Effect of stressor intensity on habituation of the adrenocortical stress response

Although it is known that the number of presentations of a stressor can influence the adrenocortical stress response, relatively little information exists on how stressor intensity affects this process. To evaluate this, we repeatedly presented rats with stressors of 3 different intensities and sampled blood for corticosterone. The first major finding was that the rat's initial adrenocortical responsiveness regardless of the stressor employed was a critical variable. Rats that showed a small corticosterone response showed no evidence of habituation or of differences due to stressor intensity. Rats that showed an initial robust response all showed partial habituation of their corticosterone response over time but the patterns varied with stressor intensity. Handled and prone restrained rats showed the same pattern but rats subjected to the more intense stressor of supine restraint showed delay in habituation and tonically elevated responses. These data indicate that individual differences in reactivity to stressors as well as stressor intensity can influence the pattern of the stress response over the course of repeated administration of the stressor.     

Habituation and sensitization of plasma catecholamine responses to chronic intermittent stress: effects of stressor intensity

Adult male Sprague-Dawley rats were exposed acutely (1 time for 30 minutes) or chronically (30 minutes per day for 27 days) to swim stress in water maintained at either 18 degrees C, 24 degrees C or 34 degrees C. Each rat was prepared with an indwelling tail artery catheter to allow for direct measures of mean arterial pressure (MAP, mmHg) and heart rate (HR, beats per minute) and for remote collection of blood samples before, during and after the 1st or 27th swim stress session. Blood samples were later analyzed for plasma levels of norepinephrine and epinephrine to serve as an assessment of sympathetic-adrenal medullary activity. Compared to handled controls, body weight gain was reduced significantly in rats exposed chronically to swim stress at any of the 3 temperatures. However, baseline values of MAP and HR and plasma levels of norepinephrine and epinephrine were similar in chronically stressed rats compared to their handled controls. The plasma norepinephrine response of rats exposed chronically to either 18 degrees C or 24 degrees C swim stress was significantly greater than that of matched controls stressed for the first time. In contrast, the plasma epinephrine response of chronically stressed rats from these two groups was slightly but not significantly reduced compared to matched controls. For swim stress at 34 degrees C, the plasma norepinephrine and epinephrine responses of chronically stressed rats were reduced significantly compared to controls stressed for the first time. These findings demonstrate that stressor intensity affects sensitization and habituation of plasma catecholamine responses in laboratory rats exposed to chronic intermittent stress.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of amygdalectomy on habituation and CER in rats.

Amygdalectomized, Sham operates and unoperated rats received 20 habituation trials followed by CER training during which the habituation stimulus was made a CS for 10 CS-UCS pairings. Although no reliable differences in rate of magnitude of habituation as measured by suppression ratio magnitudes and ITI durations were apparent between any groups, these same measures indicated that substantial deficits in conditioned suppression were produced by bilateral amygdalectomy. In addition the lesion produced a reliable deficit in unconditioned suppression and a slight but reliable reduction in the number of ccs of water consumed in a 24 hour period. These results are more consistent with the hypothesis that amygdalectomy interferes with the arousal of fear than with the proposals that this lesion produces deficits in habituation or in response inhibition.     

Aerobic exercise intensity and time of stressor administration influence cardiovascular responses to psychological stress

This study examined cardiovascular responses as a function of time following exercise in which participants were exposed to a laboratory stressor. Ninety (42 women) young (18-35 years old) nonsmoking normotensive participants engaged in 30 min of high and low intensity (75-80% and 50-55% VO(2) max) aerobic exercise and a sedentary control condition. Participants were randomly assigned to a laboratory stressor 5, 30, or 60 min following the exercise bout. Results indicate that low and high intensity exercise significantly reduce heart rate (HR) and systolic and diastolic blood pressure reactivity and HR recovery values. An inverse relationship between intensity of exercise and subsequent cardiovascular reactivity was found. These findings suggest attenuated stress responses following acute exercise depend both on exercise intensity and the time of exposure to psychological stress following exercise.     

Classical conditioning,sensitization and habituation in the spinal cat

Changes in the flexion reflex of the tibialis anterior muscle of acute spinal cats were examined during conditioning, sensitization and habituation paradigms. Experimental animals were classically conditioned by pairing electrical stimulation of the saphenous nerve (CS) with stimulation of the superficial peroneal nerve (US). Recordings from these nerves assured known and constant stimulus inputs. The response observed was an increase in the magnitude of the reflex response to the CS over training. Habituation (CS only) and sensitization (CS and US presentations, unpaired) control animals exhibited no such reflex facilitation. The results of post-tetanic potentiation studies indicated that the intertrial intervals used were not a factor in the differences observed between experimental and control groups. The results give positive support to the concept of spinal conditioning and emphasize the potential of this model system for the study of neural correlates of learning.     

Influence of stressor predictability and behavioral control on lymphocyte reactivity, antibody responses and neuroendocrine activation in rats

The present experiments were designed to study the influence of prediction and control of electric shocks on various aspects of immune function, and the possible intermediate role of glucocorticoid hormones. After two sessions of inescapable footshocks, the reactivity of splenocytes to concanavalin A was reduced by one third. This effect was completely reversed when each shock was preceded by a warning stimulus, even though the adrenocortical response was the same in both conditions. In another experiment, rats were submitted to ten sessions of continuous avoidance in a shuttle-box and a group of yoked animals received the same footshocks without any relationship to their shuttling behavior. Although yoked rats displayed a reduced reactivity of splenocytes to lectins, animals of the avoidance group had a reduced antibody response to sheep erythrocytes. In contrast, no difference was observed in the corticosterone or prolactin response. These data further support the importance of psychological factors on stress-induced changes in immune functions. Furthermore, they demonstrate that various aspects of the immune system are differentially affected by behavioral factors and the results argue against a major role for the adrenocortical system in mediating these changes.     

Sensitization of sympathetic-adrenal medullary responses to a novel stressor in chronically stressed laboratory rats

Studies from this and other laboratories have demonstrated that sympathetic-adrenal medullary responses are reduced in laboratory rats exposed to chronic intermittent stress. To extend these findings, we have conducted three experiments in which laboratory rats were exposed daily for 26 consecutive days to the same (homotypic) stressor and then challenged with a novel (heterotypic) stressor. Each stress session lasted for 30 minutes and the following homotypic/heterotypic treatments were included: Experiment 1--restraint/footshock; Experiment 2--cold swim/restraint; and Experiment 3--footshock/cold swim. Controls were exposed to the heterotypic stressor only. Plasma levels of norepinephrine (NE) and epinephrine (EPI) were measured in blood samples taken from control and chronically stressed rats to provide an assessment of sympathetic-adrenal medullary activity. In each experiment, chronically stressed rats gained significantly less weight than handled controls. Basal plasma levels of NE and EPI were similar for chronically stressed and control rats. In contrast, plasma catecholamine responses to the heterotypic stressor were significantly greater in stressed rats compared to controls that were stressed for the first time. This pattern of exaggerated responses was especially evident for the cold swim/restraint and footshock/cold swim groups. These findings provide evidence for a sensitization of sympathetic-adrenal medullary responses to a novel stressor in animals previously exposed to chronic intermittent stress.     

Involvement of corticosterone in cardiovascular responses to an open-field novelty stressor in freely moving rats

The aim of the present study was to investigate the modulatory action of different concentrations of circulating corticosterone occupying either predominantly mineralocorticoid receptors (MR) or both MR and glucocorticoid receptors (GR) in control of cardiovascular responses to a novelty stressor. Six groups of rats were instrumented with radiotelemetry transmitters: sham-operated controls, adrenalectomised (ADX) controls, ADX with chronic implantation of a 20-mg corticosterone pellet, ADX with chronic implantation of a 100-mg corticosterone pellet, ADX receiving acute bolus injection of 0.25 mg/kg of corticosterone, and ADX with both implantation of a 20-mg corticosterone pellet and bolus treatment. Exposure to the novelty of an open field caused an increase in blood pressure, heart rate, body temperature and exploratory locomotor activity. The pressor response was dose-dependently increased in ADX rats implanted with a corticosterone pellet. Bolus injection of corticosterone at 10 min prior to novelty had no effect. The tachycardia was reduced in ADX rats compared to sham-operated controls, and this effect was restored by implantation of a 20-mg, but not 100-mg, corticosterone pellet. Bolus injection of corticosterone facilitated the return of heart rate towards baseline levels. The increase in body temperature was reduced in ADX rats, a deficit that was normalised by implantation of either corticosterone dose but not by acute bolus treatment. Locomotor activity was not different between the groups except for a slightly more rapid decline of locomotor activity in both groups treated with a bolus injection of corticosterone. These data show an important role of putative brain MR in maintaining adequate cardiovascular and behavioural responsiveness to a mild psychological stressor, while additional acute or chronic occupation of GR has further differential and sometimes opposing effects.     

Effects of a varying stimulus context on habituation and sensitization of the OR

Twelve male subjects were tested in an experiment which investigated the habituation and sensitization processes of the orienting response (OR) in a varying stimulus context. Visual stimuli which varied along information and contour dimensions were presented in four blocks of 10 stimuli each. A tone served as the sensitizing stimulus and was presented before the fourth stimulus in each block. In two blocks the 5th, 6th and 7th stimuli were omitted (“blanks” condition) while in the other two blocks all stimuli were presented(“no blanks” condition). OR frequency and skin conductance levels indicated that the tone produced sensitization. Despite this, there was no evidence of dishabituation to the following stimuli. OR frequency to the 8th stimulus was significantly higher if the preceding three stimuli had been omitted. This result does not support a dual-process theory of habituation. However, rather than being contradictory, these findings are interpreted as further demonstration that laws governing habituation processes during repeated presentations of an identical stimulus do not apply in a context of repeated presentations of a varying stimulus.     

Effects of current intensity on behavioral and autonomic heat-loss responses in intracranial self-stimulating rats

We investigated the effects of current intensity on behavioral and autonomic heat-loss responses in intracranial self-stimulating (ICSS) rats. At an ambient temperature (Ta) of 22 degrees C, the tail vasodilation during ICSS behavior began at higher hypothalamic temperature (Thy) at higher current intensity. At a Ta of 36 degrees C, body extension, a typical heat-loss response in rats, appeared during ICSS behavior and frequently interrupted lever pressing. When the body extension first began, Thy was elevated if current intensity was high. In experiments in which current intensity was varied between two levels at a Ta of 22 degrees C, if current intensity was lowered after tail vasodilation began and Thy reached a steady level, the rat interrupted pressing the lever to lose heat through grooming or body extension. Rectal temperature (Tre) dropped in the process. If, on the other hand, current intensity was raised, the rate of lever pressing increased and Tre rose with tail vasoconstriction. The data suggest that the magnitude of the reward, as reflected in current intensity, affects both behavioral and autonomic heat-loss responses.     

Behavioural and glucocorticoid responses of dogs (Canis familiaris) to kennelling: Investigating mitigation of stress by prior habituation

We have explored the validity of urinary cortisol/creatinine ratios (C/C) and behavioural measures as indicators of acute psychological stress in the domestic dog, by monitoring 1 year old male Labrador Retrievers (N=31) prior to and following their introduction to novel kennels in a training establishment. Baseline early morning urine samples were taken in the dogs' original homes and then urine samples and remote recordings of behaviour were taken for ten consecutive days after transfer to the kennels. The impact of this potential stressor was manipulated by previously habituating half of the subjects to confinement in a kennel. We hypothesised that stress levels would increase upon introduction to the training establishment, but that the response would be mitigated by kennel habituation. C/C increased in all dogs when they entered the training establishment, this increase was significantly higher in the non-habituated group, and in this group C/C remained above baseline even 12 weeks after transfer. Despite the homogeneity of the subjects, the behaviour measured showed very little correlation to the C/C ratios, and the changes in behaviour that were observed, such as decreases in whining and time spent at the front of the kennel, could equally be attributed to dogs learning the most effective strategies for gaining human attention in the kennels as to attenuation of stress. We conclude that urinary C/C is a valuable indicator of acute stress and hence welfare status in dogs, but that behavioural measures need to be interpreted with caution due to individuality in coping strategies.     

Effect of voluntary contraction intensity on the H-reflex and V-wave responses

This study examined the evolution of H-reflex and V-wave responses of soleus muscle during maximal voluntary plantar-flexor contraction. We also investigated the relationship between the V response and force level and between V-wave during maximal voluntary contraction (MVC) and the maximal H reflex at rest. The H-reflex and the V-wave responses are measures of motoneuron excitability and also reflect the magnitude of presynaptic inhibition on Ia afferents and the magnitude of descending motor drive. Both may be influenced by postsynaptic inhibition. Twenty male subjects participated in the study and were assigned to one of two groups. The maximal M wave (Mmax) was evoked at rest in the 20 subjects, who then performed 10 maximal voluntary contraction. During MCV performance, a stimulus was delivered at supra-maximal intensity, which allowed us to record the superimposed M wave (Msup) and V wave of the soleus muscle. These parameters were also recorded during sub-maximal contractions (20, 40, 60, 80% of one MVC) in 10 subjects. The maximal H reflex (Hmax), was evoked at rest in the other 10 subjects. These subjects then performed 10 MVC and the Hsup (superimposed H, evoked by means of stimulus at Hmax intensity) was recorded. The results show that the amplitude of maximal M wave increased during MVC (gain 44.52 +/- 10.71%). No significant difference between Hmax/Mmax at rest and the Hsup/Msup ratios during MVC was observed, while an effect of force level on the V/Msup ratio was found. V/Msup and Hmax/Mmax were linearly correlated (r2 = 0.81), but V/Msup was significantly lower (P < 0.01) than Hmax/Mmax. In conclusion, the present study shows that maximal voluntary contractions potentiate some reflex responses. The V wave, which reflects motoneuron excitability presynaptic inhibition of Ia afferents and the magnitude of descending central motor drive to spinal motoneurons, may be a relatively simple method to analyse the modulation adaptive neural alterations at spinal and supraspinal level during voluntary contractions.     

Serum cholesterol levels and stressor controllability in rats

Whether an organism can control a stressful event is often an important variable determining the impact of the event on physiology and behavior. Numerous behavioral and physiological variables are more adversely affected by uncontrollable stress. The present experiment with rat subjects compared the effect of controllable stress (escape conditioning) or uncontrollable stress (yoked control group) vs. home cage controls on total cholesterol, as well as high-density lipoprotein (HDL) and low/very-low density lipoprotein (LDL/VLDL) serum cholesterol. Results indicated that both stressed groups had higher total and LDL/VLDL cholesterol levels than home cage controls. No group differences were observed with HDL cholesterol. The escape and yoked control subjects did not differ from each other in any dependent measure. Results are discussed in terms of the probable mediators of stress-induced cholesterol increases, and the fact that these mediators may be insensitive to stressor controllability.     

Cardiovascular and sympathetic nervous system responses to an acute stressor in borderline hypertensive rats (BHR)

The present study examined cardiovascular and plasma catecholamine responses to acute footshock stress in adult male Wistar-Kyoto (WKY) normotensive, borderline hypertensive (BHR), and spontaneously hypertensive (SHR) rats. Basal mean arterial pressure and heart rate were equivalent for SHRs and BHRs, and levels for both groups were elevated compared to WKYs. Following transfer to the footshock chamber, blood pressure increased to a greater degree in SHRs than in WKYs or BHRs. However, the tachycardia was exaggerated in both BHRs and SHRs compared to WKYs. In response to intermittent footshock stress, all groups had comparable heart rate increases while maintaining blood pressure near baseline levels. SHRs demonstrated a sympathetic hyperresponsiveness to footshock stress, with greater increases in plasma norepinephrine and epinephrine levels than WKYs immediately following footshock. At 5 minutes postfootshock, plasma catecholamines remained elevated in SHRs over both WKYs and BHRs. Plasma catecholamine increases following footshock were comparable at all time points between WKYs and BHRs. The present results demonstrate that sympathetic responsiveness of BHRs to acute footshock stress is more similar to normotensive WKYs than to hypertensive SHRs.     

Initial habituation or sensitization of the GSR depends on magnitude of first response

Habituation and sensitization of the galvanic skin response (GSR) to shock were studied in college males. One would expect that at any given time individuals vary on a continuum in the degree of their initial GSR responsiveness to a stimulus. Here it is shown that those on the higher end of the continuum are more likely to show a response decrement between trials 1 and 2 when this stimulus is presented intermittently (i.e., habituate), while subjects at the lower end of the continuum are more likely to show a response increment between trials 1 and 2 (i.e., sensitize) to the same intermittent stimulus. Following these initial and opposite changes on the first two trials, which bring all subjects to approximately the same level of responsiveness, they all now behave similarly and continue to show habituation. Previous work on subhuman species showed that habituation and sensitization curves had certain characteristic similarities across species. The present work shows that some of these similarities also occur in humans. One may speculate, that as with other characteristics that are similar across species, they have survival value.     

Comparison of frequency, intensity, and duration of aggressive responses in rats

Three judges were trained to evaluate the frequency, intensity, and duration of aggressive responses in rats. Intrajudge and interjudge reliability ranged from .84 to .99. Correlation coefficients between frequency, intensity, and duration of aggressive responses ranged from .89 to .99. All three measures were found to be highly reliable methods of measuring aggression.