Behavioural effects of (-)naloxone in mice from four inbred strains

Dissociation of heroin-induced emotional dysfunction from psychomotor activation and physical dependence among inbred mouse strains

RATIONALE

Opiate addiction is a brain disorder emerging through repeated intoxication and withdrawal episodes. Epidemiological studies also indicate that chronic exposure to opiates may lead in susceptible individuals to the emergence of depressive symptoms, strongly contributing to the severity and chronicity of addiction. We recently established a mouse model of heroin abstinence, characterized by the development of depressive-like behaviors following chronic heroin exposure.

OBJECTIVES

While genetic factors regulating immediate behavioral responses to opiates have been largely investigated, little is known about their contribution to long-term emotional regulation during abstinence. Here, we compared locomotor stimulation and physical dependence induced by heroin exposure, as well as emotional dysfunction following abstinence, across mice strains with distinct genetic backgrounds.

METHODS

Mice from three inbred strains (C57BL/6J, Balb/cByJ, and 129S2/SvPas) were exposed to an escalating chronic heroin regimen (10-50 mg/kg). Independent cohorts were used to assess drug-induced locomotor activity during chronic treatment, naloxone-precipitated withdrawal at the end of chronic treatment, and emotional-like responses after a 4-week abstinence period.

RESULTS

Distinct behavioral profiles were observed across strains during heroin treatment, with no physical dependence and low locomotor stimulation in 129S2/SvPas. In addition, different behavioral impairments developed during abstinence across the three strains, with increased despair-like behavior in 129S2/SvPas and Balb/cByJ, and low sociability in 129S2/SvPas and C57BL/6J.

CONCLUSIONS

Our results indicate that depressive-like behaviors emerge during heroin abstinence, whatever the severity of immediate behavioral responses to the drug. In addition, inbred mouse strains will allow studying several aspects of mood-related deficits associated with addiction.

Naloxone's suppression of spontaneous and food-conditioned locomotor activity is diminished in mice lacking either the dopamine D(2) receptor or enkephalin

Mice lacking the D2 dopamine receptor (D2(-/-)) and congenic to the C57BL/6J background were tested for opioid-mediated locomotor activity to examine the involvement of the D2 dopamine receptor in opioid pharmacology. Morphine-stimulated locomotor activity did not significantly differ between the two genotypes. The opioid antagonist naloxone dose-dependently decreased spontaneous motor activity in wild-type mice but was without significant effect in D2(-/-) mice. The magnitude of food-conditioned increases in locomotor activity in wild-type mice and D2(-/-) mice was similar but naloxone did not decrease conditioned motor activity in D2(-/-) mice. Spontaneous locomotor activity of mice lacking the endogenous opioids beta-endorphin and/or enkephalin was also tested and we found that naloxone did not reduce activity in mice specifically lacking enkephalin. We suggest that the D2 dopamine receptor is necessary for modulation of spontaneous locomotor activity stimulated by the endogenous opioid enkephalin.

SEE locomotor behavior test discriminates C57BL/6J and DBA/2J mouse inbred strains across laboratories and protocol conditions

Conventional tests of behavioral phenotyping frequently have difficulties differentiating certain genotypes and replicating these differences across laboratories and protocol conditions. This study explores the hypothesis that automated tests can be designed to quantify ethologically relevant behavior patterns that more readily characterize heritable and replicable phenotypes. It used SEE (Strategy for the Exploration of Exploration) to phenotype the locomotor behavior of the C57BL/6 and DBA/2 mouse inbred strains across 3 laboratories. The 2 genotypes differed in 15 different measures of behavior, none of which had a significant genotype-laboratory interaction. Within the same laboratory, most of these differences were replicated in additional experiments despite the test photoperiod phase being changed and saline being injected. Results suggest that well-designed tests may considerably enhance replicability across laboratories.

The effect of naloxone on spontaneous and evoked dopamine release in the central and peripheral nervous systems

A number of studies have reported that the opiate antagonist naloxone (NX) inhibits behaviors dependent upon central dopamine (DA) release. However, equally compelling evidence from other studies suggests that NX excites a facilitatory effect. The present review was undertaken to resolve the issue by critically evaluating the effects of NX on DA release; the substrate subserving these behaviors. Included are studies reporting an effect of NX on spontaneous as well as drug altered DA release in various central regions. In the preponderant majority of these studies, NX was found to significantly enhance DA release in the virtually every major DA pathway, irrespective of whether DA release was initially stimulated or inhibited by various agents. It is concluded that NX most probably enhances behaviors induced by DA release, especially when administered in low, specific doses. Studies finding an inhibitory effect of NX on such behaviors may inadvertently produce conditions which mask the stimulatory effects of NX on DA release-dependent behaviors.

Naloxone eliminates passive avoidance retention deficits produced by pretest exposure to novelty in rats

Pretest exposure to novelty or injections of beta-endorphin can enhance passive avoidance (PA) retention (e.g., Izquierdo & McGaugh, 1985). Enhanced retention may result from a "state-dependent" match between the CNS state during test and the novelty-induced beta-endorphin state that is obtained during training in a novel apparatus. Our Experiment 1 suggests that, unlike PA, Pavlovian fear conditioning in a conditioned lick suppression (CLS) paradigm may be beta-endorphin "state-independent." Rats were given one tone-shock pairing in a novel environment. Baseline lick rates and CLS tested 48 h later in a familiar environment were not affected by pretest exposure to novelty and/or injections of 3.33 mg/kg naloxone HCl. In Experiment 2, the same rats were PA trained/tested in a new apparatus. Saline or naloxone injections and various exposure (novel, familiar, none) conditions preceded (1h) the 24-h retention test. Pretest exposure to novelty reduced retention and naloxone eliminated that deficit. In Experiment 3, naive rats given pretest exposure to novelty also showed a PA retention deficit. The results of Experiments 2 and 3 may complement rather than contradict previous findings. Pretest induction of a beta-endorphin state by novelty may either enhance state-dependent retrieval of a "weak" memory trace or make a "strong/well consolidated" training memory more vulnerable to retroactive interference from "new learning" during the pretest exposure period.

Genetic control of hippocampal cholinergic and dynorphinergic mechanisms regulating novelty-induced exploratory behavior in house mice

Neurobehavioral genetics endeavors to trace the pathways from genetic and environmental determinants to neuroanatomical and neurophysiological systems and, thence, to behavior. Exploiting genetic variation as a tool, the behavioral sequelae of manipulating these neuronal systems by drugs and antisera are analyzed. Apart from research in rats, this paper deals mainly with the genetically-influenced regulation in mice of exploratory behaviors that are adaptive in novel surroundings and are hippocampally-mediated. Special attention is paid to neuropeptidergic, GABAergic, and cholinergic synaptic functions in the mouse hippocampus. The behaviorally different inbred mouse strains C57BL/6 and DBA/2 show opposite reactions (reductions and increases, respectively, in exploration rates) to peripheral and intrahippocampal injections with agents that interfere with peptidergic, cholinergic, and GABAergic neurotransmission. These findings can be explained by an interdependent over-release of opioids, arrested GABA release, and excess acetylcholine in the hippocampal neuronal network of DBA/2 mice, as compared to C57BL/6 mice where these systems are functionally well balanced. Very similar results have been obtained with the lines SRH and SRL, derived from C57BL/6 and DBA/2, and genetically selected for rearing behavior. Most probably, the opioids act to disinhibit exploratory responses. An additional genetic approach is mentioned, in which four inbred mouse strains and one derived heterogeneous stock are used for estimating genetic correlations between structural properties of the hippocampal mossy fibers and levels of hippocampal dynorphin B, on the one hand, and frequencies of exploratory responses to environmental novelty, on the other.

Effects of naltrexone on food preference and concurrent behavioral responses in food-deprived rats

Naltrexone (0.05-5.0 mg/kg, SC) was administered to food-deprived rats prior to a 15-min food-preference test. Total food intake and feeding duration was reduced following administration of the opiate antagonist. However, while naltrexone reduced the consumption of the initially-preferred chocolate-coated cookies, the ingestion of the nonpreferred standard laboratory chow pellets was significantly enhanced. These data cannot be explained in terms of a general anorexic effect and nonspecific suppression of feeding responses. Instead, they indicate that naltrexone reduced preference for the highly palatable cookies, so that a feeding response to the chow pellets emerged. Under the conditions of test-familiarity, naltrexone did not reduce grooming, locomotion or rearing duration. An increase in locomotion may have been secondary to the reduction in feeding. The results agree with previous data from animal and human studies in suggesting that endogenous opioid peptide activity is involved in the palatability of preferred foods.

Role of hippocampal Met-enkephalin in the genotype-dependent regulation of exploratory behavior in mice

Intrahippocampal microinjections with anti-Met-enkephalin antiserum enhanced novelty-induced vertically oriented exploratory acts and horizontal locomotor activity in inbred mouse strain DBA/2 and reduced these behaviors in C57BL/6 so that strain differences originally present between the normal serum controls were eliminated after antiserum treatment. These opposite effects suggest that hippocampal Met-enkephalin participates in the genotype-dependent control of mouse exploration.

Design and interpretation of opiate antagonist trials in dementia

In view of the reports of possible beneficial effects of naloxone in dementia, rationales and strategies for studying endogenous opiate systems are reviewed. Important considerations in the design and interpretation of clinical investigations using naloxone are also reviewed. The nature and distribution of endogenous opiate systems are summarized from an historical perspective. Endogenous opiate systems are distributed throughout the central nervous system and play important roles in a variety of brain functions, including memory and learning. In view of this, several rationales are evident for studying endogenous opiate systems in dementia, since it is a syndrome in which structures known to contain opiate systems are disturbed, functions modulated by opiate systems are disturbed, and other neurotransmitter systems (functionally linked to endogenous opiate systems) are disturbed. Different strategies for studying endogenous opiate systems are reviewed, including examination of body fluids and pharmacologic challenge studies. Naloxone hydrochloride, a competitive opiate receptor antagonist, is a commonly used pharmacologic agent. The design of a multidose naloxone study of 12 dementia patients is discussed, with reference to the pharmacokinetics, pharmacodynamics, and specificity of naloxone as well as to the nature of the dependent measures selected for this study. No cognitive benefit was observed in this study. Behavioral arousal was observed at naloxone doses, with more evident psychomotor retardation at higher doses. These findings are contrasted with the results of naloxone challenges in other studies. The varying effects of naloxone within and across populations can be conceptualized in terms of the basic and clinical considerations previously discussed. The importance of dose-finding studies is stressed for this and other drug trials.

Social behaviour in pairs of C57BL/6 mice of both sexes in the open field: effects of saline drinking and of naloxone

We have previously found that saline drinking increases fighting in male pairs and decided to test this treatment (0.9% NaCl for 24 hours before test day 1; SAL) on social behaviour of both males and females. Paired C57BL/6 mice (same-sex pairs) were observed in the open field in daily sessions for three days. One member of each pair (test mouse) was given either SAL treatment, a control injection of saline (SI), an injection of naloxone (1 mg/kg IP; NLX) or a combination of both treatments (NLX + SAL). NLX alone had previously been found to increase aggression in resident/intruder tests at the dose used. Open field testing is not associated with aggressive encounters in our experience. SAL had little effect on (unaggressive) social behaviour in males, but increased social contact seeking in females. The NLX and NLX + SAL treatments had essentially the same effects, irrespective of sex; the treated animals showed behavioural inhibition (reduced social and ambulatory behaviour), while their untreated partners showed significantly more than normal interest in the naloxone treated mice. The results are discussed in terms of opioid involvement in social behaviour.

Substrate soiled by an unfamiliar conspecific modifies opioid activity in mice placed in novel environments

Naloxone induces behavioural changes in rodents exposed to novel environments, indicating the involvement of endogenous opioid mechanisms in these situations. The present study investigated whether soiled sawdust substrate from the cage of an unfamiliar, isolated, male conspecific modifies the effect of naloxone (0.5 or 12.5 mg/kg) upon behaviour of mice in an open field test situation. There was little difference between the effects of naloxone upon the frequency of acts or postures shown in the soiled and unsoiled environments. Cluster analysis of the activities according to their position and frequency in behavioural sequences, revealed variations in behavioural organisation in these two situations in control animals, and differential responses to naloxone administration. The data are discussed in terms of an involvement in behaviour of opioid mechanisms which can be modified by non-painful, biologically-relevant, aversive stimuli such as unfamiliar, conspecific-soiled substrates.

Naltrexone fails to suppress spontaneous locomotor activity in hamsters

The increased spontaneous locomotor activity (SLMA) of rats exposed to a novel environment is decreased by opiate antagonists. In the present study, naltrexone (1.0-40 mg/kg) failed to reduce the SLMA of hamsters exposed to the novel environment of activity cages. The SLMA of another group of untreated hamsters declined following 4 consecutive exposures to the activity cages. Thus, the novelty-induced increase in hamster SLMA is not sensitive to opiate antagonism. The differential sensitivity of rats and hamsters to opiate effects on activity and feeding may be due to the presence of an opiate-sensitive hibernation system in hamsters.

A genetically controlled hippocampal transmitter system regulating exploratory behavior in mice

Male C57BL/6 and DBA/2 mice were intrahippocampally microinjected with muscimol (0.5 microgram), given 15 min prior to individual 20-min exploration tests in a novel environment, and compared to saline controls. The GABA agonist reduced various exploratory acts and locomotor activity in strain C57BL/6 and increased the scores in DBA/2. In conjunction with similar opposite effects previously found with intra-hippocampal methylscopolamine and naloxone, these findings suggest that the opioid modulation of the hippocampal cholinergic mechanism which facilitates behavioral responses to novelty in mice is effectuated indirectly through an inhibitory GABAergic system. The functioning of these regulatory systems appears to depend on genotype.

Effects of intrahippocampally-injected naloxone and morphine upon behavioural responses to novelty in mice from two selectively-bred lines

To examine the peptidergic regulation of behavioural responses to novelty, 5-month-old male mice from the inbred selection lines SRH (selected for rearing frequency: high) and SRL (selected for rearing frequency: low) were intrahippocampally micro-injected (0.5 microliter) with either the opiate antagonist naloxone (0.3 microgram), or the opiate agonist morphine (1.0 microgram), or saline vehicle alone, given 15 min prior to individual exposure to 20-min exploration tests in a novel environment. Naloxone exerted opposite effects upon various exploratory acts and locomotor activity in the two strains, that is, it decreased the scores in SRH and augmented them in SRL, while morphine depressed the scores in both. It is suggested that an excess of opioids in SRL, as compared to SRH, is attenuated by this dose of naloxone. In addition to previously obtained evidence of a genotype-dependent cholinergic mechanism in the mouse dorsal hippocampus controlling exploratory responses to novelty, these findings indicate that hippocampal opioid peptides are also involved in the genotype-dependent regulation of exploration.

Possible DA agonist properties of naloxone

Naloxone is widely if not universally considered to be a pharmacologically 'pure' opiate (mu) receptor antagonist virtually devoid of agonist action when administered in moderate dosages. However, naloxone (NX) appears to possess a striking number of DA agonist properties. Thus, some investigators have found NX capable of inducing stereotyped rearing and locomotor activity in habituated rats (a controversial finding), and decrease serum prolactin levels, improve Parkinsonism, enhance copulatory performance in sexually sluggish animals, and increase striatal HVA levels, in mimicry of centrally acting DA agonists. NX can also significantly potentiate the central effects induced by DA agonists including DA agonist (d-amphetamine) induced 3H-dopamine release, and antagonize a number of the central effects elicited by DA release inhibiting agents. Finally, virtually all of the central effects of morphine reversible by NX have also been found to be antagonized by a variety of dopamine agonists; while DA release inhibiting agents can abolish the ability of NX to antagonize morphine induced effects. Thus, NX may be exerting its central effects through a dopaminergic mechanism. Since NX does not bind DA receptors, it is quite likely that NX may ultimately antagonize the central effects of morphine by enhancing DA release from DA terminals upon which opiate receptors are localized. The same opiate receptors, shown to be localized on DA nerve terminals, have already been implicated in opiate mediated modulation of DA release.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Effects of naloxone and diprenorphine on spontaneous activity in rats and mice

The narcotic antagonist naloxone has been reported to decrease locomotor activity in the rat, presumably blocking endogenous opiate systems. Naloxone has a greater affinity for receptors which preferentially bind morphine and other opiate alkaloids as compared to receptors that bind endogenous opioid peptides. Diprenorphine, another pure opiate antagonist, binds with equal affinity to both receptor subtypes. Therefore, the effects of the narcotic antagonists naloxone and diprenorphine on spontaneous activity were compared in rats and mice, tested individually and in pairs. Only naloxone (10 mg/kg) affected spontaneous activity in rats tested individually, decreasing both gross and fine activity. In rats tested in pairs, naloxone (1.0 and 10 mg/kg) decreased both fine and gross activity, while diprenorphine (10 mg/kg) produced significant decreases only in fine activity. In mice tested individually, naloxone produced modest (nonsignificant) decreases in activity while diprenorphine (10 mg/kg) significantly enhanced activity. Neither opiate antagonist produced consistent effects on activity in paired mice. These results illustrate the species and situation dependence of the effects of opiate antagonists and point out the need for testing more than one narcotic antagonist in research designed to provide inferential information concerning possible physiological functions of endogenous opioid peptides.

Behavioural responses to novelty in two inbred mouse strains after intrahippocampal naloxone and morphine

Male C57BL/6 and DBA/2 mice were injected intrahippocampally with either naloxone (0.5 microgram) or morphine (1.0 microgram), or saline vehicle alone and, after 15 min, some 12 behavioural components carried out in a novel environment were recorded for 20 min. Naloxone reduced exploratory rearing responses, wall-leaning and object-sniffing in strain C57BL/6 and augmented these behaviours in strain DBA/2, while morphine depressed the scores in both. In conjunction with previously obtained evidence that the mouse hippocampus contains a genotype-dependent cholinergic mechanism which regulates responses to novelty, these findings support the hypothesis that hippocampal opioid peptides modulate the cholinergic control of exploration in mice, possibly indirectly through GABAergic pathways. In contrast, locomotor activity, defaecation and tail elevation remained practically unaffected by the two drugs, and grooming showed another kind of genotype-treatment interaction, that is to say, after morphine.