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The role of corticosterone in the blockade of tolerance to morphine analgesia by formalin-induced pain in the rat
Neuroscience Letters 232 (1997) 139–142
The role of corticosterone in the blockade of tolerance to morphine analgesia by formalin-induced pain in the rat Anthony L. Vaccarino*, William L. Nores, R. Denis Soignier, Richard D. Olson Department of Psychology, University of New Orleans, New Orleans, LA 70148, USA Received 8 July 1997; accepted 1 August 1997
Abstract We previously reported that morphine fails to produce analgesic tolerance when administered in the presence of formalin-induced pain, which may be related to activity of the hypothalamic-pituitary-adrenal axis. In the present study, we examined whether suppression of corticosterone secretion during pain prevents the blockade of tolerance to morphine analgesia. Male Long-Evans rats were injected with morphine (20 mg/kg) or saline for 4 consecutive days in the presence or absence of formalin-induced pain. To suppress corticosterone activity, some animals were injected daily with the corticosterone synthesis inhibitor, metyrapone (100 mg/kg), 24 h and 30 min before formalin injections. The analgesic effect of a test dose of morphine (10 mg/kg) was then measured in the tail-flick test 24 h after tolerance induction (i.e. day 5). The presence of pain during tolerance induction prevented the development of analgesic tolerance. Furthermore, inhibition of corticosterone synthesis by metyrapone prevented the blockade of tolerance by pain. These results suggest that the blockade of tolerance to morphine analgesia by formalin-induced pain depends on stress-induced corticosterone increases. 1997 Elsevier Science Ireland Ltd. Keywords: Metyrapone; Glucocorticoids; Tail-flick; Stress; Hypothalamic-pituitary-adrenal axis
Tolerance to morphine analgesia has been examined experimentally using various animal models of pain. In models of phasic, threshold-level pain, such as the tailflick or hot-plate tests, the development of tolerance to morphine analgesia has been well established [12]. The development of tolerance in models of persistent pain, however, is less clear. While some studies have reported that opiates produce tolerance, others have reported that opiates fail to produce tolerance [1]. The reasons for these discrepancies are unclear, and may be related to a number of methodological differences, including the type of pain test used, and the dose and route of morphine administration [1]. An additional key factor to consider is the presence or absence of pain during morphine delivery. For example, tolerance to morphine analgesia does not develop when morphine is administered in the presence of pain induced by subcutaneous formalin [14,15,20,21], but does when morphine is administered in the absence of formalin* Corresponding author. Tel.: +1 504 2806771; fax: +1 504 2806049; e-mail: [email protected]
induced pain [3,20,21]. Similarly, tolerance to analgesia produced by the narcotic, fentanyl, is blocked if rats are exposed to a prolonged noxious mechanical stimulus applied during tolerance induction [2]. Many previous studies that have shown tolerance to morphine analgesia have administered morphine when the animal was in a pain-free state [3,12]. The hypothalamic-pituitary-adrenal (HPA) axis is known to respond to stressful stimuli [18], including formalin injection [13]. There is evidence that the HPA axis is involved in the development of tolerance to morphine analgesia. For example, adrenocorticotropin hormone (ACTH) has been found to prevent the development of tolerance to morphine analgesia [7], and stress blocks the development of morphine tolerance in intact mice, but not adrenalectomized mice [19]. Both adrenalectomy [22] and hypophysectomy [8] have been shown to potentiate the magnitude of opiate tolerance, with the effects of hypophysectomy being reversed by replacement of ACTH [8]. Taken together, the above results raise the possibility that blockade of tolerance development by formalin-induced pain is dependent
0304-3940/97/$17.00 1997 Elsevier Science Ireland Ltd. All rights reserved PII S0304- 3940 (97 )0 0616- 2
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upon HPA activity. In support of this notion, we recently showed that genetically determined differences in stressinduced HPA activity may contribute to a differential development of tolerance to morphine analgesia during pain [20]. The Lewis strain of rat has been identified to lack typical stress-induced elevations in corticotropin releasing factor (CRF), ACTH and corticosterone, as compared with Fischer rats [18]. Consistent with the hypothesis that the blockade of tolerance by pain is related to HPA activity, we found that formalin-induced pain prevented tolerance development in the Fischer rats, but not in the Lewis rats [20]. The present study was designed to examine further the role of the HPA axis in the blockade of tolerance by formalin-induced pain by examining the effects of the corticosterone synthesis inhibitor, metyrapone, on the development of tolerance. Male Long-Evans rats weighing 300–400 g at the time of testing served as subjects. Rats were housed individually with free access to food and water, and were maintained on a 12-h light cycle (light onset at 0700 h). Experiments were carried out between 1100 h and 1300 h. Morphine sulphate (Malincrodt, USA) was dissolved in physiological saline and administered i.p. in a volume of 0.1 ml/100 g. Metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone, Sigma, USA) was dissolved in a 1:6 ratio of DMSO and saline and administered in a volume of 0.1 ml/50 g. Equal volumes of the appropriate vehicle were used for control injections. To examine the effects of morphine administration in the presence or absence of pain, rats received 20 mg/kg morphine or saline 10 min after a s.c. injection of 50 ml of 2.5% formalin or saline into one hind paw [20,21]. Formalin injections were given into a different paw site on each of the 4 days to minimize tissue damage (day 1: right/dorsal; day 2: left/ventral; day 3: right/ventral; day 4: left/dorsal). To examine the effects blocking corticosterone synthesis, half the animals from each group were pretreated with 100 mg/kg metyrapone 24 h before the first formalin injection and daily 30 min before formalin injection. There was a total of 8 groups (2 × 2 × 2 factorial design: metyrapone/ vehicle × pain/no pain × morphine/saline) with n = 6 per group (except the vehicle/pain/morphine group in which one animal died of unknown causes, n = 5). The rats were randomly assigned to one of the eight conditions, which were counterbalanced using a Latin Square design. Twenty-four hours after tolerance induction (day 5), the rats were taken from the vivarium to the testing room. Following a 1-h habituation period, the rats were tested for analgesia in the tail-flick test. With the animal gently wrapped in a cloth, the distal portion of the tail was immersed in water maintained at 52°C. The latency to flick its tail was recorded, and a 10-s cut-off period was imposed to avoid tissue damage. Rats were tested for analgesia before (baseline) and 30 min after injection of a test dose of morphine (10 mg/kg). The data were expressed as latency to tail-flick (s), or as percentage of maximum possible effect (%MPE =
(test latency − baseline latency) × 100/(cut-off value − baseline latency); where 0% indicates no change from baseline and 100% indicates maximum possible change). Data were analyzed using SPSS for Windows 7.5 (1997) statistical program. No violations of normality or homogeneity of variance were found. Analysis of baseline tail-flick latencies revealed no significant differences (mean baseline tail-flick latencies ± SEM = 2.70 ± 0.09 s). A 2 × 2 × 2 (metyrapone/ vehicle × pain/no pain × morphine/saline) between groups factorial ANOVA was performed on the data (%MPE), which revealed a significant three-way interaction (F1,39 = 7.57, P , 0.05, with power = 0.77 and partial h2 = 0.16, Fig. 1). Analysis of simple effects revealed that the nature of this three-way interaction was due to a significant metyrapone/vehicle × morphine/saline interaction in the presence of pain (F1,19 = 27.18, P , 0.05, with power = 0.99 and partial h2 = 0.59), but not in the absence of pain (F1,20 = 0.66, n.s.). Further analysis revealed that tolerance developed to morphine analgesia when administered in the absence of pain (vehicle/no pain/saline vs. vehicle/no pain/morphine, F1,10 = 32.97, P , 0.05, with power = 0.99 and partial h2 = 0.77, see Fig. 1), but not when morphine was administered in the presence of pain (vehicle/pain/morphine vs. vehicle/no pain/morphine, F1,9 = 25.37, P , 0.05, with power = 0.99 and partial h2 = 0.74). Moreover, metyrapone prevented the blockade of tolerance to morphine analgesia by pain (metyrapone/ pain/morphine vs. vehicle/pain/morphine, F1,9 = 42.50, P , 0.05, with power = 0.99 and partial h2 = 0.83). No other significant results were obtained. In the present study, it was found that morphine failed to produce analgesic tolerance when administered in the presence of formalin-induced pain. This is consistent with previous studies [14,15,20,21]. Administration of metyrapone, which prevents the synthesis of corticosterone, was found to prevent the blockade of tolerance by formalin-induced pain. Metyrapone blocks elevations of corticosterone during stressful events, without affecting basal levels of corticosterone [4]. These results suggest that the blockade of tolerance by formalin-induced pain depends on stress-induced increases in corticosterone activity. The mechanism of action in which corticosterone modulates tolerance to morphine analgesia is not known. Corticosterone binds to both glucocorticoid (GC) and mineralocorticoid (MC) receptors in the brain [17]. Since corticosterone has a lower affinity for GC than MC receptors, stress-induced levels of corticosterone are required to activate both MC and GC receptors, whereas basal levels activate predominately MC receptors [16]. Therefore, because metyrapone blocks stress-induced corticosterone levels, without affecting basal levels [4], it is likely that the effects of metyrapone in the present study are due to reduced binding at GC receptors [16]. Alternately, corticosterone also influences the activity of several neurotransmitter systems, including serotonin, dopamine, GABA,
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Fig. 1. Percentage MPE (± SEM) in the tail-flick test following morphine (10 mg/kg, i.p.). Rats received saline or morphine (20 mg/kg, i.p.) (x-axis) for 4 consecutive days in the absence of pain (NO PAIN-left panel) or presence of pain (PAIN-right panel). To examine the role of corticosterone, animals were treated with metyrapone (grey bars) or vehicle (open bars) prior to formalin injection. See text for significance levels.
opioids, and excitatory amino acids [17], which may influence tolerance to morphine analgesia. Indeed, Rahman et al. [15] reported that the blockade of tolerance by formalininduced pain was prevented by the GABA receptor agonist, muscimol. Corticosterone is also necessary for stressinduced activation of tryptophan hydroxylase, which is responsible for the metabolism of tryptophan [17]. Interestingly, kynurenic acid is an endogenous metabolite of tryptophan that acts as a post-synaptic NMDA antagonist [5]. Administration of kynurenic acid (and other NMDA antagonists) during morphine exposure prevents the development of analgesic tolerance [10]. Therefore, the antagonistic effect of pain on morphine tolerance may also be mediated by indirect influences of corticosterone at one of these systems. Glucocorticoids and stress have been shown to interfere with memory and cognition [11]. It is possible, therefore, that the antagonistic effect of pain on morphine tolerance is related to a pain/stress-mediated disruption of associative mechanisms during morphine administration. The development of tolerance to morphine analgesia has been shown to involve both associative and non-associative mechanisms [9]. Associative or conditioned tolerance involves learning an association between the environment/drug delivery (i.e. handling, injection procedure, etc.) and the effects of a drug. This is suggested to result in the development of an antagonistic response to the drug’s effect, thus reducing its effectiveness [9]. The repeated i.p. injections of morphine used in this study would be expected to favor the development of associative tolerance. Similarly, Rahman et al. [14] found
that pain induced by Freund’s Adjuvant or formalin injection attenuated the development of tolerance to morphine analgesia induced by repeated i.p. morphine injections. On the other hand, Gutstein et al. [6] found that Freund’s Adjuvant did not block the development of tolerance to morphine analgesia induced by pellet implant, which minimizes the role of associative cues in the development of tolerance, and thus involves non-associative mechanisms. Therefore, it is possible that the effects of pain on tolerance development may also depend on associative cues present during morphine administration. We are currently examining the effects of formalin-induced pain on the development of non-associative morphine tolerance. In summary, animal studies have shown that significant analgesic tolerance develops following exposure to morphine in the absence of pain [3,12]. There is emerging evidence, however, that pain [2,14,21], or the stress associated with pain [15,20], may also activate a parallel system that prevents the development of analgesic tolerance. The findings of the present study indicate that the blockade of tolerance by formalin-induced pain depends on corticosterone activity, such that increases in corticosterone triggered by the ‘stressfulness’ of pain act to attenuate the development of tolerance. [1] Cleary, J. and Backonja, M., Translating opioid tolerance research, APS Bull., March (1996) 4–7. [2] Colpaert, F.C., Niemegeers, C.R.E., Janssen, P.A.J. and Maroli, A.N., The effects of prior fentanyl administration and of pain on fentanyl administration: tolerance to and enhancement of narcotic analgesia, J. Pharmacol. Exp. Ther., 213 (1980) 418–426.
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[3] Detweiller, D.J., Rhode, D.S. and Basbaum, A.I., The development of opioid tolerance in the formalin test in the rat, Pain, 63 (1995) 251–254. [4] Freo, U., Holloway, H.W., Kalogeras, K., Rapoport, S.I. and Soncrant, T.T., Adrenalectomy or metyrapone-pretreatment abolishes cerebral metabolic responses to serotonin agonist 1-(2,5,dimethoxy-4-iodophenyl)-2-aminopropane (DOI) in the hippocampus, Brain Res., 586 (1992) 256–264. [5] Ganog, A.H., Lanthorn, T.H. and Cotman, C.W., Kynurenic acid inhibits synaptic and acidic amino acid-induced responses in the rat hippocampus and spinal cord, Brain Res., 273 (1983) 170–173. [6] Gutstein, H.B., Trujillo, K.A. and Akil, H., Does chronic nociceptive stimulation alter the development of morphine tolerance?, Brain Res., 680 (1995) 173–179. [7] Hendrie, C.A., ACTH: a single pretreatment enhances the analgesic efficiency of and prevents the development of tolerance to morphine, Physiol. Behav., 42 (1988) 41–45. [8] Holaday, J.W., Dallman, M.F. and Loh, H.H., Effects of ACTH and hypophesectomy on opiate tolerance and dependence, Life Sci., 24 (1979) 771–782. [9] Macrae, J.R., Scole, M.T. and Siegal, S., The contribution of Pavlovian conditioning in drug tolerance and dependence, Br. J. Addict., 82 (1987) 371–380. [10] Marek, P., Ben-Eliyahu, S., Gold, M. and Liebeskind, J.C., Excitatory amino acid antagonists (kynurenic acid and MK-801) attenuate the development of morphine tolerance in the rat, Brain Res., 547 (1991) 77–81. [11] McEwen, B.S. and Sapowlsky, R.M., Stress and cognitive function, Curr. Opin. Neurobiol., 5 (1995) 205–216. [12] Mucha, R.F., Kalant, H. and Linseman, J.A., Quantitative relationships among measures of morphine tolerance and physical dependence, Pharm. Biochem. Behav., 10 (1979) 397–405. [13] Pacak, K., Palkovits, M., Kvetnansky, R., Yadid, G., Kopin, I.J. and Goldstein, D.S., Effects of various stressors on in vivo norepinephrine release in the hypothalamic paraventricular nucleus and on the
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The role of corticosterone in the blockade of tolerance to morphine analgesia by formalin-induced pain in the rat Anthony L. Vaccarino*, William L. Nores, R. Denis Soignier, Richard D. Olson Department of Psychology, University of New Orleans, New Orleans, LA 70148, USA Received 8 July 1997; accepted 1 August 1997
Abstract We previously reported that morphine fails to produce analgesic tolerance when administered in the presence of formalin-induced pain, which may be related to activity of the hypothalamic-pituitary-adrenal axis. In the present study, we examined whether suppression of corticosterone secretion during pain prevents the blockade of tolerance to morphine analgesia. Male Long-Evans rats were injected with morphine (20 mg/kg) or saline for 4 consecutive days in the presence or absence of formalin-induced pain. To suppress corticosterone activity, some animals were injected daily with the corticosterone synthesis inhibitor, metyrapone (100 mg/kg), 24 h and 30 min before formalin injections. The analgesic effect of a test dose of morphine (10 mg/kg) was then measured in the tail-flick test 24 h after tolerance induction (i.e. day 5). The presence of pain during tolerance induction prevented the development of analgesic tolerance. Furthermore, inhibition of corticosterone synthesis by metyrapone prevented the blockade of tolerance by pain. These results suggest that the blockade of tolerance to morphine analgesia by formalin-induced pain depends on stress-induced corticosterone increases. 1997 Elsevier Science Ireland Ltd. Keywords: Metyrapone; Glucocorticoids; Tail-flick; Stress; Hypothalamic-pituitary-adrenal axis
Tolerance to morphine analgesia has been examined experimentally using various animal models of pain. In models of phasic, threshold-level pain, such as the tailflick or hot-plate tests, the development of tolerance to morphine analgesia has been well established [12]. The development of tolerance in models of persistent pain, however, is less clear. While some studies have reported that opiates produce tolerance, others have reported that opiates fail to produce tolerance [1]. The reasons for these discrepancies are unclear, and may be related to a number of methodological differences, including the type of pain test used, and the dose and route of morphine administration [1]. An additional key factor to consider is the presence or absence of pain during morphine delivery. For example, tolerance to morphine analgesia does not develop when morphine is administered in the presence of pain induced by subcutaneous formalin [14,15,20,21], but does when morphine is administered in the absence of formalin* Corresponding author. Tel.: +1 504 2806771; fax: +1 504 2806049; e-mail: [email protected]
induced pain [3,20,21]. Similarly, tolerance to analgesia produced by the narcotic, fentanyl, is blocked if rats are exposed to a prolonged noxious mechanical stimulus applied during tolerance induction [2]. Many previous studies that have shown tolerance to morphine analgesia have administered morphine when the animal was in a pain-free state [3,12]. The hypothalamic-pituitary-adrenal (HPA) axis is known to respond to stressful stimuli [18], including formalin injection [13]. There is evidence that the HPA axis is involved in the development of tolerance to morphine analgesia. For example, adrenocorticotropin hormone (ACTH) has been found to prevent the development of tolerance to morphine analgesia [7], and stress blocks the development of morphine tolerance in intact mice, but not adrenalectomized mice [19]. Both adrenalectomy [22] and hypophysectomy [8] have been shown to potentiate the magnitude of opiate tolerance, with the effects of hypophysectomy being reversed by replacement of ACTH [8]. Taken together, the above results raise the possibility that blockade of tolerance development by formalin-induced pain is dependent
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upon HPA activity. In support of this notion, we recently showed that genetically determined differences in stressinduced HPA activity may contribute to a differential development of tolerance to morphine analgesia during pain [20]. The Lewis strain of rat has been identified to lack typical stress-induced elevations in corticotropin releasing factor (CRF), ACTH and corticosterone, as compared with Fischer rats [18]. Consistent with the hypothesis that the blockade of tolerance by pain is related to HPA activity, we found that formalin-induced pain prevented tolerance development in the Fischer rats, but not in the Lewis rats [20]. The present study was designed to examine further the role of the HPA axis in the blockade of tolerance by formalin-induced pain by examining the effects of the corticosterone synthesis inhibitor, metyrapone, on the development of tolerance. Male Long-Evans rats weighing 300–400 g at the time of testing served as subjects. Rats were housed individually with free access to food and water, and were maintained on a 12-h light cycle (light onset at 0700 h). Experiments were carried out between 1100 h and 1300 h. Morphine sulphate (Malincrodt, USA) was dissolved in physiological saline and administered i.p. in a volume of 0.1 ml/100 g. Metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone, Sigma, USA) was dissolved in a 1:6 ratio of DMSO and saline and administered in a volume of 0.1 ml/50 g. Equal volumes of the appropriate vehicle were used for control injections. To examine the effects of morphine administration in the presence or absence of pain, rats received 20 mg/kg morphine or saline 10 min after a s.c. injection of 50 ml of 2.5% formalin or saline into one hind paw [20,21]. Formalin injections were given into a different paw site on each of the 4 days to minimize tissue damage (day 1: right/dorsal; day 2: left/ventral; day 3: right/ventral; day 4: left/dorsal). To examine the effects blocking corticosterone synthesis, half the animals from each group were pretreated with 100 mg/kg metyrapone 24 h before the first formalin injection and daily 30 min before formalin injection. There was a total of 8 groups (2 × 2 × 2 factorial design: metyrapone/ vehicle × pain/no pain × morphine/saline) with n = 6 per group (except the vehicle/pain/morphine group in which one animal died of unknown causes, n = 5). The rats were randomly assigned to one of the eight conditions, which were counterbalanced using a Latin Square design. Twenty-four hours after tolerance induction (day 5), the rats were taken from the vivarium to the testing room. Following a 1-h habituation period, the rats were tested for analgesia in the tail-flick test. With the animal gently wrapped in a cloth, the distal portion of the tail was immersed in water maintained at 52°C. The latency to flick its tail was recorded, and a 10-s cut-off period was imposed to avoid tissue damage. Rats were tested for analgesia before (baseline) and 30 min after injection of a test dose of morphine (10 mg/kg). The data were expressed as latency to tail-flick (s), or as percentage of maximum possible effect (%MPE =
(test latency − baseline latency) × 100/(cut-off value − baseline latency); where 0% indicates no change from baseline and 100% indicates maximum possible change). Data were analyzed using SPSS for Windows 7.5 (1997) statistical program. No violations of normality or homogeneity of variance were found. Analysis of baseline tail-flick latencies revealed no significant differences (mean baseline tail-flick latencies ± SEM = 2.70 ± 0.09 s). A 2 × 2 × 2 (metyrapone/ vehicle × pain/no pain × morphine/saline) between groups factorial ANOVA was performed on the data (%MPE), which revealed a significant three-way interaction (F1,39 = 7.57, P , 0.05, with power = 0.77 and partial h2 = 0.16, Fig. 1). Analysis of simple effects revealed that the nature of this three-way interaction was due to a significant metyrapone/vehicle × morphine/saline interaction in the presence of pain (F1,19 = 27.18, P , 0.05, with power = 0.99 and partial h2 = 0.59), but not in the absence of pain (F1,20 = 0.66, n.s.). Further analysis revealed that tolerance developed to morphine analgesia when administered in the absence of pain (vehicle/no pain/saline vs. vehicle/no pain/morphine, F1,10 = 32.97, P , 0.05, with power = 0.99 and partial h2 = 0.77, see Fig. 1), but not when morphine was administered in the presence of pain (vehicle/pain/morphine vs. vehicle/no pain/morphine, F1,9 = 25.37, P , 0.05, with power = 0.99 and partial h2 = 0.74). Moreover, metyrapone prevented the blockade of tolerance to morphine analgesia by pain (metyrapone/ pain/morphine vs. vehicle/pain/morphine, F1,9 = 42.50, P , 0.05, with power = 0.99 and partial h2 = 0.83). No other significant results were obtained. In the present study, it was found that morphine failed to produce analgesic tolerance when administered in the presence of formalin-induced pain. This is consistent with previous studies [14,15,20,21]. Administration of metyrapone, which prevents the synthesis of corticosterone, was found to prevent the blockade of tolerance by formalin-induced pain. Metyrapone blocks elevations of corticosterone during stressful events, without affecting basal levels of corticosterone [4]. These results suggest that the blockade of tolerance by formalin-induced pain depends on stress-induced increases in corticosterone activity. The mechanism of action in which corticosterone modulates tolerance to morphine analgesia is not known. Corticosterone binds to both glucocorticoid (GC) and mineralocorticoid (MC) receptors in the brain [17]. Since corticosterone has a lower affinity for GC than MC receptors, stress-induced levels of corticosterone are required to activate both MC and GC receptors, whereas basal levels activate predominately MC receptors [16]. Therefore, because metyrapone blocks stress-induced corticosterone levels, without affecting basal levels [4], it is likely that the effects of metyrapone in the present study are due to reduced binding at GC receptors [16]. Alternately, corticosterone also influences the activity of several neurotransmitter systems, including serotonin, dopamine, GABA,
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Fig. 1. Percentage MPE (± SEM) in the tail-flick test following morphine (10 mg/kg, i.p.). Rats received saline or morphine (20 mg/kg, i.p.) (x-axis) for 4 consecutive days in the absence of pain (NO PAIN-left panel) or presence of pain (PAIN-right panel). To examine the role of corticosterone, animals were treated with metyrapone (grey bars) or vehicle (open bars) prior to formalin injection. See text for significance levels.
opioids, and excitatory amino acids [17], which may influence tolerance to morphine analgesia. Indeed, Rahman et al. [15] reported that the blockade of tolerance by formalininduced pain was prevented by the GABA receptor agonist, muscimol. Corticosterone is also necessary for stressinduced activation of tryptophan hydroxylase, which is responsible for the metabolism of tryptophan [17]. Interestingly, kynurenic acid is an endogenous metabolite of tryptophan that acts as a post-synaptic NMDA antagonist [5]. Administration of kynurenic acid (and other NMDA antagonists) during morphine exposure prevents the development of analgesic tolerance [10]. Therefore, the antagonistic effect of pain on morphine tolerance may also be mediated by indirect influences of corticosterone at one of these systems. Glucocorticoids and stress have been shown to interfere with memory and cognition [11]. It is possible, therefore, that the antagonistic effect of pain on morphine tolerance is related to a pain/stress-mediated disruption of associative mechanisms during morphine administration. The development of tolerance to morphine analgesia has been shown to involve both associative and non-associative mechanisms [9]. Associative or conditioned tolerance involves learning an association between the environment/drug delivery (i.e. handling, injection procedure, etc.) and the effects of a drug. This is suggested to result in the development of an antagonistic response to the drug’s effect, thus reducing its effectiveness [9]. The repeated i.p. injections of morphine used in this study would be expected to favor the development of associative tolerance. Similarly, Rahman et al. [14] found
that pain induced by Freund’s Adjuvant or formalin injection attenuated the development of tolerance to morphine analgesia induced by repeated i.p. morphine injections. On the other hand, Gutstein et al. [6] found that Freund’s Adjuvant did not block the development of tolerance to morphine analgesia induced by pellet implant, which minimizes the role of associative cues in the development of tolerance, and thus involves non-associative mechanisms. Therefore, it is possible that the effects of pain on tolerance development may also depend on associative cues present during morphine administration. We are currently examining the effects of formalin-induced pain on the development of non-associative morphine tolerance. In summary, animal studies have shown that significant analgesic tolerance develops following exposure to morphine in the absence of pain [3,12]. There is emerging evidence, however, that pain [2,14,21], or the stress associated with pain [15,20], may also activate a parallel system that prevents the development of analgesic tolerance. The findings of the present study indicate that the blockade of tolerance by formalin-induced pain depends on corticosterone activity, such that increases in corticosterone triggered by the ‘stressfulness’ of pain act to attenuate the development of tolerance. [1] Cleary, J. and Backonja, M., Translating opioid tolerance research, APS Bull., March (1996) 4–7. [2] Colpaert, F.C., Niemegeers, C.R.E., Janssen, P.A.J. and Maroli, A.N., The effects of prior fentanyl administration and of pain on fentanyl administration: tolerance to and enhancement of narcotic analgesia, J. Pharmacol. Exp. Ther., 213 (1980) 418–426.
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