Kappa-opioid receptors behind the blood-brain barrier are involved in the anti-hypertensive effects of systemically administered kappa-agonists in the conscious spontaneously hypertensive rat

Difelikefalin, a peripherally restricted KOR (kappa opioid receptor) agonist, produces diuresis through a central KOR pathway

Difelikefalin is a peripherally restricted kappa opioid receptor (KOR) agonist that was recently approved by the FDA to treat pruritis in dialysis patients. Here, we investigated the cardiovascular and renal responses to difelikefalin, and using the KOR antagonist norbinaltorphimine (norBNI), examined whether any difelikefalin-induced changes in the renal excretion of water and/or electrolytes were mediated through a central or peripheral KOR pathway. The effects of norBNI pretreatment on nalfurafine, a KOR agonist that crosses the blood-brain barrier, were also examined. We hypothesized that difelikefalin would alter urine output differently than nalfurafine, given that KOR agonists produce diuresis via activating central KORs to inhibit vasopressin release. Following catheterization, conscious Sprague-Dawley rats were infused i.v. with isotonic saline and pretreated with norBNI centrally via an intracerebroventricular (ICV) cannula or peripherally via an intravenous catheter. After stabilization, difelikefalin or nalfurafine was administered i.v. and urine output, heart rate and mean arterial pressure (MAP) were recorded for 90 min. Difelikefalin produced a significant increase in urine output, and significant decrease in urinary sodium and potassium excretion, urine osmolality, and MAP. ICV norBNI pretreatment markedly attenuated the increase in urine output caused by difelikefalin and nalfurafine but did not inhibit the electrolyte effects. However, IV norBNI pretreatment prevented all responses to difelikefalin and nalfurafine. Together, these findings demonstrate that difelikefalin and nalfurafine utilize central KOR pathways to elicit diuresis and a decrease in MAP but enhance renal tubular electrolyte reabsorption through a peripheral KOR pathway, providing important insight into two clinically useful KOR agonists.

Blockade of dorsal hippocampal kappa-opioid receptors increases blood pressure in normotensive and isolation-induced hypertensive rats

Previous research in our laboratory has established a restraining role of the hippocampal kappa-opioid receptor system in the neural control of blood pressure. Chronic or acute hippocampal administration of kappa-agonists has been shown to reduce blood pressure. Isolation of male Sprague-Dawley rats provokes an increase in blood pressure, which has been proposed as a valid model of mild emotionally induced hypertension. It was the purpose of this study to investigate the blood pressure effects of dorsal hippocampal administration of nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist; in conscious male Sprague-Dawley rats subjected to isolation-induced hypertension. Chronic bi-hippocampal microinjection of nor-BNI (10 nmol per side, twice a day for 13 days) caused increases in systolic blood pressure in grouped rats from a mean of 125 to 148 mmHg, and in isolated rats from a mean of 125 to 151 mmHg. This hypertension was similar in magnitude to the increase observed after rats were isolated for 7 days and treated with vehicle in a similar fashion (mean systolic blood pressure 144 mmHg). The increases in blood pressure were accompanied by bradycardia. No significant responses were seen in the blood pressure of grouped rats treated with vehicle. The hypotensive response to a single hippocampal microinjection of the kappa-agonist U62, 066E (10 or 20 nmol) was prevented in anesthetized rats treated previously with nor-BNI, indicating that in the nor-BNI treated rats there was effective blockade of dorsal hippocampal receptors. These data suggest that the rat hippocampal kappa-opioid receptor system and the endogenous neuropeptide dynorphin may be involved in the central neural regulation of blood pressure.

Prevention of isolation-induced hypertension by intrahippocampal administration of a nonpeptide kappa-opioid receptor agonist

Previous research in this laboratory showed that hypertension in the spontaneous hypertensive rat (SHR) appears to correlate to insufficient production of hippocampal dynorphins, and that blood pressure could be reduced by intrahippocampal administration of dynorphins and nonpeptide kappa agonists. The purpose of the present study was to investigate whether kappa agonists could prevent the development of hypertension in a different hypertensive model, i.e., the isolated male rat model of hypertension (IHR). Isolation of young male rats for 5-7 days in standard rat cages caused an increase in systolic blood pressure from a mean of 132 to 184 mmHg. The blood pressures of rats grouped 3 per cage remained stable. Rats received the nonpeptide kappa agonist U62, 066E, (Spiradoline, Upjohn), 10 nmoles/0.2 microl or drug vehicle bilaterally into the the hippocampus for 3 days prior to and during isolation or grouping. Animals treated with U62, 066E did not develop hypertension as compared to isolated animals treated with vehicle. The isolation procedure used in these studies appears to induce anxietal stress, as indicated by reduced time spent by the rats in the open arms of the elevated-plus maze. This time is increased by U62, 066E, suggesting that the drug possesses anxiolytic properties and may reduce hypertension in part, by blocking an anxiety/stress component. These data strengthen our previous findings that opioids in the hippocampus may be important in restraining increased blood pressure provoked by environmental stimuli such as isolation.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Hippocampal administration of kappa-opioid receptor antisense exacerbates isolation-induced hypertension

Isolation of young male Sprague Dawley rats for 7 days provoked hypertension which was exacerbated by hippocampal administration of an antisense oligodeoxynucleotide targeted to the kappa-opioid receptor (1 microg/0.5 microL bilaterally, twice daily, during the isolation or grouping period). Systolic blood pressure rose from a mean of 134 to 162 mmHg in isolated rats treated with antisense and only 139 to 151 mmHg in grouped rats treated with antisense. In grouped rats treated with either vehicle or missense, isolation caused a mean increase in systolic blood pressure of only 16 and 14 mmHg respectively. Neither the missense oligodeoxynucleotide nor the vehicle had any significant effects upon systolic pressure in grouped rats, which had not been isolated. Pharmacological studies indicated that rats previously treated with the antisense had no significant depressor response to U62, 066E (a non-peptide kappa agonist known to reduce blood pressure acutely when administered into the hippocampus), however rats previously treated with vehicle or missense exhibited a significant hypotensive response to the drug. This implies that the antisense had reduced the density or activity of the kappa receptors within the hippocampal formation. These data are in accordance with our previous studies, i.e. in several rat models of hypertension, the increase in blood pressure may be modulated via hippocampal kappa opioid receptors.