uptake in the VTA, via the FR tract, with significantly increased Mn2+ accumulation in the VTA, the dorsomedial part of the striatum, and the PFC.

CONCLUSIONS

MEMRI successfully visualizes disruptions in the multisynaptic pathway as the consequences of repeated MA exposure. MEMRI is potentially an important method in the future to investigate functional changes within a specific pathway under the influences of pharmacological agents, given its excellent functional, in vivo, spatial, and temporal properties.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Current status of functional MRI on small animals: application to physiology, pathophysiology, and cognition

This review aims to make the reader aware of the potential of functional MRI (fMRI) in brain activation studies in small animal models. As small animals generally require anaesthesia for immobilization during MRI protocols, this is believed to be a serious limitation to the type of question that can be addressed with fMRI. We intend to introduce a fresh view with an in-depth overview of the surprising number of fMRI applications in a wide range of important research domains in neuroscience. These include the pathophysiology of brain functioning, the basic science of activity, and functional connectivity of different sensory circuits, including sensory brain mapping, the challenges when studying the hypothalamus as the major control centre in the central nervous system, and the limbic system as neural substrate for emotions and reward. Finally the contribution of small animal fMRI research to cognitive neuroscience is outlined. This review avoids focusing exclusively on traditional small laboratory animals such as rodents, but rather aims to broaden the scope by introducing alternative lissencephalic animal models such as songbirds and fish, as these are not yet well recognized as neuroimaging study subjects. These models are well established in many other neuroscience disciplines, and this review will show that their investigation with in vivo imaging tools will open new doors to cognitive neuroscience and the study of the autonomous nervous system in experimental animals.

A comparison between spontaneous electroencephalographic activities induced by morphine and morphine-related environment in rats

Previous studies demonstrated that drug cues could elicit drug-like or withdrawal-like effect, both subjectively and physiologically. However, few studies have compared the central activities induced by a drug-related environment and the drug itself. The aim of this study was to observe and compare electroencephalographic (EEG) changes induced by acute morphine administration and by the morphine-related environment. EEG activities were recorded via twelve skull electrodes scattered on the left and right cortex in conscious, freely moving rats, either after acute morphine administration or after successful training of conditioned place preference. Acute administration of morphine (0.1, 0.5, 1, 5, 10, 20 mg/kg, i.p.) produced an increase in absolute EEG power in the delta, theta, alpha1, alpha2, beta1, and beta2 bands, as well as a decrease in the gamma band. Topographic mapping revealed a maximal increase in the lateral leads in the theta band and a maximal change in the centro-frontal region in the remaining bands. After place conditioning training, the morphine-related environment induced a diffuse decrease in absolute power in the delta, theta, alpha1, alpha2, beta1, and beta2 bands, which was opposite to the changes induced by acute morphine administration. In addition, the changes in relative power induced by the two situations also diverged. These results indicate that the central mechanisms underlying the motivation of morphine-induced place preference may be somehow different from those underlying the reward effects produced by acute morphine administration.