Postoperative analgesia induced by intrathecal neostigmine or bethanechol in rats.

1. Cholinergic agonists and acetylcholinesterase inhibitors, such as neostigmine, produce a muscarinic receptor-mediated antinociception in several animal species that depends on activation of spinal cholinergic neurons. However, neostigmine causes antinociception in sheep only in the early, and not late, postoperative period. 2. In the present study, a model of postoperative pain was used to determine the antinociceptive effects of bethanechol (a muscarinic agonist) and neostigmine administered intrathecally 2, 24 or 48 h after a plantar incision in a rat hind paw. Changes in the threshold to punctate mechanical stimuli were evaluated using an automated electronic von Frey apparatus. 3. Mechanical hyperalgesia was obtained following plantar incision, the effect being stronger during the immediate (2 h) than the late post-surgical period. Bethanechol (15-90 microg/5 microL) or neostigmine (1-3 microg/5 microL) reduced incision-induced mechanical hyperalgesia, the effects of both drugs being more intense during the immediate (2 h) than the late post-surgical period. 4. The ED(50) for bethanechol injected at 2, 24 and 48 h was 5.6, 51.9 and 82.5 microg/5 microL, respectively. The corresponding ED(50) for neostigmine was 1.62, 3.02 and 3.8 microg/5 microL, respectively. 5. The decline in the antinociceptive potency of neostigmine with postoperative time is interpreted as resulting from a reduction in pain-induced activation of acetylcholine-releasing descending pathways. However, the similar behaviour of bethanechol in the same model points to an additional mechanism involving intrinsic changes in spinal muscarinic receptors.

Comparison of pre- versus post-incision administration of intraplantar indomethacin and MK886 in a rat model of postoperative pain.

The amplification of pain long after the initial stimulus may be avoided if the treatment of pain is introduced before its initiation. However, conflicting evidence exists about the efficacy of such preemptive analgesia for the management of postoperative pain. This study compares the efficacy of intraplantar administration of indomethacin (a non-selective inhibitor of cyclooxygenase) and MK886 (an inhibitor of 5-lipoxygenase-activating protein), separately or in combination to produce preemptive analgesia in a model of surgical incisional pain in male Wistar rats. All incised rats (5 to 6 rats per group) had allodynia at 2, 6, and 24 h after surgery as evaluated using von Frey filaments. MK886, but not indomethacin (50 to 200 microg/paw), reduced the allodynia when injected either 1 h before or 1 h after surgery. The effect of preoperative MK886 (160 microg/paw) against incisional allodynia had a magnitude apparently similar to that produced by postoperative MK886. Pre-, but not postoperative MK886 (80 microg/paw) reduced the allodynia but the effect was seen only at 6 h after surgery. In contrast, MK886 (40 microg/paw) intensified the allodynia observed 2 h after the incision either injected before or after surgery. MK886 or indomethacin alone did not provide preemptive analgesia in the model of incisional pain. In contrast, the combination of MK886 with indomethacin reduced the allodynia more effectively when used before than after surgery, thus fulfilling the criteria for preemptive analgesia. In conclusion, preoperative inhibition of the local generation of both prostaglandins and leukotrienes by surgical incision may be an alternative to provide preemptive analgesia.

Nanocarriers for optimizing the balance between interfollicular permeation and follicular uptake of topically applied clobetasol to minimize adverse effects.

The treatment of various hair disorders has become a central focus of good dermatologic patient care as it affects men and women all over the world. For many inflammatory-based scalp diseases, glucocorticoids are an essential part of treatment, even though they are known to cause systemic as well as local adverse effects when applied topically. Therefore, efficient targeting and avoidance of these side effects are of utmost importance. Optimizing the balance between drug release, interfollicular permeation, and follicular uptake may allow minimizing these adverse events and simultaneously improve drug delivery, given that one succeeds in targeting a sustained release formulation to the hair follicle. To test this hypothesis, three types of polymeric nanocarriers (nanospheres, nanocapsules, lipid-core nanocapsules) for the potent glucocorticoid clobetasol propionate (CP) were prepared. They all exhibited a sustained release of drug, as was desired. The particles were formulated as a dispersion and hydrogel and (partially) labeled with Rhodamin B for quantification purposes. Follicular uptake was investigated using the Differential Stripping method and was found highest for nanocapsules in dispersion after application of massage. Moreover, the active ingredient (CP) as well as the nanocarrier (Rhodamin B labeled polymer) recovered in the hair follicle were measured simultaneously, revealing an equivalent uptake of both. In contrast, only negligible amounts of CP could be detected in the hair follicle when applied as free drug in solution or hydrogel, regardless of any massage. Skin permeation experiments using heat-separated human epidermis mounted in Franz Diffusion cells revealed equivalent reduced transdermal permeability for all nanocarriers in comparison to application of the free drug. Combining these results, nanocapsules formulated as an aqueous dispersion and applied by massage appeare to be a good candidate to maximize follicular targeting and minimize drug penetration into the interfollicular epidermis. We conclude that such nanotechnology-based formulations provide a viable strategy for more efficient drug delivery to the hair follicle. Moreover, they present a way to minimize adverse effects of potent glucocorticoids by releasing the drug in a controlled manner and simultaneously decreasing interfollicular permeation, offering an advantage over conventional formulations for inflammatory-based skin/scalp diseases.

A new role for the renin-angiotensin system in the rat periaqueductal gray matter: angiotensin receptor-mediated modulation of nociception.

Renin-angiotensin (Ang) system (RAS) peptides injected into the periaqueductal gray matter (PAG) elicit antinociception. Saralasin blocks Ang II-elicited antinociception. Thus, it is possible that endogenous RAS peptides could participate on the modulation of nociception in the PAG. This possibility was tested here injecting, in the PAG, the specific Ang type 1 and type 2 receptor (AT1 receptor and AT(2 receptor) antagonists losartan and CGP42,112A, respectively, either alone or before Ang II. The effects of Ang II, losartan and CGP42,112A on nociception were measured using the tail flick test and the model of incision allodynia. Ang II increased tail-flick latency, an effect inhibited by both losartan and CGP42,112A. Ang II reduced incisional allodynia. Either losartan or CGP42,112A alone increased incision allodynia, suggesting that endogenous Ang II and/or an Ang-peptide participates in the control of allodynia by the PAG. AT1 and AT2 receptors were immunolocalized in neuronal cell bodies and processes in the ventrolateral PAG. Taken together, the antinociceptive effect of Ang II injection into the ventrolateral PAG, the increase of allodynia elicited by injecting either losartan or CGP42,112A alone in the PAG, and the presence of AT1 and AT2 receptors in neurons and neuronal processes in the same region, represent the first evidence that part of the tonic nociceptive control mediated by the PAG is carried out locally by endogenous Ang II and/or an Ang-peptide acting on AT1 and AT2 receptors.

Descending mechanisms activated by the anterior pretectal nucleus initiate but do not maintain neuropathic pain in rats.

BACKGROUND: The anterior pretectal nucleus (APtN) activates descending mechanisms of pain control. This study evaluated whether the APtN also controls neuropathic pain in rats. METHODS: The hypersensitivity to mechanical stimulation with an electronic von Frey apparatus and the number of Fos-immunoreactive (Fos-ir) neurons in the APtN were evaluated in rats before and after chronic constriction injury of the sciatic nerve. RESULTS: The tactile hypersensitivity was characterized by an initial phase (the 2 days following the injury) and a maintenance phase (the subsequent 7 days). The injection of 2% lidocaine (0.25 µL) or N-methyl-D-aspartate (2.5 µg/0.25 µL) into the APtN intensified the tactile hypersensitivity observed 2 days after injury but did not alter the tactile hypersensitivity observed 7 and 14 days after injury. The injection of naloxone (10 ng/0.25 µL) or methysergide (40 pg/0.25 µL) but not atropine (100 ng/0.25 µL) into the APtN also intensified the tactile hypersensitivity observed 2 days after the injury. A significant increase in the number of Fos-ir cells was found in the contralateral APtN 2 days but not 7 or 14 days after the injury. Electrical stimulation of the APtN reduced the tactile hypersensitivity at 2, 7 and 14 days after the nerve ligation. CONCLUSION: APtN exerts a tonic inhibitory influence on persistent pain. The results point out to an important role of opioid and serotonergic mediation into the APtN to inhibit hyperalgesia during the initial phase of neuropathic pain.

Involvement of the anterior pretectal nucleus in the control of persistent pain: a behavioral and c-Fos expression study in the rat.

The anterior pretectal nucleus (APtN) participates in nociceptive processing and in the activation of central descending mechanisms of pain control. In this study we used behavioral tests (incisional pain and carrageenan-induced inflammatory pain) and c-Fos expression changes to examine the involvement of the APtN in the control of persistent pain in rats. A 1cm longitudinal incision through the skin and fascia of the plantar region (large incision), or a 0.5cm longitudinal incision through the skin only (small incision) was used, and the postoperative incisional allodynia was evaluated with von Frey filaments. The hyperalgesia produced by the intraplantar administration of carrageenan (25 or 50 microg/100 microl) into a hind paw was evaluated by a modified paw pressure test. The electrolytic lesion of the contralateral, but not ipsilateral, APtN significantly intensified the allodynia produced by a large incision of the hind paw. The incisional allodynia and the carrageenan-induced hyperalgesia were intensified by the microinjection of 2% lidocaine into the contralateral, but not ipsilateral APtN, the effect being significantly stronger when a large incision or a higher carrageenan concentration was utilized. A significant increase in the number of c-Fos positive cells was found in the ipsilateral, and mainly in the contralateral APtN of rats submitted to a large incision. The number of positive cells in the superficial or deep laminae of the contralateral spinal cord of control and incised rats was not significantly different. Positive cells in the superficial or deep laminae of the ipsilateral spinal cord were significantly more numerous than in control, the effect being significantly more intense in rats with large incision. The microinjection of 0.5% bupivacaine into the APtN contralateral to the incised hind paw reduced the number of positive cells bilaterally in the APtN, but the effect was significant in the contralateral nucleus only. The number of positive cells in the superficial and deep laminae of the contralateral spinal cord of incised and non-incised animals was not significantly changed by the neural block of the contralateral APtN. In the ipsilateral spinal cord, the incision-induced increase in the number of positive cells was significantly reduced in the superficial lamina and significantly increased in the deep lamina of animals previously treated with bupivacaine in the contralateral APtN. In conclusion, the integrity of the APtN is necessary to reduce the severity of the responses to persistent injury. The results also are in agreement with the current notion that persistent noxious inputs to the APtN tonically activate a descending mechanism that excites superficial cells and inhibits deep cells in the spinal dorsal horn.

Analgesic effect of subarachnoid neostigmine in two patients with cancer pain.

Two patients suffering with severe pain due to metastatic abdominal neoplasm were selected to examine whether subarachnoid neostigmine provided effective pain relief. Neostigmine was injected through a catheter introduced into the subarachnoid space at L4-L5. Patients were monitored for changes in arterial blood pressure, cardiac and respiratory rates, body temperature, level of consciousness and neurologic change. Pain was classified by the patients on a verbal four-grade scale, and analgesia was classified on a verbal three-grade scale. Complete pain relief was obtained 2 h after neostigmine (0.2 mg) in one patient and 4 h after neostigmine (0.1 mg) in the second patient. Pain of mild intensity returned 20 and 22 h after drug administration, respectively. Gastrointestinal discomfort was observed in both cases, but nausea and vomiting occurred only in the patient treated with the highest dose of neostigmine. No significant change in the monitored parameters was observed, except for a 6-h period of decreased blood pressure in the patient treated with the lower dose of neostigmine which required no specific treatment. The results obtained in these anecdotal cases indicate that subarachnoid neostigmine may provide analgesia in patients with pain arising from neoplasia, but further studies using controlled trials are needed before the drug is brought into clinical use.

Antinociception induced by intraperitoneal injection of gentamicin in rats and mice.

Intraperitoneal administration of gentamicin sulfate (5-800 micrograms/kg), but not gentamicin base (23-92 micrograms/kg) produced antinociception in rats and mice, as assessed by the tail-flick, carrageenan-induced articular incapacity tests, and hot-plate tests. The AD50 s in rats (tail-flick test) and mice (hot-plate test) were 11.48 and 147.9 micrograms/kg, respectively, but doses of 200-800 micrograms/kg were required to reduce the hyperalgesia induced in rats by carrageenan. In both species, bell-shaped dose-response curves were obtained, indicating that high doses of gentamicin had little or no effect. Non-effective doses of gentamicin failed to produce a significant increase in morphine antinociception in either rodent species. The possible involvement of N-type voltage-sensitive Ca2+ channels in the mechanism of antinociception induced by gentamicin is considered.

Pharmacological and neuroanatomical evidence for the involvement of the anterior pretectal nucleus in the antinociception induced by stimulation of the dorsal raphe nucleus in rats.

Several studies have shown that the anterior pretectal nucleus (APtN) is involved in descending inhibitory pathways that control noxious inputs to the spinal cord and that it may participate in the normal physiological response to noxious stimulation. Among other brain regions known to send inputs to the APtN, the dorsal column nuclei (DCN), pedunculopontine tegmental nucleus (PPTg), deep mesencephalon (DpMe), and dorsal raphe nucleus (DRN) are structures also known to be involved in antinociception. In the present study, the effects of stimulating these structures on the latency of the tail withdrawal reflex from noxious heating of the skin (tail flick test) were examined in rats in which saline or hyperbaric lidocaine (5%) was previously microinjected into the APtN. Brief stimulation of the PPTg, DpMe or DRN, but not the DCN, strongly depressed the tail flick reflex. The antinociceptive effect of stimulating the DRN, but not the PPTg or DpMe was significantly reduced, but not abolished, by the prior administration of the local anaesthetic into the APtN. The antinociception induced by stimulation of the PPTg or DpMe, therefore, is unlikely to depend on connections between these structures and the APtN. Similar inhibition of the effect of stimulating the DRN was obtained from rats previously microinjected with naloxone (2.7 nmol) or methysergide (2 nmol) into the APtN. Strongly labelled cells were identified in the DRN following microinjection of the fluorescent tracer Fast Blue into the APtN. These results indicate that the APtN may participate as a relay station through which the DRN partly modulates spinal nociceptive messages. In addition, they also indicate that endogenous opioid and serotonin can participate as neuromodulators of the DRN-APtN connection.

Antinociception from a stereospecific action of morphine microinjected into the brainstem: a local or distant site of action?

Morphine (1 microgram) was microinjected into rats in the midline medullary nucleus raphe magnus (NRM); 1 mm lateral into nucleus reticularis paragigantocellularis (NRPG); 2 mm lateral into the VIIth nerve nucleus and 3 mm lateral into the Vth nerve nucleus. The time course of changes in the sensitivity to noxious heat was followed by the tail flick test. Significant and prolonged antinociception was seen following microinjection into NRPG. At sites 1 mm from NRPG very weak effects were seen and at 2 mm from NRPG no antinociception occurred. It is concluded that 1 microgram of morphine microinjected into the brainstem is unlikely to cause antinociception by entering the circulation and having effects at remote sites. The distance diffused by morphine to cause significant antinociception after microinjection of 1 microgram is less than 1 mm. Levorphanol (1 microgram) had very similar effects to morphine but dextrorphan and saline were ineffective. It is concluded that although the concentrations achieved following microinjections may be high, they are not excessive as the effects show stereospecificity. The concentrations of endogenous substances released into the synaptic cleft may also be high.

The effects of intraperitoneal administration of antagonists and development of morphine tolerance on the antinociception induced by stimulating the anterior pretectal nucleus of the rat.

1 The effects of intraperitoneal administration of antagonists to morphine, 5-hydroxytryptamine (5-HT), noradrenaline and dopamine have been studied on the antinociceptive effects of electrical stimulation of the anterior pretectal nucleus (APtN) of the rat. 2 A 15 s period of 35 microA sine wave stimulation of APtN significantly increased the latency of the tail flick reflex to noxious heat for periods up to 1 h. 3 Naloxone (0.25-1.0 mg kg-1) attenuated the effects of APtN stimulation in a dose-dependent manner. In rats made tolerant to morphine by daily administration of morphine, the antinociceptive effects of APtN stimulation were significantly reduced. 4 The 5-HT receptor antagonists methysergide (5 mg kg-1) and ketanserin (1 mg kg-1), the dopamine receptor antagonist haloperidol (1 mg kg-1) and the beta-adrenoceptor antagonist propranolol (1 mg kg-1) had little effect on the antinociceptive effects of stimulating the APtN. 5 alpha-Adrenoceptor antagonists caused a dose-dependent antagonism of the response. The order of potency was; idazoxan greater than prazosin greater than phenoxybenzamine, the respective ED50 for each drug being 0.08: 0.45: 1.5 mg kg-1. 6 It is concluded that antagonism at opioid receptors and alpha-adrenoceptors but not beta-adrenoceptors, dopamine or 5-HT receptors reduces the antinociceptive effects of APtN stimulation. This differs from the reported effects of these antagonists on the antinociception caused by stimulating other sites in the brain.

Antinociceptive effect of agonists microinjected into the anterior pretectal nucleus of the rat.

Electrical stimulation at many sites within the pretectal complex and adjacent structures of the rat dorsomedial thalamus yields antinociception. It is documented that no other site in this region evokes antinociception longer lasting than that obtained by stimulation of the anterior pretectal nucleus (APtN). The effects of agonists injected into different nuclei of the dorsomedial thalamus on the tail-flick reflex of rats in response to noxious heat was examined. All animals were submitted to intracerebral electrical stimulation and microinjection of agonists. It was confirmed that strong and long lasting antinociception followed brief (15 s), low intensity (35 microA rms) stimulation of the APtN. In addition, L-glutamate (3.5 and 7.0 micrograms), morphine (1.0 and 5.0 micrograms), and 5-hydroxytryptamine (5-HT; 2.5 and 5.0 micrograms), but not acetylcholine (5.0 micrograms), carbachol (2.5 micrograms), norepinephrine (5.0 micrograms), or dopamine (5.0 micrograms), induced dose-dependent antinociception when microinjected into the APtN. The effect of 5-HT was fully depressed by pretreating animals with methysergide (5 mg/kg, i.p.). A survey of the sites from which morphine and 5-HT induce antinociception revealed that in no site of the dorsomedial thalamus did the effect last longer than after microinjection into the APtN. It is concluded that antinociception evoked by stimulation of the APtN depends on the activation of neuronal cell bodies in the nucleus, and that 5-HT and endogenous opioids may play a physiological role as neurotransmitters mediating antinociception in the APtN.

Antinociception and behavioral changes induced by carbachol microinjected into identified sites of the rat brain.

The sites of the rat brain in which intracerebral administration of carbachol (0.4 microgram/0.5 microliter) elevates the nociceptive threshold to thermic (tail-flick test) and mechanical (calibrated-pinch test) noxious stimuli were examined. An extensive mapping (510 sites) ranging from AP + 10.5 to AP-0.1 mm revealed that antinociception was obtained from 119 sites (23%) widely scattered in the brain, and reached structures distant from, or within the immediate vicinity of the ventricular system. The effects from most placement were demonstrated using the tail-flick test, whereas a smaller proportion (approximately 13%) of sites was effective in reducing the response to mechanical stimuli only. Structures containing sensitive sites include the dorsal raphe nucleus, lateral border of the superior cerebellar peduncle, caudal portion of the superior colliculus, medial geniculate body, habenular complex, amygdala, temporal pole of the ventral hippocampus, rostral aspect of the dorsal hippocampus, lateral septal area, and triangular nucleus of the septum. Analysis of the distribution of responsive sites indicated that they are poorly superposed to the known distribution of opiate-sensitive areas. Most of the structures found to be responsive to carbachol are also known to possess cholinergic receptors and to evoke antinociception following focal electrical stimulation. In various placements, particularly in limbic structures, microinjection of carbachol evoked jumping to mechanical noxious stimulation, hyperexcitability to non-noxious stimuli, convulsive reactions, and other less frequent reactions. On few occasions, however, these changes were accompanied by antinociception.

Antinociceptive effects of carbachol microinjected into different portions of the mesencephalic periaqueductal gray matter of the rat.

The changes in tail-flick latency (TFL) to noxious heating of the skin produced by the microinjection of carbachol (CCh) into the dorsal (dPAG), lateral (lPAG), and ventral (vPAG) portions of the mesencephalic periaqueductal gray matter (PAG) were studied in the rat. A significant increase in TFL was produced by CCh (0.2 microgram/0.5 microliter) microinjected into sites widely distributed within the PAG. The effect of CCh was stronger in the most caudal portion of the DPAG. Smaller effects were obtained after injection of CCh into the aqueduct, indicating that drug diffusion from the injection sites to the aqueduct lumen is unlikely to cause the antinociceptive effect of CCh. Dimethyl-phenyl-piperazinium (0.35 microgram/0.5 microliter), but not bethanechol (0.22 and 0.44 microgram/0.5 microliter), produced effects similar to CCh (0.2 microgram/0.5 microliter), when injected into the dPAG. The effects of CCh were inhibited by the previous administration of mecamylamine (1 microgram/0.5 microliter), but not atropine (1 microgram/0.5 microliter) or naloxone (1 microgram/0.5 microliter), into the dPAG. These results are indicative that antinociception produced by CCh from the dPAG depends on nicotinic, but not muscarinic or opioid mechanisms within the dPAG. The intraperitoneal administration of phenoxybenzamine (1 mg/kg) or mecamylamine (1 mg/kg), but not naloxone (1 mg/kg), methysergide (1 mg/kg), or atropine (1 mg/kg), inhibited the effects of CCh injected into the dPAG. In contrast, a higher dose of intraperitoneal phenoxybenzamine (5 mg/kg) was ineffective against the antinociception evoked by CCh when injected into the vPAG. Therefore, the effects of CCh from the dPAG may depend on the activation of centrifugal pathways involving both nicotinic and alpha-adrenergic mechanisms. In addition, the results indicate that different cholinergic substrates in the PAG may mediate both alpha-adrenergic and non-alpha-adrenergic descending pain mechanisms activated by the dPAG and vPAG, respectively.

The dual effect of a nitric oxide donor in nociception.

Low intrathecal (i.t.) doses of the nitric oxide (NO)-donor 3-morpholinosydnonimine (SIN-1) (0.1-2.0 microg/10 microl) reduced, while higher doses had no effect (5 or 100 microg/10 microl) or increased (10 and 20 microg/10 microl) the mechanical allodynia induced by chronic ligature of the sciatic nerve in rats. SIN-1 (0.1-100 microg/10 microl; i.t.) produced only antinociceptive effect in the rat tail flick test. The inhibitor of guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (4 microg/10 microl; i.t.), abolished the antinociceptive effects of SIN-1 in both tests and reduced the effect of high doses of SIN-1 in neuropathic rats. Hemoglobin (100 microg/10 microl; i.t.), a NO scavenger, inhibited the effect of low dose of SIN-1 and reduced the effect of high dose of SIN-1 in neuropathic rats. 8-Bromo-cGMP (125-500 microg/10 microl; i.t.), reduced the mechanical allodynia in neuropathic rats. The NO-synthase inhibitors, NG-nitro-L-arginine (L-NOARG) and NG-monomethyl-L-arginine (L-NMMA) (75-300 microg/10 microl; i.t.) reduced the mechanical allodynia evoked by nerve injury and increased the tail-flick latency, respectively. These effects were reduced and inhibited, respectively, by previous i.t. ODQ. The effect of L-NOARG was enhanced in a non-significant manner by hemoglobin. These results indicate that SIN-1 and NO-synthase inhibitors reduce pain through a spinal mechanism that involves activation of guanylate cyclase. The effects of SIN-1 vary depending on the dose and pain model utilized, but its most sensitive effect seems to be antinociception. However, high doses of the NO-donor can intensify ongoing pain.

Pharmacological evidence for a periaqueductal gray-nucleus raphe magnus connection mediating the antinociception induced by microinjecting carbachol into the dorsal periaqueductal gray of rats.

A previous study demonstrated that microinjection of carbachol (CCh) into the dorsal periaqueductal gray matter (dPAG) of rats increases the latency for the tail flick reflex. Several other studies have implicated the raphe magnus (NRM) and the reticularis paragigantocellularis (NRPG) nuclei as relay stations through which descending pathways from the PAG project to the spinal cord via the dorsolateral funiculus (DLF). In the present study, the effects of microinjecting CCh into the dPAG on the tail flick test were examined in rats in which the ipsilateral DLF was previously lesioned, or saline or lidocaine (2%) was microinjected into the NRM or ipsilateral NRPG. The DLF lesion did not change the baseline threshold of the animals in the test, but abolished the CCh-induced increase in the tail flick latency from the dPAG. The neural block of the NRM or NRPG with lidocaine also did not change significantly the latency for the tail flick reflex. The increase in the tail flick latency produced by CCh from the dPAG was not changed by the neural block of the NRPG, but was significantly reduced by the neural block of the NRM. These results are interpreted as indicative that the central antinociceptive mechanisms activated by CCh from the dPAG depend on a descending pathway that projects to the spinal cord via DLF utilizing at least the NRM, but not the NRPG, as an intermediary relay station.

An assessment of the antinociceptive and aversive effects of stimulating identified sites in the rat brain.

At many sites in the brain electrical stimulation with low current intensity is both aversive and causes antinociception. In view of the well documented antinociception caused by various types of stress and pain it is possible that in some parts of the brain the antinociception is secondary to the stress of the stimulation. At 114 sites in the rat brain the intensity of stimulation required to evoke an aversive response has been compared with the antinociceptive current intensity. Only stimulation in the dorsal hippocampus and pretectal area caused antinociception without significant aversion. Strong aversion resulted from stimulation of 46% of the sites including the central gray and nucleus raphe magnus. Antinociception was significantly correlated with the aversiveness of the stimulation although in 15% of the stimulation sites strong aversion was seen with no antinociception. It is concluded that there can be little justification in assigning a primary antinociceptive role to a brain area which evokes strong escape reactions when stimulated.

Antinociception elicited by electrical or chemical stimulation of the rat habenular complex and its sensitivity to systemic antagonists.

The effects of intraperitoneal administration of antagonists to morphine, norepinephrine, acetylcholine, dopamine and 5-hydroxytryptamine (5-HT) have been studied on the antinociceptive effect of electrical stimulation of the rat habenular complex (HbC). The antinociceptive effect of agonists microinjected into the HbC was also examined. A 15-s period of 53 microA rms sine-wave stimulation of the HbC significantly increased the latency of the tail-flick reflex to noxious heat for periods of up to 15 min. This effect was significantly attenuated by pretreating rats with naloxone (1 mg/kg) or phenoxybenzamine (5 mg/kg). Methysergide (5 mg/kg), haloperidol (5 mg/kg), atropine (1 mg/kg), and mecamylamine (1 mg/kg) had little effect on the antinociceptive effect of HbC stimulation. L-Glutamate (3.5 and 7.0 micrograms), morphine (1.0 and 5.0 micrograms), and carbachol (0.4 and 0.8 micrograms), but not 5-HT (5 micrograms), dopamine (5 micrograms) or norepinephrine (5 micrograms), induced a dose-dependent increase in the tail-flick latency when microinjected into the HbC. The effect of carbachol was significantly attenuated in rats previously treated with intraperitoneal administration of atropine or mecamylamine and fully depressed in rats previously treated with a combination of these two cholinergic antagonists. It is concluded that antagonists of opiate receptors and alpha-adrenoceptors, but not dopamine or cholinergic receptors, reduce the antinociceptive effects of HbC stimulation. These observations differ from the reported effects of these antagonists on the antinociception caused by stimulating the periaqueductal gray, but resemble the antinociception caused by stimulating the ventrolateral medulla and locus coeruleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Antinociception induced by opioid or 5-HT agonists microinjected into the anterior pretectal nucleus of the rat.

The changes in the latency for tail withdrawal in response to noxious heating of the skin induced by microinjection of opioid or serotonergic agonists into the anterior pretectal nucleus (APtN) was studied in rats. The mu-opioid agonist DAMGO (78 and 156 picomol), but not the delta-opioid agonist DADLE (70 and 140 pmol), the kappa-opioid agonist bremazocine (0.24 and 0.48 nanomol) or the sigma-opioid agonist N-allylnormetazocine (0.54 nanomol), produced a dose-dependent antinociceptive effect. The 5-HT1 agonist 5-carboxamidotryptamine (19 and 38 nanomol) and the 5-HT1B agonist, CGS 12066B (1.12 and 2.24 nanomol), but not the non-selective 5-HT agonist m-CPP (41 to 164 nanomol), 5-HT2 agonist alpha-methylserotonin (36 and 72 nanomol) and 5-HT3 agonist 2-methylserotonin (36 and 72 nanomol), produced a dose-dependent antinociceptive effect. These results indicate that the antinociceptive effects of opioid or serotonergic agonists microinjected into the APtN depend on drug interaction with local mu or 5-HT1B receptors, respectively.

Antinociception induced by stimulation of the habenular complex of the rat.

The changes in the tail-flick latency to noxious heat stimulation and in the threshold for defensive/affective reactions to noxious pressure of the skin were studied following electrical stimulation of the habenular complex (HbC) and adjacent brain structures in the male rat. Single brief (15 s), low intensity (53 microA r.m.s.) stimulation of the HbC caused no significant increase in the locomotor activity or motor deficit but induced a potent and short-lasting antinociception as revealed by both algesimetric tests. Animals stimulated in the HbC also displayed poor avoidance learning in a conditioned place preference paradigm, thus suggesting that aversion is unlikely to determine antinociception. Rats daily stimulated in the HbC became tolerant to the antinociception induced by HbC stimulation or to a high systemic dose of morphine (6 mg/kg i.p.). These results indicate that stimulation of the HbC may increase the thresholds of spinally and supraspinally integrated reflexes, thus supporting the hypothesis that this nucleus may play a role in pain control, probably involving an opioid-dependent mechanism.