Central mechanism of indomethacin analgesia

The modern pharmacology of paracetamol: therapeutic actions, mechanism of action, metabolism, toxicity and recent pharmacological findings

Paracetamol is used worldwide for its analgesic and antipyretic actions. It has a spectrum of action similar to that of NSAIDs and resembles particularly the COX-2 selective inhibitors. Paracetamol is, on average, a weaker analgesic than NSAIDs or COX-2 selective inhibitors but is often preferred because of its better tolerance. Despite the similarities to NSAIDs, the mode of action of paracetamol has been uncertain, but it is now generally accepted that it inhibits COX-1 and COX-2 through metabolism by the peroxidase function of these isoenzymes. This results in inhibition of phenoxyl radical formation from a critical tyrosine residue essential for the cyclooxygenase activity of COX-1 and COX-2 and prostaglandin (PG) synthesis. Paracetamol shows selectivity for inhibition of the synthesis of PGs and related factors when low levels of arachidonic acid and peroxides are available but conversely, it has little activity at substantial levels of arachidonic acid and peroxides. The result is that paracetamol does not suppress the severe inflammation of rheumatoid arthritis and acute gout but does inhibit the lesser inflammation resulting from extraction of teeth and is also active in a variety of inflammatory tests in experimental animals. Paracetamol often appears to have COX-2 selectivity. The apparent COX-2 selectivity of action of paracetamol is shown by its poor anti-platelet activity and good gastrointestinal tolerance. Unlike both non-selective NSAIDs and selective COX-2 inhibitors, paracetamol inhibits other peroxidase enzymes including myeloperoxidase. Inhibition of myeloperoxidase involves paracetamol oxidation and concomitant decreased formation of halogenating oxidants (e.g. hypochlorous acid, hypobromous acid) that may be associated with multiple inflammatory pathologies including atherosclerosis and rheumatic diseases. Paracetamol may, therefore, slow the development of these diseases. Paracetamol, NSAIDs and selective COX-2 inhibitors all have central and peripheral effects. As is the case with the NSAIDs, including the selective COX-2 inhibitors, the analgesic effects of paracetamol are reduced by inhibitors of many endogenous neurotransmitter systems including serotonergic, opioid and cannabinoid systems. There is considerable debate about the hepatotoxicity of therapeutic doses of paracetamol. Much of the toxicity may result from overuse of combinations of paracetamol with opioids which are widely used, particularly in USA.

Indomethacin/prochlorperazine/caffeine: a review of its use in the acute treatment of migraine and in the treatment of episodic tension-type headache

The indomethacin/prochlorperazine/caffeine fixed combination (Difmetré®) combines the NSAID indomethacin with the phenothiazine antiemetic prochlorperazine and caffeine. It is currently available as two oral (effervescent tablet and coated tablet) and two rectal (suppository and low-dose suppository) formulations. Oral and rectal formulations of indomethacin/prochlorperazine/caffeine were effective and generally well tolerated in the treatment of migraine and episodic tension-type headache (TTH) in adult patients participating in randomized, multicentre, active-comparator controlled studies. For the most part, the efficacy of oral indomethacin/prochlorperazine/caffeine did not significantly differ from that of oral sumatriptan in patients with migraine and oral nimesulide in patients with episodic TTH. With rectal administration, indomethacin/prochlorperazine/caffeine was, in general, significantly more effective than sumatriptan in patients with migraine. Thus, oral and rectal formulations of indomethacin/prochlorperazine/caffeine provide a further option in the acute treatment of migraine and in the treatment of episodic TTH in adult patients.

The central analgesia induced by antimigraine drugs is independent from Gi proteins: superiority of a fixed combination of indomethacin, prochlorperazine and caffeine, compared to sumatriptan, in an in vivo model

A hypofunctionality of Gi proteins has been found in migraine patients. The fixed combination of indomethacin, prochlorperazine and caffeine (Indoprocaf) is a drug of well-established use in the acute treatment of migraine and tension-type headache. The aim of this study was to investigate if Indoprocaf was able to exert its central antinociceptive action when Gi proteins activity is abolished by pertussis toxin (PTX), compared to its single active ingredients and to sumatriptan. The mice model of abdominal constriction test induced by an i.p. injection of a 0.6% solution of acetic acid was used. The study showed that Indoprocaf (a fixed combination of indomethacin 1 mg/kg, prochlorperazine 1 mg/kg and caffeine 3 mg/kg, s.c.) and sumatriptan (20 mg/kg, s.c.) exert their central antinociceptive action independently from the Gi proteins. In addition, the antinociceptive efficacy of Indoprocaf in this study was statistically superior to that of sumatriptan. This study also showed that the single active ingredients of Indoprocaf, indomethacin (1 mg/kg, s.c.), prochlorperazine (1 mg/kg, s.c.) and caffeine (3 mg/kg, s.c.), were able to exert their central antinociceptive action independently from the Gi proteins. However, Indoprocaf at analgesic doses was able to abolish almost completely the abdominal constrictions, with a statistically higher efficacy compared to the single active ingredients, showing an important synergic effect of Indoprocaf. This synergic effect was evident not only when Gi proteins activity was abolished by PTX, but also under control condition, when Gi proteins were active. This study suggests that the central antinociceptive action induced by antimigraine drugs is independent from Gi proteins.

A surgical ankle sprain pain model in the rat: effects of morphine and indomethacin

Ankle sprain is a frequent injury in humans that results in pain, swelling and difficulty in walking on the affected side. Currently a suitable animal model resembling human ankle sprain is lacking. Here, we describe an animal ankle sprain model induced by ankle ligament injury (ALI) in rats. Cutting combinations of the lateral ankle ligament complex produced pain, edema and difficulty of weight bearing, thereby mimicking severe (grade III) ankle sprain in humans. Analgesic compounds, morphine and indomethacin, significantly reversed the reduced weight bearing, thus indicating that reduction of weight bearing is partially due to pain. The ALI model is a new ankle sprain model that may be useful for the study of ankle sprain pain mechanisms and treatments, as well as for the screening of new analgesic drugs.

DP-155, a lecithin derivative of indomethacin, is a novel nonsteroidal antiinflammatory drug for analgesia and Alzheimer's disease therapy

DP-155 is a lipid prodrug of indomethacin that comprises the latter conjugated to lecithin at position sn-2 via a 5-carbon length linker. It is cleaved by phospholipase A2 (PLA)(2) to a greater extent than similar compounds with linkers of 2, 3, and 4 carbons. Indomethacin is the principal metabolite of DP-155 in rat serum and, after DP-155 oral administration, the half-life of the metabolite was 22 and 93 h in serum and brain, respectively, compared to 10 and 24 h following indomethacin administration. The brain to serum ratio was 3.5 times higher for DP-155 than for indomethacin. In vitro studies demonstrated that DP-155 is a selective cyclooxygenase (COX)-2 inhibitor. After it is cleaved, its indomethacin derivative nonselectively inhibits both COX-1 and -2. DP-155 showed a better toxicity profile probably due to the sustained, low serum levels and reduced maximal concentration of its indomethacin metabolite. DP-155 did not produce gastric toxicity at the highest acute dose tested (0.28 mmol/kg), while indomethacin caused gastric ulcers at a dose 33-fold lower. Furthermore, after repeated oral dosing, gastrointestinal and renal toxicity was lower (10- and 5-fold, respectively) and delayed with DP-155 compared to indomethacin. In addition to reduced toxicity, DP-155 had similar ameliorative effects to indomethacin in antipyretic and analgesia models. Moreover, DP-155 and indomethacin were equally efficacious in reducing levels of amyloid ss (Ass)42 in transgenic Alzheimer's disease mouse (Tg2576) brains as well as reducing Ass42 intracellular uptake, neurodegeneration, and inflammation in an in vitro AD model. The relatively high brain levels of indomethacin after DP-155 administration explain the equal efficacy of DP-155 despite its low systemic blood concentrations. Compared to indomethacin, the favored safety profile and equal efficacy of DP-155 establish the compound as a potential candidate for chronic use to treat AD-related pathology and for analgesia.

Efficacy of dosing and re-dosing of two oral fixed combinations of indomethacin, prochlorperazine and caffeine compared with oral sumatriptan in the acute treatment of multiple migraine attacks: a double-blind, double-dummy, randomised, parallel group, multicentre study

AIMS AND METHODS

In this double-blind, double-dummy, randomised, parallel group, multicentre study, the efficacy of dosing and re-dosing of a fixed combination of indomethacin, prochlorperazine and caffeine (Indoprocaf) was compared with encapsulated sumatriptan in the acute treatment of two migraine attacks. Additionally, in the group taking Indoprocaf, two different oral formulations were tested: effervescent tablets and encapsulated coated tablets.

RESULTS

Of 297 patients randomised (150 assigned to Indoprocaf and 147 to sumatriptan), 281 were included in the intention-to-treat efficacy analysis. The initial dosing of Indoprocaf and sumatriptan was similarly effective with pain-free rates higher than 30% (95% CI of odds-ratio: 0.57-1.28) and headache relief rates of about 60% (95% CI of odds-ratio: 0.82-1.84) with both the drugs. The efficacy of re-dosing of Indoprocaf as rescue medication was more effective than that of sumatriptan with pain-free values of 47% vs. 27% in the total attacks with a statistically significant difference in the first migraine attack in favour of Indoprocaf. The efficacy of re-dosing to treat a recurrence/relapse was very high without differences between the drugs (pain-free: 60% with Indoprocaf and 50% with sumatriptan in the total attacks). Indoprocaf and sumatriptan were well-tolerated.

CONCLUSION

The study demonstrated that the efficacy of the initial dosing of Indoprocaf was not higher than that of sumatriptan, but that the strategy to use the lowest effective dose as soon as the headache occurred, followed by a second dose if the headache has not relieved or to treat a relapse, was very effective, especially with Indoprocaf.

Efficacy of a fixed combination of indomethacin, prochlorperazine, and caffeine in the treatment of episodic tension-type headache: a double-blind, randomized, nimesulide-controlled, parallel group, multicentre trial

In this double-blind, randomized, parallel group, multicentre study the efficacy of a fixed combination of indomethacin, prochlorperazine, and caffeine (IndoProCaf) and nimesulide were compared over an 8-h period in the treatment of two consecutive episodes of tension-type headache (TTH). Both drugs were administered orally. Of 54 randomized patients, 40 were compliant to the protocol. More patients on IndoProCaf than on nimesulide were pain-free at 2 h post-dose (45% vs. 10%; P<0.05), reached a pain reduction of at least 50% at 2 (75% vs. 30%; P<0.05) and 4 h post-dose (90% vs. 58%; P<0.05), and had a statistically significant lower mean time to a 50 and 100% pain reduction in the second TTH episode. A higher percentage of patients reached a 50 or 100% pain reduction at 2 h post-dose with IndoProCaf compared with nimesulide, in two of two treated TTH episodes. A clinically and statistically significant change within each treatment group over time was found for the severity of pain, the headache intensity difference (HID), the sum of headache intensity difference (SHID), the maximum headache intensity difference (MAXHID), the headache relief (HER), the sum of total headache relief (TOTHER) and the maximum headache relief (MAXHER). In conclusion, IndoProCaf showed to be superior, but globally not statistically different from nimesulide in the treatment of episodic TTH. Both drugs were very effective and well tolerated.

Indomethacin, alone and combined with prochlorperazine and caffeine, but not sumatriptan, abolishes peripheral and central sensitization in in vivo models of migraine

Recently it has been proposed that the throbbing pain of migraine is mediated by sensitization of peripheral trigeminovascular neurons, and that cutaneous allodynia of migraine is mediated by sensitization of central trigeminovascular neurons, and, moreover, that the triptans are less effective in aborting a migraine attack if the central sensitization is already established. The combination of indomethacin, prochlorperazine, and caffeine (IndoProCaf) is a drug of well-established use in the acute treatment of migraine. The aim of this study was to investigate whether the 3 active principles of IndoProCaf, alone and combined, compared to sumatriptan, were able to abolish the peripheral sensitization induced by kainic acid and the central sensitization induced by N-methyl-D-aspartate (NMDA) in in vivo models of hyperalgesia. The study showed that indomethacin or IndoProCaf is able to abolish both the kainic acid-induced and the NMDA-induced hyperalgesia. If administered at different times, IndoProCaf was always effective in reversing the kainic acid-induced hyperalgesia. Sumatriptan was not able to reverse either the kainic acid-induced or the NMDA-induced hyperalgesia. The efficacy of indomethacin, alone and combined with prochlorperazine and caffeine, in abolishing peripheral and central sensitization in in vivo models of hyperalgesia is a further explanation of the clinical efficacy of IndoProCaf in the treatment of migraine.

PERSPECTIVE

This study suggests that, although triptans were shown to be able to abort migraine attacks only if given before the establishment of cutaneous allodynia and central sensitization, IndoProCaf should be able to abort migraine attacks independently from the time of administration, because it is able to abolish an already established peripheral and central sensitization.

Efficacy of a fixed combination of indomethacin, prochlorperazine, and caffeine versus sumatriptan in acute treatment of multiple migraine attacks: a multicenter, randomized, crossover trial

OBJECTIVE

To compare the efficacy of a fixed combination of indomethacin, prochlorperazine, and caffeine suppositories with sumatriptan suppositories in the treatment of 2 consecutive migraine attacks of moderate or severe intensity in a multicenter, randomized, crossover study.

BACKGROUND

A fixed combination of indomethacin, prochlorperazine, and caffeine is the most commonly used drug for the acute treatment of migraine in Italy. No studies have been published comparing the efficacy of this combination with sumatriptan, the most widely prescribed of the triptans.

METHODS

One hundred twelve patients with migraine with or without aura according to the diagnostic criteria of the International Headache Society were randomized to treat 2 migraine attacks with a fixed combination of indomethacin, prochlorperazine, and caffeine and 2 migraine attacks with sumatriptan. Both drugs were rectally administered in a single dose for each attack. Patients were asked to take study medication as soon as possible at the onset of a headache.

RESULTS

Of the 112 patients, 88 were compliant to the protocol. More attacks became pain-free at 2 hours postdose (primary end point) on the combination than on sumatriptan (49% versus 34%; P<.01), while there was no difference in the relief of headache at 2 hours postdose (71% versus 65%). The combination was statistically superior to sumatriptan in the time to a pain-free response (a higher percentage of attacks became pain-free from 0.5 hours postdose to 5 hours postdose), in alleviation of nausea, and in a sustained pain-free response (pain-free at 2 hours postdose with no use of rescue medication or relapses within 48 hours). Moreover, a significant consistent response was achieved for the combination compared with sumatriptan across (higher percentage of patients pain-free at 2 hours postdose in the first, second, third, and fourth treated attack) and within patients (pain-free in 2 of 2 treated attacks in 35% of patients taking the combination and 20% of patients on sumatriptan). Both drugs were well-tolerated.

CONCLUSIONS

This study, analyzed according to the more recent guidelines for controlled trials in migraine, showed that a fixed combination of indomethacin, prochlorperazine, and caffeine is significantly more effective than sumatriptan in the acute treatment of migraine attacks. It is notable that the combination is less expensive than sumatriptan per unit dose.

Indomethacin, caffeine and prochlorperazine alone and combined revert hyperalgesia in in vivo models of migraine

The combination of indomethacin, caffeine, and prochlorperazine (hereinafter IndoProCaf) represents an effective antimigraine drug available on the Italian market. The aim of this study was to test the efficacy of the three active principles alone and in combination in reverting hyperalgesia. Hyperalgesia was induced by morphine withdrawal in mice treated with morphine for 15 days and then made hyperalgic by morphine substitution with water. This study showed that indomethacin 0.3 mg kg(-1), i.p.; caffeine 0.1 and 0.3 mg kg(-1), i.p.; and prochlorperazine 0.1 mg kg(-1), i.p.; as well as the combination of the three active principles, were able to revert morphine withdrawal induced hyperalgesia, causing a statistically significant increase of pain threshold in hyperalgic mice. In a second model, hyperalgesia was induced by the i.p. injection of a 0.3% solution of acetic acid in mice and was evaluated counting the number of abdominal constrictions. Indomethacin (0.1 mg kg(-1), i.p.), caffeine (0.3 mg kg(-1), i.p.), and prochlorperazine (0.1 mg kg(-1), i.p.) reduced the number of abdominal constrictions, while the combination of the three active principles was able to abolish almost completely the abdominal constrictions, with a significantly higher efficacy compared to the single active principles. In both models, indomethacin, caffeine, and prochlorperazine reverted hyperalgesia at dosages 10 times lower than the corresponding analgesic ones. These data provide the pharmacologic evidence of the efficacy of IndoProCaf in reverting hyperalgesia, a condition of reduction of pain threshold similar to that occurring in migraine.

Vigor of visceromotor responses to urinary bladder distension in rats increases with repeated trials and stimulus intensity

This methodological study characterized visceromotor responses (VMRs) as abdominal contractile responses to urinary bladder distension (UBD) in the female rat. Electromyographic activity of the abdominal musculature was used as a measure of the VMR. Similar to previously characterized cardiovascular responses to UBD, VMRs to UBD demonstrated an initial sensitization period whereby repeated presentation of UBD stimuli led to increase vigor of the VMR. Graded UBD produced graded VMRs, therefore stimulus-response functions could be constructed. The intravenous administration of the opioid fentanyl produced a reduced vigor of the VMR in a fashion consistent with its analgesic effect. The present report supports the utility of this model for studies of urinary bladder nociception.

Evaluation of the analgesic effect of 0.1% indomethacin solution on corneal abrasions

PURPOSE

The authors wished to verify the analgesic action of 0.1% indomethacin in a water-based solution on patients affected by traumatic corneal abrasions.

METHODS

347 patients affected by traumatic corneal abrasions, having been randomly divided into 2 groups on the basis of the administration of indomethacin, were evaluated at 30 min, 12 h and 24 h after the initial treatment of the abrasion. The level of pain experienced was evaluated on a verbal pain scale and the healing time was evaluated relative to the dimension of the abrasion.

RESULTS

The pain level was initially overwhelming for both groups: p = 0.737; at successive check-ups it was possible to verify a reduction of the symptomatology, with a more pronounced decrease in pain in the group treated with indomethacin (p < 0.0001), which also demonstrated a lower sensitivity to pain in the case of larger lesions (p < 0.0001). There was no difference in the healing time between groups, and the reduction of pain is not correlated with corneal anesthesia and healing time.

CONCLUSIONS

Our study highlighted the efficacy of indomethacin as a pain reducer for acute corneal pathology and suggested that the medication may act on the corneal nociceptors in a qualitative way.

NSAID-induced cyclooxygenase inhibition differentially depresses long-lasting versus brief synaptically-elicited responses of rat spinal dorsal horn neurons in vivo

This electrophysiological study examined the effects of NSAID administration on synaptically-elicited responses of rat single spinal dorsal horn neurons to natural stimulation of peripheral receptive fields. Nociceptive responses consisted of a fast initial discharge during the stimulus followed by a slowly-decaying afterdischarge. The cyclooxygenase inhibitor, indomethacin (2.0-8.0 mg/kg, i.v.), was without effect on the on-going rate of discharge but dose-dependently inhibited synaptically-elicited responses to noxious cutaneous mechanical stimulation (fast initial discharge: n = 3/3 with 2 mg/kg, 5/8 with 4 mg/kg, 5/6 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/3 with 2 mg/kg, 6/8 with 4 mg/kg, 6/6 with 8 mg/kg) and thermal (fast initial discharge: n = 7/9 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/4 with 4 mg/kg, n = 7/9 with 8 mg/kg). The inhibitory effect of indomethacin started within 2-4 min and lasted up to 120 min. To eliminate any effect of indomethacin via cutaneous sensory receptors it was tested on the responses of some neurons to high intensity electrical stimulation of the sciatic nerve; indomethacin depressed these evoked responses (fast initial discharge: n = 5/6 with 2 mg/kg, n = 7/7 with 4 mg/kg; slowly-decaying afterdischarge: n = 6/6 with 2 mg/kg, n = 7/7 with 4 mg/kg). The brief excitatory responses to innocuous pressure (fast initial discharge: n = 2/3 with 2 mg/kg, n = 6/8 with 4 mg/kg, n = 4/6 with 8 mg/kg) and hair (n = 2/7 with 2 and 4 mg/kg, respectively) stimulation in both non-nociceptive and wide dynamic range neurons were also depressed but to a lesser extent. However, the prolonged excitation of three wide dynamic range neurons to continuous hair stimulation was almost entirely inhibited by indomethacin. Overall, inhibition of the afterdischarge and the excitatory effect of long-lasting synaptic input were greater than inhibition of the fast synaptic input-evoked initial discharge. The evidence supports the suggestion that systemically-administered indomethacin has an effect in the spinal cord and demonstrates an action specifically in the dorsal horn. The data are interpreted to suggest that sensory inputs are more involved than input-independent excitation of dorsal horn neurons in leading to de novo synthesis of eicosanoids and that the time course of this synthesis brings the levels to a point where COX inhibition can have an observable effect during prolonged excitation. Although the data suggest that COX inhibition differentially inhibits nociceptive versus non-nociceptive mechanisms at the cellular level, irrespective of the modality of the stimulus, this is the first direct demonstration that prolonged activation of synaptic mechanisms are preferentially inhibited. According to this it would be predictable that NSAIDs would be more effective on nociceptive types of pain characterized by time or prolonged inputs of primary afferents.

Models of Visceral Nociception

Pains arising from the viscera constitute a large portion of clinically treated pains. They are characterized by poor localization; immobility with tonic increases in muscle tone; and vigorous but nonspecific changes in autonomic function, such as changes in respiration, heart rate, and blood pressure. Tissue-damaging stimuli do not reliably produce visceral pain, so the study of visceral nociception in nonhuman animals requires identification of appropriate stimuli and responses. This article defines "noxious" visceral stimuli as those that produce pain in humans, result in aversive behaviors in animals, and evoke responses that are inhibited by manipulations known to be analgesic in humans. To be valid, the measured responses must be reliable, inhibited by known analgesics, and not inhibited by nonanalgesics. Using these criteria as measures of validity, the author examined several visceral pain models. The writhing test (application of intraperitoneal irritants) failed to meet these criteria; however, responses to small bowel distension, colonic-rectal distension, artificial ureteral calculosis, urinary tract distension, and the intravesical application of irritants met most, if not all, of the criteria. Other models, such as responses to biliary system distension, to reproductive organ stimulation, to the focal application of algesic agents onto various viscera, and to ischemic stimuli, met some of these criteria. This information should assist readers in decisions related to the use of visceral pain models.

The effects on the central nervous system of nitroglycerin--putative mechanisms and mediators

Nitroglycerin is an organic nitrate that has been used as a vasodilator in the treatment of cardiac diseases for over a century. Only recently it has been demonstrated that the vasodilator effect of this drug depends upon the formation of nitric oxide in the blood vessel wall. However, clinical and research data gathered during the last decades have suggested that nitroglycerin possesses, besides its peripheral vasodilator effect, additional, puzzling biological activities. This organic nitrate compound provokes reflex cardiovascular activities via its interaction with the central sympathetic system. Its cerebrovascular effect, on the other hand, is probably mediated by the local release of neuropeptides. The direct application of nitroglycerin onto brain nuclei causes a prompt increase in the neuronal discharge rate. From a neurological point of view, nitroglycerin consistently induces a specific headache attack in patients suffering from migraine. Because of its temporal pattern and clinical characteristics, nitroglycerin-induced headache cannot be solely ascribed to the a drug-induced vasorelaxation. The demonstration that systemic nitroglycerin administration activates a widespread set of vegetative, nociceptive and neuroendocrine structures in the central nervous system seems to further support the occurrence of central mechanisms in the biological activity of nitroglycerin. Double labeling immunocytochemical and neuropharmacological studies have provided information on the putative neurotransmitters and neurochemical mechanisms involved in nitroglycerin-induced neuronal activation.

Neurochemical mechanisms of nitroglycerin-induced neuronal activation in rat brain: a pharmacological investigation

Nitroglycerin is a nitric oxide donor which induces sustained expression of Fos protein, a marker of neuronal activation, in specific neuronal groups in the central nervous system. The mechanisms which underlie nitroglycerin-induced neuronal activation are elusive at this time, although a precise role has been suggested for the pool of neurons containing nitric oxide synthase as well as for catecholaminergic and peptidergic pathways. The aim of this study was to provide further details on the central effect of nitroglycerin by means of a pharmacological manipulation of nitroglycerin-induced neuronal activation with inhibitors of the nitric oxide synthase, modulators of the sympathetic drive and mediators of pain perception. Adult male Sprague-Dawley rats received L-NGnitro-arginine methyl ester, 7-nitro-indazole, ephedrine sulfate, indomethacin, capsaicin or vehicle before the subcutaneous injection of nitroglycerin (10 mg/kg b.w.). They were sacrificed 4 hr after nitroglycerin administration and brain sections were processed for immunocytochemical visualization of Fos. All the pharmacological treatments administered before injecting nitroglycerin selectively influenced Fos expression in the different brain nuclei. The data obtained suggest that nitroglycerin-induced neuronal activation is mediated by nociceptive and barosensitive mechanisms. Nitric oxide seems to represent the most important mediator of this phenomenon. The sympathetic system and prostaglandin synthesis are also likely to be involved.