Stimulation of accumbal GABAA receptors inhibits delta2-, but not delta1-, opioid receptor-mediated dopamine efflux in the nucleus accumbens of freely moving rats

Adolescence as a critical period for nandrolone-induced muscular strength in relation to abuse liability, alone and in conjunction with morphine, using accumbal dopamine efflux in freely moving rats

Nandrolone, an anabolic androgenic steroid, is included in the prohibited list of the World Anti-Doping Agency. Drugs of abuse activate brain dopamine neurons and nandrolone has been suspected of inducing dependence. Accordingly, possible critical periods for the effects of nandrolone on muscular strength and dopaminergic activity have been investigated, including the effects of chronically administered nandrolone alone and on morphine-induced increases in dopamine efflux in the nucleus accumbens. Six- or 10-week-old male Sprague-Dawley rats were used. Treatment with nandrolone was initiated in adolescent (6-week-old) and young adult (10-week-old) rats. Nandrolone (5.0 mg/kg s.c.) or sesame oil vehicle was given once daily, on six consecutive days per week, for 3 weeks and then once per day for 4 consecutive days. Nandrolone enhanced the developmental increase in grip strength of 6- but not 10-week-old rats, without altering the developmental increase in body weight of either age group. Using in vivo microdialysis in freely moving 6-week-old rats given nandrolone for 4 weeks, basal accumbal dopamine efflux was unaltered, while the increase in dopamine efflux induced by acute administration of morphine (1.0 mg/kg s.c.) was reduced. The present study provides in vivo evidence that adolescence constitutes a critical period during which repeated administration of nandrolone enhances increases in muscular strength without influencing increases in body weight. Though repeated administration of nandrolone during this period of adolescence did not stimulate in vivo mesolimbic dopaminergic activity, it disrupted stimulation by an opioid, the drug class that is most commonly coabused with nandrolone.

In vivo microdialysis reveals that blockade of accumbal orexin OX2 but not OX1 receptors enhances dopamine efflux in the nucleus accumbens of freely moving rats

The nucleus accumbens contains orexinergic neural inputs and orexin OX₁ -and OX₂ -receptors. Behavioural studies suggest that accumbal orexin receptors modulate accumbal dopaminergic activity-dependent locomotion in rats. We studied the effects of intra-accumbal injection of orexin receptor ligands on accumbal extracellular dopamine levels in freely moving rats, using in vivo microdialysis, and analysed the roles of OX₁ - and OX₂ -receptors in the regulation of basal accumbal dopamine efflux. The orexin receptor ligands were applied intra-accumbally though a microinjection needle attached with a dialysis probe. Neither the non-selective OX₁ - and OX₂ -receptor agonist orexin-A nor the preferential OX₂ -receptor agonist orexin-B (500.0 pg and 5.0 ng) altered accumbal dopamine levels. The non-selective OX₁ - and OX₂ -receptor antagonist MK-4305 (suvorexant, 500.0 pg, 2.5 and 5.0 ng) enhanced dopamine efflux. A 2-h tetrodotoxin infusion into nucleus accumbens through the probe or co-administration of orexin-A (500.0 pg) strongly inhibited MK-4305 (5.0 ng)-induced accumbal dopamine efflux. The selective OX₂ -receptor antagonist EMPA (90.0 and 900.0 pg, 9.0 ng) increased dopamine efflux. Intra-accumbal infusion of tetrodotoxin abolished EMPA (9.0 ng)-induced dopamine efflux. The selective OX₁ -receptor antagonist SB-334867 (10.0 and 20.0 ng) failed to alter dopamine efflux. Co-administration of orexin-B (500.0 pg) inhibited both EMPA (9.0 ng)- and MK-4305 (5.0 ng)-induced dopamine efflux. Intraperitoneal injection of MK-4305 (10.0 mg/kg) did not affect accumbal dopamine efflux. The present study provides in vivo neuropharmacological evidence that accumbal OX₂ - but not OX₁ -receptors exert inhibitory regulation of basal accumbal dopamine efflux and that blockade of accumbal OX₂ -receptors enhances dopamine efflux in nucleus accumbens of freely moving rats.

Integrative opioid-GABAergic neuronal mechanisms regulating dopamine efflux in the nucleus accumbens of freely moving animals

The nucleus accumbens (NAc) is a terminal region of mesocorticolimbic dopamine (DA) neuronal projections from the ventral tegmental area. Accumbal DA release is integrated by afferents from other brain regions and by interneurons, which involve a diversity of neurotransmitters and neuropeptides. These integrative processes, implicated in the pathobiology of neuropsychiatric disorders, are mediated via receptor subtypes whose relative roles in the regulation of accumbal DA release are poorly understood. Such complex interactions are exemplified by how selective activation of opioid receptor subtypes enhances accumbal DA efflux in a manner that is modulated by changes in neural activity through GABA receptor subtypes. This review delineates the roles of GABA_A and GABA_B receptors in GABAergic neural mechanisms in NAc that participate in delta- and mu-opioid receptor-mediated increases in accumbal DA efflux in freely moving rats, focusing on studies using in vivo brain microdialysis. First, we consider how endogenous GABA exerts inhibition of accumbal DA efflux through GABA receptor subtypes. We also consider possible intra-neuronal source of the endogenous GABA that inhibits accumbal DA efflux. As NAc contains GABAergic neurons that express delta- or mu-opioid receptors, inhibition of accumbal GABAergic neurons is a candidate for mediating delta- or mu-opioid receptor-mediated increases in accumbal DA efflux. Therefore, we provide a detailed analysis of the effects of GABA receptor subtype ligands on delta- and mu-opioid receptor-mediated accumbal DA efflux. Finally, we present an integrative model to explain the mechanisms of interaction among delta- and mu-opioid receptors, GABAergic neurons and DAergic neurons in NAc.

Nucleus accumbens shell: A potential target for drug-resistant epilepsy with neuropsychiatric disorders

The nucleus accumbens (NAc) is an important component of the ventral striatum, involving motivational and emotional processes, limbic-motor interfaces. Recently, experimental and clinical data have shown that NAc, particularly NAc shell (NAcs), participates in ictogenesis and epileptogensis in drug-resistant epilepsy (DRE). Therefore, we summarize the existing literature on NAcs and potential role in epilepsy, from the bench to the clinic. Connection abnormalities between NAcs and remainings, degeneration of NAc neurons, and an aberrant distribution of neuroactive substances have been reported in patients with DRE. These changes may be underlying the pathophysiological mechanism of the involvement of NAcs in DRE. Furthermore, alterations in NAcs may also be involved in neuropsychiatric disorders in patients with DRE. These observational studies demonstrate the multiple properties of NAcs and the complex relationship between the limbic system and DRE with neuropsychiatric disorders. NAcs can be a potential target for DBS and stereotactic lesioning to manage DRE with neuropsychiatric disorders. Future studies are warranted to further clarify the role of NAcs in epilepsy.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Stimulation of accumbal GABAB receptors inhibits delta1- and delta2-opioid receptor-mediated dopamine efflux in the nucleus accumbens of freely moving rats

The nucleus accumbens contains delta-opioid receptors that may decrease inhibitory neurotransmission. As GABA_B receptors inhibit dopamine release, decrease in activation of GABA_B receptors may be a mediator of delta-opioid receptor-induced accumbal dopamine efflux. If so, accumbal dopamine efflux induced by delta-opioid receptor activation should be suppressed by stimulating GABA_B receptors. As delta-opioid receptors are further subdivided into delta1- and delta2-opioid receptors, we analysed the effects of the GABA_B receptor agonist baclofen on delta1- and delta2-opioid receptor-mediated accumbal dopamine efflux in freely moving rats using in vivo microdialysis. Drugs were applied intracerebrally through the dialysis probe. Doses of compounds show total amount administered (mol) during 25-50 min infusions. Baclofen (2.5 and 5.0 nmol), which did not alter basal dopamine levels, inhibited the delta1-opioid receptor agonist DPDPE (5.0 nmol)-induced dopamine efflux. Baclofen (2.5 and 5.0 nmol) also inhibited the delta2-opioid receptor agonist deltorphin II (25.0 nmol)-induced dopamine efflux. A low dose of the GABA_B receptor antagonist 2-hydroxysaclofen (100.0 pmol), which failed to alter basal accumbal dopamine levels, counteracted the inhibitory effects of baclofen (5.0 nmol) on DPDPE (5.0 nmol)- and deltorphin II (25.0 nmol)-induced dopamine efflux. The present results show that reduction in accumbal GABA_B receptor-mediated inhibition of accumbal dopaminergic activity facilitates activation of delta1- and delta2-opioid receptor-induced increases in accumbal dopamine efflux. This study suggests that activation of delta1- and delta2-opioid receptors on the cell bodies and/or terminals of accumbal GABAergic interneurons inhibits GABA release and, accordingly, decreases GABA_B receptor-mediated inhibition of dopaminergic terminals, resulting in enhanced accumbal dopamine efflux.