5-HT agonist induced analgesia modulated by central but not peripheral noradrenaline depletion in rats

Neonatal DSP-4 treatment modifies antinociceptive effects of the CB1 receptor agonist methanandamide in adult rats

To study the influence of the central noradrenergic system on antinociceptive effects mediated by the CB₁-receptor agonist methanandamide, intact rats were contrasted with rats in which noradrenergic nerves were largely destroyed shortly after birth with the neurotoxin DSP-4 [N-(-2-chloroethyl)-N-ethyl-2-bromobenzylamine (50 mg/kg sc × 2, P1 and P3); zimelidine (10 mg/kg sc, 30 min pretreatment, selective serotonin reuptake inhibitor). When rats attained 10 weeks of age, monoamine and their metabolite concentrations were determined in the frontal cortex, thalamus, and spinal cord by an HPLC/ED method. Antinociceptive effects after methanandamide (10 mg/kg ip) apply were evaluated by a battery of tests. In addition, immunohistochemistry and densitometric analysis of the cannabinoid CB₁ receptor in the rat brain was performed. DSP-4 lesioning was associated with a reduction in norepinephrine content of the frontal cortex (>90 %) and spinal cord (>80 %) with no changes in the thalamus. Neonatal DSP-4 treatment produced a significant reduction in the antinociceptive effect of methanandamide in the tail-immersion test, hot-plate test and writhing tests. In the paw pressure and formalin hind paw tests results were ambiguous. These findings indicate that the noradrenergic system exerts a prominent influence on analgesia acting via the cannabinoid system in brain, without directly altering CB₁ receptor density in the brain.

The role of serotonergic receptors in the effects of mu opioids in squirrel monkeys responding under a titration procedure

This experiment was conducted to determine whether drugs acting on brain serotonin modulate the effects of the mu opioid, morphine, as measured by the squirrel monkey shock titration procedure and, if so, whether serotonergic modulation is mediated via specific 5HT receptor subtypes. Under this procedure, electric shock was delivered to the monkey's tail and scheduled to increase once every 15 s from 0.01 to 2.0 mA in 30 steps. Five responses on a lever during the 15-s shock period terminated the shock for 15 s, after which the shock resumed at the next lower intensity. The intensity below which monkeys maintained shock 50% of the time (median shock level or MSL) and rate of responding (RR) in the presence of shock were determined under control conditions and after administration of morphine alone and in combination with various serotonergic compounds. Morphine increased median shock level and decreased rate of responding in a dose-dependent manner. These effects of morphine was attenuated by the 5HT1A receptor agonists, 8-OH-DPAT [(+)-8-hydroxy-2(di-n-propylamino tetralin HBr] and ipsapirone. The effects of morphine were not altered by the 5HT1A receptor antagonist, NAN-190 [1-(2-methoxyphenyl-4-[4-(2-phthalimido) butyl] piperazine HBr], and 5HT2 receptor antagonist, ketanserin, the 5HT3 receptor antagonist, MDL 72222 [3-tropanyl-3,5-dichlorobenzoate], the alpha 2 adrenergic antagonist, yohimbine, or the alpha2 adrenergic agonist, clonidine. These results suggest that 5HT1A receptors may be involved in the effects of morphine in the shock titration procedure, whereas 5HT2, 5HT3 and alpha 2 adrenergic receptors do not appear to play a role in morphine's effects in this procedure.

Intrathecal coadministration of clonidine with serotonin receptor agonists produces supra-additive visceral antinociception in the rat

The intrathecal (i.t.) coadministration of sub-antinociceptive doses of clonidine, an alpha 2-adrenoceptor agonist, with DOI or RU-24969 (5-HT2 or 5-HT1B receptor agonists, respectively) produced dose-dependent supra-additive antinociceptive effects in a model of visceral pain. The enhanced attenuation of responses to noxious colorectal distension produced by the coadministration of these drugs is evidenced by significant leftward shifts in the dose-response curves as compared to those of each drug alone and by isobolographic analysis. The supra-additive antinociceptive effects produced following the i.t. coadministration of clonidine with RU-24969 were antagonized by i.t. pretreatment with phentolamine; the coadministration of phentolamine with methysergide produced no greater antagonism of effects. The supra-additive antinociceptive effects produced by i.t. coadministration of clonidine with DOI were antagonized by i.t. pretreatment with methysergide; the coadministration of methysergide with yohimbine produced no greater antagonism of effects. These data suggest that receptors acted upon by descending bulbospinal neurons interact to modulate the rostrad transmission of visceral nociceptive transmission.