Identification and Functional Characterisation of Novel Opioid Receptor Subtypes in the Gastrointestinal Tract of Camels in Northern Nigeria

The opioid receptor system modulates gastrointestinal motility, secretion, and pain processing in mammals, and pharmacological targeting of these receptors underpins the clinical use of opioid analgesics and anti-motility drugs in veterinary practice. Camels, which are critical livestock species in Borno, Yobe, and Katsina states in northern Nigeria, possess unique gastrointestinal anatomy and physiology adapted to desert conditions, but the molecular pharmacology of their gastrointestinal opioid receptor subtypes has never been investigated. This study aimed to identify and functionally characterise opioid receptor subtypes in the gastrointestinal tract of dromedary camels using combined molecular biology, radioligand binding, and functional assay approaches. Gastrointestinal tissue samples including forestomach, small intestine, and colon were collected from eight camels at Maiduguri abattoir. Opioid receptor gene expression was characterised by quantitative RT-PCR using degenerate primers designed to conserved regions of published opioid receptor sequences. Novel receptor sequences were deposited in GenBank and compared phylogenetically with known mammalian opioid receptors. Radioligand binding with tritiated naloxone, DAMGO, and U69593 determined receptor density and affinity. Functional assays assessed receptor-mediated inhibition of smooth muscle contraction in isolated tissue preparations. Three receptor subtypes corresponding to mu, delta, and kappa families were identified, with a novel kappa subtype splice variant unique to dromedary camels showing distinct pharmacological properties and higher intestinal density than other species. Functional assays confirmed differential inhibitory potency of selective opioid ligands. These findings establish the first pharmacological characterisation of camel gastrointestinal opioid receptors and open pathways for targeted analgesic development in this medically important species. Keywords: opioid receptors, camel, gastrointestinal, pharmacology, Northern Nigeria.