Peripheral, but not local or intracerebroventricular, administration of benzodiazepines attenuates evoked activity of locus coeruleus neurons

To determine whether benzodiazepine receptors in the locus coeruleus (LC) regulate the firing of LC neurons, the effects of systemic, intracerebroventricular, and local administration of various benzodiazepines on LC activity were compared. Systemic administration of diazepam, chlordiazepoxide, and alprazolam in anesthetized rats markedly attenuated sensory-evoked activity of LC neurons while also suppressing, but to a lesser degree, spontaneous LC firing rates. When microinfused into the LC region, diazepam, chlordiazepoxide, and alprazolam reduced spontaneous LC firing rates to the same extent observed following systemic administration; however, unlike systemic administration, infusion of benzodiazepines into the LC failed to attenuate evoked LC activity. Similarly, intracerebroventricular administration of diazepam and chlordiazepoxide suppressed spontaneous, but not evoked, LC activity. These findings indicate that benzodiazepines suppress spontaneous LC firing by stimulating benzodiazepine receptors in the LC while attenuating evoked LC activity by stimulating benzodiazepine receptors that are neither in the LC nor accessible to benzodiazepines introduced into the ventricular system.

Behavioral and neural influences on cellular immune responses: effects of stress and interleukin-1

A series of experiments examined effects of stressful conditions on several cellular immune responses and attempted to elucidate the physiological mechanisms underlying these effects. Initial studies showed that stressful conditions can profoundly suppress immune responses of blood and splenic lymphocytes, including T-cell mitogenesis, natural killer cell activity, production of interleukin-2 (IL-2) and interferon and IL-2 receptor expression. Subsequent studies found that (1) multiple physiological pathways mediate stress-induced suppression of these responses; (2) stress-induced suppression of these responses is produced, at least in part, by a peptide with molecular weight greater than 10 kilodaltons, which stressed animals release into circulation; (3) whereas most stressful conditions suppress immune responses, stressful conditions of moderate intensity can enhance cellular immune responses; and (4) extremely small quantities of interleukin-1 (IL-1) acting in the brain (e.g., 3.1-12.4 X 10(-15) moles) bring about suppression of cellular immune responses very rapidly and for a prolonged period of time. The relationship between the newly-discovered immunosuppressive influence of IL-1 in the brain and immunosuppression produced by stressful conditions remains to be determined.

Altered activity of the locus coeruleus in an animal model of depression

In rats showing depression of active behavior, locus coeruleus (LC) neurons were not inhibited as they are normally; in particular, electrophysiologic recording showed LC neurons were hyperresponsive to excitatory input. Moreover, the degree to which LC neurons of individual animals were hyperresponsive correlated positively with the degree to which active behavior was depressed. Recently, hyperresponsiveness of LC neurons has been found to occur when the inhibitory influence of alpha 2-receptors on LC firing is blocked. Pharmacologic blockade of alpha 2-receptors in animals showing behavioral depression was not able to increase responsiveness of LC neurons as in normal animals, indicating that alpha 2-receptors were subnormally activated in behavioral depression. These results show that activity of the principle noradrenergic cell group in the brain, the locus coeruleus, is altered in behavioral depression in an animal model, and suggest that abnormalities in this system and its regulatory elements, such as alpha 2-receptors, may be present in some types of clinical depression.

Pertussis toxin blocks the effects of alpha 2-agonists and antagonists on locus coeruleus activity in vivo

This study assessed the effects of pertussis toxin, which is known to inactivate G proteins and therefore to block receptors linked to G proteins, on electrophysiological activity of the locus coeruleus in vivo. Pertussis toxin was injected into the lateral cerebral ventricle of rats, and locus coeruleus activity was then recorded. Compared to vehicle-injected control animals, pretreatment with pertussis toxin markedly increased the spontaneous firing rate of locus coeruleus neurons. In addition, the alpha 2-antagonist idazoxan was no longer able to augment either spontaneous or evoked locus coeruleus activity after pretreatment with pertussis toxin. Finally, pretreatment with pertussis toxin made locus coeruleus neurons resistant to inhibition by the alpha 2-agonist clonidine. These results are consistent with the view that pertussis toxin blocks alpha 2-receptors, receptors linked to G proteins, in vivo.

Transthoracic needle aspiration biopsy following negative fiberoptic bronchoscopy in solitary pulmonary nodules

To evaluate the utility of transthoracic needle aspiration biopsy (TTNAB) following negative fiberoptic bronchoscopy (FOB) in a patient with a solitary pulmonary nodule (SPN), we reviewed the records from 262 patients who had undergone TTNAB over an eight-year period. Fifty-eight patients had a SPN and met the criteria for inclusion in this series. Twenty-five of these (43 percent) were diagnosed by TTNAB; 24 had malignant lesions; one had M tuberculosis. Of the remaining 33 patients, 18 went on to definitive diagnostic procedures (surgery, repeat FOB or TTNAB). Nine of these patients had a malignancy. Fifteen patients were followed long-term; one was diagnosed with carcinoma two years after the initial work-up. Carcinoma was not found in any patient under 40 years of age. A benign lesion was diagnosed by TTNAB in only two patients. We conclude that TTNAB is a valuable procedure in the evaluation of patients with a SPN and negative FOB. While a negative FOB and TTNAB do not assure that the lesion is benign, a complex of variables influences the decision regarding thoracotomy vs careful follow-up.

Neurochemical and electrophysiological events underlying stress-induced depression in an animal model

This paper has described an animal (rodent) model of depression in which depression-like characteristics are produced by exposure of animals to stressful events that they cannot control. This model, called "stress-induced depression," appears to be mediated by stress-induced changes in brain norepinephrine (NE), and evidence now indicates that a large depletion of NE in the Locus Coeruleus region of the brain stem (LC) is critical for producing the behavioral disturbance seen in this model. To explain the functional significance of NE depletion in the LC, it has been suggested that this change results in decreased stimulation, (i.e., functional blockade) of alpha-2 receptors that normally inhibit firing of LC neurons. Consequently, LC neurons should be disinhibited in stress-induced depression. The final part of this paper describes findings from electrophysiological measurement of LC activity. Prior to measuring changes occurring in stress-induced depression, studies examined the basic role played by alpha-2 receptors in LC activity. These receptors were found to regulate the responsivity of LC neurons to excitatory stimuli, so that pharmacological blockade of these receptors increased the firing of LC neurons to excitatory input. When LC activity was then measured in stressed animals showing behavioral depression, responsivity of LC neurons to excitatory input was elevated in comparison to animals not exposed to stress. Also, administration of an alpha-2 blocking drug could not increase responsivity of LC neurons in stressed animals, which further indicated that alpha-2 receptors in the LC region were functionally blocked in these animals. Finally, the amount that LC neurons increased their firing to an excitatory stimulus (i.e., an index of alpha-2 receptor blockade) was highly correlated with the amount of behavioral depression seen in an activity test conducted just prior to electrophysiological measurement. These electrophysiological findings indicate that LC neurons are disinhibited in stress-induced depression, and that this disinhibition is particularly characterized by increased responsivity of LC neurons to excitatory input, which is indicative of alpha-2 receptor blockade. These findings further support the view that the LC is involved in stress-induced depression, and are consistent with a proposed mechanism that attributes behavioral disturbance in the model to disinhibition of LC neurons arising from subnormal activation of inhibitory alpha-2 receptors.

Alpha-2 receptor blockade increases responsiveness of locus coeruleus neurons to excitatory stimulation

This study presents evidence that alpha 2-receptors in the locus coeruleus (LC) regulate the responsiveness of LC neurons to excitatory stimuli. In the first experiment, intravenous administration of the alpha 2-adrenergic antagonist idazoxan markedly potentiated the responses of LC neurons to the excitatory stimulus of contralateral hind paw compression (PC). Increased responsiveness of LC neurons to PC was seen with doses of idazoxan far below those that altered spontaneous activity of the LC. In the second experiment, increased responsiveness of LC neurons to PC was seen when low doses of idazoxan were infused directly into the LC, thereby indicating that increased responsiveness of LC neurons resulted from blockade of alpha 2-receptors in the LC region and not from greater stimulus input to the LC resulting from blockade of alpha 2-receptors elsewhere. In the third experiment, another alpha 2-adrenergic antagonist, yohimbine, also increased the responsiveness of LC neurons to PC. Finally, the response of the LC to another excitatory stimulus, peripheral injection of nicotine, was also found to be increased by idazoxan. Results obtained prior to these studies had indicated that alpha 2-receptors in the LC regulate the period of quiescence that follows a burst of LC firing. However, more recent studies suggest that this quiescence results primarily from changes in ionic conductance of the membrane following directly from depolarization. The present findings indicate that, in addition to whatever role alpha 2-receptors play in regulating postfiring quiescence, these receptors in the LC appear to play a major role in regulating the responsiveness of LC neurons to excitational influences.

Infusion of a monoamine oxidase inhibitor into the locus coeruleus can prevent stress-induced behavioral depression

Behavioral depression produced by exposing animals to a stressor that they cannot control (uncontrollable shock) was reversed by infusion of the monoamine oxidase (MAO) inhibitor pargyline into the locus coeruleus (LC) region of the brain stem. Following exposure to uncontrollable shock, rats were infused through bilateral cannulas implanted in the LC region with either pargyline or vehicle. At 110 min after infusion, animals were tested for behavioral activity in a swim tank. Immediately following the behavioral test, animals were sacrificed for determination of the monoamines [norepinephrine (NE), dopamine (DA), serotonin (5-HT)], as well as 5-hydroxy-indoleacetic acid (5-HIAA) in various brain regions. The results showed that animals exposed to uncontrollable shock and then infused with vehicle exhibited significantly less activity in the swim test than animals not exposed to shock and similarly infused with vehicle; thus, the usual behavioral depression following exposure to uncontrollable shock was observed. On the other hand, shocked animals infused with pargyline did not show reduced activity in the swim test. Unshocked animals infused with pargyline showed no more activity than did shocked animals infused with pargyline or unshocked animals infused with vehicle, which demonstrated that the infusion of pargyline into shocked animals did not eliminate the shock-induced depression of activity simply by generally stimulating motor activity. Measurement of the concentration of NE, DA, 5-HT, and 5-HIAA present in seven brain regions at the conclusion of the swim test showed that pargyline infusion into the LC eliminated the large depletion of NE in the LC that is normally observed after exposure to uncontrollable shock while having no effect on NE levels in the other brain regions examined. The level of 5-HT in the LC was also raised by infusion of pargyline into the LC, but again, there was no effect of pargyline infusion on 5-HT levels in any of the other brain regions. In conclusion, infusion of pargyline into the LC region of the brain eliminated both the large depletion of NE in the LC region and the behavioral depression that otherwise results from exposure of animals to uncontrollable shock.

Reversal of behavioral depression by infusion of an alpha-2 adrenergic agonist into the locus coeruleus

This experiment demonstrated that behavioral depression produced by exposure of rats to strong uncontrollable shocks could be reversed by infusion of the alpha-2 adrenergic agonist clonidine into the region of the locus coeruleus (LC). A 20-min infusion, through bilateral cannulae, into the locus coeruleus of clonidine, piperoxane (alpha-2 antagonist) or inactive vehicle (0.85% saline), was given beginning 70 min after the animals were removed from the stress situation. The dose and volume of drug given in the infusion (0.16 microgram/microliter, 0.1 microliter/min) had been previously shown to produce effects specific to the locus coeruleus (Weiss, Simson, Hoffman, Ambrose, Cooper and Webster, 1986; Neuropharmacology 25: 367-384). At the conclusion of the infusion, active behavior of animals was measured in a 15-min swim test. Results showed that stressed animals infused with vehicle exhibited significantly less active behavior in the swim test than did non-stressed animals infused with vehicle, thereby showing the usual behavioral depression seen after exposure to an uncontrollable stress. Stressed animals infused with clonidine showed no difference in active behavior in comparison to non-stressed animals infused with vehicle and showed significantly more activity than did the stressed animals infused with vehicle. Stressed animals infused with piperoxane showed no significant difference in activity in comparison to the stressed animals infused with vehicle and were significantly less active than either the non-stressed animals infused with vehicle or the stressed animals infused with clonidine. Thus, infusion into the locus coeruleus of the alpha-2 agonist clonidine, but not the alpha-2 antagonist piperoxane, eliminated behavioral depression.(ABSTRACT TRUNCATED AT 250 WORDS)

Infusion of adrenergic receptor agonists and antagonists into the locus coeruleus and ventricular system of the brain. Effects on swim-motivated and spontaneous motor activity

These studies examined how pharmacological stimulation and blockade of alpha receptors would affect active motor behavior in rats. In experiment I, alpha-2 receptor antagonists (piperoxane, yohimbine) and agonists [clonidine, norepinephrine (NE)] were infused into various locations in the ventricular system of the brain, including the locus coeruleus region, and motor activity was measured. Activity was measured principally in a swim test but spontaneous (ambulatory) activity was also recorded while drugs were being infused. When infused into the locus coeruleus region, small doses of the antagonists piperoxane and yohimbine depressed activity in the swim test while infusion of the agonists clonidine and NE had the opposite effect of stimulating activity. These effects were highly specific to the region of the locus coeruleus, since infusions of these drugs into other nearby locations in the ventricular system or use of larger doses had different, often opposite effects. This was especially true of clonidine and NE which profoundly depressed activity when infused posterior to the locus coeruleus, particularly over the dorsal vagal complex. Infusion of small doses of these drugs into the lateral ventricle had effects similar to infusion into the locus coeruleus region, though less pronounced. Changes in spontaneous motor activity were also observed, but this measure differentiated the groups less well than did the swim test. In experiment II, the predominantly postsynaptic receptor agonists isoproterenol (beta agonist) and phenylephrine (alpha-1 agonist) were infused into the ventricular system. Since infusions of piperoxane and yohimbine into the locus coeruleus that decreased activity in experiment I increase the release of NE by blocking alpha-2 inhibitory receptors on cell bodies and dendrites of the locus coeruleus, experiment II tested whether ventricular infusion of predominantly postsynaptic receptor agonists would also decrease activity in the swim test. Both isoproterenol and phenylephrine produced this effect, but did so selectively with respect to dose and location of infusion in the ventricular system. These findings are consistent with recent results relating to the mechanism that underlies stress-induced depression of active behavior.

Ion exposure chambers for small animals

Ion exposure chambers that have been designed and tested for use in biological and behavioral research with small animals are described in this report. The chambers exhibit an acceptable degree of uniformity in ion concentration, current density, and electric field within the exposure area. Gaseous by-products of corona discharge (O3 and NO2) have been measured and found to be less than .01 ppm and less than .1 ppm, respectively. Filtered air is fed to the individual exposure chambers, and temperature and humidity are well controlled. Noise due to corona and the air delivery system has been measured.

[The surfactant system of the lung as a functional unit]

Morphological, biochemical and biophysical data indicate that the pulmonary surfactant system of the lung has to be considered as a functional unit from the alveoli to the trachea. In the nonciliated part of the air passages the surfactant assures the stability of the small airways and contributes to the transport processes. In the ciliated part of the air passages the surfactant supports indirectly the optimal transport of mucus due to its anti-glue properties. Any deterioration in the surfactant system results in functional consequences for lung mechanics and bronchial clearance mechanisms as discussed for RDS and COLD. Stimulation of surfactant synthesis and surfactant replacement can be regarded as new aspects for therapy.

Total deafness fromaminoglycoside overdosage: Histopathologic case study

A 57-year-old patient became totally deaf two days after receiving excessive doses of the aminoglycosidic antibiotic lividomycin parenterally for 14 days; she died four and a half months later. Her temporal bones were examined by microdissection, surface preparation, and serial sectioning of the modiolus. Loss of inner and outer cochlear hair cells was virtually complete. Refractile concretions were scattered along the atrophic stria vascularis, especially in the middle turn. The distal half of the radial cochlear nerve fibers in the osseous spiral lamina had degenerated, but closer to the modiolus they appeared to be intact. The spiral ganglion in the basal turn showed partial loss of neurons. Scanning electron microscopy revealed hair cell loss from the vestibular end-organs, more severe in the ampullar cristae than in the utricular macula. The dark cells of the utricular wall appeared to be altered.

Stress-induced suppression of immunity in adrenalectomized rats

Stress-induced suppression of lymphocyte stimulation by phytohemagglutinin was demonstrated in Isolated lymphocytes and in cultures of whole blood from adrenalectomized rats. The results demonstrate that corticosteroid independent mechanisms participate in the suppression of lymphocyte function by stressors. Stress-induced lymphopenia, however, was found to be adrenal dependent, indicating that the modulation of immunity by stress is complex and multidetermined.

Suppression of immunity by stress: effect of a graded series of stressors on lymphocyte stimulation in the rat

In rats a graded series of stressors produced progressively greater suppression of lymphocyte function, as measured by the number of circulating lymphocytes and by phytohemagglutinin stimulation of lymphocytes in whole blood and isolated cultures. This evidence suggests that stress suppresses immunity in proportion to the intensity of the stressor.

A comparison of ornithine decarboxylases from normal and SV40-transformed 3T3 mouse fibroblasts

Ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) has been purified from 3T3- and SV40-transformed 3T3 mouse fibroblasts by affinity chromatography, and the physicochemical properties of the two enzymes compared. Measured properties include molecular weight of the active species, subunit molecular weight and specific activity of the purified enzymes, kinetic parameters, thermostability, degradation rate in vivo and immunological cross-reactivity. Although crude extracts of the transformant possess more ornithine decarboxylase activity per mg of protein than the parent strain, there is no evidence for the appearance of an altered form of the enzyme in these cells. The results reported in the present paper indicate that the increased ornithine decarboxylase activity in the transformed cells is the result of higher enzyme biosynthesis de novo.

Biochemical evidence for an interaction of ethanol and stress: preliminary studies

Studies were carried out to examine the interaction of stress and ethanol. Ethanol (0.5 g/kg)-treated, stressed (electric foot shocks) rats exhibited less marked elevations in plasma levels of nonesterified fatty acids and of corticosterone compared to saline-treated stressed subjects. Stress alone markedly elevated these two parameters in plasma. Ethanol treatment alone also resulted in small elevations in both parameters. Results provide support for a significant interaction of ethanol and stress, and might provide some basis for the purported stress-relieving effect of low doses of ethanol.

Stress-induced depression of motor activity correlates with regional changes in brain norepinephrine but not in dopamine

This experiment examined how inescapable tail shock alters the level of dopamine and norepinephrine within various brain regions of the rat and the relationship of these changes to the depression of motor activity produced by the shock. Following exposure to tail shock that is known to interfere with acquisition of active behavioral tasks, animals were briefly tested for spontaneous motor activity and then sacrificed for neurochemical measures. Norepinephrine and dopamine levels in the frontal cortex, brain stem, striatum, olfactory tubercle, hypothalamus, hippocampus, septum, and amygdala were measured by a sensitive radioenzymatic technique. Exposure to 45 min of tail shock did not alter motor activity significantly, but shock sessions of 60 and 75 min duration produced a marked decrease in motor activity. Levels of dopamine were found to be very little changed in all brain regions studied except for the hypothalamus, in which a substantial rise in dopamine level was observed. Norepinephrine levels, in contrast, fell in many brain regions in response to shock. The fall in norepinephrine levels observed in two brain regions was significantly correlated with the decline in motor activity (brain stem r = +0.70, hypothalamus r = +0.60). These data suggest that deficits in active motor behavior produced by shock parameters similar to those used in this study may reflect concomitant disturbances of noradrenergic function in specific brain regions.

Effect of ACTH 4-10 on passive avoidance of rats lacking vasopressin (Brattleboro strain)

The hypothesis that the effects of ACTH 4-10 on avoidance are mediated via the release of endogenous vasopressin was investigated. To test this hypothesis, we observed the effect of ACTH 4-10 on the passive avoidance of Brattleboro rats with diabetes insipidus resulting from a total genetic deficiency of vasopressin (DI) and Brattleboro rats without diabetes insipidus (HE). Normal Long-Evans rats (LE) were also included for comparison purposes. The results did not support the hypothesis. ACTH 4-10 did influence the passive avoidance of DI rats; this should not have occurred if the release of endogenous vasopressin is necessary for ACTH 4-10 to influence avoidance.

Predicting success in psychiatric training for foreign medical graduates: II. Patterns in course

Faculty ratings of 22 foreign medical graduates ('FMGs'), all of whom entered psychiatric residency training at the University of Missouri-Columbia from 1966 through 1973, were compared with those of a similar group of North American medical graduates ('AMGs'). An on-going evaluation programme provided data for a variety of parameters ranging from 'theoretical knowledge' to 'clinical skills'. Results indicated that in most of performance the median FMG started at a level substantially lower than that of the median AMG and very slowly caught up. When achievement criteria were utilized it became apparent that, at least by the third year of training, 'superior' FMGs could equal or surpass the median performance of 'superior' AMGs, whereas 'marginal' residents, whether foreign or native-born, seldom attained even competence in most major spheres of functioning. Implications of these findings and those in related studies are discussed.

Malignant tumors of the testis: analysis of treatment results and sites and causes of failure

A total of 150 patients with germinal testicular neoplasms treated at the University of California, San Francisco, were reviewed. The 5-year actuarial survival rate was 95% for 57 patients with seminoma and 56% for 75 patients with carcinoma. Treatment modalities were compared with respect to sites and causes of failure and complications. The most common sites of failure in the carcinoma patients were lung, and supraclavicular or mediastinal lymph nodes. Scrotal orchiectomy, elevated gonadotropins, elements of choriocarcinoma, and bulky abdominal disease were all correlated with high treatment failure rate.

Attenuation of gastric lesions by psychological aspects of aggression in rats

Rats that fought with each other in response to electric shock showed reduced gastric lesions in comparison with animals that received the same shocks alone so that fighting behavior did not occur. Also, gastric lesions were similarly reduced in animals that fought even though they could not physically contact one another because of a barrier between them, so that the "protective" effect of fighting derived from release of, or display of, fighting behavior and did not require physical combat. A second experiment showed that animals that received shock together but did not engage in fighting behavior showed no reduction of gastric lesions, so that the protective effect of fighting was not an artifact of animals receiving shock together. Possible explanations for why fighting behavior reduces gastric lesions are discussed.

Effects of chronic exposure to stressors on avoidance-escape behavior and on brain norepinephrine

A single exposure to a severe stressor (either cold swim or inescapable shock) impairs subsequent performance in a shuttle avoidance-escape task (1), a deficit attributed to reduction in brain noradrenergic activity produced by these stressors. In the present paper, two experiments are described which examine how repeated exposure to such stressors affects (a) shuttle avoidance-escape performance (Experiment 1), and (b) aspects of brain norepinephrine metabolism (Experiment 2). Experiment 1 showed that, whereas subjects receiving the single exposure to cold swim or shock showed a large avoidance-escape deficit, subjects that received repeated exposure to these stressors for 14 days performed similarly to the control group that received no stressor. Experiment 2 showed that, whereas subjects that received one session of the inescapable shock stressor showed a lower level of norepinephrine in hypothalamus and cortex than did subjects that received no shock, subjects that received repeated exposure to inescapable shock or cold swim showed neurochemical "habituation." Subjects that received repeated shock showed elevated tyrosine hydroxylase activity and no depletion of norepinephrine level, and both repeated shock and cold swim caused a decrease in uptake of 3H-norepinephrine by slices of cortex in vitro. Thus, it is concluded that the behavioral and neurochemical changes that were observed after the stressful conditions studied are consistent with the hypothesis that changes in avoidance-escape responding following exposure to these stressful events are due to changes in brain noradrenergic activity.

Effects of acute exposure to stressors on subsequent avoidance-escape behavior

This paper attempts to show that severely stressful conditions can lead to a deficit in avoidance-escape responding by reducing noradrenergic activity in the brain. It is argued that this stress-induced neurochemical change explains more adequately an avoidance-excape deficit seen previously in dogs after severe inescapable shock than does the "learned helplessness" explanation originally offered for this effect. A series of six experiments are described that test the stress-neurochemical change explanation, which is called the "motor activation deficit" hypothesis. The first experiment showed that a brief exposure to cold swim, which reduced central noradrenergic activity but did not induce "learned helplessness," produced an avoidance-escape deficit similar to that observed after inescapable shock in the original "learned helplessness" studies. Subsequent experiments demonstrated that the deficit induced by cold swim was also similar to the original deficit in that it too dissipated with the passage of time and was reduced by pretraining animals in the correct response. Further experiments then showed that the deficit induced by cold swim was (a) aggravated by raising the height of the barrier between compartments in the shuttle box, and (b) did not occur if the avoidance-escape task used required little motor activity, both of the findings being predicted by the "motor activation deficit" hypothesis. Finally, an avoidance-escape deficit was produced by inescapable shock, and it was found that this stressor likewise did not impair avoidance-escape responding if the task required little skeletal activity, a result not predicted by the "learned helplessness" hypothesis. The paper concludes by discussing how the results presented in this paper, as well as in succeeding papers, support the "motor activation deficit" explanation for certain avoidance-escape deficits, defining the cases to which the explanation applies and discussing the neurotransmitters involved.

Monamines as mediators of avoidance-escape behavior

A series of three experiments were carried out to test the hypothesis ("motor activation deficit" hypothesis) that the avoidance-escape deficits observed following certain highly stressful conditions result from changes in activity of noradrenergic (or other monominergic) neural systems. These studies indicate that: (1) Depletion of monamines by a single injection of tetrabenazine produces an active avoidance-escape deficit when the avoidance-escape response involves a relatively high degree of motor activity but not when a minimum of motor activity is required. This parallels results found when animals are acutely exposed to a stressor prior to avoidance-escape testing. (2) Daily injections of tetrabenazine for a period of two weeks, like daily exposure to inescapable shock for the same period of time, markedly attenuates the magnitude of the avoidance-escape deficit produced by either a single injection of tetrabenazine or single session of inescapable shock. (3) Decreasing the stress-induced depletion of monamines by the use of an MAO inhibitor serves to protect the animals from the effects of inescapable shock, markedly reducing the avoidance-escape deficit produced by such shock. It is concluded that these results are consistent with the motor activation deficit hypothesis.