C‐Fos Responses Produced by the Central Microinjection of Salvinorin A in Conscious Rats

Abstract

Intracerebroventricular (ICV) injection of the kappa opioid agonists produces a marked diuresis in rats via inhibition of vasopressin secretion. Salvinorin A (SalA) is a plant‐derived high affinity agonist for the kappa opioid receptor (KOR). Previous studies have proposed that SalA has similar potency and efficacy as other synthetic kappa agonists in mediating the CNS effects of KOR activation. The present study examines the expression of the proto‐oncogene c‐fos in several forebrain regions of rats treated with SalA as compared to either acetonitrile or saline vehicle treated animals. SalA is a highly lipophilic drug and past studies utilized drug dissolving vehicles (e.g., DMSO) that produced significant cardiovascular and renal effects. In this study, we dissolved SalA in acetonitrile/saline (1:9 dilution) to minimize solvent viscosity and cardiovascular effects.MethodsAnimals were habituated to a metabolic cage and injected with either SalA, acetonitrile or saline. Urine samples were collected during two 15 min control periods and six 15 min periods beginning 15 min after injection into the lateral ventricle. After 90 minutes, animals were perfused with a phosphate buffer and PFA. Brains were removed and cut on a cryostat (40 μm thick). Brain sections were processed for c‐fos using a commercially available antibody. Sections were also double labeled with oxytocin to define anatomically specific forebrain regions.ResultsSalA decreased c‐fos expression in the parvocellular PVN, magnocellular PVN, supraoptic (SON) and median pre‐optic (MnPO) nucleus relative to the acetonitrile group. However, only in the parvocellular PVN was the decrease in c‐fos expression statistically significant (p<0.001). In contrast, there was no difference in c‐fos expression between SalA and saline vehicle treated rats, with the exception of the MnPO in which c‐fos expression was markedly increased (P<0.001).ConclusionThe ability of kappa opioid receptor agonists to increase diuresis and decrease osmolality may be associated with changes in c‐fos expression in the PVN and lamina terminalis. Previous studies in our lab, demonstrated a small increase in urine excretion rate following ICV administration of SalA and is consistent with the present findings in which there is a concomitant decrease in c‐fos expression in the magnocellular PVN and SON forebrain regions in SalA treated animals. However, the decrease was not a prominent as observed with U50488H, a synthetic KOR agonist. These results support the findings that the diuretic effects of SalA are not as potent as synthetic kappa opioid agonists.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.