Antinociceptive effects of the novel anxiolytic buspirone in three pain tests in rats

The Buspirone-dependent Abdominal Pain Transmission Within the Nucleus Tractus Solitarius in the Rat

Buspirone, a partial agonist of the 5-HT_1aR, due to potential antinociceptive properties can be useful for abdominal pain treatment in IBS patients. Pain-related effects of buspirone can be mediated by the 5-HT_1aRs, located within the nucleus tractus solitarius. The 5-HT_1aR involvement in pain transmission within the NTS is unclear. The objective of our study was to evaluate the involvement of the 5-HT_1aR in abdominal pain transmission within the NTS. Using a model of abdominal pain on urethane-anesthetized rats, two types of NTS pain-related neurons responding to the noxious colorectal distension (CRD) with excitatory and inhibitory sustained patterns of evoked activity were revealed. Buspirone (1.0-4.0 mg kg^-1, iv) has complex time- and dose-depended action on the CRD-induced NTS neuron responses. Buspirone inhibits the responses of the excitatory neurons and inverts the responses of the inhibitory pain-related neurons but at a dose of 4.0 buspirone, the effect on NTS pain-related neurons attenuates. The inhibitory effect of buspirone on the CRD-evoked responses of the excitatory NTS neurons is completely blocked by an intra-cerebroventricular administration of buspirone agonist WAY100,635. The inhibitory responses do not change by this agonist. The inhibitory action of buspirone is mediated by supraspinal 5-HT_1a receptors however, its excitatory effect on inhibitory neurons does not dependents on these receptors. We proposed that the NTS pain-related neurons could be involved in anti- or pronociceptive effects of buspirone on abdominal pain.

The central effects of buspirone on abdominal pain in rats

BACKGROUND

Buspirone, a partial agonist of the 5-HT_1a receptor (5-HT_1a R), owing to potential antinociceptive properties may be useful in treatment of abdominal pain in IBS patients. The pain-related effects of buspirone are mediated via the 5-HT_1a Rs, specifically located within the ventrolateral medulla (VLM). The most animal studies of the 5-HT_1a R involvement in pain control have been carried out with somatic behavioral tests. The 5-HT_1a R contribution in visceral pain transmission within the VLM is unclear. The objective of our study was to evaluate the 5-HT_1a R contribution in abdominal pain transmission within the VLM.

METHODS

Using animal model of abdominal pain (urethane-anaesthetized rats), based on the noxious colorectal distension (CRD) as pain stimulus we studied effects of buspirone (1.0-4.0 mg kg^-1 , iv) on the CRD-induced VLM neuron and blood pressure responses as markers of abdominal pain before and after the 5-HT_1a R blockade by antagonist, WAY 100,635.

RESULTS

The CRD induced a significant increase in VLM neuron activity up to 201.5 ± 18.0% and depressor reactions up to 68 ± 1.8% of baseline. Buspirone (1.0-4.0 mg kg^-1 , iv) resulted in an inhibition of the CRD-induced neuron responses which were changed inversely with dose increase and decreased depressor reactions directly with dose increase. These effects were antagonized by intracerebroventricular WAY 100,635.

CONCLUSION

Buspirone exerts complex biphasic action on the pain-related VLM neuron activity inversely depending on dose. The final effect of buspirone depends on the functional balance between of activation the pre- and postsynaptic 5-HT_1a Rs in mediating pain control networks.

Pronociceptive effect of 5-HT(1A) receptor agonist on visceral pain involves spinal N-methyl-D-aspartate (NMDA) receptor

The functional role of serotonergic 5-HT(1A) receptors in the modulation of visceral pain is controversial. The objective of this study was to systematically examine the mechanism and site of action of a selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (DPAT) on visceral pain. In the behavioral model of visceral pain, systemic injection (5-250 μg/kg) of DPAT produced a significant increase in the viscero-motor response (VMR) to colorectal distension (CRD) and this effect was blocked by the selective 5-HT(1A) receptor antagonist WAY-100135 (5 mg/kg, s.c.). Similarly, intrathecal (i.t.) injection (5 μmol) of DPAT into the lumbo-sacral (L6-S1) spinal cord produced a significant increase in VMR. The administration of N-methyl D-aspartate (NMDA) receptor antagonist AP5 (50 μg/kg) prior to DPAT injection completely blocked the pronociceptive effect of DPAT. Similarly, DPAT failed to increase VMR in rats chronically treated with NR1 subunit-targeted antisense oligonucleotide (ON), whereas the drug increased VMR in rats treated with mismatched-ON. Chronic i.t. injection of allylglycine (AG), a γ-amino decarboxylase (GAD) enzyme inhibitor, produced significant increase in VMRs, suggesting that the inhibition of GABA synthesis produces pronociception. In AG-treated rats, i.t. injection of DPAT failed to further increase in VMR, suggesting that the DPAT action is linked to GABA release. Similarly, WAY-100135 failed to attenuate VMR in AG-treated rats, suggesting that unlike DPAT, AG action is not via the activation of 5-HT(1A) receptors. In electrophysiology experiments, DPAT (50 μg/kg) significantly increased the responses of spinal neurons to CRD, but did not influence the mechanotransduction property of CRD-sensitive pelvic nerve afferent fibers. The effect of DPAT on spinal neurons remained unaffected when tested in spinal-transected (C1-C2) rats. These results indicate that the 5-HT(1A) receptor agonist DPAT produces pronociceptive effects, primarily via the activation of presynaptic 5-HT(1A) receptors in GABAergic neuron to restrict GABA release and thereby disinhibits the excitatory glutamatergic neurons in the spinal cord.

Antinociceptive activity of Tilia americana var. mexicana inflorescences and quercetin in the formalin test and in an arthritic pain model in rats

Tilia species are well known around the world for their properties in traditional medicine. Antinociceptive activity of hexane, methanol and aqueous extracts from Tilia americana var. mexicana inflorescences was evaluated in the pain-induced functional impairment model in rats (PIFIR). A preliminar 300 mg/kg dosage of aqueous extracts i.p., but not the same dose of methanol or hexane extract, produced an antinociceptive response in rats similar to that of tramadol (17.8 mg/kg i.p.). A dose-response curve from aqueous extract allowed the determination of ED(50) = 364.97 mg/kg in comparison to ED(50) = 10.35 mg/kg for tramadol in this model. A previous HPLC-DAD analysis corroborated by an HPLC-MS technique in this study demonstrated the flavonoid composition in this Tilia aqueous extract revealing the presence of glycosides mainly derived from quercetin. Thus, Tilia aqueous extract and quercetin were tested at 30 and/or 100 mg/kg dosages i.p. in the PIFIR and formalin models producing a significant and dose-dependent antinociceptive response resembling that produced by a total and a partial agonist of 5-HT(1A) receptors like 8-OH-DPAT (0.1 mg/kg, s.c.) and buspirone (5 mg/kg, i.p.), respectively. In all the treatments, antinociceptive response was inhibited in the presence of WAY 100635 (0.12 mg/kg, i.p.). Our results support the analgesic activity of T. americana var. mexicana inflorescences attributed by folk medicine; they also indicate that quercetin is partly responsible for this pharmacological activity that is likely mediated by serotonin 5-HT(1A) receptors.

Buspirone-induced antinociception is mediated by L-type calcium channels and calcium/caffeine-sensitive pools in mice

RATIONALE

Previous studies have shown that buspirone, a partial 5-HT(1A) receptor agonist, produces antinociceptive effects in rats and mice; Ca(2+) plays a critical role as a second messenger in mediating nociceptive transmission. 5-HT(1A) receptors have been proven to be coupled functionally with various types of Ca(2+) channels in neurons, including N-, P/Q-, T-, or L-type. It was of interest to investigate the involvement of extracellular/intracellular Ca(2+) in buspirone-induced antinociception.

OBJECTIVES

To determine whether central serotonergic pathways participate in the antinociceptive processes of buspirone, and investigate the involvement of Ca(2+) mechanisms, particularly L-voltage-gated Ca(2+) channels and Ca(2+)/caffeine-sensitive pools, in buspirone-induced antinociception.

METHODS

Antinociception was assessed using the hot-plate test (55 degrees C, hind-paw licking latency) in mice treated with either buspirone (1.25-20 mg/kg i.p.) alone or the combination of buspirone and fluoxetine (2.5-10 mg/kg i.p.), 5-HTP (25 mg/kg i.p.), nimodipine (2.5-10 mg/kg i.p.), nifedipine (2.5-10 mg/kg i.p.), CaCl(2) (25-200 nmol per mouse i.c.v.), EGTA (5-30 nmol per mouse i.c.v.), or ryanodine (0.25-2 nmol per mouse i.c.v.).

RESULTS

Buspirone dose dependently increased the licking latency in the hot-plate test in mice. This effect of buspirone was enhanced by fluoxetine, 5-HTP, nimodipine, and nifedipine. Interestingly, central administration of Ca(2+) reversed the antinociceptive effects of buspirone. In contrast to these, ryanodine or EGTA administered centrally potentiated buspirone-induced antinociception.

CONCLUSIONS

Decreasing neuronal Ca(2+) levels potentiated buspirone-induced antinociception; conversely, increasing intracellular Ca(2+) abolished the antinociceptive effects of buspirone. These results suggest that Ca(2+) influx from extracellular fluid and release of Ca(2+) from Ca(2+)/caffeine-sensitive microsomal pools may be involved in buspirone-induced antinociception.

5-HT1A agonists induce central cholinergic antinociception

The antinociceptive effects of the 5-HT1A agonists buspirone [3 mg/kg intraperitoneally (i.p.)], gepirone (3-6 mg/kg i.p.), and 8-OH-DPAT [3-5 mg/kg i.p.; 1-3 micrograms per mouse intracerebroventricularly (i.c.v.)] were examined in mice by using the hot-plate (thermal stimulus) and abdominal constriction (chemical stimulus) tests. Buspirone, gepirone, and 8-OH-DPAT produced significant antinociception, which was prevented by atropine (5 mg/kg i.p.), the ACh depletor hemicholinium-3 (1 microgram per mouse i.c.v.), and the 5-HT1A antagonist NAN 190 (0.5 microgram per mouse i.c.v.), but not by naloxone (1 mg/kg i.p.), the GABAB antagonist CGP 35348 (100 mg/kg i.p.), and pertussis toxin (0.25 microgram per mouse i.c.v.). NAN 190 which totally antagonized buspirone, gepirone, and 8-OH-DPAT antinociception, did not modify the analgesic effect of morphine (5 mg/kg subcutaneously). In the antinociceptive dose range, none of the 5HT1A agonists impaired mouse performance evaluated by rota-rod and hole board tests. On the basis of these data, it can be postulated that buspirone, gepirone, and 8-OH-DPAT exert an antinociceptive effect mediated by a central amplification of cholinergic transmission.

Effects of K+ channel blockers and openers on antinociception induced by agonists of 5-HT1A receptors

The modulation by K+ channel-acting drugs of the antinociceptive effect of several 5-HT1A receptor agonists was examined with the hot plate test in mice. All the 5-HT1A receptor agonists tested induced dose-dependent antinociception, the order of potency being (+/-)-8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT) > buspirone > or = lesopitron > or = tandospirone. The blockers of ATP-sensitive K+ channels (KATP) gliquidone and glipizide (1-4 and 16-64 micrograms/mouse i.c.v., respectively) reduced the antinociceptive effect of 8-OH-DPAT, whereas cromakalim (32-64 micrograms/mouse i.c.v.), an opener of KATP channels, enhanced the effect. In contrast, 4-aminopyridine (25-250 ng/mouse i.c.v.) and tetraethylammonium (10-20 micrograms/mouse i.c.v.), which antagonize several non-ATP-dependent K+ conductances, were inactive. The same results were found with other agonists of 5-HT1A receptors (lesopitron, buspirone and tandospirone): gliquidone inhibited whereas cromakalim increased their antinociceptive effects. None of the K+ channel-acting drugs modified the binding of [3H]8-OH-DPAT to hippocampal membranes, whereas all the 5-HT1A receptor agonists displaced the ligand. These results suggest that ATP-sensitive K+ conductances are involved in the antinociception induced by agonists of 5-HT1A receptors.

Different functions of spinal 5-HT1A and 5-HT2 receptor subtypes in modulating behaviour induced by excitatory amino acid receptor agonists in mice

The modulating effects of 5-HT1A and 5-HT2 receptor agonists on behaviour spinal excitatory amino acid (EAA) agonists were examined. Intrathecal (i.th.) administration of both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) produce a behavioural syndrome of caudally directed biting and scratching. Serotonin (5-HT) agonists were coadministered with either NMDA or AMPA, and changes in EAA-induced behaviour were scored. All drugs were administered i.th. The 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) (15-60 nmol) reduced both NMDA (0.25 nmol) and AMPA (0.06 nmol) induced behaviour in a dose-dependent manner, and preadministration of the 5-HT1A receptor antagonist, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine hydrobromide (NAN-190) (20 nmol) reversed this effect. The administration of the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.7-28 nmol) produced a dose-dependent behavioural syndrome similar to the EAA agonists. This was reversed by preadministration of ritanserin (10 nmol), a 5-HT2 antagonist. When DOI was coadministered with NMDA (0.25 nmol) or MAPA (0.06 nmol) there was an increase in the behaviour recorded and this effect was antagonised by ritanserin. The results of this study implicate that in the spinal cord subtypes of 5-HT receptors have different effects on modulation of behaviour induced by activation of the NMDA or the AMPA receptors; the activated 5-HT1A receptors have an inhibitory effect whereas activation of the 5-HT2 receptors enhance the induced behaviour.

The formalin test: an evaluation of the method

The formalin test for nociception, which is predominantly used with rats and mice, involves moderate, continuous pain generated by injured tissue. In this way it differs from most traditional tests of nociception which rely upon brief stimuli of threshold intensity. In this article we describe the main features of the formalin test, including the characteristics of the stimulus and how changes in nociceptive behaviour may be measured and interpreted. The response to formalin shows an early and a late phase. The early phase seems to be caused predominantly by C-fibre activation due to the peripheral stimulus, while the late phase appears to be dependent on the combination of an inflammatory reaction in the peripheral tissue and functional changes in the dorsal horn of the spinal cord. These functional changes seem to be initiated by the C-fibre barrage during the early phase. In mice, the behavioural response in the late phase depends on the ambient temperature. We argue that the peripheral tissue temperature as well as other factors influencing the peripheral inflammation may affect the response, possibly confounding the results obtained with the test. Furthermore, we discuss the methods of recording the response and the value of observing more than one aspect of behaviour. Scoring of several behavioural variables provides a means of assessing motor or sensorimotor function as possible causes for changes in behaviour. In conclusion, the formalin test is a valuable addition to the battery of methods available to study nociception.

Role of spinal serotonin1 receptor subtypes in thermally and mechanically elicited nociceptive reflexes

The ability of 5-HT1A and 5-HT1B agonists to alter a spinal animal's nociceptive threshold was examined using two analgesiometric tests. In the spinal withdrawal reflex test, administration of the selective 5-HT1A agonists ipsapirone, gepirone and PAPP resulted in significant dose-dependent increases in receptive field (RF) area for withdrawal reflexes when compared to predrug baseline values, indicating an increase in nociceptive sensitivity. The average overall percent maximal increase in RF area following administration of 5-HT1A selective compounds was: 80 +/- 16% for the ventroflexion reflex, 90 +/- 6% for the dorsiflexion reflex and 87 +/- 8% for the lateral flexion reflex. Similar to the effects noted with 5-HT1A agonists, administration of 5-HT1B agonists RU24969, mCPP and TFMPP resulted in a hyperalgesic response with an overall percent maximal increase of 43 +/- 6% for the ventroflexion reflex, 51 +/- 6% for the dorsiflexion reflex and 38 +/- 9% for the lateral flexion reflex. In the tail-flick analgesiometric test, administration of the 5-HT1A agonists 8-OH-DPAT and ipsapirone and the 5-HT1B agonists RU24969 and mCPP resulted in a significant dose-dependent increase in tail-flick latencies when compared to predrug baseline values, indicating a decrease in nociceptive sensitivity to noxious thermal stimuli. No differences in magnitude of the effect of the two receptor subtypes were found, indicating that stimulation of either 5-HT1A or 5-HT1B receptors was equipotent in producing the antinociceptive tail-flick response.(ABSTRACT TRUNCATED AT 250 WORDS)

Supraspinal and spinal monoamine-modified function and the expression of opioid antinociception

Opioids are highly valuable clinical agents for the treatment of pain which are thought to act both at the spinal and supraspinal level. During the course of their actions, they have complex interactions with monoamine systems. These include 5-hydroxytryptamine (5-HT) and noradrenaline (NA), so this topic is discussed using these two transmitter systems, their locations and receptor sub-types, as prime candidates for modulating nociceptive and antinociceptive processes. Several classes of 5-HT receptors, as well as α(2)-adrenoceptors, appear to be clearly involved in antinociception and the functions of systems carrying these receptors may be modified using psychotropic agents. In particular, some antidepressants may acutely augment opioid antinociception and this property may be exploited to delay the onset of opioid tolerance in the sub-acute situation.

Effects of systemically administered monoamine reuptake blocking agents on patterns of buspirone-induced analgesia in rats

We have recently shown the serotonin 5-HT1A receptor agonist buspirone to produce analgesia in several pain tests in rats. As a 5-HT1A agonist, buspirone may change serotonergic (5-HT) tone to alter the balance of central monoaminergic (MA) systems that function in analgesia. MA-reuptake blocking drugs have been shown to produce analgesia, both when given alone and in combination with a variety of other agents, presumably via their action upon MA neurochemistry. The present study was undertaken to examine the effect of systemic administration of the 5-HT-reuptake blocker amitriptyline (AMI: 10 mg/kg), NE-reuptake blocker desipramine (DMI: 10 mg/kg) or DA-reuptake blocker GBR-12909 (7.5 mg/kg) on patterns of analgesia produced by buspirone (1-5 mg/kg) in thermal and mechanical pain tests in rats. Neither reuptake blocking agents or buspirone, when administered alone or in combination, produced overt changes in motor activity at the doses tested. AMI alone was not analgesic in either thermal or mechanical pain tests. In both assays, AMI reduced the analgesic action of buspirone, with greater effects seen in the thermal test. When administered alone, DMI produced significant analgesia against thermal and mechanical pain. DMI significantly attenuated the analgesic action of all doses of buspirone in both pain tests. Alone, GBR-12909 did not affect nociception in thermal or mechanical tests. GBR-12909 decreased buspirone-induced analgesia at all doses in the thermal test, while having no effect on buspirone-induced analgesia against mechanical pain. These results demonstrate that facilitation of 5-HT, NE and DA function with reuptake blocking drugs did not enhance the analgesic action of buspirone. These data indicate against the adjuvant use of reuptake blocking compounds and buspirone as analgesics. Furthermore, such findings may suggest other possible non-MA substrates of buspirone-induced analgesia.