Cholinergic changes during conditioned suppression in rats

Depression: The predisposing influence of stress

Aversive experiences have been thought to provoke or exacerbate clinical depression. The present review provides a brief survey of the stress-depression literature and suggests that the effects of stressful experiences on affective state may be related to depletion of several neurotransmitters, including norepinephrine, dopamine, and serotonin. A major element in determining the neurochemical changes is the organism's ability to cope with the aversive stimuli through behavioral means. Aversive experiences give rise to behavioral attempts to cope with the stressor, coupled with increased utilization and synthesis of brain amines to contend with environmental demands. When behavioral coping is possible, neurochemical systems are not overly taxed, and behavioral pathology will not ensue. However, when there can be no behavioral control over the stressful stimuli, or when the aversive experience is perceived as uncontrollable, increased emphasis is placed on coping through endogenous neurochemical mechanisms. Amine utilization increases appreciably and may exceed synthesis, resulting in a net reduction of amine stores, which in turn promotes or exacerbates affective disorder. The processes governing the depletions may be subject to sensitization or conditioning, such that exposure to traumatic experiences may have long-term repercussions when the organism subsequently encounters related stressful stimuli. With continued uncontrollable stimulation, adaptation occurs in the form of increased activity of synthetic enzymes, and levels of amines approach basal values. It is suggested that either the initial amine depletion provoked by aversive experiences or a dysfunction of the adaptive processes, resulting in persistent amine depletion, contributes to behavioral depression. Aside from the contribution of behavioral coping, several organismic, experiential, and environmental variables will influence the effects of aversive experiences on neurochemical activity, and may thus influence vulnerability to depression.

CNS–immune system interactions: Conditioning phenomena

Converging data from different disciplines indicate that central nervous system processes are capable of influencing immune responses. This paper concentrates on recent studies documenting behaviorally conditioned suppression and enhancement of immunity. Exposing rats or mice to a conditioned stimulus previously paired with an immunomodulating agent results in alterations in humoral and cell-mediated immune responses to antigenic stimuli, and unreinforced reexposures to the conditioned stimuli result in extinction of the conditioned response. Although the magnitude of such conditioning effects has not been large, the phenomenon has been independently verified under a variety of experimental conditions. The biological impact of conditioned alterations in immune function is illustrated by studies in which conditioning operations were applied in the pharmacotherapy of autoimmune disease in New Zealand mice. In conditioned animals, substituting conditioned stimuli for active drugs delays the onset of autoimmune disease relative to nonconditioned animals using a dose of immunosuppressive drug that, by itself, is ineffective in modifying the progression of disease. The hypothesis that such conditioning effects are mediated by elevations in adrenocortical steroid levels receives no support from available data. Despite its capacity for self-regulation, it appears that the immune system is integrated with other psychophysiological processes and subject to modulation by the brain.

Neurochemical changes associated with the action of acute administration of diazepam in reversing the behavioral paradigm conditioned emotional response (CER)

Neurotransmitter turnover of biogenic monoamines (dopamine, norepinephrine, and serotonin) and amino acids (glutamate, aspartate, and gamma-aminobutyric acid) was evaluated in rats exposed to the conditioned emotional response (CER) paradigm in the absence (total suppression) or presence of acute 5 mg/kg i.p. diazepam (which reversed suppression and restored normal responding). Based on previous studies of CER, with controls for shock and stimulus histories, the results with respect to the anxiolytic could be divided into several categories: changes in turnover which are associated only with the CER behavior; changes associated only with the drug, diazepam; changes which augmented the effects of the behavior; or changes which were the reverse of those associated with the behavior. Due to the multitude and complexity of the results, not all observations have clear explanations at this time. However, for the CER behavior per se, it is apparent that a combination of neurotransmitters, including some implications about acetylcholine, act in concert to bring about the behavioral suppression. The action of diazepam is more complex, involving the full spectrum of neurotransmitters to bring about its direct and indirect effects.

Clinical and biochemical manifestations of depression. Relation to the neurobiology of stress (2)

Thousands of studies have been conducted of the functioning of the many neurotransmitter systems in order to explore the biologic basis of major depressive disorder. Instead of reviewing this literature exhaustively, we have attempted to propose a model that accommodates the clinical observation that chronic stress early in life in vulnerable persons predisposes them to major depression with contemporary observations of the potential consequences of repeated central nervous system exposure to effectors of the stress response. This model accords with current clinical judgment that major depression is best treated with a combination of psychopharmacologic agents and psychotherapy. Accordingly, whereas psychopharmacologic intervention may be required to resolve an active episode of major depression and to prevent recurrences, psychotherapy may be equally important to lessen the burden of stress imposed by intense inner conflict and counterproductive defenses.

Catecholamine levels in the brain of SART (repeated cold)-stressed rats

1 The catecholamine levels in the brains of SART (specific alternation of rhythm in temperature)-stressed (repeated cold-stressed) rats with vagotonic-type dysautonomia, were examined by high performance liquid chromatography with electrochemical detection techniques. 2 The cerebral cortex, hypothalamus and hippocampus of the SART-stressed rats had increased levels of noradrenaline. All brain areas examined showed increased levels of dopamine. 3 These increased catecholamine levels were still maintained by day 10 of SART stress. 4 Among brain areas examined, the hypothalamus showed most rapid change. 5 Cold-stressed rats showed increased noradrenaline levels only in the basal ganglia and dopamine levels in the hippocampus. 6 Rats suffering from restraint and water immersion stress showed decreased noradrenaline levels and increased dopamine levels. 7 These results suggest that SART-stressed animals are in a disease state differing from that of other so-called stressed animals, and changes in the hypothalamus give rise to the various symptoms in SART-stressed animals.

Total acetylcholine content, and activities of choline acetyltransferase and acetylcholinesterase in brain and duodenum of SART-stressed (repeated cold-stressed) rat

The cholinergic activities in SART (specific alternation of rhythm in temperature)-stressed (repeated cold-stressed) rats, which are diseased rats with vagotonic-type dysautonomia, were examined with the following results. A decreased content of total acetylcholine (T-ACh) and increased activities of choline acetyltransferase (CAT) and acetylcholinesterase (ACh) in the basal ganglia and an increase in the T-ACh content and decrease in the AChE activity in the duodenum of SART-stressed rats reached the respective plateaus on day 5 of stress, which were maintained thereafter. CAT activity, however, in the hypothalamus was activated most on day 2. These changes in SART-stressed rats were different from those in simple cold-stressed rats. Subdiaphragmatic vagotomy inhibited the appearance of the changes in the duodenum, but not those in the hypothalamus of SART-stressed rats. The sedative analgesic Neurotropin prevented all the changes in SART-stressed rats described above. These results suggest that cholinergic neurons may be activated in both the hypothalamus and basal ganglia of the brain of SART-stressed rats, and the characteristic peripheral changes of the cholinergic system in the duodenum of SART-stressed rats may be under the control of the parasympathetic center.

Limbic acetylcholine turnover rates correlated with rat morphine-seeking behaviors

Acetylcholine (ACh) turnover rates were measured in fourteen brain regions of rats intravenously self-administering morphine and in yoked-morphine and yoked-vehicle infused littermates to identify cholinergic neuronal pathways potentially involved in opiate reinforcement processes. Rats receiving chronic passive administration of morphine had increased ACh turnover rates in the frontal cortex and diagonal band and decreased rates in the medial septum. The significant changes in animals self-administering the drug were prominent in limbic regions with increases in the frontal cortex and decreases in the pyriform cortex, nucleus accumbens, amygdala and ventral tegmental area. Some components of opiate reinforcement may be mediated by increases in the activity of cholinergic ventral pallidal and diagonal band fibers innervating the frontal cortex and by decreases in activity of cholinergic fibers innervating the ventral tegmental area. These data and turnover rates for dopamine, norepinephrine, serotonin, aspartate, glutamate and gamma-aminobutyric acid previously determined in similarly treated animals are consistent with two neuronal circuits that may be involved in opiate seeking behaviors and opiate reinforcement processes.

Changes in biogenic amine and benzodiazepine receptors correlated with conditioned emotional response and its reversal by diazepam

Groups of littermate rats were trained to respond for food reinforcement on a variable interval one-min (VI 1) schedule, after which they were classically conditioned to associate a conditioned stimulus (CS) with footshock (conditioned emotional response; conditioned suppression; CER). Two control groups received yoked footshock (no CS) or the visual-auditory stimulus only (no footshock). On test day, a group of the CER conditioned animals received injections of either vehicle or diazepam prior to exposure to the VI 1 food-reinforced schedule. After 30 min of the VI 1 schedule, the CS was presented continuously for 15 min, after which the animals were decapitated, the brains removed, membranes prepared and in vitro receptor binding evaluated. During the CS, the CER animals suppressed responding and exhibited conditioned fear (emotional) behavior, while the control groups, and animals given acute diazepam, maintained normal responding. [3H]Diazepam binding was reduced in the CER animals, yet acute benzodiazepine administration did not effect this binding. [3H]QNB binding was reduced by CER and increased by diazepam administration. Adrenergic, serotonergic and dopaminergic systems were also evaluated. Traditional biogenic amine systems may respond to CER and diazepam administration in some compensatory manner.

Biogenic monoamine turnover in discrete rat brain regions is correlated with conditioned emotional response and its conditioning history

The content and turnover of dopamine, norepinephrine and 5-hydroxytryptamine (serotonin), and the content of their respective major metabolites were evaluated in 19 discrete brain areas of rats exposed to conditioned emotional response (CER), and in control groups which received either equivalent yoked shock (shock only) or compound stimulus presentation (tone only). On test day, CER animals suppressed responding and exhibited forms of emotional behavior after presentation of the conditioned stimulus (CS); while shock only and tone only control groups, and CER animals which received an acute dose of diazepam prior to testing, did not suppress. Few changes were observed in content of the biogenic amines or their metabolites, suggesting that the behavioral manipulations were acting within normal physiological limits. On the other hand, numerous changes were observed in the utilization of the 3 biogenic monoamines, which were correlated with the conditioning-anxiety (comparisons of CER vs shock only) and the shock history (comparison of shock only vs tone only). These observations are consistent with putative neural pathways in the frontal cortex, septum, nucleus accumbens, amygdala, striatum, hippocampus and brain stem (which utilize specific monoamines), and with discrete brain areas which have been implicated in classical conditioning and CER-related phenomena. These observations suggest roles for biogenic monoamines in mediating or responding to the classical conditioning and emotional components of the paradigm.