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

Abstract

Intracerebroventricular (ICV) injection of kappa opioid agonists produces a marked diuresis in rats via inhibition of vasopressin secretion. Salvinorin B (Sal B) is a plant‐derived high‐affinity ligand for the kappa opioid receptor (KOR). Previous studies have proposed that Sal B has similar potency and efficacy as other synthetic kappa agonists in mediating the CNS effects of KOR activation. The present study examines the effects of Sal B on renal function and the expression of the proto‐oncogene c‐Fos in several hindbrain regions. Sal B effects were compared to either acetonitrile or saline vehicle treated animals. Sal B 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 Sal B in acetonitrile/saline (1:9 dilution) to minimize solvent viscosity and cardiovascular effects.MethodsAnimals were habituated to a metabolic cage and injected with either Sal B, acetonitrile or saline. Urine samples were collected during the habituation period and used as an additional control. On the day of the experiment, urine was collected during two 15 min control periods followed by six 15 min periods beginning 15 min after the ICV injection into the lateral ventricle. After 90 minutes, animals were perfused with 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 double labeled with dopamine beta‐hydroxylase (DBH; CY3 Anti‐mouse) to define anatomically specific hindbrain regions.ResultsICV Sal B failed to produce any significant changes to renal function (urine volume, sodium and potassium excretion rate) as compared to control vehicle. Sal B decreased c‐Fos expression in the medial nucleus of the solitary tract (NTS‐m) and rostral ventrolateral medulla (RVLM), and noradrenergic cell group (A5).ConclusionPrevious studies suggested that Sal B has affinity for the KOR; however, our results indicate that Sal B lacks efficacy and does not produce diuretic or anti‐natriuretic effects associated to synthetic kappa opioid agonists. In addition, KOR agonists increase renal sympathetic nerve activity via changes to cardiovascular controlling centers in the brain. Sal B decreased c‐Fos expression in certain hindbrain regions, but further studies are necessary to determine the role Sal B plays in these regions.