John C. Liebeskind

Analgesia from Electrical Stimulation in the Brainstem of the Rat

Stimulation at several mesencephalic and diencephalic sites abolished responsiveness to intense pain in rats while leaving responsiveness to other sensory modes relatively unaffected. The peripheral field of analgesia was usually restricted to one-half or to one quadrant of the body, and painful stimuli applied outside this field elicited a normal reaction. Analgesia outlasted stimulation by up to 5 minutes. Most electrode placements that produced analgesia also supported self-stimulation.One placement supported self-stimulation only in the presence of pain.

Monoaminergic mechanisms of stimulation-produced analgesia

The roles played by the cerebral monoamines (dopamine, noradrenaline and serotonin) in stimulation-produced analgesia (SPA) have been investigated in the rat employing the tail flick test. SPA was elicited through bipolar electrodes chronically implanted in the mesencephalic periaqeductal gray matter, an area previously shown to yield potent and reliable analgesic effects. Four approaches were used to alter transmission in monoamine pathways. (1) Depletion of monoamines by administration of tetrabenazine (TBZ), p-chlorophenylalanine (PCPA), alpha-methyl-para-tyrosine (AMPT), or disulfiram. (2) Replacement of depleted monoamine stores by appropiate precursors (5-HTP or L-DOPA) in combination with a peripheral decarboxylase inhibitor. (3) Potentiation of monoamine systems by administration of precursors to previously untreated animals or by administration of a dopamine receptor stimulator, apomorphine. (4) Blockade of catecholamine receptors by haloperidol or of dopamine receptors by pimozide. These four approaches yielded internally consistent results. Depletion of all 3 monoamines (TBZ) led to a powerful inhibition of SPA. Original levels of SPA were restored by injection of either 5-HTP or L-DOPA. Specific depletion of serotonin (PCPA) caused a reduction in SPA, whereas elevation of serotonin levels (5-HTP) caused an increase in SPA. Dopamine receptor blockade (pimozide) decreased SPA, whereas the precursor (L-DOPA) and a dopamine receptor stimulator (apomorphine) increased SPA. On the other hand, selective depletion of noradrenaline (disulfiram) caused an increase in SPA; and at a time when noradrenaline levels are depressed and dopamine levels are elevated (AMPT + L-DOPA), SPA was seen to be particularly enhanced. thus, dopamine and serotonin appear to facilitate SPA, whereas noradrenaline appears to inhibit it. When a general catecholamine receptor blocker (haloperidol) was employed, SPA was diminished, suggesting that the influence of dopamine in SPA is greater than that of noradrenaline. Most of the drugs used in this study significantly altered SPA at doses which left baseline tail flick latency unaffected. It would appear, therefore, that SPA has a neural substrate at least partly independent of that underlying baseline pain responsiveness. Consideration is given to various ascending and descending monoamine system as possible component paths in this neural substrate of SPA. Finally, the present results are discussed in relation to studies by others on the site and mechanism of morphines analgesic action. Some striking parallels between SPA and morphine analgesia are noted. These suggest the existence of a common pain-inhibitory system in the brain activated by morphine and by focal electrical stimulation.

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.

Stress increases metastatic spread of a mammary tumor in rats: Evidence for mediation by the immune system

Causal relationships among stress, immune suppression, and enhanced tumor development have often been suggested, but direct evidence is scant. We studied stress effects in Fischer 344 rats using a tumor model in which lung metastases of a syngeneic mammary tumor (MADB106) are controlled by natural killer (NK) cells. Animals exposed to acute stress showed a substantial decrease in NK cell cytotoxicity against this tumor in an in vitro assay and, when intravenously injected with this tumor, showed a twofold increase in surface lung metastases. The critical period during which stress increases metastases appears to be the same as that during which this tumor is known to be controlled by NK cells. These findings support the hypothesis that stress can facilitate the metastatic process via suppression of the immune system.

Epileptic properties of leucine- and methionine-enkephalin: Comparison with morphine and reversibility by naloxone

Morphologically similar epileptic seizures were recorded from the cortex of rats after injections into the lateral ventricle of 100 microgram of leucine-enkephalin, methionine-enkephalin, and morphine. Seizures were either greatly attenuated or blocked completely by prior systemic administration of naloxone (10 mg/kg). These findings suggest that such seizures result from an interaction of these compounds with opiate receptors in the brain. The epileptogenic potency of the enkephalins was illustrated by the observation that seizures and other pathological manifestations could still be elicited by doses as low as 10 microgram. Leucine-enkephalin was seen to have greater epiliptic potency than methionine-enkephalin. At doses of 1 microgram both enkephalins typically evoked cortical spindles resembling those seen in drowsy animals. Enkephalin-induced analgesia was seen in only one animal at the 100 microgram dose. Results obtained with repeated injections of morphine suggest that the epileptogenic effect of opiates may be subject to either tolerance or potentiation, depending on the prior occurrence of seizures. A synthesis of the present findings with several other lines of evidence suggests both that endogenous enkephalins play some role in normal mechanisms of reward, and that, when regulatory processes are disturbed, they may contribute as well to the elaboration of certain epileptic phenomena.

Mesencephalic central gray lesions and fear-motivated behavior in rats

Abstract In Experiment 1, the performance of 11 rats with central gray lesions including complete destruction of the ventrolateral edge, 11 rats with comparable lesions but with only partial ventrolateral damage and 14 sham-operated controls was compared in an operant, shock-food conflict task. The animals with complete ventrolateral destruction accepted more shocks and accepted shocks at a higher intensity than controls, while the group with partial destruction showed intermediate results. In another test, the spontaneous activity of control animals was totally suppressed for 2 min following noxious shock but was unaltered in animals with complete ventrolateral destruction. In animals with partial lesions, post-shock activity was directly related to the amount of ventrolateral destruction. Both lesion groups explored significantly more than controls in an open field. In Experiment 2, employing different animals, 9 lesioned rats with complete or near-complete ventrolateral central gray destruction and 6 sham-operated controls were tested in a step-through passive avoidance task. The performance of the lesioned animals was greatly impaired. The open field and post-shock activity results of the first experiment were replicated in Experiment 2. When placed in the open field or on a high perch most controls but no lesioned animals defecated. Lesioned animals were significantly more active than controls during a 17-h observation period on an activity platform. It was concluded that the mesencephalic central gray matter, and in particular its ventrolateral aspect, is critical to the normal expression of fear in rats. Several alternative hypotheses were discussed.

Evidence for opioid and non-opioid forms of stimulation-produced analgesia in the rat.

This study compares stimulation-produced analgesia (SPA) elicited from two different midline regions of the midbrain of the rat. Dorsal electrode placements were in the caudal periaqueductal gray matter; ventral placements lay within or subjacent to the dorsal raphe n. SPA thresholds were measured by the tail-flick method both during and immediately after the period of brain stimulation. Thresholds were consistently higher in the post-stimulation test. SPA from dorsal and ventral regions differed in the following ways: (1) Post-stimulation analgesia was significantly more difficult to obtain in ventral than in dorsal regions, whereas during-stimulation analgesia did not vary as a function of electrode location; (2) Although a continuous distribution of thresholds was seen for ventral placements, thresholds for dorsal placements tended to be either high or low on both during- and post-stimulation tests; (3) Naloxone (0.01--10 mg/kg) reliably elevated SPA thresholds for ventral but not dorsal stimulation placements. We conclude that different substrates of SPA lie in close proximity to one another in the medial midbrain of the rat. This portion of the midbrain appears to mediate both opioid and non-opioid mechanisms of analgesia.

Analgesia from rostral brain stem stimulation in the rat

Rats implanted with bipolar stimulating electrodes in the rostral medial brain stem were tested for brain stimulation-produced analgesia using tail-flick, pinch and hot-plate tests. Potent analgesia across all three tests was obtained from stimulation of sites in the gray matter surrounding the aqueduct and the caudal portion of the third ventricle, the posterior hypothalamus, the midline area of the caudal thalamus and the pretectal region of the meso-diencephalic junction. The analgesia obtained from these sites was comparable to that produced by stimulation of the previously studied caudal periaqueductal gray matter: it outlasted the period of brain stimulation, was not due to a generalized motor debilitation of the animal, and was not correlated with changes in electrographic activity. Stimulation of sites in the caudal thalamus and pretectal area yielded analgesia without stimulation-induced aversive reactions, confirming the potential of these sites for use in the relief of clinical pain in man.

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)