A novel anxiogenic role for the delta opioid receptor expressed in GABAergic forebrain neurons.

BACKGROUND: The delta opioid receptor (DOR) is broadly expressed throughout the nervous system; it regulates chronic pain, emotional responses, motivation, and memory. Neural circuits underlying DOR activities have been poorly explored by genetic approaches. We used conditional mouse mutagenesis to elucidate receptor function in GABAergic neurons of the forebrain. METHODS: We characterized DOR distribution in the brain of Dlx5/6-CreXOprd1(fl/fl) (Dlx-DOR) mice and tested main central DOR functions through behavioral testing. RESULTS: The DOR proteins were strongly deleted in olfactory bulb and striatum and remained intact in cortex and basolateral amygdala. Olfactory perception, circadian activity, and despair-like behaviors were unchanged. In contrast, locomotor stimulant effects of SNC80 (DOR agonist) and SKF81297 (D1 agonist) were abolished and increased, respectively. The Dlx-DOR mice showed lower levels of anxiety in the elevated plus maze, opposing the known high anxiety in constitutive DOR knockout animals. Also, Dlx-DOR mice reached the food more rapidly in a novelty suppressed feeding task, despite their lower motivation for food reward observed in an operant paradigm. Finally, c-fos protein staining after novelty suppressed feeding was strongly reduced in amygdala, concordant with the low anxiety phenotype of Dlx-DOR mice. CONCLUSIONS: We demonstrate that DORs expressed in the forebrain mediate the described locomotor effect of SNC80 and inhibit D1-stimulated hyperactivity. Our data also reveal an unanticipated anxiogenic role for this particular DOR subpopulation, with a potential novel adaptive role. In emotional responses, DORs exert dual anxiolytic and anxiogenic roles, both of which may have implications in the area of anxiety disorders

Non-nociceptive roles of opioids in the CNS: opioids' effects on neurogenesis, learning, memory and affect.

Mortality due to opioid use has grown to the point where, for the first time in history, opioid-related deaths exceed those caused by car accidents in many states in the United States. Changes in the prescribing of opioids for pain and the illicit use of fentanyl (and derivatives) have contributed to the current epidemic. Less known is the impact of opioids on hippocampal neurogenesis, the functional manipulation of which may improve the deleterious effects of opioid use. We provide new insights into how the dysregulation of neurogenesis by opioids can modify learning and affect, mood and emotions, processes that have been well accepted to motivate addictive behaviours

Double Fluorescent Knock-In Mice to Investigate Endogenous Mu-Delta Opioid Heteromer Subcellular Distribution

International audienceThe heteromerization of mu (MOP) and delta (DOP) opioid receptors has been extensively studied in heterologous systems. These studies demonstrated significant functional interaction of MOP and DOP evidenced by new pharmacological properties and intracellular signaling in transfected cells co-expressing the receptors. Due to the lack of appropriate tools for receptor visualization, such as specific antibodies, the pharmacological and functional properties of MOP-DOP heteromers in cells naturally expressing these receptors remain poorly understood. To address endogenous MOP-DOP heteromer trafficking and signaling in vivo and in primary neuronal cultures, we generated a double knock-in mouse line expressing functional fluorescent versions of DOP and MOP receptors. This mouse model has successfully been used to map the neuroanatomic distribution of the receptors and to identify brain regions in which the MOP-DOP heteromers are expressed. Here, we describe a method to quantitatively and automatically analyze changes in the subcellular distribution of MOP-DOP heteromers in primary hippocampal culture from this mouse model. This approach provides a unique tool to address specificities of endogenous MOP-DOP heteromer trafficking

Neuropeptidomics Mass Spectrometry Reveals Signaling Networks Generated by Distinct Protease Pathways in Human Systems

Neuropeptides regulate intercellular signaling among cells of the central brain and peripheral nervous systems, and the endocrine system. Diverse neuropeptides of distinct primary sequences of various lengths, often with post-translational modifications, coordinate and integrate regulation of physiological functions. Mass spectrometry-based analyses of the diverse neuropeptide structures in neuropeptidomics research is necessary to define the full complement of neuropeptide signaling molecules. Human neuropeptidomics has notable importance in defining normal and dysfunctional neuropeptide signaling in human health and disease. Neuropeptidomics has great potential for expansion in translational research opportunities for defining neuropeptide mechanisms of human diseases, providing novel neuropeptide drug targets for drug discovery, and monitoring neuropeptides as biomarkers of drug responses. In consideration of the high impact of human neuropeptidomics for health, an observed gap in this discipline is the few published articles in human neuropeptidomics compared to, for example, human proteomics and related mass spectrometry disciplines. Focus on human neuropeptidomics will advance new knowledge of the complex neuropeptide signaling networks participating in the fine control of neuroendocrine systems. This commentary review article discusses several human neuropeptidomics accomplishments that illustrate the rapidly expanding diversity of neuropeptides generated by protease processing of pro-neuropeptide precursors occurring within the secretory vesicle proteome. Of particular interest is the finding that human-specific cathepsin V participates in producing enkephalin and likely other neuropeptides, indicating unique proteolytic mechanisms for generating human neuropeptides. The field of human neuropeptidomics has great promise to solve new mechanisms in disease conditions, leading to new drug targets and therapeutic agents for human diseases

Diversity of Neuropeptide Cell-Cell Signaling Molecules Generated by Proteolytic Processing Revealed by Neuropeptidomics Mass Spectrometry

Neuropeptides are short peptides in the range of 3-40 residues that are secreted for cell-cell communication in neuroendocrine systems. In the nervous system, neuropeptides comprise the largest group of neurotransmitters. In the endocrine system, neuropeptides function as peptide hormones to coordinate intercellular signaling among target physiological systems. The diversity of neuropeptide functions is defined by their distinct primary sequences, peptide lengths, proteolytic processing of pro-neuropeptide precursors, and covalent modifications. Global, untargeted neuropeptidomics mass spectrometry is advantageous for defining the structural features of the thousands to tens of thousands of neuropeptides present in biological systems. Defining neuropeptide structures is the basis for defining the proteolytic processing pathways that convert pro-neuropeptides into active peptides. Neuropeptidomics has revealed that processing of pro-neuropeptides occurs at paired basic residues sites, and at non-basic residue sites. Processing results in neuropeptides with known functions and generates novel peptides representing intervening peptide domains flanked by dibasic residue processing sites, identified by neuropeptidomics. While very short peptide products of 2-4 residues are predicted from pro-neuropeptide dibasic processing sites, such peptides have not been readily identified; therefore, it will be logical to utilize metabolomics to identify very short peptides with neuropeptidomics in future studies. Proteolytic processing is accompanied by covalent post-translational modifications (PTMs) of neuropeptides comprising C-terminal amidation, N-terminal pyroglutamate, disulfide bonds, phosphorylation, sulfation, acetylation, glycosylation, and others. Neuropeptidomics can define PTM features of neuropeptides. In summary, neuropeptidomics for untargeted, global analyses of neuropeptides is essential for elucidation of proteases that generate diverse neuropeptides for cell-cell signaling. Graphical Abstract ᅟ