Opioids affect acquisition of LiCl-induced conditioned taste aversion: involvement of OT and VP systems

Aversive properties of lithium chloride (LiCl) are mediated via pathways co mprising neurons of the nucleus of the solitary tract (NTS) and oxytocin (O T) and vasopressin (VP) cells in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Because opioids act on brain regions that mediate effects of LiCl, we evaluated whether administration of opioids shortly be fore LiCl in rats influences 1) development of conditioned taste aversion ( CTA) and 2) activation of NTS neurons and OT/VP cells. Neuronal activation was assessed by applying c-Fos immunohistochemical staining. Three opioids were used: morphine (MOR), a m-agonist, butorphanol tartrate (BT), a mixed mu/kappa-agonist, and nociceptin/orphanin FQ (N/OFQ), which binds to an ORL 1 receptor. BT and N/OFQ completely blocked acquisition of CTA. MOR allevia ted but did not eliminate the aversive effects. Each of the opioids decreas ed LiCl-induced activation of NTS neurons as well as OT and VP cells in the PVN and SON. We conclude that opioids antagonize aversive properties of Li Cl, presumably by suppressing activation of pathways that encompass OT and VP cells and NTS neurons.