Chronic stress reduces fighting behavior of rats: the effect of antidepressants

Potential roles of zinc in the pathophysiology and treatment of major depressive disorder

Incomplete response to monoaminergic antidepressants in major depressive disorder (MDD), and the phenomenon of neuroprogression, suggests a need for additional pathophysiological markers and pharmacological targets. Neuronal zinc is concentrated exclusively within glutamatergic neurons, acting as an allosteric modulator of the N-methyl D-aspartate and other receptors that regulate excitatory neurotransmission and neuroplasticity. Zinc-containing neurons form extensive associational circuitry throughout the cortex, amygdala and hippocampus, which subserve mood regulation and cognitive functions. In animal models of depression, zinc is reduced in these circuits, zinc treatment has antidepressant-like effects and dietary zinc insufficiency induces depressive behaviors. Clinically, serum zinc is lower in MDD, which may constitute a state-marker of illness and a risk factor for treatment-resistance. Marginal zinc deficiency in MDD may relate to multiple putative mechanisms underlying core symptomatology and neuroprogression (e.g. immune dysfunction, monoamine metabolism, stress response dysregulation, oxidative/nitrosative stress, neurotrophic deficits, transcriptional/epigenetic regulation of neural networks). Initial randomized trials suggest a benefit of zinc supplementation. In summary, molecular and animal behavioral data support the clinical significance of zinc in the setting of MDD.

Stress induces rapid changes in central catecholaminergic activity in Anolis carolinensis: Restraint and forced physical activity

Immobilization stress and physical activity separately influence monoaminergic function. In addition, it appears that stress and locomotion reciprocally modulate neuroendocrine responses, with forced exercise ameliorating stress-induced serotonergic activity in lizards. To investigate the interaction of forced physical activity and restraint stress on central dopamine (DA), norepinephrine (NE), and epinephrine (Epi), we measured these catecholamines and their metabolites in select brain regions of stressed and exercised male Anolis carolinensis lizards. Animals were handled briefly to elicit restraint stress, with some lizards additionally forced to run on a track until exhaustion, or half that time (50% of average time to exhaustion), compared to a control group that experienced no restraint or exercise. Norepinephrine concentrations in the hippocampus and locus ceruleus decreased with restraint stress, but returned to control levels following forced exhaustion. Levels of NE in the raphé nuclei and area postrema, and epinephrine in raphé became elevated following restraint stress, and returned to control levels following forced physical activity to 50% or 100% exhaustion. Striatal DA increased as animals were exercised to 50% of exhaustion, and returned to baseline with exhaustion. At exhaustion, striatal Epi levels were diminished, compared with controls. In the area postrema, exhaustion reversed a decline in epinephrine levels that followed forced physical activity. These results suggest that stress stimulates a rapid influence on central catecholamines. In addition, forced exercise, and even exhaustion, may alleviate the effects of restraint stress on central monoamines.

Resolving Apparent Contradictions Concerning the Relationships Among Fear or Anxiety and Aggression During Lactation: Theoretical Comment on D'Anna, Stevenson, and Gammie (2005)

Pregnancy and lactation involve dramatic changes in behavior and physiology necessary for the successful production and sustenance of offspring. This requires numerous processes to emerge and be coordinated in a finely tuned manner. Reduced corticotropin releasing factor (CRF) receptor activity and hypothalamic-pituitary-adrenal function are thought to be involved in numerous functions during lactation, such as reducing fear and anxiety, which in turn, is proposed to disinhibit aggressive behaviors. K. L. D'Anna, S. A. Stevenson, and S. C. Gammie (2005) reported that central infusion of urocortin 1 and 3, anxiogenic peptides that bind to CRF receptors, reduce maternal aggression in lactating mice and alter neural c-fos expression. However, the relationship between stress hormones and where they act, emotional state, and aggression during lactation is complicated, and our understanding of how these factors influence each other might benefit from a comprehensive reevaluation of these literatures. ((c) 2005 APA, all rights reserved).

Examining novel concepts of the pathophysiology of depression in the chronic psychosocial stress paradigm in tree shrews

Despite decades of research on psychiatric disorders, the aetiology and precise biological mechanisms that underlie depressive diseases are still poorly understood. There is increasing evidence that psychiatric disorders not only have a neurochemical basis but are also associated with morphological alterations in central nervous neurons and/or glial cells. Antidepressants may act by restoring structure as well as function of neural networks, meaning that they may, as a fundamental principle, affect neural plasticity underlying normal brain functioning. To examine these novel concepts of the pathophysiology of depression and antidepressant medication we have carried out a series of experiments using the chronic psychosocial stress paradigm in male tree shrews, an animal model with a high validity for the pathophysiology of depressive disorders, in which the animals were treated with the tricyclic antidepressant compound clomipramine. We found that one month of stress reduced cell proliferation in the dentate gyrus, and decreased the total hippocampal volume. Gene transcription analysis revealed that, under these experimental conditions, expression of genes known to be involved in processes of cell differentiation is suppressed. These effects of social conflict on hippocampal cells, including gene transcription, and on the entire hippocampal volume could be counteracted by chronic treatment with the antidepressant clomipramine. Stress also induced a constant hyperactivity of the hypothalamic-pituitary-adrenal axis, and suppressed both motor and marking behaviour. These neuroendocrine and behavioural stress-induced changes were also re-normalized by clomipramine.

Central monoamines in free-ranging lizards: differences associated with social roles and territoriality

During the breeding season different social classes of field-active lizards, Sceloporus jarrovi, exhibit regionally specific changes in central monoaminergic activation. Changes in serotonergic content and turnover between lizards from different social classes are seen in forebrain structures (telencephalon and diencephalon) and reflect events associated with reproductive behaviors, stress and aggression. Males without territories (satellite males) exhibit higher forebrain serotonin (5-HT) system activation compared to territorial males and adult females. This serotonergic activation includes increased 5-hydroxyindoleacetic acid (5-HIAA) and 5-HIAA/5-HT ratio, suggesting increased release and catabolism. Satellite males also exhibit higher 5-HIAA/5-HT ratios (serotonergic turnover) compared to territorial males following agonistic interactions. Territorial males, immediately following aggressive defense of territories against intruder males, exhibit increased 5-hydroxytryptophan (5-HTP) levels, higher 5-HIAA levels and 5-HIAA/5-HT ratio, higher epinephrine levels, greater MHPG/NE, more DOPAC and larger DOPAC/DA ratio compared to territorial males that did not have an aggressive encounter. These differences suggest activation of 5-HT, norepinephrine (NE), and dopamine (DA) systems by the synthesis and release of more 5-HT and the release of more NE and DA during aggressive defense of territory. The highest activity of serotonergic system is exhibited by satellite males compared to territorial males, perhaps reflecting stress in subordinate animals from social and ecological sources.

c-fos expression, behavioural, endocrine and autonomic responses to acute social stress in male rats after chronic restraint: modulation by serotonin

The effects in male rats of serotonin depletion (using the neurotoxin 5,7-dihydroxytryptamine) on the cross-sensitization of an acute social stress (defeat by a larger resident male) by previous repeated restraint stress (10 days, 60 min per day) was studied. Previous restraint increased freezing responses during social defeat in sham-operated rats, but this was not observed in those with depleted serotonin (83% or more in different regions of the brain). In contrast, neither heart rate (tachycardia) nor core temperature responses (hyperthermia) were accentuated in previously restrained rats (i.e. neither showed heterotypical sensitization), and neither adapted to repeated restraint (there is a hypothermic core temperature response during restraint). Corticosterone levels, which did adapt, nevertheless did not show accentuated responses to social defeat in previously restrained rats, though samples could only be taken 60 min after defeat. c-fos expression in the central nucleus of the amygdala 60 min after social defeat was increased by previous restraint. No other areas examined in the hypothalamus (e.g., paraventricular nucleus) or brainstem (e.g., solitary nucleus) showed differences related to previous restraint. Serotonin depletion reduced the expression of c-fos in the frontal cortex, lateral preoptic area, medial amygdala, central gray, medial and dorsal raphe, and locus coeruleus after social stress, but this was not altered by previous restraint. These results show that serotonin depletion has selective effects on the cross-sensitization of responses in previously stressed rats to a heterotypical stressor.

Regional and temporal separation of serotonergic activity mediating social stress

Stressful aggressive interaction stimulates central serotonergic activation in telencephalon as well as brainstem. Social roles can be distinguished by monoamine activity following aggression. Pairs of male lizards, Anolis carolinensis, were allowed to fight and form dominant/subordinate relationships. In micropunched regions of telencephalon, the greatest serotonergic changes occur in subordinate males. In hippocampal cortex and nucleus accumbens, subordinate males have increased 5-hydroxyindoleacetic acid/serotonin at 1 h following the fight. In these areas the ratio gradually decreases over a week of cohabitation, as was previously reported for brainstem. Medial and lateral amygdala develop increased serotonergic activity more slowly, with the greatest increase being evident following a week of interaction. Turnover, serotonin and 5-hydroxyindoleacetic acid levels in amygdala escalate over the first week of interaction in subordinate males, and return to baseline by one month. In dominant males, the pattern is accelerated, with the most extensive serotonin system activity present at 1 h, then decreasing over a month. The patterns of serotonergic activation are so similar in hippocampus, nucleus accumbens and brainstem that a co-ordinated response may be involved in mediating short-term social stress and aggression. Similarly, medial and lateral amygdala exhibit corresponding, but delayed patterns in subordinate males, suggesting a co-ordinated response in these regions mediating longer-term stress responses. These data are consistent with rapid neuroendocrine stress modulation in dominant individuals, and delayed serotonergic activity changes in subordinate males.

Greater behavioral effects of stress in immature as compared to mature male mice

The effect of sexual maturity on behavioral effects of stress was examined in male mice. Immature (4-week-old) or mature (8-week-old) animals were subjected to either social stress (exposure to an isolated adult male) or restraint stress for 5 days and examined for body weight, food intake, or plus-maze behavior. Social stress reduced food intake, body weight, and open-arm entries in 4-week-old but not 8-week-old mice. Restraint reduced body weight in 4-week-old but not 8-week-old mice. It is concluded that immature male mice show greater behavioral disturbances after stress than their mature counterparts. The findings are in agreement with much anecdotal evidence that children are more vulnerable to stress than adults.

Female social reproductive roles affect central monoamines

Central monoamines display a variety of activation patterns in different social groups, and among males and females. We addressed three social conditions for female lizards of the species Anolis carolinensis: Isolated, paired with a mate, and in a group of 5 competing for one mate. Among those in a group, only 1 or 2 females exhibited recrudescing ovaries. Individuals paired with a mate (for one month) exhibited ovarian growth, isolated animals (initial controls) had quiescent ovaries. Reproductively dominant females had significantly greater telencephalic 5-HIAA, and serotonergic activation, as indicated by the ratio of 5-HIAA to 5-HT. Telencephalic HVA as well as the HVA/DA ratio were also significantly greater in dominant females compared to all other groups. In contrast, serotonergic activation in brainstem was elevated in subordinate females only. These results suggest that serotonergic activation in telencephalon, found only in dominant females, not in other reproductively active females, is a function of the unique social role of a dominant female, possibly combining submissive behaviors toward a male with dominance over other females and competition for access to that male. Dopaminergic activation in telencephalon, also found only in dominant females, may be related to aggressive interactions with other females. Activation of serotonin in brainstem, found in this study in subordinate females and previously in males [C.H. Summers and N. Greenberg, Activation of central biogenic amines following aggressive interaction in male lizards, Anolis carolinensis, Brain Behav. Evol., 45 (1995) 339-349], may be associated with subordinate social status. Monoamines, involved in social behaviors, appear to be regionally specialized for dominant and subordinate social roles, in males [C.H. Summers and N. Greenberg, Activation of central biogenic amines following aggressive interaction in male lizards, Anolis carolinensis, Brain Behav. Evol., 45 (1995) 339-349][T.R. Summers, E.T. Larson, A.L. Hunter, K.J. Renner, N. Greenberg and C.H. Summers, Amygdalar serotonin mediates long-term social roles following aggressive interaction, Soc. Neurosci. Abs., 22 (1996) 1147] and females. Dominant females exhibit unique social position, behavior and monoamine profile whereas subordinate females and males have a similar serotonergic response in this species.

Loss of social status: preliminary evaluation of a novel animal model of depression

Stable dominance hierarchies were determined in pairs of male Lister hooded rats, by repeated observation of agonistic behaviour at the onset of the dark phase of the light-dark cycle. No lasting alterations in dominance behaviour were caused by subjecting the dominant member of the pair either to restraint stress or to defeat by another dominant animal. However, defeat of the dominant animal by a male of the aggressive Tryon Maze Dull (TMD) strain caused a loss of dominant status in the home cage which lasted at least 7 days. Repeated weekly defeat by TMD animals decreased both home cage dominance behaviour and consumption of a palatable sucrose solution, relative to non-defeated animals; both behaviours were normalized in defeated animals by 3 weeks of treatment with the tricyclic antidepressant imipramine.

Effects of chronic mild stress on performance in behavioural tests relevant to anxiety and depression

Chronic exposure to mild unpredictable stress (CMS) has previously been found to depress the consumption of a weak (1%) sucrose solution by rats. This effect was confirmed in each of three experiments in the present study, following which behaviour was examined in other tests relevant to either depression or anxiety. CMS did not significantly affect behaviour in the social interaction test and caused an anxiolytic-like profile in the elevated plus-maze. CMS increased submissive behaviour in the resident-intruder test, and decreased male sexual behaviour. The latter effect was more pronounced in animals reared in isolation from the time of weaning; isolation rearing did not influence sexual behaviour in nonstressed animals. Isolation rearing also potentiated the effect of CMS on sucrose drinking, in both male and female rats. These results support the relevance of the CMS procedure as a potential animal model of depression.

Effects of stress on defensive aggression and dominance in a water competition test

Water-deprived rats in a pair competing for a single source of water quickly establish a firm relationship during which one rat drinks consistently more (dominant) than the other (submissive) animal. This relationship is formed during the first competition and is very stable during subsequent tests. Exposure of dominant rats to a severe (18 hrs immobilization), but not a mild (2 hrs immobilization), stressor reduced markedly aggressive behavior and inverted transiently the dominant submissive relationship of the pairs. Exposure of submissive rats to the severe stressor resulted in only minor reductions of aggressive behavior in these animals. Prestress anxiety predicted stress effects in the dominant animals in that high-anxious animals lost more dominant behavior and weight during stress as compared with low-anxious rats. Thus, severe stress can transiently reduce dominant but not submissive behavior during water competition and high-anxious rats are more prone to lose their aggressive behavior.

Seven-day variable-stress regime alters cortical beta-adrenoceptor binding and immunologic responses: reversal by imipramine

Rats were submitted daily to a variable stressor for 1 week with or without concurrent imipramine (IMI) administration. One day after the last injection or stressful event, binding of cortical beta-adrenoceptors was determined in all experimental groups. Another group of chronically stressed animals with or without concurrent IMI administration were sacrificed 24 h following the last stress or injection treatment, and several immunologic parameters were evaluated. Chronically stressed rats showed an enhanced number of cortical beta-adrenergic sites without changes in their affinity. This effect was not present following concurrent administration with the antidepressant. In addition, a decreased percentage of T lymphocytes and a reduced delayed-type hypersensitivity reaction was also observed in stressed animals. Both responses were no longer evident when stressed rats were previously administered IMI. A possible link between behavioral, neurochemical, and immunologic alterations due to the stress regime is discussed.

Effect of regularity of exposure to chronic immobilization stress on the circadian pattern of pituitary adrenal hormones, growth hormone, and thyroid stimulating hormone in the adult male rat

Circadian variation of serum levels of adrenocorticotropin hormone (ACTH), corticosterone, growth hormone (GH), and thyroid-stimulating hormone (TSH) were studied in three groups of adult male rats exposed to chronic intermittent immobilization stress (IMO) for 2 hr daily under different schedules. IMO resulted in reduced food intake, body weight loss, and increased adrenal weight. ACTH levels were not affected but corticosterone levels were increased in all IMO rats as compared to control ones during the diurnal phase of the circadian cycle. IMO decreased serum GH and TSH levels but the circadian pattern of secretion was influenced in a complex way depending on the specific pattern of daily exposure to IMO. Differences observed between the IMO groups were not caused by differences in food intake because its circadian rhythm was very similar in all IMO groups. These results suggest that regularity of exposure to immobilization alters in a complex fashion circadian GH and TSH rhythms.