Intra-accumbens Microinfusion of the Dopamine D3 Receptor Partial Agonist (±)VK4-40 Does Not Affect Basal Locomotion in Mice

The opioid epidemic remains a pressing public health crisis, prompting the search for alternative pharmacotherapies for Opioid Use Disorder (OUD). This study explores the potential of the dopamine D3 receptor (D3R) partial agonist, (±)VK4-40, as a novel treatment option. We investigated the impact of intra-nucleus accumbens (NAc) microinfusion of VK4-40 on basal locomotion in mice. Results indicate that VK4-40 did not significantly alter basal locomotion, suggesting that its therapeutic effects may not be mediated through disruptions in generalized motor function. Future research will focus on elucidating the neuropharmacological mechanisms underlying VK4-40\u27s therapeutic actions and exploring its effects on psychostimulant-induced hyperlocomotion. These findings provide valuable insights into potential D3R-targeted pharmacotherapies for managing OUD

Fos Expression in Lateral Hypothalamus/Perifornical Area is Correlated with Psychosocial Stress-Induced Cocaine-Seeking Behavior in a Sex-Specific Manner

Cocaine Use Disorder persists as a significant public health concern in the United States. Recent epidemiological data indicate that rates of cocaine-involved overdose deaths are rising, and treatment of Cocaine Use Disorder is challenging due to a lack of FDA-approved medications to help patients achieve abstinence and avoid relapse. Stress can precipitate cocaine craving and trigger relapse episodes, however the underlying neural circuitry by which stressors drive cocaine seeking is not completely understood. Our laboratory has recently identified the potential involvement of the rostrolateral aspect of the periaqueductal gray (rlPAG) in psychosocial stress-induced cocaine-seeking behavior using a rodent model of cocaine relapse. Neuroanatomical evidence points to a dense monosynaptic input from the lateral hypothalamus (LH) to the rlPAG, yet whether activity within the LH is also associated specifically with psychosocial-stress induced cocaine seeking has not been previously explored. In this study, adult male and female Long-Evans rats were allowed to self-administer cocaine (0.5 mg/kg/inf, IV) in 2-h daily sessions for 20 sessions. During sessions 11, 14, 17, and 20, a tactile cue (perforated polycarbonate enclosure) was placed within the operant chamber, and these sessions were immediately followed by either social defeat stress (SDS, n=16; 8M, 8F) or a no-stress empty-cage control condition (EC, n=12; 6M, 6F). Beginning on day 21, animals underwent extinction training during which lever-presses were not reinforced. Once responding was extinguished, rats were re-exposed to the tactile cue that signaled their assigned stress/no-stress stimulus, and reinstatement of cocaine-seeking was measured for 2 h under extinction conditions. Immediately after the reinstatement test, animals were sacrificed, and brains collected and processed via immunohistochemistry for expression of the immediate early gene c-Fos and the neuropeptide orexin within the LH and neighboring Perifornical Area (PfA). Despite no significant difference between groups in the number of Fos+ or orexin+ cells in the LH/PfA, there was a significant positive correlation between Fos expression in non-orexinergic LH/PfA cells and stress-induced cocaine-seeking magnitude specifically within males. Additionally, Fos expression in these cells was also significantly and positively correlated with Fos expression in the rostrolateral periaqueductal gray. Collectively, these findings suggest that a LH --\u3e rlPAG projection may be selectively engaged during psychosocial stress-induced cocaine seeking behavior in male rats

Effects of Sex and Estrous Cycle on Intravenous Oxycodone Self-Administration and the Reinstatement of Oxycodone-Seeking Behavior in Rats

The increasing misuse of both prescription and illicit opioids has culminated in a national healthcare crisis in the United States. Oxycodone is among the most widely prescribed and misused opioid pain relievers and has been associated with a high risk for transition to compulsive opioid use. Here, we sought to examine potential sex differences and estrous cycle-dependent effects on the reinforcing efficacy of oxycodone, as well as on stress-induced or cue-induced oxycodone-seeking behavior, using intravenous (IV) oxycodone self-administration and reinstatement procedures. In experiment 1, adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone according to a fixed-ratio 1 schedule of reinforcement in daily 2-hr sessions, and a dose-response function was subsequently determined (0.003-0.03 mg/kg/inf). In experiment 2, a separate group of adult male and female Long-Evans rats were trained to self-administer 0.03 mg/kg/inf oxycodone for 8 sessions, followed by 0.01 mg/kg/inf oxycodone for 10 sessions. Responding was then extinguished, followed by sequential footshock-induced and cue-induced reinstatement tests. In the dose-response experiment, oxycodone produced a typical inverted U-shape function with 0.01 mg/kg/inf representing the maximally effective dose in both sexes. No sex differences were detected in the reinforcing efficacy of oxycodone. In the second experiment, the reinforcing effects of 0.01-0.03 mg//kg/inf oxycodone were significantly attenuated in females during proestrus/estrus as compared to metestrus/diestrus phases of the estrous cycle. Neither males nor females displayed significant footshock-induced reinstatement of oxycodone seeking, but both sexes exhibited significant cue-induced reinstatement of oxycodone seeking at magnitudes that did not differ either by sex or by estrous cycle phase. These results confirm and extend previous work suggesting that sex does not robustly influence the primary reinforcing effects of oxycodone nor the reinstatement of oxycodone-seeking behavior. However, our findings reveal for the first time that the reinforcing efficacy of IV oxycodone varies across the estrous cycle in female rats

A short history of the 5-HT2C receptor: from the choroid plexus to depression, obesity and addiction treatment

This paper is a personal account on the discovery and characterization of the 5-HT2C receptor (first known as the 5- HT1C receptor) over 30 years ago and how it translated into a number of unsuspected features for a G protein-coupled receptor (GPCR) and a diversity of clinical applications. The 5-HT2C receptor is one of the most intriguing members of the GPCR superfamily. Initially referred to as 5-HT1CR, the 5-HT2CR was discovered while studying the pharmacological features and the distribution of [3H]mesulergine-labelled sites, primarily in the brain using radioligand binding and slice autoradiography. Mesulergine (SDZ CU-085), was, at the time, best defined as a ligand with serotonergic and dopaminergic properties. Autoradiographic studies showed remarkably strong [3H]mesulergine-labelling to the rat choroid plexus. [3H]mesulergine-labelled sites had pharmacological properties different from, at the time, known or purported 5-HT receptors. In spite of similarities with 5-HT2 binding, the new binding site was called 5-HT1C because of its very high affinity for 5-HT itself. Within the following 10 years, the 5-HT1CR (later named 5- HT2C) was extensively characterised pharmacologically, anatomically and functionally: it was one of the first 5-HT receptors to be sequenced and cloned. The 5-HT2CR is a GPCR, with a very complex gene structure. It constitutes a rarity in theGPCR family: many 5-HT2CR variants exist, especially in humans, due to RNA editing, in addition to a few 5-HT2CR splice variants. Intense research led to therapeutically active 5-HT2C receptor ligands, both antagonists (or inverse agonists) and agonists: keeping in mind that a number of antidepressants and antipsychotics are 5- HT2CR antagonists/inverse agonists. Agomelatine, a 5-HT2CR antagonist is registered for the treatment of major depression. The agonist Lorcaserin is registered for the treatment of aspects of obesity and has further potential in addiction, especially nicotine/ smoking. There is good evidence that the 5-HT2CR is involved in spinal cord injury-induced spasms of the lower limbs, which can be treated with 5-HT2CR antagonists/inverse agonists such as cyproheptadine or SB206553. The 5-HT2CR may play a role in schizophrenia and epilepsy. Vabicaserin, a 5-HT2CR agonist has been in development for the treatment of schizophrenia and obesity, but was stopped. As is common, there is potential for further indications for 5-HT2CR ligands, as suggested by a number of preclinical and/or genome-wide association studies (GWAS) on depression, suicide, sexual dysfunction, addictions and obesity. The 5-HT2CR is clearly affected by a number of established antidepressants/antipsychotics and may be one of the culprits in antipsychotic-induced weight gain