Chlordiazepoxide antinociception: cross-tolerance with opiates and with stress

Automated analysis of foot-shock sensitivity and concurrent freezing behavior in mice

Foot-shock is used in a variety of behavioral tasks either as a motivational stimulus, a way to characterize different rodents, or to test various pharmacological agents for their antinociceptive or analgesic effects. All these procedures need to assess foot-shock sensitivity either to rule out possible differences (when the shock is used as a motivational stimulus) or to use the differences to compare animals or treatments. In addition, many of the procedures that utilize foot-shock as a motivational stimulus evaluate freezing as an index of anxiety or fear. In the present study, data obtained by an automated computer system was compared with data obtained by human observers to validate the automated system for examining foot-shock sensitivity in mice. The different computer measures obtained for foot-shock sensitivity exhibited high correlations with human scoring at shock levels as low as 0.2 mA. The computer controlled analysis provided a non-subjective, quantifiable measurement of the foot-shock-induced response as well as freezing behavior. Automated data collection is an improvement over the methods of human visual observation in that the data collection is more standardized, efficient and consistent.

Enhancements in swim stress-induced hypothermia, but not analgesia, following amygdala lesions in rats

Lesions placed in the rat amygdala significantly reduce analgesic responses induced either by conditioning or exposure to a cat. Such lesions have alternatively reduced or failed to affect unconditioned foot shock analgesia. The present study expanded the situational determinants by examining whether lesions placed in the amygdala altered analgesia or hypothermia elicited by exposure to either continuous (CCWS) or intermittent (ICWS) cold-water swims. Lesion extent included the central, medial cortico-medial, baso-lateral, baso-medial and lateral amygdaloid nuclei. Basal jump thresholds, but not core body temperatures were significantly increased by unilateral and bilateral amygdala lesions. In contrast, the hypothermic, but not the analgesic responses following CCWS and ICWS were significantly enhanced by unilateral and bilateral amygdala lesions. These data support a hypothesis suggesting that these lesions are effective in reducing those classes of analgesic responses related to the signals of stressors than to the stressors themselves.

Effect of chronic maternal diazepam treatment on the development of stress-induced antinociception in young rats

The use and abuse of benzodiazepines is widespread and we have begun to address whether maternal exposure to these drugs influences the development of opioid systems. We have studied the effect of maternal diazepam treatment on stress-induced antinociception in the neonatal offspring. Diazepam (1 or 10 mg/kg) was administered twice daily to mothers from conception. Pre- and postweanling rat pups were assessed for opioid-mediated stress-induced antinociception by 3-min swimming and measuring nociception using the tail immersion test. In preweanling rats there was stress-induced antinociception in both vehicle- and diazepam-treated animals but in diazepam-treated groups (1 and 10 mg/kg) this was insensitive to reversal by the opioid antagonist naloxone, suggesting that nonopioid systems are operating this response. In postweanling rats a similar insensitivity to naloxone was observed in 1 mg/kg diazepam-treated groups; with 10 mg/kg diazepam there was no significant antinociception. The results suggest that maternal diazepam treatment interferes with the development of stress-mediated responses and that part of this toxicity is due to actions on opioid systems in the CNS.

Naloxone-induced hypoalgesia: lack of involvement of the GABA-benzodiazepine receptor complex

Previous evidence has demonstrated that repeated daily administration of the opiate receptor antagonist naloxone prior to assessment of pain sensitivity provokes the development of a nonopioid form of hypoalgesia. The present experiments assessed whether the GABA-benzodiazepine receptor complex may be involved in the mediation of this effect. Male Wistar rats were administered 10 mg/kg naloxone prior to hot-plate tests (48.5 degrees C) for pain sensitivity for 8 consecutive days. Control animals were administered saline prior to, and naloxone 2-4 h after, assessment of pain reactivity. Beginning on the fourth or fifth day of this regimen, animals tested under the influence of naloxone displayed longer paw-lick latencies than controls. Preadministration of the GABAA agonist muscimol (1.0-5.0 mg/kg) and GABAA antagonist bicuculline (0.25-1.0 mg/kg) failed to affect paw-lick latencies in naloxone-tested and control rats. The GABAB receptor agonist baclofen (1.0-5.0 mg/kg) and the benzodiazepine receptor agonist diazepam (1.0-5.0 mg/kg) both elevated paw-lick latencies to the same degree in both groups of animals. These results suggest that the GABA-benzodiazepine receptor complex is not involved in the mediation of naloxone-induced hypoalgesia.

Spontaneously hypertensive Wistar-derived male rats are more aggressive than those of their normotensive progenitor strain

We compared a group of spontaneously hypertensive rats (SHRs) to a group of Wistar-Kyoto (WKY) rats on each of the three most commonly studied forms of aggressive behavior in rats: muricide, intraspecific aggression, and shock-induced fighting (SIF). A significantly higher proportion of SHRs were muricidal; they also fought more at the lowest shock level. A trend for a higher incidence of intraspecific offense behaviors by SHRs was not significant. SHR flinch and jump thresholds were lower than the respective WKY thresholds. Although there were no significant correlations between shock thresholds and any aspects of SIF, the possibility that strain differences in shock sensitivity may contribute to differences in SIF cannot be ruled out. Within strains, there were no correlations among the different forms of aggression. Several different inherited characteristics may be associated with the accentuation of different forms of aggression in SHRs.

Aggressive behaviors in adult rats deprived of playfighting experience as juveniles

Matched litter mates were reared in one of three conditions: in pairs or in isolation with or without one hour of daily playfighting experience from 20 to 50 days of age. The rats were then regrouped within condition so that they lived with identically reared cagemates for a month. This regrouping eliminated the transient effects of isolation such as increased fearfulness. When tested as adults, there was no effect of early rearing condition on the probability of intraspecific aggression or muricide, although isolation-reared rats were less likely to retrieve the mice. However, isolation rearing reduced the latency to initiate shock-induced defensive aggression and increased both its frequency and intensity. Isolated animals which had been given daily playfighting during development did not show the effects of early social deprivation. The mechanisms through which playfighting experience shapes later defensive behavior remain to be determined.

Gender-specific and gonadectomy-specific effects upon swim analgesia: role of steroid replacement therapy

Both gender-specific and gonadectomy-specific effects have been observed for the analgesic responses following continuous and intermittent cold-water swims (CCWS and ICWS respectively): female rats display significantly less analgesia than males, and gonadectomized rats display significantly less analgesia than sham-operated controls. The present study evaluated the effects of steroid replacement therapy with testosterone propionate (TP: 2 mg/kg, SC) upon CCWS and ICWS analgesia on the tail-flick and jump tests and hypothermia in sham-operated or gonadectomized male and female rats. Thirty days following surgery, rats received either no treatment, a sesame oil vehicle or TP for 14 days prior to, and then during testing. Relative to the no treatment condition, repeated vehicle injections in sham-operated rats eliminated the gender-specific, but did not affect the gonadectomy-specific effects upon CCWS and ICWS analgesia. TP reversed the deficits in CCWS and ICWS analgesia observed in both castrated and ovariectomized rats on both pain tests. TP only potentiated CCWS analgesia in sham-operated males on the tail-flick test. TP potentiated CCWS and ICWS hypothermia in gonadectomized rats and in male sham-operated rats. These data indicate that gonadal steroids play a major modulatory role in the etiology of swim analgesia, and that the observed gender effects are sensitive to possible adaptational variables.

Differential control of hypothalamically elicited flight behavior by the midbrain periaqueductal gray in the cat

The purpose of the present study was to determine the role of the midbrain periaqueductal gray (PAG) and thalamic centrum medianum-parafascicular complex (CM-Pf) in the regulation of hypothalamically elicited flight behavior in the cat. The experimental paradigm involved a comparison of the differences in response latencies between single stimulation of the hypothalamus and concurrent stimulation of the hypothalamus and sites in the PAG or the CM-Pf. Dual stimulation of the ventral and dorsal aspects of the PAG resulted in differential modulation of flight behavior. Stimulation of the dorsal PAG suppressed hypothalamically elicited flight behavior while stimulation of the ventral aspects of the PAG facilitated flight behavior. Facilitation of flight behavior was also found from stimulation of ventral portions of the CM-Pf.

Fear is not critical to classically conditioned analgesia: the effects of periaqueductal gray lesions and administration of chlordiazepoxide

A high correlation between fear and analgesia classically conditioned to footshock in rats has been reported in the literature. However, it has never been directly tested whether or not fear is in fact causal to the production of conditioned analgesia. We therefore tested whether conditioned analgesia could be elicited in the absence of fear by employing two independent methods of fear suppression. First, areas of the periaqueductal gray (PAG) previously implicated in fear were selectively lesioned. Lesions of the dorsolateral PAG significantly attenuated conditioned analgesia and markedly decreased fear responses. Second, fear was attenuated via administration of chlordiazepoxide (CDP). Rats which had been conditioned while in the presence of CDP showed no reduction in conditioned pain inhibition. These results demonstrate that: (1) fear is not causal to classically conditioned analgesia and (2) the anatomical substrates for fear and conditioned analgesia are distinct but partially overlapping. The fact that fear is not a critical antecedent for classically conditioned analgesia suggests that classical conditioning techniques may be applied clinically to increase the effectiveness of some analgesic manipulations.

Convulsant and anticonvulsant effects of opioids: relationship to GABA-mediated transmission

The convulsant benzodiazepine Ro 5-3663, bicuculline and picrotoxin induced electroencephalographic (EEG) and behavioural convulsions. In rabbits, the EEG modifications consisted, with increasing doses, of three different patterns: slow waves in the optic lead, spike- and wave-complexes in the sensorimotor cortex, and grand-mal generalized seizures. These EEG effects were terminated by administration of diazepam (1 mg/kg) and morphine (0.25-1.0 mg/kg). Naloxone, in doses of 5-10 mg/kg, potentiated the effects of the three convulsant drugs. This potentiating phenomenon was also antagonized by the administration of diazepam and morphine. In membrane preparations, obtained from rat cortex, deprived of endogenous modulators of [3H]GABA binding, naloxone but not morphine, was able to inhibit [3H]GABA binding to its specific recognition sites. These data agree with previous findings indicating a GABA-antagonistic effect of naloxone, and support the hypothesis that the anticonvulsant effect of morphine might be, at least in part, due to an increase in GABAergic activity at the synaptic level.

Neuroleptic and analgesic interactions upon pain and activity measures

Previous data in rats indicate that while dopamine receptor blockers like haloperidol (HAL) potentiate opiate analgesia, dopamine receptor stimulants like apomorphine reduce cold-water swim (CWS) and 2-deoxy-D-glucose (2-DG) analgesia. Yet recently, HAL and chlorpromazine (CBZ) have been shown to reduce heat and immobilization analgesia. To address these differences, the present study investigated whether HAL (10, 50, 100 microgram/kg) or CPZ (1, 3, 5 mg/kg) would potentiate or reduce the effects of morphine (MOR), CWS, 2-DG and chlordiazepoxide (CDP) upon analgesia and activity. While HAL increased jump thresholds in a dose-dependent manner, CPZ doses exerted erratic effects. MOR analgesia was potentiated by the two higher CPZ doses and by the highest HAL dose. 2-DG analgesia was potentiated by only the highest HAL dose while CDP analgesia was potentiated by the moderate CPZ dose. While all CPZ doses potentiated CWS-induced increases in jump thresholds, the lowest HAL dose reduced this effect. These effects are considered in terms of the analgesic manipulation and its magnitude of effect, the neuroleptic and its dose, the pain test, and possible concurrent effects upon activity.