Effects of clonidine and yohimbine, alone and in combination with morphine, on supraspinal analgesia

Modulation of formalin-induced pain-related behaviour by clonidine and yohimbine in the Speke's hinged tortoise (Kiniskys spekii)

The study was designed to investigate the involvement of noradrenergic and serotonergic receptor systems in the modulation of formalin-induced pain-related behaviour in the Speke's hinged tortoise. Intradermal injection of 100 μL of formalin at a dilution of 12.5% caused pain-related behaviour (hindlimb withdrawal) that lasted for a mean time of 19.28 min (monophasic response). Intrathecal administration of clonidine (α₂ -adrenergic receptor agonist) and yohimbine (α₂ -adrenergic receptor antagonist) at a dose of 40 μg/kg and 37.5 μg/kg or 50 μg/kg, respectively, caused a highly significant reduction in the duration of the formalin-induced pain-related behaviour. The effect of clonidine was reversed by intrathecal administration of yohimbine at a dose of 26.7 μg/kg. The effect of yohimbine at a dose of 50 μg/kg was reversed by intrathecal injection of 20 μg/kg of the serotonergic receptor antagonist methysergide maleate. When performing antagonistic reactions, the administration of the antagonist was followed immediately by that of the agonist. The study indicates that for experimental purposes, intrathecal route of drug administration through the atlanto-occipital joint is effective in tortoises. The data also suggest that testudines have noradrenergic and serotonergic systems that appear to play a role in the modulation of pain in this species.

Nociceptive threshold and analgesic response to morphine in aged and young adult rats as determined by thermal radiation and intracerebral electrical stimulation

The present experiment compared the nociceptive threshold and analgesic response to morphine in young (4-5 months) and aged (24 months) rats using peripheral thermal stimulation and intracerebral electrical stimulation. Responses to thermal stimuli were assessed using both the classical tail-flick procedure in which latency of response is the dependent variable and a new method in which threshold in calories of heat is the dependent variable. In the intracerebral nociceptive threshold procedure, electrical stimuli were delivered via an electrode implanted in the mesencephalic reticular formation (MRF), a pain pathway, and the animals were trained to terminate the stimulation by turning a cylindrical manipulandum embedded in one wall of the experimental chamber. For the classical tail-flick method, the aged rats required a greater intensity of stimulation to produce a basal response latency that was between 2.5 and 3.5 s. Using the new psychophysical method for determining the tail-flick threshold, the aged rats' basal thresholds were significantly higher than that of the young rats. However, the basal thresholds obtained by direct stimulation of the MRF failed to show a significant age effect, suggesting that the registration of pain is not different between young and aged rats. These age-related differences in baseline tail-flick response may be due to changes in the spinal reflex associated with aging. Although, there was no difference in the analgesic effects of morphine between young and aged rats using the latency of the tail-flick response, evidence for decreased analgesic response was seen using the tail-flick threshold measure and the intracerebral stimulation threshold method.

Fangchinoline inhibited the antinociceptive effect of morphine in mice

Fangchinoline (FAN), a non-specific calcium antagonist, is a major alkaloidal component of the creeper Stephania tetrandra S. Moore (or fenfangji). It has been shown to possess antagonistic activity on morphine-induced antinociception in mice. This study was undertaken to assess the antagonistic mechanism. The results demonstrated that FAN (IP) attenuated morphine (SC)-induced antinociception in a dose-dependent manner with significant effect at doses of 30 and 60mg/kg body wt. (IP) in the tail-flick test but not the tail-pinch tests, carried out in mice. This antagonism was abolished by pretreatment with a serotonin precursor, 5-hydroxytryptophan (5-HTP, IP), but not by pretreatment with a noradrenaline precursor, L-dihydroxyphenylalanine (L-DOPA, IP) in the tail-flick test. On the other hand, the development of morphine-induced analgesic tolerance was not prevented by FAN. These results suggest that the serotonergic pathway may be involved in the antagonism of morphine-induced antinociception by FAN and, in agreement with other reports, also indicates the possible dissociation of the morphine analgesic effect from its tolerance-development mechanism.

Effects of yohimbine on the antinociceptive and place conditioning effects of opioid agonists in rodents

1. The pharmacological modulation of opioid actions by drugs acting on heterologous mechanisms could be useful to overcome some of the main problems associated with the use of opiate agonists. Based on previous findings on the interactions between yohimbine and opioid drugs, we have further studied the effects of yohimbine on the antinociceptive and positive-negative reinforcing effects of morphine (mu opioid receptor-preferring agonist), U-50,488 (kappa agonist) and SNC80 (delta agonist). 2. Pretreatment with yohimbine completely blocked the antinociception provided by the three opioid agonists in the mouse tail-immersion test. 3. A similar blockade of SNC80 and U-50,488-induced antinociception was observed with yohimbine in the mouse hot plate test at the same doses. In this paradigm, the effect of the kappa agonist was very slight and the actions of yohimbine rather variable. 4. In place conditioning experiments with SD (Sprague -- Dawley) male rats, yohimbine alone was inactive but it limited the preference induced by morphine and SNC80 and the aversive effect of U-50,488. Impaired novelty preference was also observed with the combination of yohimbine and U-50,488. 5. It is concluded that yohimbine tends to limit opioid antinociception and the addictive potential of mu and delta opioid agonists. More selective drugs could help to understand the mechanisms involved in these actions.

alpha-Adrenoceptor and opioid receptor modulation of clonidine-induced antinociception

1. The antinociceptive action of clonidine (Clon) and the interactions with alpha 1, alpha 2 adrenoceptor and opioid receptor antagonists was evaluated in mice by use of chemical algesiometric test (acetic acid writhing test). 2. Clon produced a dose-dependent antinociceptive action and the ED50 for intracerebroventricular (i.c.v.) was lower than for intraperitoneal (i.p.) administration (1 ng kg-1 vs 300 ng kg-1). The parallelism of the dose-response curves indicates activation of a common receptor subtype. 3. Systemic administration of prazosin and terazosin displayed antinociceptive activity. Pretreatment with prazosin produced a dual action: i.c.v. Clon effect did not change, and i.p. Clon effect was enhanced. Yohimbine i.c.v. or i.p. did not induce antinonciception, but antagonized Clon-induced activity. These results suggest that alpha 1- and alpha 2-adrenoceptors, either located at the pre- and/or post-synaptic level, are involved in the control of spinal antinociception. 4. Naloxone (NX) and naltrexone (NTX) induced antinociceptive effects at low doses (microgram kg-1 range) and a lower antinociceptive effect at higher doses (mg kg-1 range). Low doses of NX or NTX antagonized Clon antinociception, possibly in relation to a preferential mu opioid receptor antagonism. In contrast, high doses of NX or NTX increased the antinociceptive activity of Clon, which could be due to an enhanced inhibition of the release of substance P. 5. The results obtained in the present work suggest the involvement of alpha 1-, alpha 2-adrenoceptor and opioid receptors in the modulation of the antinociceptive activity of clonidine, which seems to be exerted either at spinal and/or supraspinal level.

Chronic escapable footshock causes a reduced response to morphine in rats as assessed by local cerebral metabolic rates

The 2-deoxy-D-[14C]glucose (2-DG) method was used to examine the effects of morphine sulfate (MS) on local cerebral metabolic rates for glucose (LCMRglu) in male F-344 rats required to turn a wheel manipulandum in order to escape from nociceptive footshock. This nociceptive stimulus was identical with that utilized in a previous 2-DG study from this laboratory [15] except that animals were exposed to 15 daily 30 min sessions of footshock prior to the 2-DG testing day rather than a single footshock exposure. This allows a direct comparison of the effects of morphine in chronic and acute pain. Unlike the acute footshock study, morphine in chronic footshock rats did not have a significant effect compared with chronic footshock alone in any of the 73 measured brain structures, including limbic and midline thalamic structures previously shown to be important in morphine-induced analgesia during acute pain [15]. Whereas 93% of measured cerebral structures showed decreases in LCMRglu following morphine administration in the acute footshock rats, morphine given to chronic footshock rats caused decreases in only 56% of the structures as compared with chronic footshock plus saline. It is hypothesized that these differential effects of morphine are due in part to a habituation to the chronic stressor such that chronic footshock rats are less stressed than acute footshock rats. Additionally, it is suggested that chronic exposure to pain produces a constant elevation of opioid peptides leading to opioid receptor downregulation and consequently morphine tolerance. These results demonstrate that, even in the presence of the same nociceptive stimulus, morphine can have widely disparate effects on brain metabolism if there are differences in the pain history of the animal.

Histaminergic mechanisms in clonidine induced analgesia in rat tail-flick test

The role of neural histamine in clonidine-analgesia and in clonidine-induced potentiation of stress analgesia was studied. Pretreatment of rats with alpha-fluoromethylhistidine (FMH) (200 ug icv/rat; daily for five days) increased the analgesic effect of the alpha 2-agonist clonidine on the spinal reflex of the tail-flick test. Rats subjected to cold-restraint stress (30 min at 4 degrees C) showed increased latency compared to the unstressed rats. The analgesic efficacy of clonidine was significantly greater in rats subjected to cold-restraint with respect to unstressed rats. However, the inhibition of histamine biosynthesis by FMH significantly reduced cold-restraint analgesia in saline-controls, and consistently increased the analgesic efficacy of the alpha 2-agonist, showing a maximum latency. Yohimbine exhibited high affinity as an antagonist for alpha 2-receptors, inducing hyperalgesic effects and antagonizing clonidine analgesia and clonidine-induced potentiation of cold stress analgesia. In FMH-pretreated rats, yohimbine failed to reverse clonidine analgesia and did not block the increased analgesic efficacy of clonidine in cold-restrained FMH-pretreated rats. Results of this study suggest that inhibition of histamine release through alpha 2-adrenoceptors on histaminergic axons may contribute to the analgesic efficacy of systemically injected clonidine, also confirming that neural histaminergic pathways are implicated in the mediation of pain response in particular conditions of stress.

The local cerebral metabolic effects of morphine in rats exposed to escapable footshock

The 2-deoxy-D-[1-14C]glucose (2-DG) method was used to examine the effects of morphine sulfate (MS) on local cerebral metabolic rates for glucose (LCMRglu) in male F-344 rats required to turn a wheel manipulandum in order to escape from nociceptive footshock. Four groups of rats were studied: control-saline, control-MS, footshock-saline and footshock-MS. All animals were administered MS (4 mg/kg, s.c.) or saline 7 days, 3 days and 10 min prior to the start of the 2-DG experiment. In agreement with its well-known effect on the emotional component of pain, MS administered to rats exposed to footshock caused a significant decrease in LCMRglu compared to footshock-saline rats in limbic structures such as the diagonal band of Broca, lateral septum, bed nucleus of the stria terminalis, horizontal limb of the diagonal band, habenular complex and medial amygdala. Additionally, two components of the midline thalamus with extensive connections with the limbic system, the paraventricular and paratenial thalamic nuclei, were similarly affected by morphine. Footshock caused an overall increase in cerebral metabolism as 52 of 73 measured structures demonstrated increases in activity compared to saline control; however, statistically significant effects in specific structures were limited. These results identify limbic and midline thalamic structures important in morphine-induced analgesia and indicate that footshock tends to have a generalized stimulatory effect on LCMRglu.