Electric muscle stimulation in the spontaneously hypertensive rat induces a post-stimulatory reduction in activity: role of different opioid receptors

Somatic nerve stimulation and cholera-induced net fluid secretion in the small intestine of the rat: evidence for an opioid effect

The effects of somatic nerve stimulation on cholera toxin induced secretion was investigated in vivo in anaesthetised rats. Small intestinal secretion was induced with cholera toxin and measured by a gravimetric technique. Afferent stimulation (pulse frequency within train; 100 Hz; train duration: 50 ms; train frequency: 3 Hz) of the sciatic nerve over 30 min significantly reduced the net fluid secretion both during (P < 0.05) and after cessation of the stimulation (P < 0.01). The greatest effect was obtained immediately after the termination of the nerve stimulation when the secretion was reversed to net fluid absorption. The opioid receptor antagonist naloxone (10 mg kg(-1) i.v.) administrated during the stimulation, significantly inhibited the antisecretory effect seen after the stimulation, thus no significant difference was seen between the control period and the periods after cessation of the stimulation. The opioid receptor antagonist naloxone methiodide (10 mg kg(-1) i.v.), which does not cross the blood-brain barrier, partly inhibited the antisecretory effects but not with the same magnitude as naloxone, thus the net fluid secretion was still significantly inhibited after the stimulation (P < 0.05). We conclude that afferent stimulation of the sciatic nerve strongly inhibits the cholera toxin induced secretion in the small intestine. This inhibition involves primarily a central opioid mechanism and to a lesser extent peripheral opioid mechanism.

Proenkephalin gene expression in the brainstem regulates post-exercise hypotension

The opioid receptor antagonist naloxone reverses the reduction in blood pressure following exercise. We have previously demonstrated that compared to genetically matched controls, spontaneously hypertensive rats (SHR) have decreased proenkephalin mRNA levels in the nucleus tractus solitarius (NTS), the caudal (CVLM) and rostral ventrolateral medulla (RVLM) of the brainstem. We hypothesized that in SHR an acute bout of exercise would increase proenkephalin mRNA in the NTS and RVLM. Female 12-week-old SHR (n = 4/group) were randomly assigned to exercise and control groups. Mean arterial pressure (MAP) and heart rate were recorded at rest and every 5 min for 30 min following: (1) 40 min of treadmill running at 30 m/min, 10% grade; or (2) 40 min of rest on the treadmill. Rats were sacrificed 30 min post-exercise or post-rest. Exercise induced increases in MAP and heart rate, approximately 18 mmHg and approximately 140 beats/min, respectively, P < 0.001. There were no differences in pre-exercise/rest MAP between groups, or in control rats following rest on the treadmill, 162.5 +/- 3 vs. 163.1 +/- 4 mmHg, control and control after treadmill rest, respectively; NS P > 0.05. The pre- to post-exercise reduction in MAP after 40 min of treadmill running was from 164 +/- 5.1 to 146 +/- 2.0 mmHg (P < 0.001) as recorded 30 min post-exercise. At 30 min post-exercise proenkephalin mRNA levels in the NTS, CVLM and RVLM were increased: 97, 198 and 227%, respectively, P < 0.01. These data reconfirm the existence of post-exercise hypotension in SHR and suggest that increases in enkephalin synthesis and release in the NTS, CVLM, and RVLM may be involved in regulating post-exercise hypotension.

The effects of selective opioid antagonists on somatosensory evoked potentials during relative cerebral ischemia in rats

Hemorrhagic hypotension in spontaneously hypertensive rats induces attenuation of somatosensory evoked potentials. In this model of relatively mild cerebral ischemia, our previous studies have shown that naloxone stereospecifically enhances the evoked potentials, without changes in cortical blood flow. The high dose of naloxone needed to enhance the evoked potentials suggests that the attenuation is mediated by low affinity opioid receptors (delta or kappa). In the present study, we used this model to study the effects of naloxone-methobromide (5 mg kg-1, a quaternary derivative of naloxone with selective peripheral action when injected intravenously), MR 2266 (1 mg kg-1, a kappa receptor antagonist), and naloxone (5 mg kg-1) as well as saline injection (as control) in four different groups of rats. Following injection, we examined the changes in somatosensory evoked potentials, cortical blood flow and heart rate for 15 min while mean arterial pressure was held constant by a pressure-regulating reservoir. Only naloxone changed the somatosensory evoked potential amplitude significantly compared with the saline group in which no effect was seen. However, there was a tendency for a delayed effect of naloxone-methobromide on the evoked potentials, possibly indicating that the substance slowly passes the blood-brain barrier. Naloxone and MR 2266 caused a transient decrease in heart rate, while following naloxone-methobromide injection there was a slight increase in heart rate. Our results thus indicate that the beneficial effects of naloxone on somatosensory evoked potentials during relative cerebral ischemia may be centrally mediated by a non-kappa mechanism.