Excessive high fat diet (HFD) consumption can induce food addiction which is believed to involve the communication between the hypothalamus and mesolimbic dopaminergic neurons, originating in the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAc). These brain regions are densely populated with opioid receptors, raising the possibility that these receptors, and particularly mu opioid receptors (MORs), are involved in mediating reward induced by palatable foods. This study sought to investigate the involvement of MORs in high fat diet (HFD)-induced reward and if there is any difference between male and female subjects in this response. We also assessed if exposure to HFD would potentiate the rewarding action of oxycodone, a relatively selective MOR agonist. The place conditioning paradigm was used to determine if conditioning for a short time (STC, 2 h) or long time (LTC, 16 h) with HFD induces reward or alters the rewarding action of oxycodone. Male and female C57BL/6J mice as well as MOR knockout and wildtype mice of both sexes were tested for basal place preference on day 1 and then conditioned with HFD in one chamber and regular chow diet (RCD) in another chamber and tested for place preference again after three sets of STC and again after a set of LTC. Each set consisted of two conditioning with RCD and two conditioning with HFD. Controls were conditioned with RCD in both conditioning chambers. Following the last place preference test, mice were treated with oxycodone and conditioned in the HFD-paired chamber and saline in the RCD-paired chamber for one hour once a day to explore the possibility if the HFD could alter oxycodone reward. The result showed that HFD induced conditioned place preference (CPP) in both male and female subjects following the LTC. This response was further potentiated after oxycodone conditioning. The latter response was mediated via MORs, as it was blunted in MOR knockout mice. However, HFD-induced CPP was observed following LTC in only female MOR knockout mice, suggesting for sexual dimorphism in this response.

As variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continue to emerge, assessment of vaccine immunogenicity remains a critical factor to support continued vaccination. To this end, an in vitro microneutralization (MN50) assay was validated to quantitate SARS-CoV-2 neutralizing antibodies against ancestral and variant strains (Beta, Delta, Omicron BA.1, Omicron BA.5 and XBB.1.5) in human serum. For ancestral strain, the MN50 assay met acceptance criteria for inter-/intra-assay precision, specificity, linearity, and selectivity. The assay was robust against changes to virus/serum incubation time, cell seeding density, virus content per well, cell passage number, and serum interference. Analyte in serum samples was stable up to 5 freeze/thaw cycles and for up to 12 months of storage at –80 ± 10 °C. Similar results were observed for the variant-adapted MN50 assays. The conversion factor to convert assay result units to WHO international standard units (IU/mL) was determined to be 0.62 for the ancestral prototype strain. This MN50 assay will be useful for vaccine immunogenicity analyses in clinical trial samples, enabling assessment of vaccine immunogenicity for ancestral and variant strains as variant-adapted vaccines are developed.