An adrenal-mediated, naloxone-reversible increase in reaction time in the tail-flick test following intrathecal administration of substance P at the lower thoracic spinal level in the rat

Substance P and enkephalin immunoreactivities in axonal boutons presynaptic to physiologically identified dorsal horn neurons. An ultrastructural multiple-labelling study in the cat

A combination of intracellular electrophysiological recording and injection of horseradish peroxidase with ultrastructural immunocytochemistry was used to investigate the synaptic interplay between substance P- and enkephalin-immunoreactive axonal boutons and three types of functionally characterized dorsal horn neurons in the cat spinal cord. The dorsal horn neurons were classified as nociceptive specific, wide dynamic range and non-nociceptive based on their responses to innocuous and noxious stimuli. Most of the nociceptive neurons (either nociceptive specific or wide dynamic range) contained enkephalin immunoreactivity, but none of the non-nociceptive neurons were positive for enkephalin. Three types of immunoreactive boutons were found in contact with the functionally characterized dorsal horn neurons. These boutons were positive for either substance P, enkephalin, or substance P+enkephalin. Quantitative analysis revealed that the percentages of substance P-immunoreactive boutons apposed to the cell bodies, proximal dendrites and distal dendrites of nociceptive neurons were significantly higher than those of non-nociceptive neurons. Furthermore, the percentages of substance P+enkephalin-immunoreactive axonal boutons apposed to the distal dendrites of nociceptive neurons were significantly higher than those of non-nociceptive neurons and the percentages of enkephalin-immunoreactive boutons apposed to the cell bodies and proximal dendrites of nociceptive neurons were significantly higher than in non-nociceptive neurons. Finally, neither enkephalin-immunoreactive nor substance P+enkephalin-immunoreactive boutons were ever seen presynaptic to substance P-immunoreactive boutons. These results provide evidence of an anatomical substrate within the dorsal horn for the interaction of substance P-mediated with enkephalin-mediated mechanisms. The data support the idea that the modulation of nociceptive input in the dorsal horn by enkephalinergic neurons occurs mainly via a postsynaptic mechanism, and thus suggest that dorsal horn enkephalinergic neurons participate in a local inhibitory feedback loop in a distinct pathway from the previously postulated opioid-mediated depression of substance P release from primary afferent terminals.

Mechanisms underlying antinociception provoked by heterosegmental noxious stimulation in the rat tail-flick test

Physiological studies were conducted to examine the effects of noxious stimulation of one hindpaw or one forepaw on the latency of the withdrawal reflex in the tail-flick test in lightly anesthetized spinally intact or transected rats. Male Sprague-Dawley rats were anesthetized with an intraperitoneal injection of a mixture of Na-pentobarbital (20 mg/kg) and chloral hydrate (120 mg/kg). After baseline readings were taken in the tail-flick test, the effects of various noxious stimuli applied to remote body regions were assessed. The noxious stimuli included unilateral or bilateral hindpaw or unilateral forepaw thermal (immersion in water at 55 degrees C for 90 s), unilateral or bilateral chemical (subcutaneous hindpaw injection of 50 microliters of 5% formalin) and unilateral or bilateral mechanical (pinch with clamp exerting a force of 14.75 or 27 N) stimulation. Bilateral chemical and thermal, and unilateral thermal stimulation induced an antinociceptive response, consisting of an increase in tail-flick latency, peaking at 30 s after stimulation. Recovery to baseline levels occurred over the next 3-6 min. The antinociceptive effect of noxious thermal stimulation was attenuated or absent in chronically spinalized animals (T6/7) following hindpaw or forepaw immersion, respectively. Noxious mechanical stimulation had no effect on tail-flick latency. The data provide evidence that a noxious thermal or chemical stimulus produces a heterosegmental antinociceptive effect which is mediated in part via a supraspinal mechanism and in part via a local spinal mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Combined use of cerebral spinal fluid drainage and naloxone reduces the risk of paraplegia in thoracoabdominal aneurysm repair

PURPOSE

This report summarizes our experience with the use of cerebral spinal fluid drainage (CSFD) and naloxone for prevention of postoperative neurologic deficit (paraplegia or paraparesis).

METHODS

We reviewed 110 consecutive patients with 86 thoracoabdominal aneurysms and 24 thoracic aneurysms. The status of 47 patients (43%) was acute (rupture or dissection), and the status of 52 (47%) was Crawford type I or II. None of the patients had intercostal artery reimplantation. There were two patient groups for analysis of neurologic deficit risk. Group A (61 patients) received naloxone and CSFD, and group B (49 patients) did not.

RESULTS

One deficit occurred in group A and 11 deficits occurred in group B (p = 0.001). By multiple logistic regression analysis, the variables acute status, Crawford type II, or group B classification were significant factors for deficit risk. Use of the same logistic regression analysis on the subgroup of 47 patients with acute aneurysms and 33 patients with Crawford type 2 aneurysms confirmed the protective effect of combined CSFD and naloxone (group A) and that clinical presentation and extent of aorta replaced are the primary risk factors for development of deficit. To test this conclusion we developed a highly predictive model (correlation coefficient 0.997 with 16 series of thoracoabdominal aneurysms) for neurologic deficit. We applied our data to this model. Group B had the predicted number of deficits, and group A had substantially fewer deficits than predicted.

CONCLUSIONS

We conclude that the combined use of CSFD and naloxone offers significant protection from neurologic deficits in patients undergoing thoracoabdominal and thoracic aortic replacement.

Neural mediation of the cardiovascular responses to intrathecal administration of substance P in the rat: slowing of the cardioacceleration by an adrenal opioid factor

Substance P, given intrathecally at the second (T2) or ninth (T9) thoracic level in the anesthetized rat, increased heart rate, arterial pressure and circulating catecholamines. At T9 in adrenalectomized animals and at T2 in intact animals, the cardioacceleration was more abrupt than in intact animals injected at T9 suggesting that the adrenals are not necessary for the cardiovascular responses and that the adrenals may have released a factor which slows the neurally mediated cardioacceleration. As opioids are co-released with catecholamines from the adrenals, naloxone (10 mg/kg i.v.) or nalorphine HCl (which does not cross the blood-brain barrier; 10 mg/kg s.c.) was given 5 min before administration of substance P at T9 in intact rats. In both groups the cardioacceleration was similar to that elicited in adrenalectomized animals, indicating that the adrenal factor was opioid and that its action was peripheral rather than central. When propranolol (10 mg/kg i.v.) was given 3 or 15 min before, substance P increased arterial pressure but heart rate was unchanged, indicating that the opioid factor was not slowing the cardioacceleration by a direct effect on the heart. The results indicate that intrathecal administration of substance P produces a neurally mediated increase in arterial pressure and heart rate and induces the release of an adrenal opioid factor which slows the neurally-mediated cardioacceleration by an action in the periphery. This indicates a functional interaction between humoral and neural sympathetic mechanisms regulating the cardiovascular system.

Effects of sympathectomy on a rat model of peripheral neuropathy

The aim of this study was to determine the effects of sympathectomy on our previously developed animal model for neuropathic pain. The neuropathy was produced by a unilateral tight ligation of the L5 and L6 spinal nerves in 81 rats, all of which showed a marked increase in frequency of paw lifting in response to innocuous mechanical stimuli and a shortened latency of paw withdrawal in response to noxious radiant heat stimuli on the affected limb. We interpreted these as behavioral signs of mechanical allodynia and heat hyperalgesia. Surgical sympathectomy was performed by removing the sympathetic chain bilaterally from the L2 to L6 levels at 1 week prior to and 1, 3 and 5 weeks after nerve injury. In addition, the effect of sympathetic block was tested by systemically injecting guanethidine or phentolamine. Surgical sympathectomy relieved the signs of both mechanical allodynia and heat hyperalgesia. The effect of sympathectomy for mechanical allodynia is estimated to be almost fully expressed within 30 min after the operation. Sympathetic block by chemical agents reversibly relieved the mechanical allodynia. These data suggest that the rats in our model exhibit behavioral signs of neuropathic pain that are sympathetically maintained.

Adenosine receptor link in an adrenal opioid-induced antinociception in the rat tail-flick test

Intrathecal administration of substance P at the lower thoracic spinal level has an antinociceptive effect on reaction time in the tail-flick test; this response is blocked by naloxone i.v. but not by i.v. administration of opiate antagonists which do not cross the blood-brain barrier. As morphine-induced analgesia is blocked by adenosine antagonists, to determine whether this substance P-induced, opioid-mediated antinociception also includes a purine link, the adenosine receptor antagonist, caffeine, was given systemically 10 min prior to substance P administration. In control rats pretreated with saline, substance P (6.5 nmol) produced an increase in reaction time to about 160% of preadministration values at one min after intrathecal injection. The effect could also be observed at 6 min after this injection. Pretreatment with 16 or with 32 mg/kg of caffeine i.p. blocked the response to substance P, and produced a hyperalgesia similar to that reported in studies at the lumbo-sacral spinal level. These results indicate that the adrenal opioid-induced antinociception observed upon intrathecal administration of substance P at the lower thoracic level occurs via an adenosine link. This is the first demonstration of a purine link in the expression of antinociceptive effects of an endogenously released opioid.

Treatment of reflex sympathetic dystrophy with topical capsaicin. Case report

A 31-year-old woman with intractable reflex sympathetic dystrophy experienced nearly complete, though temporary, resolution of pain following 3 weeks of topical capsaicin. We propose that capsaicin may be a useful treatment for reflex sympathetic dystrophy, either by depleting substance P from primary afferent neurons that mediate allodynia, or by modulating sympathetic efferent activity.

Spinal mediation of the increases in arterial pressure and heart rate in response to intrathecal administration of bicuculline

The present experiments were designed to determine the mechanisms by which the intrathecal administration of the GABAA antagonist, bicuculline (2.2 and 8.8 nmol), at the second thoracic spinal segment (T2) affects cardiovascular function in the anaesthetized rat. Bicuculline produced a dose-related, transient increase in arterial pressure and heart rate which peaked at 5-7 min and persisted for 30 min or more, depending on dose. There was a mutually reversible interaction between bicuculline and intrathecal administration of the GABAA agonist, muscimol (8.8 nmol), which alone decreased arterial pressure and heart rate. Bicuculline was given intrathecally at the third lumbar spinal level (8.8 nmol) and intravenously (8.8 nmol), but in these cases it failed to affect these cardiovascular parameters. Pretreatment with intrathecal infusion of 15 microliters of 1% lidocaine or with intravenous injection of hexemathonium (10 mg/kg) prevented the responses to intrathecal administration of 8.8 nmol of bicuculline at T2. These results demonstrate that the effects of bicuculline on arterial pressure and heart rate are due to an action in the spinal cord on GABAA receptors, and the data may be interpreted as indicating that there is a tonic GABAergic inhibition of sympathetic outflow at the spinal level.

Central but not peripheral opiate receptor blockade prolonged pituitary-adrenal responses to stress

Evidence from pharmacological studies suggest that opiate systems may serve either inhibitory or stimulatory functions on stress-induced responses of the hypothalamic-pituitary-adrenocortical (HPA) axis. The objective of these experiments was to determine whether these discrepant findings may result, in part, from differential effects of central or peripheral opiate receptor blockade on HPA axis responses. To this effect, groups of rats received injections of either saline, naltrexone (NHCl) or the quaternary analogue naltrexone methobromide (NMBr). The animals were then exposed to 30 min of a motion stressor and blood samples were obtained from each rat for analysis of ACTH, corticosterone, and prolactin. The data showed that resting and stress-induced levels of prolactin were decreased by NHCl only. Although neither drug affected the magnitude of the stress-induced ACTH and corticosterone responses, treatment with NHCl, but not NMBr, delayed the poststress decline of these responses. Hence, we concluded that central opiate mechanisms may be important for cessation of HPA axis activity, after exposure to stressful situations.