Przemyslaw Marek

Excitatory amino acid antagonists (kynurenic acid and MK-801) attenuate the development of morphine tolerance in the rat

To investigate the possible role of excitatory amino acids (EAAs) in the mechanisms of morphine tolerance, rats were treated either with the wide-spectrum EAA antagonist, kynurenic acid (150 mg/kg), or the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist. MK-801 (0.05 mg/kg), during a four-day induction period of morphine tolerance. Morphine was given once daily at a dose of 15 mg kg. On the fifth day rats were injected only with morphine (15 mg/kg), and analgesia was assessed using the hot-plate test. Morphine tolerance was significantly reduced by both EAA antagonists. Control experiments showed that at the same doses neither acute nor chronic administration of these antagonists affected morphine analgesia itself in a manner that can explain these findings. The possible involvement of EAAs in the mechanisms of morphine tolerance is discussed.

Sex differences in the antagonism of swim stress-induced analgesia: effects of gonadectomy and estrogen replacement

&NA; Sex differences in the neurochemical mediation of swim stress‐induced analgesia (SSIA) were examined in Swiss‐Webster mice. Intact and gonadectomized adult mice of both sexes were tested for their analgesic response (hot‐plate test) to 3 min of forced swimming in 15°C and 20°C water. SSIA resulting from 15°C swim was previously shown to be naloxone‐insensitive (i.e., non‐opioid) whereas SSIA resulting from 20°C swim produced an analgesia that was partially reversible by naloxone (i.e., mixed opioid/non‐opioid). The non‐opioid components of these SSIA paradigms were attenuated by the N‐methyl‐d‐aspartate (NMDA) receptor antagonist, dizocilpine (MK‐801). We now report that in males, but not females, dizocilpine (0.075 mg/kg i.p.) and naloxone (10 mg/kg i.p.) antagonized the non‐opioid and opioid components of SSIA, respectively. After ovariectomy, females displayed a pattern of antagonism similar to males such that dizocilpine attenuated non‐opioid SSIA, although naloxone remained ineffective in antagonizing 20°C SSIA. Thus, SSIA in intact females was neither opioid‐ nor NMDA‐mediated, yet it was of similar magnitude to the SSIA displayed by intact males. In separate experiments, estrogen replacement (estrogen benzoate; 5.0 &mgr;g/day, i.p.) administered to ovariectomized mice over a 6–8 day period reinstated the dizocilpine‐insensitivity of 15°C SSIA characteristic of intact females. However, a similar estrogen regimen administered to both intact and castrated males did not compromise the sensitivity to dizocilpine previously noted in male mice. These findings suggest the existence of a novel female‐specific estrogen‐dependent mechanism of SSIA and highlight the need to consider gender as a factor in basic and clinical research in this area.

The NMDA receptor antagonist MK-801 prevents long-lasting non-associative morphine tolerance in the rat

Several studies have demonstrated that the N-methyl-D-aspartate (NMDA) antagonist MK-801 attenuates the development of morphine tolerance and withdrawal. These studies employed repeated morphine injections to induce tolerance, a procedure in which learning has been suggested to play a significant role in tolerance development. MK-801 has been reported to block some types of learning, and it is unclear, therefore, whether the effect of MK-801 on tolerance development is due to its antagonism of associative (learning) or non-associative factors. Moreover, previous studies have tested the effects of MK-801 on morphine tolerance only up to 48 h after its induction; yet morphine tolerance can persist for many months, and it is not known whether MK-801 can block long-lasting tolerance. In the present study, therefore, we adopted a model of morphine tolerance in which the involvement of learning is minimized by using a single injection of morphine in a sustained-release preparation, and we tested tolerance for up to 56 days. In the first experiment, simultaneously administering MK-801 (0.2 mg/kg, s.c.) and morphine (60 mg/kg, s.c.), each in a sustained-release preparation, abolished tolerance that lasted at least 12 days. Analgesia was measured in the hot-plate test following a test dose of morphine (15 mg/kg, i.p.). In the second experiment, delivering MK-801 and morphine as before, the duration of morphine-induced catalepsy and analgesia was prolonged. Nevertheless, 24 h later one symptom of naloxone-precipitated withdrawal was significantly attenuated in these same animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Delayed application of MK-801 attenuates development of morphine tolerance in rats

To investigate the possible involvement of enduring or delayed changes at the N-methyl-D-aspartic acid (NMDA) receptor in the mechanisms of morphine tolerance, rats were treated with the specific NMDA receptor antagonist, MK-801 (0.15 mg/kg) 2 h after morphine injection (20 mg/kg) during a 4-day induction period of tolerance. On the fifth day rats were injected only with morphine (15 mg/kg), and analgesia was assessed using the hot-plate test. Morphine tolerance was significantly reduced by MK-801. These findings suggest that long-lasting or delayed changes at the NMDA receptor underlie the development of morphine tolerance. Moreover, because MK-801 was delivered 2 h after morphine and therefore could not serve as a cue for morphine administration, these findings indicate that the attenuating effect of MK-801 on the development of morphine tolerance is not attributable to state-dependent learning.

NMDA receptor antagonists, MK-801 and ACEA-1011, prevent the development of tonic pain following subcutaneous formalin ☆

Subcutaneous injection of formalin produces a biphasic pain response: an early, transient phase followed by a late tonic phase. The present study examined the involvement of the N-methyl-D-aspartic acid (NMDA) receptor in the development of the late pain produced following subcutaneous injection of formalin into the hind paw in mice. Blockade of the NMDA receptor by its non-competitive antagonist, MK-801, prior to formalin injection, but not after, reduced pain during the late phase. Similarly, blockade of the NMDA receptor allosteric site by the novel glycine site antagonist, ACEA-1011, also reduced the pain response in the late phase. These results suggest that the development of the late phase of formalin pain is due to NMDA-mediated activity during the early phase.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress induced analgesia. II. Comparison across three swim stress paradigms in selectively bred mice.

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (dizocilpine, 0.075 mg/kg, i.p.) on swim-stress-induced analgesia (SSIA) were studied in control (C) mice and in mice selectively bred for high (HA) or low (LA) SSIA. In three consecutive experiments, animals were subjected to forced swimming at water temperature of 20 degrees C, 32 degrees C and 15 degrees C and the resulting analgesia (hot-plate test) was found to be mixed opioid/non-opioid, opioid and non-opioid, respectively, as a function of the degree of antagonism by naloxone (10 mg/kg, i.p.). The major finding of this study is that MK-801 attenuated 15 degrees C SSIA, against which naloxone was ineffective, but had no effect on 32 degrees C SSIA, which naloxone blocked completely. A combination of naloxone and MK-801 significantly attenuated 20 degrees C SSIA in C and HA mice and in HA mice this attenuation was significantly larger than that produced by either drug alone. Morphine analgesia (10 mg/kg, i.p.) was unaffected by MK-801. It is concluded that low doses of MK-801 selectively block non-opioid mechanisms of SSIA.

The role of strain/vendor differences on the outcome of focal ischemia induced by intraluminal middle cerebral artery occlusion in the rat

This study investigated the role of strain and vendor differences on the outcome of focal cerebral ischemia in the rat induced by occlusion of the middle cerebral artery by an intraluminal thread. The cortical infarct volumes (Mean +/- S.E.M.) were: 14.2 +/- 6.2 mm3 in Simonsen Laboratories Sprague-Dawley rats; 84.0 +/- 22.9 mm3 in Simonsen Laboratories Wistar rats; 223.3 +/- 23.6 mm3 in Taconic Laboratories Sprague-Dawley rats; and 239.5 +/- 30.7 mm3 in Charles River Laboratories Sprague-Dawley rats. Middle cerebral artery occlusion combined with bilateral common carotid artery occlusion for 60 min increased cortical infarct volumes to: 113.0 +/- 18.8 mm3; 152.4 +/- 21.1 mm3; 227.8 +/- 19.3 mm3; and 248.4 +/- 24.0 mm3, respectively. To control the effect of blood pressure as a variable contributing to the outcome of ischemia, additional experiments where performed in which the blood pressure in Simonsen Laboratories Sprague-Dawley rats was lowered to the level of the blood pressure in Taconic Laboratories Sprague-Dawley rats. Although this manipulation increased the cortical infarct volumes in Simonsen Laboratories Sprague-Dawley rats, the volumes were still less than those in Taconic Laboratories Sprague-Dawley rats. The results of the present study indicate that the outcome of ischemia in the intraluminal thread model may dramatically differ depending on the strain and vendor of animal used.

Morphine fails to produce tolerance when administered in the presence of formalin pain in rats

The present study examined the development of tolerance to morphine analgesia under conditions in which morphine was administered in the presence or absence of pain induced by subcutaneous injection of 50 microliters of 2.5% formalin into the hind paw of rats. Animals were injected with morphine (25 mg/kg, i.p.) or saline for 3 consecutive days either in the presence of pain (10 min after formalin injection) or in the absence of pain (6 h prior to formalin injection). On the 4th day, tolerance to the analgesic effect of test doses of morphine (6 or 10 mg/kg) was assessed in the formalin and tail-flick tests, respectively. Significant tolerance in both tests was observed in animals receiving morphine in the absence of pain during the tolerance induction period, but not in animals receiving morphine in the presence of pain.

Stimulation of the hypothalamic paraventricular nucleus produces analgesia not mediated by vasopressin or endogenous opioids

The analgesic effect of electrical stimulation of the hypothalamic paraventricular nucleus (PVN) was studied. Additionally, the involvement of vasopressin and opioid peptides in this process was examined by comparing vasopressin-deficient (Brattleboro) and Long-Evans rats and by administering the opiate antagonist naloxone. Rats were chronically implanted with a stimulating electrode in the parvocellular (PVN-Pc) and magnocellular (PVN-Mg) divisions of the PVN. At least 10 days after surgery, the analgesic effects of PVN stimulation were examined in lightly anesthetized rats, using the tail-flick method, and in unanesthetized rats, using the hot-plate test. PVN stimulation produced marked analgesia in both tests. Current threshold for analgesia was lower from PVN-Pc than from PVN-Mg. Threshold did not differ significantly between Brattleboro and Long-Evans rats and was not affected by naloxone administration. The results indicate that the PVN is part of the brains pain inhibitory system, and show that the analgesia induced by PVN stimulation is not mediated by either vasopressin or opioid peptides.

NMDA receptor antagonist MK-801 blocks non-opioid stress-induced analgesia in the formalin test

&NA; The analgesic effect of a 3‐min swim stress was assessed using the formalin test. Male Swiss mice were injected i.p. with naloxone (0.1 or 1.0 mg/kg), MK‐801 (0.075 mg/kg) or saline 15 min prior to swimming in water maintained at 20°C or 32°C. The mice were then injected with 20 &mgr;1 of 5% formalin into the plantar surface of 1 hind paw and pain behaviour (time spent licking the injected paw) was continuously monitored during the subsequent 10 min. Swim stress produced a significant reduction in pain behaviour at both 20°C and 32°C. MK‐801 completely blocked the analgesia produced by both the 20°C and 32°C swim. At a dose of 0.1 mg/kg, naloxone partially antagonized the analgesia produced by the 32°C swim but did not affect the analgesia produced by the 20°C swim. Naloxone at a dose of 1.0 mg/kg had no effect on swim stress‐induced analgesia. Neither MK‐801 nor 0.1 mg/kg naloxone altered baseline pain behaviour, although 1.0 mg/kg naloxone did significantly reduce it. It is unlikely that the effect of MK‐801 on swim stress‐induced analgesia is due to an interaction with an opioid mechanism, as MK‐801 had no effect on morphine analgesia. These results suggest that the analgesia produced by the 20°C swim stress in the formalin test is non‐opioid in nature and mediated via the NMDA receptor, whereas the 32°C swim stress‐induced analgesia has both an opioid and non‐opioid component.