Effects of naloxone on the cardiovascular responses to muscular contraction in decerebrate cats

Modulation of cardiovascular excitatory responses in rats by transcutaneous magnetic stimulation: role of the spinal cord

This study investigated the efficacy of magnetic stimulation on the reflex cardiovascular responses induced by gastric distension in anesthetized rats and compared these responses to those influenced by electroacupuncture (EA). Unilateral magnetic stimulation (30% intensity, 2 Hz) at the Jianshi-Neiguan acupoints (pericardial meridian, P 5-6) overlying the median nerve on the forelimb for 24 min significantly decreased the reflex pressor response by 32%. This effect was noticeable by 20 min of magnetic stimulation and continued for 24 min. Median nerve denervation abolished the inhibitory effect of magnetic stimulation, indicating the importance of somatic afferent input. Unilateral EA (0.3-0.5 mA, 2 Hz) at P 5-6 using similar durations of stimulation similarly inhibited the response (35%). The inhibitory effects of EA occurred earlier and were marginally longer (20 min) than magnetic stimulation. Magnetic stimulation at Guangming-Xuanzhong acupoints (gallbladder meridian, GB 37-39) overlying the superficial peroneal nerve on the hindlimb did not attenuate the reflex. Intravenous naloxone immediately after termination of magnetic stimulation reversed inhibition of the cardiovascular reflex, suggesting involvement of the opioid system. Also, intrathecal injection of delta- and kappa-opioid receptors antagonists, ICI174,864 (n=7) and nor-binaltorphimine (n=6) immediately after termination of magnetic stimulation reversed inhibition of the cardiovascular reflex. In contrast, the mu-opioid antagonist CTOP (n=7) failed to alter the cardiovascular reflex. The endogenous neurotransmitters for delta- and kappa-opioid receptors, enkephalins and dynorphin but not beta-endorphin, therefore appear to play significant roles in the spinal cord in mediating magnetic stimulation-induced modulation of cardiovascular reflex responses.

Ventrolateral medullary control of cardiovascular activity during muscle contraction

An overview of the role of ventrolateral medulla (VLM) in regulation of cardiovascular activity is presented. A summary of VLM anatomy and its functional relation to other areas in the central nervous system is described. Over the past few years, various studies have investigated the VLM and its involvement in cardiovascular regulation during static muscle contraction, a type of static exercise as seen, for example, during knee extension or hand-grip exercise. Understanding the neural mechanisms that are responsible for regulation of cardiovascular activity during static muscle contraction is of particular interest since it helps understand circulatory adjustments in response to an increase in physical activity. This review surveys the role of several receptors and neurotransmitters in the VLM that are associated with changes in mean arterial pressure and heart rate during static muscle contraction in anesthetized animals. Possible mechanisms in the VLM that modulate cardiovascular changes during static muscle contraction are summarized and discussed. Localized administration of an excitatory amino-acid antagonist into the rostral portion of the VLM (RVLM) attenuates increases in blood pressure and heart rate during static muscle contraction, whereas its administration into the caudal part of the VLM (CVLM) augments these responses. Opioid or 5-HT1A receptor stimulation in the RVLM, but not in the CVLM, attenuates cardiovascular responses to muscle contraction. Furthermore, intravenous, intracerebroventricular or intracisternal injection of an alpha 2-adrenoceptor agonist or a cholinesterase inhibitor attenuates increases in blood pressure and heart rate during static muscle contraction. Finally, the possible involvement of endogenous neurotransmitters in the RVLM and the CVLM associated with cardiovascular responses during static muscle contraction is discussed. An overview of the role of the VLM in the overall cardiovascular control network in the brain is presented and critically reviewed.

Rostral ventrolateral medullary opioid receptor activation modulates pressor response to muscle contraction

The effects of an opioid agonist, [D-Ala2]methionine enkephalinamide (DAME), administered into the rostral ventrolateral medulla (rVLM) or caudal ventrolateral medulla (cVLM) on cardiovascular responses to isometric muscle contraction were determined in anesthetized rats. A 30-s contraction evoked by tibial nerve stimulation increased mean arterial pressure (MAP) and heart rate (HR) by 34 +/- 6 mmHg and 40 +/- 7 beats/min, respectively, with a developed tension of 322 +/- 30 g, after bilateral insertion of microdialysis probes into the rVLM. Thirty-minute dialysis of DAME (10 and 100 microM) attenuated the contraction-evoked cardiovascular changes dose dependently (10 microM: MAP = 25 +/- 4 mmHg, HR = 27 +/- 3 beats/min, tension = 333 +/- 25 g; 100 microM: MAP = 14 +/- 4 mmHg, HR = 16 +/- 5 beats/min, tension = 330 +/- 34 g). Preadministration of an opioid antagonist, naloxone (100 microM), augmented contraction-evoked MAP and HR responses and blocked effects of 100 microM DAME. Microdialysis of DAME into the cVLM produced no changes in the pressor response to contraction. At end of each experiment, tibial nerve stimulation after neuromuscular blockade evoked no MAP or HR change. Results demonstrate that opioid receptor activation within the rVLM modulates cardiovascular responses to isometric muscle contraction.

Effects of naloxone on cardiovascular responses to static exercise and behavior in conscious cats

Conscious cats performing static (isometric) exercise were studied before and after administration of the opiate antagonist naloxone in order to evaluate the possible role of endogenous opiates in regulating cardiovascular responses to static exercise. Intravenous administration (0.4 mg/kg) of naloxone had no effect on resting left ventricular systolic pressure or on heart rate. In addition, similar increases in left ventricular systolic pressure and heart rate occurred during static exercise both before and after naloxone. However, naloxone reduced the number of exercise events the cats would perform per day. We conclude that moderate doses of naloxone do not modulate the cardiovascular responses to moderate static exercise in conscious cats. The results do indicate that endogenous opiates are involved in regulating the number of exercise bouts the cats will perform per day.

Spinal opiate modulation of cardiovascular reflexes in the exercising dog

The possible role of spinal opiate receptors on hind limb muscle afferents, participating in arterial blood pressure and heart rate responses to ischemic exercise, were investigated following lumbosacral intrathecal injection of morphine and naloxone in chronically instrumented dogs. Morphine markedly attenuated these responses. Naloxone had no effect alone but blocked the morphine responses. Thus, spinal opiate receptors may modulate ascending afferent information mediating cardiovascular reflexes from ischemic muscle.

Endogenous opiates: 1984

This paper is the seventh in an annual series of reviews of research involving the endogenous opiate peptides, each installment being restricted to work published during the previous year. As in the past three years, the review this year is limited to non-analgesic and behavioral studies of the opiate peptides. The specific topics this year include: stress, tolerance and dependence, consummatory responses, gastric and renal activity, alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, seizures and neurological disorders, activity, and miscellaneous other topics.