A1 adenosine receptors in the striatum play a role in the memory impairment caused by sleep deprivation through downregulation of the PKA pathway.

Sleep deprivation is known to affect memory formation, but how it interacts with different memory systems is not completely understood. Adenosine, a homeostatic regulator of sleep that has an increased extracellular concentration during sleep deprivation, is one of the neuromodulators that may be involved in this interaction. The A1 adenosine receptor is involved in both sleep regulation and memory formation. Among other pathways, the A1 receptor decreases cAMP levels in the cytosol and thus also regulates protein kinase A (PKA) and exchange protein activated by cAMP (EPAC) activity. To verify the role of the A1 receptor in the memory impairment caused by sleep deprivation, we tested the effect of 96 h of sleep deprivation (SD) and the administration of DPCPX, an A1 receptor antagonist on male Wistar rats prior to the training sessions for two memory tasks that relies on the hippocampal function: the multiple trial inhibitory avoidance (MTIA) task, which also requires the striatum, and the contextual fear conditioning (CFC) task, which does not. We also evaluated the effect of SD, DPCPX and the MTIA training session on the protein expression levels of the A1 receptor, PKA phosphorylation and EPAC activity in both the hippocampus and the striatum. Sleep deprivation impaired the performance in the test sessions of both tasks; DPCPX was able to prevent the impairment in the MTIA test but not in the CFC test. SD increased A1 receptor protein expression levels in the striatum but not in the hippocampus and also decreased PKA phosphorylation in both structures; DPCPX prevented this decrease in the striatum, but not in the hippocampus. Finally, SD had no effect on EPAC activity in either of the structures. These results indicate that the A1 adenosine receptors play a role in the memory impairment caused by sleep deprivation in tasks that involve the striatum through modulation of the cAMP/PKA pathway.

Multiple trial inhibitory avoidance acquisition and retrieval are resistant to chronic stress.

Chronic mild stress (CMS) is a widely accepted animal model relevant to depression that among other consequences, is chiefly known to induce anhedonia, often assessed as decreased preference for sucrose solution. CMS is also known to affect cognition, particularly memory tasks. In this study we have employed the multiple-trial inhibitory avoidance memory task (MTIA) to assess CMS effects on memory acquisition and retrieval. MTIA consists of repeated exposures to the unconditioned stimulus until a learning criterion is reached. Wistar rats underwent CMS for 5 weeks, and sucrose consumption was assessed once a week. At the end of CMS, animals were evaluated in the MTIA task. Overall decreased sucrose solution preference was highly variable. Further analyses showed that a subset of animals expressed resilience while another subset was sensitive to stress. CMS did not affect the number of acquisition sessions before reaching criterion or retrieval latency of MTIA task in neither sensitive nor resilient groups. Although tasks that assess learning ability in animal models relevant to depression indicate cognitive deficits, the ability to learn the association between compartment crossing and the aversive electric foot shock, which is strongly dependent on emotional aspects, was intact.

Bombesin administration impairs memory and does not reverse memory deficit caused by sleep deprivation.

Sleep deprivation impairs performance in emotional memory tasks, however this effect on memory is not completely understood. Possible mechanisms may involve an alteration in neurotransmission systems, as shown by the fact that many drugs that modulate neural pathways can prevent memory impairment by sleep loss. Gastrin releasing peptide (GRP) is a neuropeptide that emerged as a regulatory molecule of emotional memory through the modulation of other neurotransmission systems. Thus, the present study addressed the effect of intraperitoneal (IP) administration of bombesin (BB) (2.5, 5.0 and 10.0µg/kg), a GRP agonist, on the performance of Wistar rats in a multiple trail inhibitory avoidance (MTIA) task, after sleep deprivation, using the modified multiple platforms method (MMPM). Sleep deprived animals exhibited acquisition and retention impairment that was not prevented by BB injection. In addition, non-sleep deprived animals treated with BB before and after the training session, but not before the test, have shown a retention deficit. In summary, BB did not improve the memory impairment by sleep loss and, under normal conditions, produced a memory consolidation deficit.

Changes in GABAergic inputs in the paraventricular nucleus maintain sympathetic vasomotor tone in chronic heart failure.

The paraventricular nucleus (PVN) of the hypothalamus is an important region of the brain involved in the regulation of sympathetic vasomotor tone. Accumulating evidence supports the idea that a change in hypothalamic γ-aminobutyric acid (GABA)-ergic inhibitory and glutamatergic excitatory inputs contribute to the exacerbated sympathetic drive in chronic heart failure (HF). The purpose of this study was to determine whether a possible imbalance between glutamatergic and GABAergic inputs to the PVN contributes to increased sympathetic outflow in HF in two different sympathetic territories. Renal (RSNA) and splanchnic sympathetic nerve activity (SSNA), mean arterial blood pressure (MAP) and heart rate were recorded from urethane-anesthetized HF or sham rats. The NMDA-glutamate and GABA-A receptor densities within the PVN were quantified in HF and sham rats by autoradiography. Bilateral microinjection of kynurenic acid (4nmol) into the PVN decreased MAP and RSNA and SSNA in HF but not in sham rats. Furthermore, in response to GABA-A blockade in the PVN by bicuculline (400 pmol), hypertension and SSNA were reduced in HF compared to sham. The quantification of ionotropic NMDA receptors and GABA-A receptors in the PVN showed a significant reduction of GABA-A in HF rats; however, the NMDA density in the PVN did not differ between groups. Thus, this study provides evidence that the sympathoexcitation is maintained by an imbalance between GABAergic and glutamatergic inputs in the PVN in HF. The reduced GABAergic input results in relatively augmented glutamatergic actions in the PVN of HF rats.

Inflammatory markers are associated with inhibitory avoidance memory deficit induced by sleep deprivation in rats.

Sleep deprivation (SD) causes detrimental effects to the body, such as memory impairment and weight loss. SD also changes the concentration of inflammatory mediators such as cytokines, which, in turn, can affect cognitive functioning. Thus, the objective of this study was to investigate the involvement of these inflammatory mediators in inhibitory avoidance memory deficit in sleep-deprived rats. Male Wistar rats were deprived of sleep by the modified multiple platform method for 96 h, while their respective controls remained in their housing cages. To assess memory after SD, all animals underwent training, followed by the inhibitory avoidance task test 24h later. Also, the weight of each animal was recorded daily. In the first experiment, animals received an acute administration of lipopolysaccharide (LPS, 50 or 75 µg/kg i.p.) 3h before the inhibitory avoidance training. In the experiment 2, the animals received acute or chronic administration of anti-IL-6 antibody (Ab, 2 µg/kg i.p.). The acute administration was performed 3h before the inhibitory avoidance training, while the chronic treatment administrations were performed daily during the SD period. The 75 µg/kg dose of LPS, but not the 50 µg/kg dose, caused a significant attenuation of memory impairment in the sleep-deprived animals. Although the treatments with the anti-IL-6 Ab did not produce any significant changes in cognitive performance, the Ab attenuated weight loss in sleep-deprived animals. Taken together, these results suggest the involvement of inflammatory mediators in the modulation of memory deficit and weight loss that are observed in sleep-deprived rats.

Effects on prolactin secretion and binding to dopaminergic receptors in sleep-deprived lupus-prone mice.

Sleep disturbances have far-reaching effects on the neuroendocrine and immune systems and may be linked to disease manifestation. Sleep deprivation can accelerate the onset of lupus in NZB/NZWF(1) mice, an animal model of severe systemic lupus erythematosus. High prolactin (PRL) concentrations are involved in the pathogenesis of systemic lupus erythematosus in human beings, as well as in NZB/NZWF(1) mice. We hypothesized that PRL could be involved in the earlier onset of the disease in sleep-deprived NZB/NZWF(1) mice. We also investigated its binding to dopaminergic receptors, since PRL secretion is mainly controlled by dopamine. Female NZB/NZWF(1) mice aged 9 weeks were deprived of sleep using the multiple platform method. Blood samples were taken for the determination of PRL concentrations and quantitative receptor autoradiography was used to map binding of the tritiated dopaminergic receptor ligands [3H]-SCH23390, [3H]-raclopride and [3H]-WIN35,428 to D(1) and D(2) dopaminergic receptors and dopamine transporter sites throughout the brain, respectively. Sleep deprivation induced a significant decrease in plasma PRL secretion (2.58 +/- 0.95 ng/mL) compared with the control group (25.25 +/- 9.18 ng/mL). The binding to D(1) and D(2) binding sites was not significantly affected by sleep deprivation; however, dopamine transporter binding was significantly increased in subdivisions of the caudate-putamen--posterior (16.52 +/- 0.5 vs 14.44 +/- 0.6), dorsolateral (18.84 +/- 0.7 vs 15.97 +/- 0.7) and ventrolateral (24.99 +/- 0.5 vs 22.54 +/- 0.7 microCi/g), in the sleep-deprived mice when compared to the control group. These results suggest that PRL is not the main mechanism involved in the earlier onset of the disease observed in sleep-deprived NZB/NZWF(1) mice and the reduction of PRL concentrations after sleep deprivation may be mediated by modifications in the dopamine transporter sites of the caudate-putamen.

Effects on prolactin secretion and binding to dopaminergic receptors in sleep-deprived lupus-prone mice

Sleep disturbances have far-reaching effects on the neuroendocrine and immune systems and may be linked to disease manifestation. Sleep deprivation can accelerate the onset of lupus in NZB/NZWF1 mice, an animal model of severe systemic lupus erythematosus. High prolactin (PRL) concentrations are involved in the pathogenesis of systemic lupus erythematosus in human beings, as well as in NZB/NZWF1 mice. We hypothesized that PRL could be involved in the earlier onset of the disease in sleep-deprived NZB/NZWF1 mice. We also investigated its binding to dopaminergic receptors, since PRL secretion is mainly controlled by dopamine. Female NZB/NZWF1 mice aged 9 weeks were deprived of sleep using the multiple platform method. Blood samples were taken for the determination of PRL concentrations and quantitative receptor autoradiography was used to map binding of the tritiated dopaminergic receptor ligands [³H]-SCH23390, [³H]-raclopride and [³H]-WIN35,428 to D1 and D2 dopaminergic receptors and dopamine transporter sites throughout the brain, respectively. Sleep deprivation induced a significant decrease in plasma PRL secretion (2.58 ± 0.95 ng/mL) compared with the control group (25.25 ± 9.18 ng/mL). The binding to D1 and D2 binding sites was not significantly affected by sleep deprivation; however, dopamine transporter binding was significantly increased in subdivisions of the caudate-putamen - posterior (16.52 ± 0.5 vs 14.44 ± 0.6), dorsolateral (18.84 ± 0.7 vs 15.97 ± 0.7) and ventrolateral (24.99 ± 0.5 vs 22.54 ± 0.7 µCi/g), in the sleep-deprived mice when compared to the control group. These results suggest that PRL is not the main mechanism involved in the earlier onset of the disease observed in sleep-deprived NZB/NZWF1 mice and the reduction of PRL concentrations after sleep deprivation may be mediated by modifications in the dopamine transporter sites of the caudate-putamen.

Prostaglandin involvement in hyperthermia induced by sleep deprivation: a pharmacological and autoradiographic study.

AIMS: Hyperthermia is a characteristic functional effect of sleep deprivation (SD). We hypothesize here that prostaglandin E2 (PGE2) could be involved in hyperthermia induced by sleep deprivation. MAIN METHODS: To address this issue we examined the effects of a selective cyclo-oxygenase-2 inhibitor (COX-2) agent on hyperthermia induced by SD in rats. We also investigated binding to PGE2 receptors in hypothalamic brain areas of sleep-deprived rats using in vitro autoradiography. Male Wistar rats were deprived of sleep for 96 h using the platform technique. Sleep deprived and control groups received saline or Celecoxib (20, 30 and 40 mg/kg; p.o.) daily during the SD period. Colonic temperature was measured daily. KEY FINDINGS: Results indicated that core temperature of sleep-deprived rats that receiving saline increased from the first to the fourth day of SD compared to baseline and to the respective control group. However, the hyperthermia induced by SD was not blocked by COX-2 inhibitor at any dose. [(3)H]PGE2 binding did not differ significantly among the groups in any of a number of hypothalamic areas examined. SIGNIFICANCE: Although SD rats showed no response to the COX-2 inhibitor and no alterations in [(3)H]PGE2 binding, the possibility remains that other prostaglandin system and/or receptor subtypes may be altered by SD.

Paradoxical sleep deprivation and sleep recovery: effects on the hypothalamic-pituitary-adrenal axis activity, energy balance and body composition of rats.

Numerous studies indicate that sleep deprivation alters energy expenditure. However, this conclusion is drawn from indirect measurements. In the present study, we investigated alterations of energy expenditure, body composition, blood glucose levels, plasma insulin, adrenocorticotropic hormone (ACTH) and corticosterone levels immediately after 4 days of sleep deprivation or after 4 days of sleep recovery. Rats were sleep deprived or maintained in a control environment (groups sleep-deprived/deprivation and control/deprivation). One half of these animals were sacrificed at the end of the deprivation period and the other half was transported to metabolic cages, where they were allowed to sleep freely (groups sleep-deprived/recovery and control/recovery). At the end of the sleep recovery period, these rats were sacrificed. After sleep deprivation, sleep-deprived rats exhibited loss of body weight, augmented energy expenditure and reduced metabolic efficiency compared to control rats. These alterations were normalised during the sleep recovery period. The body composition of sleep-deprived rats was altered insofar as there was a loss of fat content and gain of protein content in the carcass compared to control rats. However, these alterations were not reversed by sleep recovery. Finally, plasma levels of insulin were reduced during the sleep deprivation period in both control and sleep deprived groups compared to the recovery period. After the deprivation period, plasma ACTH and corticosterone levels were increased in sleep-deprived rats compared to control rats, and although ACTH levels were similar between the groups after the sleep recovery period, corticosterone levels remained elevated in sleep-deprived rats after this period. By means of direct measurements of metabolism, our results showed that sleep deprivation produces increased energy expenditure and loss of fat content. Most of the alterations were reversed by sleep recovery, except for corticosterone levels and body composition.

Sleep deprivation reduces total plasma homocysteine levels in rats.

Hyperhomocysteinemia has been associated with pathological and stressful conditions and is a risk factor for cardiovascular disease. Since sleep deprivation is a stressful condition that is associated with disruption of various physiological processes, we investigated whether it would also be associated with increases in plasma homocysteine levels. Further, since hyperhomocysteinemia may promote oxidative stress, and we had previously found evidence of oxidative stress in brain following sleep deprivation, we also searched for evidence of systemic oxidative stress by measuring glutathione and thiobarbituric acid reactive substance levels. Rats were sleep deprived for 96 h using the platform technique. A group was killed after sleep deprivation and another two groups were allowed to undergo sleep recovery for 24 or 48 h. Contrary to expectation, plasma homocysteine was reduced in sleep-deprived rats as compared with the control group and did not revert to normal levels after 24 or 48 h of sleep recovery. A trend was observed towards decreased glutathione and increased thiobarbituric acid reactive substance levels in sleep-deprived rats. It is possible that the observed decreases in homocysteine levels may represent a self-correcting response to depleted glutathione in sleep-deprived animals, which would contribute to the attenuation of the deleterious effects of sleep deprivation.

Melatonin treatment does not prevent decreases in brain glutathione levels induced by sleep deprivation.

Recent findings from this laboratory revealed that sleep deprivation reduces total glutathione (GSH) levels in hypothalamus, suggesting an increased vulnerability to oxidative damage. Since melatonin has been shown to prevent oxidative damage in other experimental situations, the present study tested the effects of exogenous melatonin on sleep deprivation-induced GSH decreases. Rats were deprived of sleep for 96 h on small platforms, and melatonin (10 mg/kg body weight; i.p.) or vehicle was given twice a day. Hypothalamic GSH levels were significantly reduced in sleep-deprived groups, irrespective of melatonin treatment. Indeed, unexpectedly, melatonin treatment resulted in lower hypothalamic GSH levels in all groups, including cage controls. These results confirm that sleep deprivation reduces hypothalamic GSH and further indicate that melatonin treatment not only is ineffective in reversing this effect but may actually potentiate it.

Sensitization to ethanol's stimulant effect is associated with region-specific increases in brain D2 receptor binding.

RATIONALE: Stimulation of locomotor activity by low doses of ethanol (EtOH) and the potentiation of this response after repeated administration (sensitization) have been related to EtOH's rewarding and addictive properties and to altered dopaminergic activity in brain. In mice, behavioral sensitization to EtOH occurs only in a subset of treated animals, and this provides an opportunity for distinguishing general drug effects from sensitization-specific brain effects. OBJECTIVES: In view of evidence suggesting a role for dopamine D2 receptors in EtOH preference and abuse liability, the present study addressed the hypothesis that D2 binding would be altered in specific brain regions in mice showing differential sensitization responses to chronic EtOH administration. METHODS: Male albino Swiss mice received 2.4 g/kg EtOH i.p. daily for 21 days and were then separated into sensitized or non-sensitized subgroups on the basis of weekly locomotor activity tests. RESULTS: Autoradiographic analyses of [(3)H]raclopride binding to D2 sites revealed significant increases in the anterior caudate-putamen of mice in the EtOH-sensitized group when compared with either saline controls (+40%, P<0.00009) or to mice in the EtOH non-sensitized group (+32%; P<0.0003). Smaller increases were seen in the ventrolateral caudate-putamen of sensitized animals (+18% vs. control, P<0.02; and 12% vs. non-sensitized mice, P<0.07). No differences were found in other brain regions, including the nucleus accumbens, olfactory bulb and substantia nigra. CONCLUSIONS: The observed increases in D2-receptor binding in circumscribed targets of nigrostriatal projections may reflect either a pre-existing condition in sensitization-prone animals or a selective vulnerability of D2 receptors to chronic EtOH in these animals. In either case, it may be a marker for differential susceptibility to EtOH sensitization.

Treatment with dexamethasone alters yawning behavior induced by cholinergic but not dopaminergic agonist.

Because stressful manipulations have been reported to modify drug-induced yawning, the present study investigated the effects of single and repeated treatment with a synthetic glucocorticoid, dexamethasone (DEXA) on apomorphine- and pilocarpine-induced yawning in male rats. Neither single nor repeated treatment with DEXA altered apomorphine-induced yawning. Single administration of DEXA, however, resulted in an increased number of yawns induced by pilocarpine. Conversely, repeated administration of DEXA led to a decreased number of yawns induced by pilocarpine. In conclusion, the present findings show that dopaminergic and cholinergic are distinctly altered by DEXA, in terms of yawning behavior when animals received DEXA.

Immediate increase in benzodiazepine binding in rat brain after a single brief experience in the plus maze: a paradoxical effect.

A single drug-free experience in the elevated plus-maze is well documented to reduce the behavioral effects of benzodiazepines (BZs) in subsequent tests. To ascertain the possible role of altered BZ receptor binding to in this phenomenon, rats received a 5-min exposure to the elevated plus maze and were immediately sacrificed. Receptor autoradiography revealed that [3H]flunitrazepam binding was significantly elevated in several amygdaloid and hippocampal nuclei (range: 13-23%); [3H]muscimol binding in adjacent sections was not significantly altered. These results suggest that BZ receptors can change very rapidly in response to anxiogenic conditions. However, the unexpected finding that [3H]flunitrazepam binding is increased by maze exposure suggests that the subsequent loss of BZ anxiolytic effects in the plus-maze is probably unrelated to alterations in BZ binding in brain.

Increased ACTH and corticosterone secretion induced by different methods of paradoxical sleep deprivation.

The methods used to induce paradoxical sleep (PS) deprivation are believed to be stressful. In the present study, two methods were compared in regard to their ability to activate the hypothalamic-pituitary-adrenal (HPA) axis. Animals were placed on multiple large (MLP) or small (MSP) platforms or on single large (SLP) or small (SSP) platforms and blood sampled at the end of a 4-day period of PS deprivation (experiment 1) or on Days 1 (short-term) and 4 (long-term) of PS deprivation (experiment 2). ACTH and corticosterone (CORT) levels were determined by RIA. The results of experiment 1 showed that all experimental animals presented increased ACTH response, compared to controls. CORT levels, however, were only elevated in MSP animals, suggesting increased adrenal sensitivity. Experiment 2 showed that ACTH levels of MSP animals were higher than MLP and SSP animals, and that animals placed on the multiple platform tanks showed the highest ACTH levels on Day 4 of manipulation. CORT levels were elevated in the animals kept over small platforms, and these levels where higher on Day 1 than basal and further elevated on Day 4 of PS deprivation. These results indicate that the multiple platform technique induces a distinct activation of the HPA axis, and that PS deprivation may act as an additional stressor.

Sleep deprivation induces brain region-specific decreases in glutathione levels.

Rats were deprived of sleep for 96 h by the platform technique and total glutathione (GSHtau) levels were measured in seven different brain areas. Glutathione levels were found to be significantly reduced in the hypothalamus of sleep-deprived animals when compared with large platform (-18%) or home cage (-31%) controls. Deprived rats also had reduced GSHtau levels in thalamus compared with home cage controls only. Glutathione levels did not differ among the three groups in any of the other brain areas examined. These results indicate that specific brain areas may be differentially susceptible to oxidative stress after sleep deprivation. The apparent vulnerability of the hypothalamus to these effects may contribute to some of the functional effects of sleep deprivation.

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.

Absence of oxidative stress following paradoxical sleep deprivation in rats.

Paradoxical sleep deprivation was performed on rats using platform technique to investigate the oxidative process associated with it. Levels of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), total glutathione (GSH) and malondialdehyde production were measured in brain of rats under control conditions (C) and those on single large platforms (SLP), multiple large platforms (MLP), single small platforms (SSP) and multiple small platforms (MSP) groups. SOD, CAT and GPx brain activity and malondialdehyde production were not modified by any of the procedures. Brain GSH, however, was significantly reduced in both SSP and SLP groups. These results suggest that paradoxical sleep deprivation per se is not associated with oxidative damage. The observed alterations could be attributed to factors such as immobilization and social isolation present in the single platform techniques.

Atropine increases pilocarpine-induced yawning behavior in paradoxical sleep deprived rats.

Paradoxical sleep (PS) deprivation has been suggested to induce supersensitivity of postsynaptic dopamine (DA) receptors and subsensitivity of acetylcholine (ACh) receptors. Yawning behavior is reduced after PS deprivation and is believed to result from an interaction between ACh and DA systems. Concomitant treatment of PS deprived animals with DA agonists reverses PS deprivation effects on stereotypy and aggressiveness. To examine this possibility on yawning behavior, rats were treated, during the deprivation period, with atropine, methamphetamine, haloperidol or distilled water. Following PS deprivation, rats were injected with apomorphine or pilocarpine and number of yawns was recorded. Atropine increased yawning of PS deprived rats induced by pilocarpine, but not by apomorphine. Treatment with methamphetamine and haloperidol did not change PS deprivation effect on pilocarpine- and apomorphine-induced yawning. The data suggest that reversal of PS deprivation-induced yawning inhibition is mediated distinctly by both acetylcholine and dopamine systems.

Effects of stress on drug-induced yawning: constant vs. intermittent stress.

Effects of stress on drug-induced yawning: Constant vs. intermittent stress. PHYSIOL BEHAV 58(1) 181-184, 1995.--Experiment 1 tested whether chronic exposure to immobilization, foot shock or forced swimming would result in suppression of apomorphine-, pilocarpine-, and physostigmine-induced yawning. Immobilization caused suppression of yawning, whereas foot shock and swimming resulted in increased number of yawns. Since interstressor interval was long in the two latter stressors, animals could have recovered and the increase in yawning could be due to the last (acute) exposure to stress. In Experiment 2 we recorded the number of yawns induced by pilocarpine in animals exposed to 1 h of swimming or foot shock. No differences between control and acutely stressed animals were detected. These results suggest that yawning is differently altered by constant and intermittent stressors (i.e., diminished by constant and increased by intermittent stress).