Alpha-adrenoceptor number and function in rat cortex after ethanol and immobilization stress

Effects of acute, repeated and chronic variable stress on in vivo tyrosine hydroxylase activity and on alpha(2)-adrenoceptor sensitivity in the rat brain

We assessed the effects of a single tail pinch and two chronic stress regimes, repeated and variable, on in vivo tyrosine hydroxylase activity and on alpha2-adrenoceptor sensitivity in two brain regions. After administering a 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor, tyrosine hydroxylase activity, measured as the accumulation of DOPA, and noradrenaline (NA) content were determined by using high-performance liquid chromatography. A single tail pinch for 5 min induced an enhancement of DOPA content in hippocampus (28%) and hypothalamus (67%) which was still present 24 h later. This increase could account for the lack of changes in NA content in both regions after the application of this stressor. However, tyrosine hydroxylase activity was unmodified 24 h after exposure to both repeated (5 min of tail pinch, twice daily, for 14 days) and chronic variable stress (one of 5 different stressors, once daily, for 14 days) although there was an enhancement of NA levels in hippocampus (45 and 54%, respectively) and hypothalamus (24.5 and 36%, respectively). The sensitivity of the alpha2-adrenoceptors which regulate [3H]-NA release in hippocampal and hypothalamic synaptosomes was not modified by the acute or chronic stress protocols assayed. The results show that both paradigms of chronic stress had similar effects on the noradrenergic indices evaluated.

Emerging evidence for a central epinephrine-innervated alpha 1-adrenergic system that regulates behavioral activation and is impaired in depression

Currently, most basic and clinical research on depression is focused on either central serotonergic, noradrenergic, or dopaminergic neurotransmission as affected by various etiological and predisposing factors. Recent evidence suggests that there is another system that consists of a subset of brain alpha(1B)-adrenoceptors innervated primarily by brain epinephrine (EPI) that potentially modulates the above three monoamine systems in parallel and plays a critical role in depression. The present review covers the evidence for this system and includes findings that brain alpha(1)-adrenoceptors are instrumental in behavioral activation, are located near the major monoamine cell groups or target areas, receive EPI as their neurotransmitter, are impaired or inhibited in depressed patients or after stress in animal models, and are restored by a number of antidepressants. This "EPI-alpha(1) system" may therefore represent a new target system for this disorder.

Substance induced plasticity in noradrenergic innervation of the paraventricular hypothalamic nucleus

Single administration of the cytokine interleukin-1 alpha (IL-1), or the psychostimulant amphetamine, enhanced adrenocorticotropin hormone and corticosterone responses to a stress challenge weeks later. This long-lasting hypothalamic-pituitary-adrenal (HPA)-sensitization is paralleled by an increase in electrically evoked release of noradrenaline in the paraventricular hypothalamic nucleus (PVN). We hypothesized that these functional changes may be associated with morphological plasticity of noradrenergic projections to the PVN, a parameter that shows high reproducibility. Specific alterations in relative (nor)adrenergic innervation density were studied by using dopamine-alpha-hydroxylase (DBH) as a marker. An image analysis system was used to detect changes in the relative DBH innervation density of the PVN. Groups of adult male rats were given IL-1 (10 microg/kg i.p.), amphetamine (5 mg/kg i.p.), or saline. Three weeks later, IL-1 and amphetamine primed rats showed enhanced adrenocorticotropin hormone and corticosterone responses to an amphetamine challenge. In another set of experiments, the relative DBH innervation density was measured in different PVN subnuclei at four rostro-caudal levels. Single administration of either IL-1 or amphetamine causes three weeks later a selective decrease in relative DBH innervation density in those subnuclei of the PVN that contain high numbers of corticotrophin-releasing hormone (CRH) producing neurons: the dorsal parvocellular and medial parvocellular PVN. We conclude that (1) long-lasting sensitization induced by single exposure to IL-1 and amphetamine induces specific pattern of neuroplastic changes in (nor)adrenergic innervation in the PVN and (2) reduction of relative DBH innervation density in CRH-rich areas is associated with paradoxical increase of electrically evoked release of (nor)adrenaline.

Epidemiological evidence about the relationship between PTSD and alcohol abuse: the nature of the association

This article uses the Bradford Hill criteria for assessing causal associations to examine the nature of the relationship between PTSD and alcohol abuse. A series of studies are presented which examine this relationship. A cross-sectional study of 2,501 subjects in a community sample examined the relationship between at-risk drinking and 11 types of traumatic events. The traumatic events associated with at-risk drinking were involvement in life threatening accidents, witnessing severe injury, rape, being the victim of serious physical assault using the CIDI. In a longitudinal study of 469 firefighters exposed to a natural disaster, PTSD was associated with both an increase and decrease in alcohol consumption and PTSD rather than exposure accounted for the changes in drinking behaviour. In three other populations, psychiatric inpatients, motor accident victims and female prisoners, the association between PTSD and alcohol abuse emphasised the clinical and public health importance of this relationship. The available evidence does nevertheless support the causal nature of this relationship. Other risk factors are necessary to predict alcohol abuse following exposure to traumatic events, although exposure to traumatic events can be caused by alcohol abuse.

Heterogeneous effects of rapid eye movement sleep deprivation on binding to alpha- and beta-adrenergic receptor subtypes in rat brain

Quantitative receptor autoradiography was used to map alterations in binding to alpha1-, alpha2-, beta1- and beta2-adrenergic receptors throughout the brain of rats deprived of rapid eye movement sleep for 96 h. Binding of [3H]prazosin to alpha1 sites, while not significantly different in any of 46 brain regions examined, showed a clear overall tendency towards decreased values after sleep deprivation. [3H]UK-14,314-labeled alpha2 binding sites were not significantly affected by sleep deprivation in any of 91 brain regions analysed, despite a trend towards increased values. In contrast, beta-adrenergic binding was significantly reduced throughout the brain. Binding to beta1 sites labeled by [125I]iodopindolol in the presence of ICI-11855 was significantly reduced in 13 of 69 brain areas examined; binding to beta2 sites labeled by [125I]iodopindolol in the presence of CGP-20712A was likewise reduced throughout the brain and significantly so in 25 of the 72 brain areas analysed. Rank ordering of the binding changes indicated that reductions in beta1 vs beta2 binding were maximal in different brain areas. This pattern of results may reflect a particular configuration of effects specifically associated with sleep loss stress. The results are consistent with evidence of persisting noradrenergic cell activity during sleep deprivation. The observed heterogeneity of effects suggests that not all norepinephrine receptors are equally affected by rapid eye movement sleep deprivation.

The role of ethanol availability on stress-induced increases in ethanol consumption

Exposure to stressors can increase ethanol consumption and ethanol can attenuate the behavioral and biochemical effects of stressors. This study determined whether the availability of ethanol during the period of exposure to repeated restraint alters the poststress increase in ethanol intake. Seven days of restraint increased ethanol intake on the first poststress test only in animals deprived of ethanol during the restraint period. These results indicate that the availability of ethanol during exposure to restraint can attenuate the impact of restraint on ethanol intake. Ethanol intake was also positively related to novelty-induced locomotion and restraint eliminated this relationship. However, amphetamine-induced locomotion was not altered by either restraint or EtOH intake. These results indicate that voluntary ethanol intake can attenuate the impact of restraint stress and that restraint stress can alter the influence of ethanol on novelty-induced locomotion. It is suggested that this symmetrical relationship between ethanol intake and restraint stress may be involved in an interactive manner that determines stress-induced ethanol consumption.

Chronic treatment with prazosin causes a subtype-specific increase in the alpha 1-adrenoceptor density of the stressed rat cerebral cortex

The effects of chronic treatment with prazosin and of immobilization stress on the alpha 1-adrenoceptor subtypes in rat cerebral cortex have been examined. Prazosin-treated rats were allowed free access to tap water containing two different concentrations of prazosin (16 or 156 mg L-1) for 5 weeks. The mean plasma concentrations of prazosin were 5 ng mL-1 in groups treated with a low dose and 8 or 14 ng mL-1 in those treated with a high dose. Immobilization stress (2 h day-1, 2 weeks) or chronic treatment with a low dose of prazosin caused no significant change in the affinity for [3H]prazosin or in the maximum number of alpha 1-adrenoceptor sites (Bmax). However, treatment with prazosin (low dose) combined with stress increased the density of alpha 1-adrenoceptors with low affinity for prazosin. Treatment with a high dose of prazosin increased the density of alpha 1L-adrenoceptors, irrespective of stress loading. The densities of alpha 1A- and alpha 1B-adrenoceptors with high affinity for prazosin were increased only after treatment with a high dose of prazosin in combination with stress. These results indicate that three distinct alpha 1-adrenoceptor subtypes, alpha 1A, alpha 1B and alpha 1L, might be affected differently by treatment with prazosin and by stress.

Enhancement of 5-hydroxytryptamine-stimulated phosphoinositide hydrolysis in the rat cerebral cortex by repeated immobilization stress

The present study was undertaken to investigate the influence of repeated immobilization stress on phosphoinositide hydrolysis induced by 5-hydroxytryptamine (5-HT) and noradrenaline in the rat cerebral cortex. Three groups of rats subjected to stress intervention were immobilized for 2 h per day for 3, 7, and 14 days. The stress intervention of any duration did not alter noradrenaline-stimulated phosphoinositide hydrolysis. The 3- and 7-day repeated immobilization enhanced 5-HT-stimulated phosphoinositide hydrolysis, whereas the characteristics of 5-HT2 receptor binding did not change. Chronic treatment with imipramine partially, but significantly, suppressed the increase in 5-HT-stimulated phosphoinositide hydrolysis, induced by the 3-day repeated immobilization. These findings imply that modulation of 5-HT-stimulated phosphoinositide hydrolysis occurs in stressful situations and that the therapeutic effects of tricyclic antidepressant drugs might be related to the modulation of phosphoinositide hydrolysis mediated by 5-HT receptors.

Effect of repeated novel stressors on depressive behavior and brain norepinephrine receptor system in Sprague-Dawley and Wistar Kyoto (WKY) rats

This study compared the effects of repeated novel stressors on 'depressive behaviors', defined by the forced-swim and open-field tests, in Sprague-Dawley (S-D) and Wistar Kyoto (WKY) rats. Since stress appears to alter brain norepinephrine (NE) activity, this study also investigated the effects of the stressors on beta-adrenoceptors (beta-ARs), alpha 2-adrenoceptors (alpha 2-ARs) and NE transporter (NET) sites in S-D and WKY rats. Stress did not alter 125I-iodopindolol (125I-PIN) binding to beta-ARs, nor [3H]idazoxan ([3H]IDAZ) binding to alpha 2-ARs in S-D rats, compared to non-stressed controls. However, WKY-stressed rats showed a significant reduction in 125I-IPIN binding to beta-ARs in the cortex, hippocampus, amygdala and hypothalamus, and a reduction in [3H]IDAZ binding to alpha 2-ARs in the amygdala. [3H]nisoxetine ([3H]NIS) binding to NET sites in WKY-stressed rats was also reduced in the cortex, hippocampus and amygdala. When both strains were compared, the most surprising finding was a significantly higher density of NET sites in the hippocampus and amygdala in WKY rats compared to S-D rats. The results of this study indicate that stress, not only exacerbates depressive behavior in WKY rats, but also selectively alters beta-ARs, alpha 2-ARs and NET sites in limbic brain regions. Thus, the WKY strain may serve as a useful animal model for depressive behavior and for the investigation of novel antidepressant drugs.

Restraint stress in biomedical research: an update

Since the publication of our initial review of restraint stress in 1986, much work has continued with this technique, either as a tool for the investigation of other pharmacological, physiological, or pathologic phenomena or with restraint stress itself serving as the object of the study. As we noted in 1986, the major use of restraint has been for the induction of stress responses in animals and, more specifically, for the investigation of drug effects, particularly as they affect typical stress-related pathology--gastrointestinal, neuroendocrine, and immunological agents have been extensively studied. In compiling this update on restraint stress and its effects, we noted an increasing emphasis on central nervous system mechanisms in peripheral disease, especially gastrointestinal disease. In particular, many CNS-active agents have been tested for their effects on gastric and duodenal lesion formation and gastric secretion, including antidepressants, antipsychotics, anxiolytics, noradrenergic, serotonergic, dopaminergic, and peptidergic compounds. Some of these agents are especially active in the gastrointestinal tract even when administered centrally, further solidifying the concept of a brain-gut axis. The present update includes studies of: methods and procedures, pre-restraint manipulations, post-restraint/healing effects, and drug effects. In addition, a current bibliography of reports that have employed restraint is included.

Effects of ischemia on regional ligand binding to adrenoceptors in the rat brain

Changes in ligand binding to adrenoceptors ([3H]prazosin to alpha 1-receptors, [3H]idazoxan to alpha 2-receptors and [125I]cyanopindolol to beta-receptors) following transient cerebral ischemia were investigated using autoradiographic methods. The binding was quantified in brain sections from control rats, rats subjected to 15 min of 2-vessel occlusion ischemia, and rats with recirculation times of 1 h, 1 week or 4 weeks after ischemia. No significant change in alpha 1-receptor binding was observed during and immediately following ischemia, but a decrease was noted in the vulnerable hippocampal CA1 region following 1 week's survival. In the parietal cortex, the ligand binding to alpha 1-receptors increased at 4 weeks. A reduced [3H]idazoxan binding was observed 1 h after ischemia in the temporal cortex and amygdala. No change in ligand binding to beta-receptors was seen in the early phase postischemia, but a marked increase had occurred in the hippocampal CA1 region at 1 and 4 weeks after ischemia (+163% and +142%, respectively), presumably due to accumulation of macrophages expressing beta-receptors. The early postischemic changes in receptor binding may represent downregulation of the adrenoceptors by processes activated during ischemia, while neuronal degeneration, compensatory mechanisms in surviving neurons and proliferation of non-neuronal cells may account for the subsequent changes.

Invited commentary: complexities in evaluating neurobiological models for stress resistance

Professors Salmon and Stanford are to be commended for their efforts in drawing attention to noradrenergic contributions to stress resistance and the potential importance of within-subject patterns of stress response. In commenting on their paper, I would like to develop the following points: (1) self-control over stress exposure strongly influences the noradrenergic response to stress; (2) β-receptor regulation may reflect a lower sensitivity to stress under a carefully specified range of conditions; and (3) genetic and environmental factors may interact in a complex fashion that may not fit simple linear models based on β-receptor function.

Stressor-induced anhedonia in the mesocorticolimbic system

It has been suggested that uncontrollable stressors induce motivational changes in animals which are reminiscent of reward alteration in human depression. Although there is considerable support for this position, most animal models of depression do not adequately address this issue. The present review suggests that stressor-induced reductions in the rewarding value of electrical brain stimulation (ICSS) from the mesocorticolimbic system may simulate the anhedonia of human depression. The magnitude, severity and the site of these stressor-induced reward alterations within the mesocorticolimbic system vary with the strain of animal employed. The anhedonic effects of stressors are attenuated by treatments which influence mesocorticolimbic DA turnover, including systemic antidepressant and intraventricular neuropeptide administration. Although the diverse symptom profile of depression should be addressed by consideration of the constellation of behavioral disturbances induced by stressors, considerable emphasis should be devoted to an assessment of reward loss in depression. The implications of these data to the stressor depression topography and the potential role of mesocorticolimbic DA in depression and anhedonia are discussed.

Chronic ethanol feeding and diamine oxidase activity in rat brain and liver

The activity of diamine oxidase, the principal enzyme for diamine catabolism, was evaluated in brain and liver from male rats fed nutritionally complete diets, of which 36% of total calories was given as ethanol or as isocaloric carbohydrates for 4 months. Ethanol caused an increase in diamine oxidase activity in the brain and a decrease in the liver, tissues with a low and high rate of ethanol oxidation, respectively.

Immobilization and restraint effects on pain reactions in animals

Acute physical restraint represents a potent stressor in several animal species and is accompanied by a complex pattern of hormonal responses and functional changes in the central nervous system. Repeated immobilization leads to partial blunting of the behavioral and hormonal responses, with transient modifications of neurotransmitter systems in the brain. Pain reactions, as investigated by different kinds of nociceptive tests, are usually attenuated both during and immediately following acute immobilization and the analgesic effect of opiate compounds potentiated; these behavioral alterations may be attributed at least in part to activation of an endogenous opioid system. In some species, restraint may induce a reflex immobility (animal hypnosis or tonic immobility) which is also characterized by suppression of pain reactions in rabbits, probably subserved by different mechanisms. Analysis of available data suggests that pain testing in unanesthetized, restrained animals may involve alterations of the animal's reactivity to noxious stimuli.

Alpha-2-adrenoceptor function in alcohol withdrawal: a pilot study of the effects of iv. clonidine in alcoholics and normals

Central alpha-2-adrenoceptor function was studied in seven withdrawing alcoholics and compared with seven normal controls using the clonidine challenge test. The alcoholic group had significantly elevated basal levels of blood pressure and plasma 3-methoxy-4-hydroxy-phenyl glycol. The percentage of fall in blood pressure produced by clonidine was blunted in the alcoholics. There was disruption of the neuroendocrine effects of clonidine on growth hormone release. Physiological indices of withdrawal, in particular plasma 3-methoxy-4-hydroxyphenyl glycol and blood pressure correlated with measures of previous withdrawal experiences, supporting the idea that kindling might occur with repeated episodes of withdrawal.

Biological mechanisms in posttraumatic stress disorder. Relevance for substance abuse

Recent studies suggest a significant biological contribution to posttraumatic stress disorder (PTSD). In particular, central catecholamine and endogenous opioid systems have been implicated both in this syndrome and in substance abuse. We review relevant animal and human studies that support these hypotheses and suggest that this overlap may contribute to the incidence of substance abuse in PTSD. The animal studies have primarily employed the learned helpless and conditioned emotional response models and have included assessments of brain catecholamines, locus ceruleus activity, and behavioral correlates in rodents and nonhuman primates. Human studies have used only indirect measures to assess these variables. However, both therapeutic approaches and attempts at self-medication for PTSD have supported this hypothesis.

Some aspects of stress and depression

1. The role of stress in depressive illness is discussed together with utility of the "learned helplessness" model and some neuropharmacological correlates of uncontrollable shock. 2. Similarities and differences between chronic antidepressant treatment and chronic stress treatment regimes are reviewed. 3. Finally the role of adaptive process in stress on antidepressant treatments is discussed.

Foot shock induces time and region specific adrenergic receptor changes in rat brain

Rats were subjected to 1 hr or 2 hr of electric foot shock for 1 day or 7 days and adrenergic receptor binding was evaluated in the hypothalamus, brainstem and cortex. beta-Adrenergic receptor density in the hypothalamus was dramatically reduced following 1 hr of shock. Following repeated shock, alpha 2-adrenergic receptors in the cortex and brainstem were observed to increase. Cortical alpha 2-adrenergic receptors were more sensitive to stress than the alpha 2-adrenergic receptors of the brainstem, alterations in the latter only reaching statistical significance following 7 days of shock and 24 hr of recovery. alpha 1- and beta-adrenergic receptors in the brainstem and cortex were relatively resistant to stress induced changes. The significance of type of stress, duration of stress, and strain of rat for understanding the current data are discussed in the context of prior reports of stress induced receptor changes.

Clorgyline and desipramine alter the sensitivity of [3H]noradrenaline release to calcium but not to clonidine

Synaptosomes (P2) were prepared from cerebral cortices of control rats and from those which had received clorgyline (1 mg/kg/day for 21-28 days) or desipramine (10 mg/kg/day for 21-28 days). Following incubation with [3H]noradrenaline (500 nM/15 min, 37 degrees C), aliquots of the synaptosomes were gently filtered onto Whatman GF/A filters and superfused with Krebs buffer (pH 7.5, 37 degrees C) for a maximum period of 2 h. During this time, the basal efflux of tritiated materials (approximately 75% noradrenaline) together with K+-evoked release of the amine and metabolites, were measured. Chronic antidepressant drug regimens increased the K+-stimulated release, but its attenuation by clonidine was not altered. Thus, chronic antidepressant drug regimens do not apparently alter presynaptic alpha 2-adrenoceptors. These results suggest that the reported antidepressant drug induced decreases in [3H]clonidine binding, occur on sites which are postsynaptic to noradrenergic neurones. Following the chronic antidepressant drug regimens, the sensitivity of the [3H]noradrenaline release process to Ca2+ is significantly increased. This change may explain the enhanced K+-evoked release which follows the antidepressant drug regimens. It is proposed that this increased sensitivity of the [3H]noradrenaline release process may be an adaptation to the decrease in neuronal firing which have been reported following antidepressant drug treatments.

Impact of circulating corticosterone on alpha 1- and alpha 2-noradrenergic receptors in discrete brain areas

The impact of adrenalectomy (ADX) and subsequent corticosterone(CORT) replacement, on the binding of [3H]p-aminoclonidine to alpha 2-noradrenergic receptors and [3H]prazosin to alpha 1-noradrenergic receptors, was studied in 8 discrete hypothalamic and 5 extra-hypothalamic areas of rats. With little change in extra-hypothalamic receptors, ADX produced a large CORT-reversible decrease in alpha 2-receptor binding specifically within the hypothalamic paraventricular nucleus (PVN) and a CORT-reversible increase within the supraoptic nucleus. alpha 1-Noradrenergic receptors, in contrast, were generally unaffected by ADX. This and other evidence leads us to propose a potential modulatory influence of circulating CORT on hypothalamic alpha 2 receptors and a specific function for this CORT-alpha 2 receptor interaction specifically within the PVN, in the control of eating behavior.

Central serotonergic responses and behavioural adaptation to repeated immobilisation: the effect of the corticosterone synthesis inhibitor metyrapone

Rats were immobilised for 2 h/day. Twenty-four hours after the 1, 3 or 7 immobilisation periods they were injected with the 5HT agonist 5-methoxy-N,N-dimethyltryptamine (5MeODMT; 5 mg/kg i.p.) and behavioural responses (i.e. hind limb abduction, forepaw treading, head weaving, tremor, Straub tail) compared with those of a control group. As we have previously observed after 7 (but not after 1 or 3 immobilisations) forepaw treading and tremor were enhanced and the other responses unaffected. Pretreatment with metyrapone (a corticosterone synthesis inhibitor 150 mg/kg i.p., 3 h before each immobilisation) did not affect the above responses to 1 immobilisation, increased tremor after 3 immobilisations and also increased forepaw treading, hind limb abduction and Straub tail after 7 immobilisations but decreased head weaving under the latter conditions. Metyrapone without immobilisation had no effect on responses to 5MeODMT. Twenty four hours after 1 or 3 (but not 7) immobilisation periods, rats placed for the first time in an open field showed less locomotion and rearing and more defaecation than control animals. Rats also given metyrapone exhibited normal open field behaviour after only 3 immobilisations. The drug also accelerated the return to normal on repeated immobilisation of the impairment of food intake and growth rate which occurred after a single immobilisation. The results as a whole suggest that metyrapone promotes behavioural adaptation to repeated immobilisation and that this is associated with enhanced postsynaptic responses to 5HT. These findings suggest that immobilisation stress-induced changes might be relevant as an animal model for depression which incorporates reported biochemical abnormalities in the illness and is of relevance to proposals concerning its precipitation by stress.

Acute ethanol administration: effects on stress-induced gastric and duodenal ulcer in rats

Rats were given 6% ethanol (v/v) as their only source of liquid for 4 days. On the basis of ethanol consumption (g/kg/day), animals were divided into high, medium and low ethanol consuming groups. A non-ethanol exposed control group was also included. Following a 24 hr food deprivation period, animals were restrained for 3 hr. No differences in gastric ulcer frequency or severity were noted with the exception of a slight tendency toward a lower incidence among ethanol consuming rats relative to controls. An unusual observation was the high incidence of duodenal ulcer observed only among ethanol consuming rats. This ethanol-stress interaction is discussed in terms of an animal's history of ethanol exposure.

Altered characteristics of [3H]dopamine release from superfused slices of corpus striatum obtained from rats receiving ethanol in vivo

Ethanol, 50 mM, in vitro inhibited the release of [3H]dopamine ([3H]DA) induced by depolarisation with 40 mM K+ from slices of corpus striatum of the rat. In contrast, the release of [3H]DA induced by the Ca2+ ionophore (A23187) was enhanced by the presence of ethanol in vitro. When similar preparations were obtained from brains of rats which had received ethanol in vivo chronically by inhalation for 5-7 days the characteristics of release of [3H]DA were altered. Thus, the inhibitory effect of ethanol in vitro on release induced by K+-depolarisation was lost, as was the enhancing effect of ethanol on the release induced by A23187. When release of [3H]DA was studied in the absence of added ethanol the fraction of stored 3H released either by K+-depolarisation or by A23187 was increased in the preparations from animals which had received ethanol in vivo. Similar changes in release induced by A23187, though of lesser magnitude, could be seen in rats which had received ethanol acutely (3 g kg-1 i.p.; 30 min). An even greater fraction of [3H]DA was released by A23187 in preparations from rats which had been made physically dependent on ethanol. These changes in the release characteristics of [3H]DA were still apparent in animals undergoing a physical syndrome of withdrawal from ethanol. The results are discussed in relation to the cellular basis for the development of tolerance to and dependence on ethanol.

Clonidine induced sedation is not altered by repeated stress in the RHA/iop and RLA/iop strains of rats

An hypothesis that repeated stress results in central changes in alpha 2-adrenoceptor sensitivity was investigated using a behavioural test. Stressed (immobilisation for 2 h/day for 7 days) and unstressed rats from the RHA/iop and RLA/iop strains were tested for the sedative effects of the alpha 2-adrenoceptor agonist clonidine on Y-maze behaviour. The measures used were number of lines crossed, arm entries and rearing. The stressed animals showed higher scores for line crossings and rearing; but the only significant difference between the strains was for rearing, which was higher for RHA/iop. Clonidine significantly depressed all the measures of activity. However, there was no evidence of an interaction of the drug with stress for any of the measures. It is concluded that neither repeated stress nor genetic differences in the ability to cope with stress influence the behavioural effects of clonidine. This suggests that stress responses are not related to the central alpha 2-adrenoceptor system.