Beta-endorphin and central control of arterial blood pressure during challenge of circulatory homeostasis

Opioids and opiates: analgesia with cardiovascular, haemodynamic and immune implications in critical illness

Traumatic injury, surgical interventions and sepsis are amongst some of the clinical conditions that result in marked activation of neuroendocrine and opiate responses aimed at restoring haemodynamic and metabolic homeostasis. The central activation of the neuroendocrine and opiate systems, known collectively as the stress response, is elicited by diverse physical stressor conditions, including ischaemia, glucopenia and inflammation. The role of the hypothalamic-pituitary-adrenal axis and sympathetic nervous system in counterregulation of haemodynamic and metabolic alterations has been studied extensively. However, that of the endogenous opiates/opioid system is still unclear. In addition to activation of the opiate receptor through the endogenous release of opioids, pharmacotherapy with opiate receptor agonists is frequently used for sedation and analgesia of injured, septic and critically ill patients. How this affects the haemodynamic, cardiovascular, metabolic and immune responses is poorly understood. The variety of opiate receptor types, their specificity and ubiquitous location both in the central nervous system and in the periphery adds additional complicating factors to the clear understanding of their contribution to the stress response to the various physical perturbations. This review aims at discussing scientific evidence gathered from preclinical studies on the role of endogenous opioids as well as those administered as pharmacological agents on the host cardiovascular, neuroendocrine, metabolic and immune response mechanisms critical for survival from injury in perspective with clinical observations that provide parallel assessment of relevant outcome measures. When possible, the clinical relevance and corresponding scenarios where this evidence can be integrated into our understanding of the clinical implications of opiate effects will be examined. Overall, the scientific basis to enhance clinical judgment and expectations when using opioid sedation and analgesia in the management of the injured, septic or postsurgical patient will be discussed.

Peptidergic and catecholaminergic synaptic contacts onto nucleus preopticus medianus neurons projecting to the subfornical organ in the rat

The nucleus preopticus medianus (POMe) is known to be a key site in regulation of cardiovascular and body fluid homeostasis. To clarify the regulation mechanism to the POMe, the innervation pattern of synapses made by axon terminals immunoreactive to beta-endorphin, neuropeptide Y and tyrosine hydroxylase onto POMe neurons projecting to the subfornical organ (SFO) was investigated in the rat. After injection of a retrograde tracer, wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex, into the SFO, many neurons were retrogradely labeled in the POMe, more frequently in its dorsal part. Electron microscopy of the POMe revealed that beta-endorphin- and tyrosine hydroxylase-immunoreactive axon terminals formed predominantly axo-somatic synapses, and neuropeptide Y-immunoreactive axon terminals formed more axo-dendritic than axo-somatic synapses with retrogradely labeled neurons. The present localization patterns of POMe neurons retrogradely labeled from the SFO and the type of synapses of axon terminals immunoreactive to three neurochemical markers on these neurons were compared to those of POMe neurons retrogradely labeled from the paraventricular hypothalamic nucleus demonstrated in our previous report.

Naltrexone Protects Against Hypotension, Hyperthermia, and β-Endorphin Overproduction During Heatstroke in the Rat

Heat stroke is characterized by hyperthermia, arterial hypotension, decreased baroreflex sensitivity, and increased serum levels of beta-endorphin. Whereas naltrexone may have therapeutic potential in heat stroke, the underlying mechanism remains unclear. We tested the hypothesis that naltrexone may attenuate heat stroke by reducing hyperthermia, hypotension, decreased baroreceptor sensitivity, and/or increased serum levels of beta-endorphin. Heat stroke was induced by exposing the anesthetized adult Sprague-Dawley rats in an incubator at 43 degrees C. The moment in which the mean arterial pressure dropped irreversibly from the peak level was taken as the onset of heat stroke. Control rats were exposed to 24 degrees C. Mean arterial pressure, baroreceptor sensitivity, and maximal reflex bradycardia, after the onset of heat stroke, were all significantly lower than in control rats. However, rectal temperature and serum levels of beta-endorphin were all greater after the onset of heat stroke. Intravenous delivery of naltrexone (10 mg/kg) 20 min before the initiation of heat stress, but not immediately at the onset of heat stroke, significantly attenuated the above-mentioned reactions. Accordingly, naltrexone improved survival during heat stroke. These results suggest that naltrexone protects against hypotension and decrement of both baroreceptor sensitivity and maximal reflex bradycardia during heat stroke by reducing both hyperthermia and increment of serum beta-endorphin and thus improves survival.

Stress-specific opioid modulation of haemodynamic counter-regulation

1. The haemodynamic and cardiovascular responses to stress, in addition to being under control of the autonomic nervous system, are also under opiate modulation. Our studies have provided evidence for activation of the endogenous opioid system in haemorrhagic shock, sepsis and trauma. Furthermore, we have demonstrated that both central and systemic opiate administration to naïve rats result in marked alterations in haemodynamic responses, which are associated with activation of the sympathetic nervous system. 2. Because of the ubiquitous presence of opiate receptors in both the central nervous system and peripheral tissues, as well as their production and release centrally and peripherally, this facilitates an endocrine as well as a paracrine contribution to modulating vascular responses to stress, either directly or indirectly. Results from previous studies suggest that endogenous opioids are not involved in mediating the lipopolysacharide-induced hypotensive response. 3. In more recent studies, we have examined the role of opiate receptor activation in modulating the haemodynamic and neuroendocrine responses to fixed pressure haemorrhagic shock in conscious unrestrained rats. Using systemic opiate blockade (naltrexone, 15 mg/kg, i.p.) prior to haemorrhage, we have observed that blood loss required to achieve mean arterial blood pressure of 40 mmHg was higher in naltrexone-treated animals than in time-matched saline controls. Interestingly, the haemodynamic modulation exerted by naltrexone cannot be attributed to differences in circulating catecholamine levels. Haemorrhage produced an immediate and progressive increase in circulating adrenaline and noradrenaline levels, reaching values that were 50- and 20-fold higher than basal, respectively. Naltrexone pretreatment did not alter the time-course or magnitude of the rise in circulating levels of catecholamines. 4. These results indicate that endogenous opioid activation contributes to the haemodynamic dishomeostasis associated with blood loss. Our findings suggest stress-specific roles for opiate-sensitive haemodynamic counter-regulatory responses.

Beta-endorphin-, adrenocorticotrophic hormone- and neuropeptide y-containing projection fibers from the arcuate hypothalamic nucleus make synaptic contacts on to nucleus preopticus medianus neurons projecting to the paraventricular hypothalamic nucleus in the rat

The nucleus preopticus medianus is known to be situated in a key site in pathways regulating the paraventricular hypothalamic nucleus. To investigate the innervation pattern to nucleus preopticus medianus neurons by afferent fibers containing beta-endorphin, adrenocorticotrophic hormone and neuropeptide Y, a retrograde tracing method was combined with immunohistochemistry for these peptides in the rat. In the first experiment with injection of a retrograde tracer in the nucleus preopticus medianus, retrogradely labeled neurons were found in many regions throughout the brain. Among these, the arcuate hypothalamic nucleus contained a number of retrogradely labeled neurons showing immunoreactivity to the neuropeptides examined. About 20%, 20% and 40% of retrogradely labeled arcuate hypothalamic nucleus neurons showed beta-endorphin, adrenocorticotrophic hormone and neuropeptide Y immunoreactivity, respectively. About 18% and 57% of retrogradely labeled neurons in the nucleus tractus solitarius and ventrolateral medulla, respectively, were immunoreactive to neuropeptide Y. There were many more neuropeptide Y-immunoreactive projections to the nucleus preopticus medianus from the arcuate hypothalamic nucleus than those from the medulla. None of the retrogradely labeled neurons in the medulla showed immunoreactivity to beta-endorphin or adrenocorticotrophic hormone. In the second experiment with injection of a retrograde tracer in the paraventricular hypothalamic nucleus, electron microscopic observation revealed that retrogradely labeled neurons in the nucleus preopticus medianus were in synaptic contact with beta-endorphin-, adrenocorticotrophic hormone- and neuropeptide Y-immunoreactive axon terminals. The present finding indicates that nucleus preopticus medianus neurons projecting to the paraventricular hypothalamic nucleus are innervated by beta-endorphin-, adrenocorticotrophic hormone- and neuropeptide Y-containing arcuate hypothalamic nucleus neurons in addition to being innervated by neuropeptide Y-containing catecholaminergic medullary neurons which have been reported in our previous study.

Bradycardia and asystole following atropine-neostigmine administration after caesarean section in a parturient receiving methyldopa for pregnancy-induced hypertension

We report one case of bradycardia and asystole immediately after the administration of 1 mg atropine and 2 mg neostigmine after the completion of an urgent caesarean section. We attribute this adverse reaction to the treatment of pregnancy-induced hypertension with methyldopa, perhaps facilitated by other drugs employed. Similar reactions have been reported relating to beta-receptor antagonists and tricyclic antidepressants, but not to methyldopa.