Normotensive Wistar rats differ from spontaneously hypertensive and renal hypertensive rats in their cardiovascular responses to opioid agonists

The cardiovascular effects of a chimeric opioid peptide based on morphiceptin and PFRTic-NH2

MCRT (YPFPFRTic-NH(2)) is a chimeric opioid peptide based on morphiceptin and PFRTic-NH(2). In order to assess the cardiovascular effect of MCRT, it was administered by intravenous (i.v.) injection targeting at the peripheral nervous system and by intracerebroventricular (i.c.v.) injection targeting at the central nervous system. Naloxone and L-NAME were injected before MCRT to investigate possible interactions with MCRT. Results show that administration of MCRT by i.v. or i.c.v. injection could induce bradycardia and decrease in mean arterial pressure (MAP) at a greater degree than that with morphiceptin and PFRTic-NH(2). When MCRT and NPFF were coinjected, we observed a dose-dependent weakening of these cardiovascular effects by MCRT. Because naloxone completely abolished the cardiovascular effects of MCRT, we conclude that opioid receptors are involved in regulating the MAP of MCRT regardless of modes of injection. The effect of MCRT on heart rate is completely dependent on opioid receptors when MCRT was administered by i.c.v. instead of i.v. The central nitric oxide (NO) pathway is involved in regulating blood pressure by MCRT under both modes of injection, but the peripheral NO pathway had no effect on lowering blood pressure mediated by MCRT when it was administered by i.c.v. Based on the results from different modes of injection, the regulation of heart rate by MCRT mainly involves in the central NO pathway. Lastly, we observed that the cardiovascular effects of MCRT such as bradycardia and decrease of blood pressure, were stronger than that of its parent peptides. Opioid receptors and the NO pathway are involved in the cardiovascular regulation by MCRT, and their degree of involvement differs between intravenous and intracerebroventricular injection.

Differential regulation of hippocampal progenitor proliferation by opioid receptor antagonists in running and non-running spontaneously hypertensive rats

Voluntary running in mice and forced treadmill running in rats have been shown to increase the amount of proliferating cells in the hippocampus. Little is known as yet about the mechanisms involved in these processes. It is well known that the endogenous opioid system is affected during running and other forms of physical exercise. In this study, we evaluated the involvement of the endogenous opioids in the regulation of hippocampal proliferation in non-running and voluntary running rats. Nine days of wheel running was compared with non-running in spontaneously hypertensive rats (SHR), a rat strain known to run voluntarily. On the last 2 days of the experimental period all rats received two daily injections of the opioid receptor antagonists naltrexone or naltrindole together with injections of bromodeoxyuridine to label dividing cells. Brain sections from the running rats showed approximately a five-fold increase in newly generated cells in the hippocampus, and this increase was partly reduced by naltrexone but not by naltrindole. By contrast, both naltrexone and naltrindole increased hippocampal proliferation in non-running rats. In non-running rats the administration of naltrexone decreased corticosterone levels and adrenal gland weights, whereas no significant effects on these parameters could be detected for naltrindole. However, adrenal gland weights were increased in naltrexone- but not in naltrindole-administered running rats. In addition, in voluntary running rats there was a three-fold increase in the hippocampal levels of Met-enkephalin-Arg-Phe compared with non-runners, indicating an increase in opioid activity in the hippocampus during running. These data suggest an involvement of endogenous opioids in the regulation of hippocampal proliferation in non-running rats, probably through hypothalamic-pituitary-adrenal axis modulation. During voluntary running in SHR naltrexone altered hippocampal proliferation via as yet unknown mechanisms.

Depressor effect of kappa opioid agonist on hypertension induced by isolation in the rat

Previous studies by us established that peripheral and hippocampal administration of kappa opioid receptor agonists lowered blood pressure (BP) in the spontaneously hypertensive rat (SHR). The object of the present study was to determine whether U62,066E, a non-peptide kappa agonist, would lower BP in another animal model of hypertension; that produced by isolation of young male rats. After 7 days of isolation had produced a sustained hypertension of approximately 40 mmHg, drug effects were determined in the isolated hypertensive animals and group-housed normotensive rats. Two drug-treated plus vehicle control groups were used as follows: (1) 2 mg/kg intraperitoneally twice daily for 3 days in conscious animals and (2) intrahippocampal injection of from 1 to 10 nmol in animals anesthetized with chloralose and pentobarbital. In group (1) the drug lowered both systolic BP (SBP) and mean BP (MBP) nearly to pre-isolation levels, while having no significant effect on these parameters in group-housed normotensive controls. Heart rate (HR) was not significantly affected by the drug in either sub-group. In group (2) SBP, MBP and HR were reduced in both the isolated hypertensive and group-housed normotensive animals when the drug was given intrahippocampally at 5.0 nmol. The depressor response to intrahippocampal U-62,066E was dose-related in both isolated and grouped rats at doses ranging from 1 to 10 nmol. The findings support our earlier suggestions that the hippocampal kappa opioid system may be somehow involved in cardiovascular regulation and that the non-peptide kappa agonists might make effective antihypertensive drugs.

3H-naloxone binding in brain regions of normotensive Wistar, spontaneously hypertensive and renal hypertensive rats

In order to study the role of opioid receptors in two models of experimental hypertension the binding of 3H-naloxone to membranes prepared from discrete brain regions and spinal cord was determined. Renal hypertensive rats (RHR) were found to have a greater density of 3H-naloxone binding sites in the hippocampus and hypothalamus when compared to spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NR). The apparent dissociation constant (Kd) for 3H-naloxone binding did not differ between groups.

Spatial navigation learning in spontaneously hypertensive, renal hypertensive and normotensive Wistar rats

The relation between blood pressure and cognitive performance was assessed in the spatial navigation task. Spatial learning by rats with spontaneous hypertension (SHR) and Goldblatt renal hypertension (RHR) was compared with that of normotensive Wistar rats (NR). The task required the rats to escape from water by finding a submerged and hidden platform. It was found that SHR rats showed improved learning capacity in the maze task in acquisition compared to the RHR and NR groups already on Day 1 and Day 2. The performances of all tested groups reached almost similar asymptotic level on Day 4 and in the probe trial on Day 5. After a reversal training the SHR rats did not show preference to swim in the new platform position quadrant. The present results confirm earlier reports on different behavioural characteristics associated with hypertension.

Enhanced disruptive spatial learning effect after sufentanil in renal hypertensive rats versus normotensive rats

The effects of the peripherally administered sufentanil citrate (S), a potent opioid agonist with high affinity for mu receptors on the spatial navigation task, were tested in normotensive Wistar (NR) and renal hypertensive rats (RHR). Rats were injected subcutaneously once daily in doses of 0.25 or 1 microgram/kg S before the water maze training. In NR rats, weak effects of 0.25 micrograms/kg S and impairments after 1 microgram/kg S were seen, whereas in RHR 0.25 and 1 microgram/kg S showed clearcut impairments. These data from the Morris water maze task support previous reports that RHR have an increased sensitivity for opioid agonists.

Cerebrospinal fluid dynorphin-converting enzyme activity is increased by voluntary exercise in the spontaneously hypertensive rat

The activity in cerebrospinal fluid (CSF) of dynorphin-converting enzyme (DCE) has been studied after voluntary exercise in the spontaneously hypertensive (SH) rat. The exercise consisted of spontaneous running in wheels for 4-5 weeks and the average running distance during the last two weeks was 4.0 km/24h. CSF samples were obtained under anaesthesia from the cisterna magna after penetration of the atlanto-occipital membrane. DCE transforms the members of the dynorphin family of opioid peptides into Leu-enkephalin-Arg6. In the present investigation a radioimmunoassay was used for quantitation of Leu-enkephalin-Arg6 release from dynorphin A1-17 and dynorphin B1-13. The rats that were running had a DCE activity (vs. both substrates) in CSF that was approximately 6-12 times higher than in animals not given the running opportunity. A statistically significant correlation between the two prodynorphin-derived substrate peptides was found in terms of DCE activity. We therefore propose that a single enzyme activity may be responsible for the hydrolysis of dynorphin B1-13 and dynorphin A1-17. Furthermore, a significant correlation was also found between running activity and DCE activity 12-14 h before the CSF was withdrawn. Besides measurement of DCE activity by radioimmunoassay, the formation of Leu-enkephalin-Arg6 was identified by reversed-phase micro-column liquid chromatography and plasma desorption mass spectrometry. The experiment shows that voluntary exercise affects opioid peptidergic mechanisms.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.