Reversible inactivation of rostral nucleus raphe pallidus attenuates acute autonomic responses but not their habituation to repeated audiogenic stress in rats.

The medullary nucleus raphe pallidus (RPa) mediates several autonomic responses evoked by acute stress exposure, including tachycardia and hyperthermia. The present study assessed whether the RPa contributes to the decline/habituation of these responses observed during repeated audiogenic stress. Adult male rats were implanted with cannulae aimed at the RPa, and abdominal E-mitters that wirelessly acquire heart rate and core body temperature. After surgical recovery, animals were injected with muscimol or vehicle (aCSF) in the RPa region, followed by 30 min of 95-dBA loud noise or no noise control exposures on 3 consecutive days at 24-h intervals. Forty-eight hours after the third exposure, animals were exposed to an additional, but injection-free, loud noise or no noise test to assess habituation of hyperthermia and tachycardia. Three days later, rats were restrained for 30-min to evaluate their ability to display normal acute autonomic responses following the repeated muscimol injection regimen. The results indicated that the inhibition of cellular activity induced by the GABAA-receptor agonist muscimol centered in the RPa region reliably attenuated acute audiogenic stress-evoked tachycardia and hyperthermia, compared with vehicle-injected rats. Animals in the stress groups exhibited similar attenuated tachycardia and hyperthermia during the injection-free fourth audiogenic stress exposure, and displayed similar and robust increases in these responses to the subsequent restraint test. These results suggest that cellular activity in neurons of the RPa region is necessary for the expression of acute audiogenic stress-induced tachycardia and hyperthermia, but may not be necessary for the acquisition of habituated tachycardic responses to repeated stress.

Evidence for the Integration of Stress-Related Signals by the Rostral Posterior Hypothalamic Nucleus in the Regulation of Acute and Repeated Stress-Evoked Hypothalamo-Pituitary-Adrenal Response in Rat.

A likely adaptive process mitigating the effects of chronic stress is the phenomenon of stress habituation, which frequently reduces multiple stress-evoked responses to the same (homotypic) stressor experienced repeatedly. The current studies investigated putative brain circuits that may coordinate the reduction of stress-related responses associated with stress habituation, a process that is inadequately understood. Initially, two rat premotor regions that respectively regulate neuroendocrine (medial parvicellular region of the paraventricular hypothalamic nucleus [PaMP]) and autonomic (rostral medullary raphe pallidus [RPa]) responses were targeted with distinguishable retrograde tracers. Two to 3 weeks later, injected animals underwent loud noise stress, and their brains were processed for fluorescent immunohistochemical detection of the tracers and the immediate early gene Fos. A rostral region of the posterior hypothalamic nucleus (rPH), and to a lesser extent, the median preoptic nucleus, exhibited the highest numbers of retrogradely labeled cells from both the RPa and PaMP that were colocalized with loud noise-induced Fos expression. Injections of an anterograde tracer in the rPH confirmed these connections and suggested that this region may contribute to the coordination of multiple stress-related responses. This hypothesis was partially tested by posterior hypothalamic injections of small volumes of muscimol, which disrupts normal synaptic functions, before acute and repeated loud noise or restraint exposures. In addition to significantly reduced corticosterone release in response to these two distinct stressors, rPH muscimol disrupted habituation to each stressor modality, suggesting a novel and important contribution of the rostral posterior hypothalamic nucleus in this category of adaptive processes. Significance statement: Habituation to stress is a process that possibly diminishes the detrimental health consequences of chronic stress by reducing the amplitude of many responses when the same challenging conditions are experienced repeatedly. Stress elicits a highly coordinated set of neuroendocrine, autonomic, and behavioral responses that are independently and relatively well defined; however, how the brain achieves coordination of these responses and their habituation-related declines is not well understood. The current studies provide some of the first anatomical and functional results suggesting that a specific region of the hypothalamus, the rostral posterior hypothalamic nucleus, targets multiple premotor regions and contributes to the regulation of acute neuroendocrine responses and their habituation to repeated stress.

Habituation of hypothalamic-pituitary-adrenocortical axis hormones to repeated homotypic stress and subsequent heterotypic stressor exposure in male and female rats.

Understanding potential sex differences in repeated stress-induced hypothalamic-pituitary-adrenocortical (HPA) axis habituation could provide insight into the sex-biased prevalence of certain affective disorders such as anxiety and depression. Therefore in these studies, male and female rats were exposed to 30 min of either audiogenic or restraint stress daily for 10 days in order to determine whether sex regulates the extent to which HPA axis hormone release is attenuated upon repeated homotypic stressor presentation. In response to the initial exposure, both stressors robustly increased plasma concentrations of both adrenocorticotropic hormone (ACTH) and corticosterone (CORT) in both sexes. Acutely, females displayed higher ACTH and CORT concentrations following restraint stress, whereas males exhibited higher hormone concentrations following loud noise stress. HPA axis hormone responses to both stressors decreased incrementally over successive days of exposure to each respective stressor. Despite the differential effect of sex on acute hormone responses, the extent to which HPA axis hormone response was attenuated did not differ between male and female animals following either stressor. Furthermore, ACTH and CORT responses to a novel environment were not affected by prior exposure to stress of either modality in either male or female rats. These experiments demonstrate that despite the acute stress response, male and female rats exhibit similar habituation of HPA axis hormones upon repeated homotypic stressor presentations, and that exposure to repeated stress does not produce exaggerated HPA axis hormone responses to a novel environment in either female or male rats.

Central gene expression changes associated with enhanced neuroendocrine and autonomic response habituation to repeated noise stress after voluntary wheel running in rats.

Accumulating evidence indicates that regular physical exercise benefits health in part by counteracting some of the negative physiological impacts of stress. While some studies identified reductions in some measures of acute stress responses with prior exercise, limited data were available concerning effects on cardiovascular function, and reported effects on hypothalamic-pituitary-adrenocortical (HPA) axis responses were largely inconsistent. Given that exposure to repeated or prolonged stress is strongly implicated in the precipitation and exacerbation of illness, we proposed the novel hypothesis that physical exercise might facilitate adaptation to repeated stress, and subsequently demonstrated significant enhancement of both HPA axis (glucocorticoid) and cardiovascular (tachycardia) response habituation to repeated noise stress in rats with long-term access to running wheels compared to sedentary controls. Stress habituation has been attributed to modifications of brain circuits, but the specific sites of adaptation and the molecular changes driving its expression remain unclear. Here, in situ hybridization histochemistry was used to examine regulation of select stress-associated signaling systems in brain regions representing likely candidates to underlie exercise-enhanced stress habituation. Analyzed brains were collected from active (6 weeks of wheel running) and sedentary rats following control, acute, or repeated noise exposures that induced a significantly faster rate of glucocorticoid response habituation in active animals but preserved acute noise responsiveness. Nearly identical experimental manipulations also induce a faster rate of cardiovascular response habituation in exercised, repeatedly stressed rats. The observed regulation of the corticotropin-releasing factor and brain-derived neurotrophic factor systems across several brain regions suggests widespread effects of voluntary exercise on central functions and related adaptations to stress across multiple response modalities.

Stressor-specific effects of sex on HPA axis hormones and activation of stress-related neurocircuitry.

Experiencing stress can be physically and psychologically debilitating to an organism. Women have a higher prevalence of some stress-related mental illnesses, the reasons for which are unknown. These experiments explore differential HPA axis hormone release in male and female rats following acute stress. Female rats had a similar threshold of HPA axis hormone release following low intensity noise stress as male rats. Sex did not affect the acute release, or the return of HPA axis hormones to baseline following moderate intensity noise stress. Sensitive indices of auditory functioning obtained by modulation of the acoustic startle reflex by weak pre-pulses did not reveal any sexual dimorphism. Furthermore, male and female rats exhibited similar c-fos mRNA expression in the brain following noise stress, including several sex-influenced stress-related regions. The HPA axis response to noise stress was not affected by stage of estrous cycle, and ovariectomy significantly increased hormone release. Direct comparison of HPA axis hormone release to two different stressors in the same animals revealed that although female rats exhibit robustly higher HPA axis hormone release after restraint stress, the same effect was not observed following moderate and high intensity loud noise stress. Finally, the differential effect of sex on HPA axis responses to noise and restraint stress cannot readily be explained by differential social cues or general pain processing. These studies suggest the effect of sex on acute stress-induced HPA axis hormone activity is highly dependent on the type of stressor.

Auditory cortex lesions do not disrupt habituation of HPA axis responses to repeated noise stress.

Previous research has suggested that sensory areas may play a role in adaptation to repeated stress. The auditory cortex was the target of the present studies because it is a major projection area of the auditory thalamus, where functional inactivation disrupts stress habituation to repeated loud noise. Large bilateral excitotoxic lesions of the auditory cortex were made in male rats 2 weeks prior to (Experiment 1) or a few days after (Experiment 2) a 5 day 30 min repeated 95 dBA noise or no noise regimen. Blood was collected immediately after exposure on days 1, 3, and 5. Two weeks after the 5th exposure, the rats were retested with 30 min noise or no noise to determine retention of the habituated responses. Animals were killed immediately after the retest and trunk blood and brains collected for lesion verification. Plasma adrenocorticotropic hormone (ACTH) and corticosterone levels were determined. In both experiments, significant between-subjects effects were found for noise (95 dBA or no noise) but not for surgery (lesion, sham, or no surgery control rats), with lesion groups exhibiting similar levels of ACTH and corticosterone across days as the sham and no surgery control groups. All noise exposed groups displayed similar habituation rates and retention levels. A third experiment indicated that similar auditory cortex lesions significantly disrupted background noise gap detection in an acoustic startle paradigm. Overall, these data suggest that the information mediating hypothalamic-pituitary-adrenal axis response habituation to repeated loud noise exposures is not derived from the auditory cortex.

Effects of voluntary wheel running on heart rate, body temperature, and locomotor activity in response to acute and repeated stressor exposures in rats.

Stress often negatively impacts physical and mental health but it has been suggested that voluntary physical activity may benefit health by reducing some of the effects of stress. The present experiments tested whether voluntary exercise can reduce heart rate, core body temperature and locomotor activity responses to acute (novelty or loud noise) or repeated stress (loud noise). After 6 weeks of running-wheel access, rats exposed to a novel environment had reduced heart rate, core body temperature, and locomotor activity responses compared to rats housed under sedentary conditions. In contrast, none of these measures were different between exercised and sedentary rats following acute 30-min noise exposures, at either 85 or 98 dB. Following 10 weeks of running-wheel access, both groups displayed significant habituation of all these responses to 10 consecutive daily 30-min presentations of 98 dB noise stress. However, the extent of habituation of all three responses was significantly enhanced in exercised compared to sedentary animals on the last exposure to noise. These results suggest that in physically active animals, under some conditions, acute responses to stress exposure may be reduced, and response habituation to repeated stress may be enhanced, which ultimately may reduce the negative and cumulative impact of stress.

Lack of contextual modulation of habituated neuroendocrine responses to repeated audiogenic stress.

Exposure to stress reliably activates the hypothalamo-pituitary-adrenocortical (HPA) axis response in rodents, which is significantly reduced (habituated) following repeated exposures. In the current study, it was first established that HPA axis response habituation to repeated loud noise lasted for at least 4 weeks in rats. In the next experiment, a contextual extinction procedure following repeated loud noise exposures failed to restore the habituated HPA axis response. Although an additional study indicated some recovery of responses when the context was modified on a test day following habituation, this effect could be mostly attributed to the familiarity with the contextual cues. A final study confirmed that rats could distinguish between the contexts used and further indicated that context preexposures reduce acute HPA axis responses to loud noise. These studies therefore provide no support for the hypothesis that contextual cues regulate HPA axis response habituation.

Physical activity, but not environmental complexity, facilitates HPA axis response habituation to repeated audiogenic stress despite neurotrophin mRNA regulation in both conditions.

Stress exacerbates several physical and psychological disorders. Voluntary exercise can reduce susceptibility to many of these stress-associated disorders. In rodents, voluntary exercise can reduce hypothalamic-pituitary-adrenocortical (HPA) axis activity in response to various stressors as well as upregulate several brain neurotrophins. An important issue regarding voluntary exercise is whether its effect on the reduction of HPA axis activation in response to stress is due to the physical activity itself or simply the enhanced environmental complexity provided by the running wheels. The present study compared the effects of physical activity and environmental complexity (that did not increase physical activity) on HPA axis habituation to repeated stress and modulation of brain neurotrophin mRNA expression. For six weeks, male rats were given free access to running wheels (exercise group), given 4 objects that were repeatedly exchanged (increased environmental complexity group), or housed in standard cages. On week 7, animals were exposed to 11 consecutive daily 30-min sessions of 98-dBA noise. Plasma corticosterone and adrenocorticotropic hormone were measured from blood collected directly after noise exposures. Tissue, including brains, thymi, and adrenal glands was collected on Day 11. Although rats in both the exercise and enhanced environmental complexity groups expressed higher levels of BDNF and NGF mRNA in several brain regions, only exercise animals showed quicker glucocorticoid habituation to repeated audiogenic stress. These results suggest that voluntary exercise, independent from other environmental manipulations, accounts for the reduction in susceptibility to stress.

Stress rapidly increases alpha 1d adrenergic receptor mRNA in the rat dentate gyrus.

The hippocampal formation is a highly plastic brain region that is sensitive to stress. It receives extensive noradrenergic projections, and noradrenaline is released in the hippocampus in response to stressor exposure. The hippocampus expresses particularly high levels of the alpha(1D) adrenergic receptor (ADR) and we have previously demonstrated that alpha(1d) ADR mRNA expression in the rat hippocampus is modulated by corticosterone. One of the defining features of a stress response is activation of the hypothalamic pituitary adrenal (HPA) axis, resulting in the release of corticosterone from the adrenal glands. However, the effect of stress on hippocampal expression of alpha(1d) ADR mRNA has not been determined. In this study, male rats were exposed to inescapable tail shock, loud noise or restraint, and the effect on alpha(1d) ADR mRNA expression in the hippocampus was determined by semi-quantitative in situ hybridization. All three stressors resulted in a rapid upregulation of alpha(1d) ADR mRNA in the dentate gyrus, with expression peaking at approximately 90min after the start of the stressor. Physical activity has previously been reported to counteract some of the effects of stress that occur within the dentate gyrus. However, 6weeks of voluntary wheel running in rats did not prevent the restraint stress-induced increase in alpha(1d) ADR mRNA expression in the dentate gyrus. Although the function of the alpha(1D) ADR in the dentate gyrus is not known, these data provide further evidence for a close interaction between stress and the noradrenergic system in the hippocampus.

Accessory and main olfactory systems influences on predator odor-induced behavioral and endocrine stress responses in rats.

Exposures to predator odors are very effective methods to evoke a variety of stress responses in rodents. We have previously found that ferret odor exposure leads to changes in endocrine hormones (corticosterone and ACTH) and behavior. To distinguish the contributions of the main and accessory olfactory systems in these responses, studies were designed to interfere with these two systems either independently, or simultaneously. Male Sprague-Dawley rats were treated with 10% zinc sulfate (ZnSO(4)), which renders rodents anosmic (unable to smell) while leaving the accessory olfactory areas intact, or saline, in Experiment 1. In Experiment 2, the vomeronasal organs of rats were surgically removed (VNX) to block accessory olfactory processing, while leaving the main olfactory system intact. And in the third experiment both the main and accessory olfactory areas were disrupted by combining the two procedures in the same rats. Neither ZnSO(4) treatment nor VNX alone reliably reduced the increased corticosterone response to ferret odor compared to strawberry odor, but in combination, they did. This suggests that processing through the main or the accessory olfactory system can elicit the endocrine stress response to ferret odor. VNX alone also did not affect the behavioral responses to the ferret odor. ZnSO(4) treatment, alone and in combination with VNX, led to changes in behavior in response to both ferret and strawberry odor, making the behavioral results less clearly interpretable. Overall these studies suggest that both the main and accessory olfactory systems mediate the neuroendocrine response to predator odor.

Disruption of neuroendocrine stress responses to acute ferret odor by medial, but not central amygdala lesions in rats.

Investigations of the neural pathways associated with responses to predators have implicated the medial amygdala (MeA) as an important region involved in defensive behaviors. To our knowledge, however, the involvement of the MeA in neuroendocrine responses to predator odor exposure has not been investigated. Therefore, the present study examined the effects of MeA disruption in rats exposed to ferret or control odor on hypothalamo-pituitary-adrenocortical (HPA) axis activation. Bilateral lesions of the MeA were made in Sprague-Dawley rats with the neurotoxin ibotenic acid (10 microg/microl; 0.3 microl / side). As a control for regional specificity, additional groups of rats were given lesions in the central amygdala (CeA). One week after recovery, the rats were exposed to ferret or strawberry control towels in small cages to examine HPA axis responses as determined by plasma corticosterone and adrenocorticotropin hormone (ACTH) levels. Rats with complete bilateral MeA but not CeA lesions displayed significantly less corticosterone and ACTH release compared to sham-operated control rats only in the ferret odor conditions. These results suggest that the MeA is an important structure involved in the HPA axis responses to predator odors, in support of previous studies investigating behavioral responses under similar conditions.

Reversible inactivation of the auditory thalamus disrupts HPA axis habituation to repeated loud noise stress exposures.

Although habituation to stress is a widely observed adaptive mechanism in response to repeated homotypic challenge exposure, its brain location and mechanism of plasticity remains elusive. And while habituation-related plasticity has been suggested to take place in central limbic regions, recent evidence suggests that sensory sites may provide the underlying substrate for this function. For instance, several brainstem, midbrain, thalamic, and/or cortical auditory processing areas, among others, could support habituation-related plasticity to repeated loud noise exposures. In the present study, the auditory thalamus was tested for its putative role in habituation to repeated loud noise exposures, in rats. The auditory thalamus was inactivated reversibly by muscimol injections during repeated loud noise exposures to determine if brainstem or midbrain auditory nuclei would be sufficient to support habituation to this specific stressor, as measured during an additional and drug-free loud noise exposure test. Our results indicate that auditory thalamic inactivation by muscimol disrupts acute HPA axis response specifically to loud noise. Importantly, habituation to repeated loud noise exposures was also prevented by reversible auditory thalamic inactivation, suggesting that this form of plasticity is likely mediated at, or in targets of, the auditory thalamus.

Repeated ferret odor exposure induces different temporal patterns of same-stressor habituation and novel-stressor sensitization in both hypothalamic-pituitary-adrenal axis activity and forebrain c-fos expression in the rat.

Repeated exposure to a moderately intense stressor typically produces attenuation of the hypothalamic-pituitary-adrenal (HPA) axis response (habituation) on re-presentation of the same stressor; however, if a novel stressor is presented to the same animals, the HPA axis response may be augmented (sensitization). The extent to which this adaptation is also evident within neural activity patterns is unknown. This study tested whether repeated ferret odor (FO) exposure, a moderately intense psychological stressor for rats, leads to both same-stressor habituation and novel-stressor sensitization of the HPA axis response and neuronal activity as determined by immediate early gene induction (c-fos mRNA). Rats were presented with FO in their home cages for 30 min a day for up to 2 wk and subsequently challenged with FO or restraint. Rats displayed HPA axis activity habituation and widespread habituation of c-fos mRNA expression (in situ hybridization) throughout the brain in as few as three repeated presentations of FO. However, repeated FO exposure led to a more gradual development of sensitized HPA-axis and c-fos mRNA responses to restraint that were not fully evident until after 14 d of prior FO exposure. The sensitized response was evident in many of the same brain regions that displayed habituation, including primary sensory cortices and the prefrontal cortex. The shared spatial expression but distinct temporal development of habituation and sensitization neural response patterns suggests two independent processes with opposing influences across overlapping brain systems.

Chronic voluntary wheel running facilitates corticosterone response habituation to repeated audiogenic stress exposure in male rats.

Voluntary exercise is associated with the prevention and treatment of numerous physical and psychological illnesses, yet the mechanisms by which it confers this protection remain unclear. In contrast, stress, particularly under conditions of prolonged or repeated exposure when glucocorticoid levels are consistently elevated, can have a devastating impact on health. It has been suggested that the benefits of physical exercise may lie in an ability to reduce some of the more deleterious health effects of stress and stress hormones. The present series of experiments provides evidence that voluntary exercise facilitates habituation of corticosterone but not adrenocorticotropin hormone responses to repeated stress presentations. After 6 weeks of running wheel access or sedentary housing conditions, rats were exposed to 11 consecutive daily 30 min presentations of 98 dB noise stress. Similar corticosterone responses in exercised rats and sedentary controls were observed following the first, acute stress presentation. While both groups demonstrated habituation of corticosterone secretory responses with repeated noise stress exposures, the rate of habituation was significantly facilitated in exercised animals. These results suggest that voluntary exercise may reduce the negative impact of prolonged or repeated stress on health by enhancing habituation of the corticosterone response ultimately reducing the amount of glucocorticoids the body and brain are exposed to.

Long-term habituation to repeated loud noise is impaired by relatively short interstressor intervals in rats.

The phenomenon of spaced (longer intertrial interval) compared with massed (shorter intertrial interval) training leading to better long-term habituation and associative learning is well documented. However, the effects of intertrial intervals on response habituation to repeated stress exposures have not been previously examined. The present experiments found that massed (six 30-min exposures of 95 dB white noise in 6 hr) and spaced (one 30-min exposure daily for 6 days) noise exposures led to similar habituation of plasma corticosterone and ACTH responses, heart rate, and core body temperature after the 6th exposure in male Sprague-Dawley rats. However, these habituated responses were not retained in the massed group on a similar noise re-exposure 48 hr later, compared with the spaced group. The habituated responses found in the massed group after the 6 noise exposures were not due to differential hearing threshold shifts, as examined with modifications of the acoustic startle reflex. These data indicate that relatively short interstressor intervals impair long-term stress adaptation. This series of studies supports the idea of distinct short- and long-term habituation processes to stress responsiveness.

The pattern of brain c-fos mRNA induced by a component of fox odor, 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), in rats, suggests both systemic and processive stress characteristics.

Predators to rodents and their associated odors are increasingly chosen to study the neural mechanisms of stress and anxiety. Specifically, predatory odors are believed to elicit responses based on the perceived threat (psychological or processive), rather than to any direct systemic effects (pain, blood loss, infection, etc.) of the stimulus, which are mediated by distinct neural pathways. The hypothesis that a chemical component from fox feces, 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), elicits stress responses by specific activation of processive neural pathways was tested. Different amounts of TMT (range: 0-600 micromol) or the control odor butyric acid (0-1200 micromol) were presented to male Sprague-Dawley rats for 30 min. Immediately after odor presentation, rats were sacrificed, blood levels of adrenocorticotropic hormone (ACTH) and corticosterone were measured, and brains were rapidly harvested to measure regional brain c-fos mRNA induction by in situ hybridization. Presentation of TMT (> or =75 micromol), but not butyric acid (up to 1200 micromol), significantly increased ACTH and corticosterone release. TMT presentation, especially with amounts (> or =75 micromol) producing endocrine activation, induced c-fos mRNA in several brain areas, including the olfactory bulb, lateral septal nucleus, septohypothalamic nucleus, anteromedial and oval nuclei of the bed nucleus of the stria terminalis, the central nucleus of the amygdala, the anteroventral, anterodorsal, and medial preoptic nuclei, the anterior, dorsomedial, lateral, supramammillary, dorsal premammillary and paraventricular hypothalamic nuclei, the external lateral parabrachial nucleus, the locus coeruleus, and the nucleus of the solitary tract. Interestingly, these brain regions represent a mix of regional c-fos mRNA induction pattern not reported previously with any other single stressor. These results suggest that TMT elicits stress responses through a relatively unique and complex mix of brain regions associated with both processive and systemic neural pathways, unlike those seen in response to cat odors.

Dopamine overflow is increased in olfactory bulbectomized rats: an in vivo microdialysis study.

Olfactory bulbectomy (OBX) in rats produces behavioral, physiological, and neurochemical changes that resemble symptoms of depression in humans. The procedure thus serves as a rodent model of affective disorder. Many of the behavioral effects of OBX resemble psychomotor agitation. The possible role of dysregulation of ventral striatal dopamine (DA) systems in this phenomenon was investigated. Basal levels of DA, norepinephrine (NE), homovanillic acid, dihydroxyphenylacetic acid, and 5-hydroxyindoleacetic acid were examined in the striatum of OBX and sham-operated controls using in vivo microdialysis. OBX rats exhibited significantly higher basal DA levels (192%) and lower NE levels (12%) than sham-operated controls. Locomotor activity in response to novelty and footshock stress was elevated in OBX rats. The finding of higher DA levels in striatum may explain this "agitation-like" behavior, a commonly observed phenomenon in the OBX model.

Intravenous self-administration of amphetamine is increased in a rat model of depression.

Affective disorders and substance abuse frequently coexist, yet few previous studies have examined drug self-administration using animal models of depression. The olfactory-bulbectomized rat is a well-established model that exhibits a high degree of neurochemical similarity to depression. Olfactory bulbectomy (OBX) increases dopamine receptor densities in the ventral striatum, which may increase the reinforcing effects of drugs of abuse. Experiments were designed to test the hypotheses that acquisition and stable self-administration of amphetamine would be increased in bulbectomized rats. In the first experiment, rats underwent bilateral OBX or sham surgery and intravenous jugular catheters were implanted 12-14 days later. Acquisition was examined using a standard operant paradigm involving a nose-poke response for a very low dose of D-amphetamine sulfate (12 microg/infusion, IV). A separate group of rats received coinfusions of sulpiride. In a second experiment designed to minimize differences in acquisition and examine stable self-administration, lever pressing for a low (0.10 mg/kg, IV) or high (0.25 mg/kg, IV) dose of D-amphetamine sulfate was measured in rats pretrained to lever press for food. Bulbectomized rats acquired the self-administration of very low dose amphetamine faster than sham-operated rats and this effect was reversed by sulpiride coinfusion. Stable self-administration of the low dose of amphetamine was also markedly increased in bulbectomized rats. The findings reveal the utility of the OBX model for studying the neurobiological basis of depression and drug abuse comorbidity and support the hypothesis that neurochemical abnormalities associated with depression may enhance the addictive properties of some drugs of abuse.