Maurizio Sandrini

The antinociceptive action of paracetamol is associated with changes in the serotonergic system in the rat brain

The antinociceptive activity of paracetamol in the hot plate and formalin tests was studied to establish the relationship between antinociceptive activity and the central serotonergic system. Significant antinociceptive activity of paracetamol was observed in the formalin test at the dose of 300 mg/kg, while, at the dose of 400 mg/kg, the drug was active both in the formalin and in the hot-plate test. Serum paracetamol levels remained sub-toxic and the behavioral profile remained unchanged. Depletion of brain serotonin with p-chlorophenylalanine prevented the antinociceptive effect of paracetamol in the hot-plate test and in the first phase of the formalin response. Paracetamol significantly increased the serotonin content in the pontine and cortical areas (by 75 and 70%, respectively). The pretreatment with p-chlorophenylalanine reduced the 5-hydroxytryptamine (5-HT) content in cortical and pontine areas to 12 and 19% of baseline values, respectively, and prevented the enhancement induced by paracetamol. The maximum number of cortical 5-HT2 receptors was reduced by paracetamol, while the number of 5-HT1A receptors in both cortical and pontine areas was unchanged. Pre-treatment with p-chlorophenylalanine prevented the reduction in the number of 5-HT2 receptors induced by paracetamol. These results provide evidence for the involvement of the central serotonergic system in the antinociceptive effect of paracetamol in the hot plate and formalin tests.

Long-term follow-up of patients treated for chronic headache with analgesic overuse.

The study aim is to describe the long-term clinical outcome of 102 chronic headache patients with analgesic daily use. They were assessed for daily drug intake (DDI), headache index (HI) and quality of life (QoL) and compared with a parallel group of patients with active chronic daily headache but no analgesic overuse. For the primary study group, baseline 1995 DDI was 1.80 ± 1.87 and did not differ significantly in 1999. Patients who daily continued to use analgesics had a higher 1995 baseline DDI (t = 2.275, P = 0.025), a longer drug abuse history (t = 2.282, P = 0.025) and a higher DDI (t = 4.042, P < 0.001) 4 years later. At 4 years of follow-up, only one-third of patients initially treated for chronic daily headache and analgesic overuse are successful in refraining from chronic overuse. Those subjects appear to have a persistence for combination analgesic agents; however, their QoL is slightly better than that of patients who revert to episodic headache or continue with chronic daily headache but do not overuse analgesic agents. Persistent analgesic overuse seems to be linked to the length of abuse and to the number of drugs ingested.

Nitroglycerin induces hyperalgesia in rats—a time-course study

Nitroglycerin is a nitric oxide (NO) donor which activates nuclei involved in nociceptive transmission following systemic administration. The effect of nitroglycerin on the nociceptive threshold was studied in rats by means of two experimental tests that explore different modalities of pain: the tail-flick test and the formalin test. Nitroglycerin induced a significant reduction in the latency of the tail flick 2 and 4 h after its administration. Similarly, formalin-induced pain-related behaviour increased significantly 2 and 4 h after nitroglycerin administration.

Effect of acute and chronic treatment with triiodothyronine on serotonin levels and serotonergic receptor subtypes in the rat brain

Hyperthyroidism is often associated with behavioral disorders, and thyroid hormones modify receptor sensitivity as well as the synthesis and/or turnover rate of many neurotransmitters. We evaluated the influence in adult rats of triiodothyronine (T3), administered s.c. (100 micrograms/kg) acutely (once only) or chronically (once a day for 3 or 7 consecutive days), on brain serotonin concentration and on the density and affinity of two brain serotonin (5-HT) receptor subtypes mainly involved in behavioral effects. After both acute and chronic T3 treatment, serotonin levels increased in the cerebral cortex but not in the hippocampus. The density and affinity of 5-HT1A receptors (using [3H]-8-OH-DPAT as ligand) were not affected, while there was a significant decrease in the number of 5-HT2 receptors in the cerebral cortex (using [3H]ketanserin as ligand). This observation might indicate that thyroid hormones enhance 5-HT concentration in certain brain areas, thus causing a down-regulation of 5-HT2 receptors. The serotonergic system could be involved in the complex brain-neurotransmitter imbalance underlying hyperthyroidism-linked behavioral changes.

Effect of rofecoxib on nociception and the serotonin system in the rat brain

Abstract.Objective and design: The purpose of the present study was to determine whether the antinociceptive activity of rofecoxib is mediated, at least in part, through changes in the brain serotonergic system.¶Materials and subjects: Male Wistar rats weighing 180-200 g (groups of eight) were subjected to the hot-plate and formalin tests after rofecoxib treatment. Cortical areas were removed for serotonin (5-HT) level, 5-HT2 and μ-receptor evaluation.¶Treatment: Rofecoxib was administered orally at doses of 5, 10, 20 and 50 mg/kg for the time course evaluation in the hot-plate test (30, 60 and 120 min), and at the dose of 10 mg/kg for the formalin test and biochemical determinations.¶Methods: The tests performed were the hot-plate and the formalin assays. HPLC was used to determine 5-HT levels and radioligand-binding assays were utilized to evaluate the characteristics of 5-HT2 and μ-receptors. The data were analysed by ANOVA or Students t test.¶Results: The lowest active dose of rofecoxib in the hot-plate test was 10 mg/kg. The percentage of the maximum possible effect (%MPE) values were: control = 1.7 ± 3.4; treated 23.4 ± 6.5 (p<0.05). The same dose had a significant effect on both phases of the formalin test. Pretreatment with p-chlorophenylalanine (PCPA) significantly decreased the activity of rofecoxib in the hot-plate test. Rofecoxib treatment increased serotonin levels and decreased the maximum number of 5-HT2 receptors. 5-HT levels (ng/g) were: control = 240.1 ± 28.5, rofecoxib = 326.1 ± 19.9 in the frontal cortex. The characteristics of μ-receptors did not change.¶Conclusions: These results suggest that rofecoxib may exert its therapeutic effect, at least in part, through the central serotonergic system. The opioidergic system, on the other hand, seems to be unaffected.

Glutamate-mediated astrocyte-to-neuron signalling in the rat dorsal horn

By releasing neuroactive agents, including proinflammatory cytokines, prostaglandins and neurotrophins, microglia and astrocytes are proposed to be involved in nociceptive transmission, especially in conditions of persistent, pathological pain. The specific action on dorsal horn neurons of agents released from astrocytes, such as glutamate, has been, however, poorly investigated. By using patch‐clamp and confocal microscope calcium imaging techniques in rat spinal cord slices, we monitored the activity of dorsal horn lamina II neurons following astrocyte activation. Results obtained revealed that stimuli that triggered Ca2+ elevations in astrocytes, such as the purinergic receptor agonist BzATP and low extracellular Ca2+, induce in lamina II neurons slow inward currents (SICs). Similarly to SICs triggered by astrocytic glutamate in neurons from other central nervous system regions, these currents (i) are insensitive to tetrodotoxin (TTX), (ii) are blocked by the NMDA receptor (NMDAR) antagonist d‐AP5, (iii) lack an AMPA component, and (iv) have slow rise and decay times. Ca2+ imaging also revealed that astrocytic glutamate evokes NMDAR‐mediated episodes of synchronous activity in groups of substantia gelatinosa neurons. Importantly, in a model of peripheral inflammation, the development of thermal hyperalgesia and mechanical allodynia was accompanied by a significant increase of spontaneous SICs in dorsal horn neurons. The NMDAR‐mediated astrocyte‐to‐neuron signalling thus represents a novel pathway that may contribute to the control of central sensitization in pathological pain.

Streptozotocin-induced diabetes provokes changes in serotonin concentration and on 5-HT1A and 5-HT2 receptors in the rat brain

Since reduced levels of brain serotonin are known to cause behavioural abnormalities, to which diabetics are also prone, we investigated the effect, in rats, of chronic diabetes on brain serotonin concentration and on the numbers of 5-HT(1A) and 5-HT2 receptors in cerebral cortex and brainstem. Our data show that streptozotocin induces a longlasting hyperglicemia that is associated with a decrease in cerebral concentration of serotonin and with an accompanying increase in the maximum number of 5-HT(1A) and 5-HT2 receptors in the brain areas studied. Our results may suggest that changes in serotonergic transmission in the CNS play a role in diabetes-related behavioural abnormalities.

Differential involvement of central 5-HT1B and 5-HT3 receptor subtypes in the antinociceptive effect of paracetamol

AbstractObjective: We investigated the effect of pre-treatment with ondansetron or CP 93129 (a 5-HAT1B agonist) on the antinociceptive activity of paracetamol and the changes in central 5-HT3 receptors induced by paracetamol alone or co-administered with ondansetron. Materials and Subjects: Male Wistar rats (eight per group) were injected with ondansetron (2 and 4 mg/kg s.c.) or CP 93129 (0.5, 1 and 2 mg/kg s.c.) 15 min before paracetamol (400 mg/kg, i.p.). Methods: Pain threshold was evaluated in the hot-plate or in the paw pressure test 30 min after the last treatment. 5-HT3 receptor binding capacity was measured in the frontal cortex, temporal-parietal cortex and midbrain by means of radioligand binding technique. Statistical analysis was done using ANOVA followed by Student-Newman-Keuls test and 2 X 2 factorial analysis when appropriate. Results: Pre-treatment with ondansetron, at doses of 2 and 4 mg/kg, did not affect the antinociceptive activity of paracetamol in the hot-plate test and in the paw pressure test. Paracetamol did not change the characteristics of 5-HT3 receptors in all the areas investigated. Ondansetron (4 mg/kg s.c) per se significantly increased the 5-HT3 receptor number in the areas used, the effect not being modified by co-administration with paracetamol. On the other hand, CP 93129 (2 mg/kg s.c.) significantly prevented the effect of paracetamol in both algesimetric tests used. Conclusions: Our data indicate that 5-HT1B but not 5-HT3 receptors are involved in the antinociceptive effect of paracetamol in our experimental conditions.

The potentiation of analgesic activity of paracetamol plus morphine involves the serotonergic system in rat brain

Abstract.Objective and Design: We investigated the antinociceptive effect of subactive doses of paracetamol and morphine, given in combination.¶Material and Treatment: Male Wistar rats were injected with paracetamol (50 or 100 mg/kg i.p.) and morphine (2, 3 or 5 mg/kg s.c.) 10 min later and subjected to algesimetric tests 20 min thereafter.¶Methods: Pain threshold was evaluated in the hot-plate and formalin tests. 5-HT2 receptor binding capacity and 5-HT and 5-HIAA levels were measured in cortical and pontine areas of the brain by means of radioligand binding technique and by HPLC, respectively. Statistical analysis was done using Student-Neuman-Keuls test and 2 × 2 factorial analysis.¶Results: Only when given in combination, paracetamol (100 mg/kg) and morphine (2 and 3 mg/kg) were able to evoke an antinociceptive effect in both tests associated with an increase in 5-HT levels and a decrease in 5-HT2 receptors in the cortex. These effects were prevented by i.p. pretreatment with naloxone (1 mg/kg i.p.).¶Conclusions: Subactive doses of paracetamol and morphine exert an analgesic effect when given in combination in the rat and indicate an involvement of both serotonergic and opiatergic systems.

The effect of paracetamol on nociception and dynorphin A levels in the rat brain.

Male Wistar rats were administered with naloxone (1 mg/kg i.p.) or MR 2266 (5 mg/kg i.p) 15 min before paracetamol (400 mg/kg i.p.) treatment and the pain threshold was evaluated. Rats were subjected to the hot-plate and formalin tests and immunoreactive dynorphin A (ir-dynorphin A) levels were measured in the hypothalamus, hippocampus, striatum, brainstem, frontal and parietal-temporal cortex by radioimmunoassay. Pretreatment with naloxone abolished paracetamol antinociceptive activity both in hot-plate and in the first phase, but not in the second phase of the formalin test, while MR 2266 pretreatment was able to antagonise paracetamol effect either in the hot-plate test or in both phases of the formalin test. Among different brain areas investigated paracetamol significantly decreased ir-dynorphin A levels only in the frontal cortex. MR 2266 but not naloxone reversed the decrease in ir-dynorphin A levels elicited by paracetamol. Paracetamol seems to exert its antinociceptive effect also through the opioidergic system modulating dynorphin release in the central nervous system (CNS) of the rat, as suggested by the decrease in the peptide levels.