Duration-dependent effects of repeated restraint stress on cortical projections of locus coeruleus neurons

Antidepressant modulation of isolation and restraint stress effects on brain chemistry and morphology

Stress elicits adaptive responses from the brain, but it can also lead to maladaptive consequences. For example, stress can precipitate mental illness, including depression. Prolonged stress also causes damage to neurons in the hippocampus. Antidepressant drugs must be evaluated, not only for their ability to potentiate adaptive responses, but also to inhibit maladaptive consequences of stress. Ongoing research in our laboratory has compared the atypical tricyclic antidepressant, tianeptine, with the typical tricyclics, desipramine and imipramine, with respect to the effects of isolation and repeated restraint stress. Tianeptine and desipramine similarly attenuated isolation stress-induced increases in locus coeruleus and midbrain tyrosine hydroxylase mRNA levels and isolation-stress induced decreases in preproenkephalin mRNA levels in striatum and nucleus accumbens. However, tianeptine and imipramine differed in their effects in the cerebral cortex and hippocampus on 5HT2, and 5HT1A receptor levels but, surprisingly, produced similar effects on levels of the serotonin transporter labelled with [3H] paroxetine. Tianeptine also prevented stress-induced reductions in the length and number of branchpoints of dendrites of CA3 pyramidal neurons in hippocampus; comparison with effects of typical tricyclics are ongoing. Tianeptine also blocked effects of corticosterone treatment to reduce branching and length of CA3 dendrites. These actions of tianeptine may be due to interactions between 5HT and excitatory amino acids in the mossy fiber terminals on CA3 pyramidal neurons. Taken together, these results indicate that tianeptine has unique properties compared to some other antidepressant drugs, but shares in common with those drugs the ability to attenuate stress effects on tyrosine hydroxylase gene expression and on the serotonin transporter. It remains to be seen whether these actions are the basis of a common antidepressant action.

Reboxetine Improves Auditory Attention and Increases Norepinephrine Levels in the Auditory Cortex of Chronically Stressed Rats

Chronic stress impairs auditory attention in rats and monoamines regulate neurotransmission in the primary auditory cortex (A1), a brain area that modulates auditory attention. In this context, we hypothesized that norepinephrine (NE) levels in A1 correlate with the auditory attention performance of chronically stressed rats. The first objective of this research was to evaluate whether chronic stress affects monoamines levels in A1. Male Sprague-Dawley rats were subjected to chronic stress (restraint stress) and monoamines levels were measured by high performance liquid chromatographer (HPLC)-electrochemical detection. Chronically stressed rats had lower levels of NE in A1 than did controls, while chronic stress did not affect serotonin (5-HT) and dopamine (DA) levels. The second aim was to determine the effects of reboxetine (a selective inhibitor of NE reuptake) on auditory attention and NE levels in A1. Rats were trained to discriminate between two tones of different frequencies in a two-alternative choice task (2-ACT), a behavioral paradigm to study auditory attention in rats. Trained animals that reached a performance of ≥80% correct trials in the 2-ACT were randomly assigned to control and stress experimental groups. To analyze the effects of chronic stress on the auditory task, trained rats of both groups were subjected to 50 2-ACT trials 1 day before and 1 day after of the chronic stress period. A difference score (DS) was determined by subtracting the number of correct trials after the chronic stress protocol from those before. An unexpected result was that vehicle-treated control rats and vehicle-treated chronically stressed rats had similar performances in the attentional task, suggesting that repeated injections with vehicle were stressful for control animals and deteriorated their auditory attention. In this regard, both auditory attention and NE levels in A1 were higher in chronically stressed rats treated with reboxetine than in vehicle-treated animals. These results indicate that NE has a key role in A1 and attention of stressed rats during tone discrimination.

Review : Stress, Antidepressant Treatments, and Neurotrophic Factors: Molecular and Cellular Mechanisms

Repeated stress or an excess of glucocorticoids can exacerbate neuronal damage in response to insults and, in severe cases, can lead to neuronal atrophy and death. These effects are thought to be related to the actions of stress and glucocorticoids on glutamate function, neuronal metabolism, and the generation of cytotoxic free radicals. Recent studies demonstrate that the regulation of neurotrophic factors may contribute to the actions of stress on neuronal function. Acute or chronic stress decreases the expression of brain derived neurotrophic factor, the most abundant neurotrophin in the brain, in specific regions of the hippocampus, and other forebrain regions. In addition, chronic stress increases the expression of neurotrophin-3 in certain regions of the hippocampus and may, thereby, help to protect these regions from the neurotoxic effects of chronic stress. The deleterious effects of stress may contribute to psy chiatric illnesses, such as depression, that can be precipitated or worsened by stress and that are often characterized by hypercortisolism. Electroconvulsive seizure therapy, as well as antidepressant drugs, increase the expression of brain derived neurotrophic factor and its receptor, trkB, in the brain, demon strating that neurotrophins are a target of antidepressant treatments. These findings outline a role of neurotrophic factors in the etiology and treatment of certain psychiatric illnesses. The Neuroscientist 1:351-360, 1995

Altered expression of tyrosine hydroxylase in the locus coeruleus noradrenergic system in citalopram neonatally exposed rats and monoamine oxidase a knock out mice

In rodents, noradrenergic (NE) locus coeruleus (LC) neurons are well known to express tyrosine hydroxylase (TH) immunoreactivity. However, due to its very low enzyme activity, NE cortical fibers do not typically express TH immunoreactivity, thus dopamine-β-hydroxylase (DBH) immunoreactivity is commonly utilized as a marker for NE cortical fibers. In this study, we performed double and/or triple immunofluorescent staining using antibodies against TH, DBH, and/or norepinephrine transporter (NET) to investigate the altered NE TH expression of cortical fibers in citalopram (CTM)-exposed rats and monoamine oxidase (MAO) A knock out (KO) mice. We have noted the following novel findings: (1) neonatal exposure to the selective serotonin reuptake inhibitor (SSRI) CTM enhanced NE TH immunoreactive fibers throughout the entire neocortex, and a few of them appeared to be hypertrophic; (2) slightly enhanced NE cortical TH immunoreactive fibers were also noted in MAO A KO mice, and many of them revealed varicosities compared with the rather smooth NE cortical TH immunoreactive fibers in wild-type (WT) mice; (3) LC dendrites of MAO A KO mice exhibited beaded morphology compared with the smooth LC dendrites in WT mice. Our findings suggest that both genetic and environmental factors during early development may play a critical role in the regulation and proper function of NE TH expression in the neocortex.

Duration-dependent effects of the bite-raised condition on hippocampal function in SAMP8 mice

We evaluated the effect of the duration of occlusal disharmony induced chronic stress on hippocampal function by examining spatial memory in the Morris water maze and on the number of hippocampal neurons in aged senescence-accelerated prone (SAMP8) mice. The bite of SAMP8 mice was raised 0.1 mm using dental adhesive. Groups of mice were tested in the Morris water maze 8, 11, or 22 d after raising the bite. The results indicated that the longer the duration of the bite-raised condition, the greater the impairment in spatial learning ability and the greater the decrease in the number of neurons in the hippocampal CA3 subfield. Thus, behavioral and morphologic deficits induced by the bite-raised condition in aged SAMP8 mice are influenced by the duration of the occlusal disharmony.

Differential responses to anxiogenic drugs in a mouse model of panic disorder as revealed by Fos immunocytochemistry in specific areas of the fear circuitry

Sensitivity to pharmacological challenges has been reported in patients with panic disorder. We have previously validated transgenic mice overexpressing the neurotrophin-3 (NT-3) receptor, TrkC (TgNTRK3), as an engineered murine model of panic disorder. We could determine that TgNTRK3 mice presented increased cellularity in brain regions, such as the locus ceruleus, that are important neural substrates for the expression of anxiety in severe anxiety states. Here, we investigated the sensitivity to induce anxiety and panic-related symptoms by sodium lactate and the effects of various drugs (the alpha2-adrenoceptor antagonist, yohimbine and the adenosine antagonist, caffeine), in TgNTRK3 mice. We found enhanced panicogenic sensitivity to sodium lactate and an increased intensity and a differential pattern of Fos expression after the administration of yohimbine or caffeine in TgNTRK3. Our findings validate the relevance of the NT-3/TrkC system to pathological anxiety and raise the possibility that a specific set of fear-related pathways involved in the processing of anxiety-related information may be differentially activated in panic disorder.

Chronic stress increases the plasmalemmal distribution of the norepinephrine transporter and the coexpression of tyrosine hydroxylase in norepinephrine axons in the prefrontal cortex

Norepinephrine (NE) potently modulates the cognitive and affective functions of the prefrontal cortex (PFC). Deficits in NE transmission are implicated in psychiatric disorders, and antidepressant drugs that block the NE transporter (NET) effectively treat these conditions. Our initial ultrastructural studies of the rat PFC revealed that most NE axons (85-90%) express NET primarily within the cytoplasm and lack detectable levels of the synthetic enzyme tyrosine hydroxylase (TH). In contrast, the remaining 10-15% of PFC NE axons exhibit predominantly plasmalemmal NET and evident TH immunoreactivity. These unusual characteristics suggest that most PFC NE axons have an unrecognized, latent capacity to enhance the synthesis and recovery of transmitter. In the present study, we used dual-labeling immunocytochemistry and electron microscopy to examine whether chronic cold stress, a paradigm that persistently increases NE activity, would trigger cellular changes consistent with this hypothesis. After chronic stress, neither the number of profiles exhibiting NET labeling nor their size was changed. However, the proportion of plasmalemmal NET nearly doubled from 29% in control animals to 51% in stressed rats. Moreover, the expression of detectable TH in NET-labeled axons increased from only 13% of profiles in control rats to 32% of profiles in stressed animals. Despite the consistency of these findings, the magnitude of the changes varied across individual rats. These data represent the first demonstration of activity-dependent trafficking of NET and expression of TH under physiological conditions and have important implications for understanding the pathophysiology and treatment of stress-related affective disorders.

Movements restriction and alterations of the number of spines distributed along the apical shafts of layer V pyramids in motor and primary sensory cortices of the peripubertal and adult rat

The number and distribution of spines along apical shafts of rapid-Golgi-stained layer V pyramidal cells from visual, motor and somatosensory cortical areas were analyzed in control and movement-restricted (beginning at 20 days old) Wistar rats killed at 30, 40, 80 and 120 days of age (experiment A). In other group of rats, spine density was analyzed when restriction initiates on day 40 and the animals were killed at 50, 60 and 80 days postpartum, or after restriction starting on day 80 and killed at 120 days of age (experiment B). It has been found that the restriction of movements significantly reduces the total number of spines on apical shafts in the three cortical areas, when this condition starts at 20, 40 or 80 days without changing the overall distribution of spines. Also present findings indicate that the effects of movements restriction are attenuated when they were concurrent with maturational brain processes (20-40 days) than when they occurred later in life. The question remains open of which part of the measured reduction on the number of spines is due to the immobilization and which to the stress associated with this maneuver.

Contribution of the stress-induced degeneration of the locus coeruleus noradrenergic neurons to the pathophysiology of depression: a study on an animal model

A novel theory on the pathophysiology of depression would be expected to resolve a contradiction between therapeutic time lag and monoamine hypothesis. On the basis of the fact that a subgroup of depression appears during or after stress, we exposed rats to a long-term (2 weeks) forced walking stress and produced depression-model rats in one group and spontaneous recovery rats in another. The density of axon terminals of the locus coeruleus (LC) neurons in the frontal cortex stained by dopamine β-hydroxylase antiserum was lower in the depression-model rats than in the spontaneous recovery rats and in the control rats without stress. The density was higher in the model rats daily treated with imipramine than in those treated with saline. Morphological projection (MP) index (a percentage of horseradish peroxidase-positive LC cells in total number of LC cells) and electrophysiological projection index (a percentage of LC neurons activated antidromically by electrical stimulation of the cerebral cortex) were lower in the depression-model rats than in the recovery and control rats. MP index was higher in the imipramine-treated rats than that in the saline-treated rats. Electron microscopic examination of the LC disclosed such degenerative changes as low-dense areas without structure, aggregation of intracellular organs, destroyed membranes around the rough endoplasmic reticulum (rER), a decreased number of deformed subsurface cisterns, glia invaginated into the LC neurons and prominent appearance of microglia containing increased number of lipofustin or lysosome in the model rats, but not in the spontaneous recovery rats. These findings suggest that the terminals and cell bodies of the LC noradrenergic neurons degenerate in the stress-induced depression-model rats and regenerate in the imipramine-treated model rats. This degenerative change may possibly contribute to the decrease in synthesis and metabolism of noradrenaline (NA), the slowing of axonal flow, the accumulation of NA in the neurons, the decrease in discharge rate of LC neurons without stress and the increase in release of NA in response to an additional stress. It may also explain the therapeutic time lag that is required to repair the noradrenergic neurons.

Opposite morphological responses of partially denervated cortical serotonergic and noradrenergic axons to repeated stress in adult rats

We examined plastic changes in serotonin (5-HT) axons following repeated stress in the adult rat brain, and compared stress-induced changes between 5-HT and noradrenaline (NA) axons. We locally injected the specific neurotoxin to 5-HT axons or to NA axons into the frontal cortex to cause partial denervation. The animals were mildly restrained from 1 day after the neurotoxin injection and this stress was repeated daily for 20 min during the first 2 days and for 40 min during the next 11 days. On the fourteenth day after injection, the brains were removed to visualize 5-HT and NA axons by immunohistochemistry. Repeated stress did not significantly alter the denervation area of 5-HT or NA axons, but the density of 5-HT axons was increased whereas that of NA axons was decreased in cortical regions outside the denervation site. In addition, the expression of brain-derived neurotrophic factor (BDNF) was increased in cortical regions where the 5-HT axon density was increased in response to stress. These results suggest that repeated stress causes opposite changes in the morphology of partially denervated 5-HT and NA axons in the cerebral cortex. The stress-induced increase in BDNF expression may contribute to 5-HT axon sprouting following repeated stress.

Interaction between serotonergic and noradrenergic axons during axonal regeneration

The present experiments focused on the morphological interaction between serotonergic (5-HT) and noradrenergic (NA) axons during regeneration following partial axonal denervation in the cerebral cortex in adult rats. The denervation paradigm used employed two neurotoxins, one for 5-HT and one for NA axons, infused together at one cortical site while a single neurotoxin to either 5-HT or NA was infused at the symmetrical cortical site in the other hemisphere. This treatment enabled us to assess the role of 5-HT or NA axons in the regeneration of the other monoaminergic axon. 5-HT axon regeneration became apparent as early as 28 days after the toxin injection, whereas the regeneration of NA axons was not evident even at 60 days after the toxin injection. Since NA axons revealed marked regeneration in the cortical site with denervation of 5-HT axons, intact 5-HT axons may be inhibitory on the regeneration of NA axons. In contrast, since the regeneration of 5-HT axons was suppressed in the absence of NA axons, NA axons appear to exert a facilitatory effect on 5-HT axon regeneration. These results suggest that the role of 5-HT axons in the regeneration of NA axons is opposite to that of NA axons in the regeneration of 5-HT axons. In addition, the regeneration of 5-HT axons occurred much faster than that of NA axons in response to axonal damage. The differential roles of 5-HT and NA axons in axonal regeneration may play a role in a variety of physiological functions related to these monoamines and possibly in the pathophysiology of clinical depression.

Novel objects in a holeboard probe the role of the locus coeruleus in curiosity: support for two modes of attention in the rat

Idazoxan, an alpha 2 adrenoceptor antagonist (2 mg/kg), enhanced novel object investigation in a holeboard in rats as previously reported (V. Devauges & S. J. Sara, 1990). Two weeks of 10 min/day in 37 degrees C water increased dopamine-beta-hydroxylase staining density in the locus coeruleus but did not enhance novel object investigation. In contrast to idazoxan, however, the warm water treatment increased rearing, center entries, and activity, a pattern previously described during tonic infusion of norepinephrine into the hippocampus. Correlations among dopamine-beta-hydroxylase measures and behavior reinforced these tonic norepinephrine/behavior associations. The behavioral effects across the idazoxan and warm water experiments support G. Aston-Jones et al.'s (1999) 2 modes of attention hypothesis for locus coeruleus function: Phasic locus coeruleus activity promotes focused attention; tonic locus coeruleus activity promotes scanning attention.

The effects of acute and chronic stress on motor and sensory performance in male Lewis rats

Any behavioral testing induces stress to some degree. A meaningful interpretation of behavioral results can be difficult if stress, caused by handling or the testing situation, modifies the experimental outcome. Especially for neurological animal models, it is important to know how stress affects motor and sensory performance. Therefore, we investigated the effects of varying degrees of stress on several motor and sensory tasks that are frequently used to assess functional recovery after lesion-induced impairments in adult rats. Acute, subchronic, and chronic stress impaired ladder walking and prolonged the duration of grasping a bar. Stress also altered walking patterns by increasing the base of support and foot rotation and reducing stride length. Furthermore, chronic stress induced hypersensitivity to painful stimuli, but did not significantly influence the latency to remove sticky papers from the hindpaws (sticky paper test). In the light--dark (L/D) test, stress reduced the latency to enter the dark compartment and enhanced the number of transitions supporting that cold swim stress modifies the animal's level of anxiety. These data point towards a critical influence of acute or chronic stress on motor control and sensory performance of rats, suggesting that stress might be a critical intervening variable of the outcome of behavioral tests.

The effects of prenatal stress on the development of hypothalamic paraventricular neurons in fetal rats

The present experiments focused on the influence of prenatal stress on the development of neurons of the hypothalamic paraventricular nucleus in the fetal rat, including corticotropin-releasing factor-containing neurons. Prenatal stress was administered by restraining pregnant rats in a small cage for either 30 (30-min stress group) or 240 min (240-min stress group) daily for three days from embryonic day 15 to 17, and the fetal brains were taken on embryonic day 18 for later analysis. Golgi-impregnated neurons of the paraventricular nucleus in the 240-min stress group revealed that the total length of the processes was significantly shorter than in the control (unstressed) and 30-min stress groups. In addition, the 240-min stress group showed an increase in the number of apoptotic cells in the fetal paraventricular nucleus. On the other hand, Golgi-impregnated neurons of the paraventricular nucleus in the 30-min stress group had a greater degree of cell differentiation as manifested by an increase in both the number of branch points and the total length of the processes from the cell body. Furthermore, the fetal paraventricular nucleus in the 30-min stress group showed enhanced corticotropin-releasing factor messenger RNA expression, while the varicosities of corticotropin-releasing factor-containing axons at the median eminence revealed more matured morphology such as shorter intervals between the varicosities. These findings suggest the duration-dependent effects of prenatal stress on the development of fetal hypothalamic paraventricular nucleus neurons, including corticotropin-releasing factor-containing neurons: long-lasting stress causes neurotoxic changes of fetal paraventricular nucleus neurons, whereas short-lasting stress facilitates the development of these fetal brain neurons. These morphological changes induced by prenatal stress may contribute to behavioral changes of the offspring after birth.

Long-term stress degenerates, but imipramine regenerates, noradrenergic axons in the rat cerebral cortex

Exposed to a forced walking stress for 2 weeks, some rats became persistently inactive (depression-model rats), whereas others gradually recovered from exhaustion (spontaneous recovery rats). We also studied rats exposed to short-term stress, rats without stress, and the model rats treated with imipramine or saline. We examined the density of noradrenergic axons in the frontal cortex using retrograde labeling of the locus coeruleus with horseradish peroxidase injected into the cortex and immunohistochemical staining of cortical axons with dopamine beta-hydroxylase antiserum. The density was significantly lower in the depression-model rats, but tended to be higher in the recovery rats and short-term stressed rats. Chronic treatment with imipramine significantly increased the density in the model rats. There was also a correlation between the density of noradrenergic axons and the recovery rate of activity. Our results suggest that cortical noradrenergic degeneration is involved in the pathogenesis of depression.

Restraint stress reversibly enhances spatial memory performance

The effects of restraint stress on performance of a spatial memory task, the eight arm radial maze, was examined in rats. When stress was given for 6 h/day for 7 days and performance evaluated days 10-13 post stress, no effect on performance was noted; however, daily restraint stress for 13 days caused a small, but significant, enhancement of performance days 10-13 post stress. Stressed rats performed better than controls: their number of correct choices in the first 8 visits was higher than the controls, and stressed rats took fewer total choices to finish the maze than controls. Stress-dependent, enhanced performance does not appear permanent since further maze testing on days 14 and 15 post stress showed no differences between the groups. Performance of the stressed rats significantly correlated with their stress-induced, serum corticosterone levels measured after 6 h of restraint on the last day of restraint, day 13 (r = -0.63, P < 0.05); rats with higher levels of CORT took fewer choices to finish the task. Examination of hippocampal CA3c pyramidal neurons with Golgi techniques showed no effect of stress on the basal or apical dendritic arbors. Since our previous study showed that 21 days of restraint stress is associated with impaired spatial memory performance (10), these results suggest that the duration of stress may differentially affect learning/memory with shorter periods of stress serving an adaptive function while longer durations causing maladaptive changes.

Stress and antidepressants differentially regulate neurotrophin 3 mRNA expression in the locus coeruleus

The mechanisms by which stress and anti-depressants exert opposite effects on the course of clinical depression are not known. However, potential candidates might include neurotrophic factors that regulate the development, plasticity, and survival of neurons. To explore this hypothesis, we examined the effects of stress and antidepressants on neurotrophin expression in the locus coeruleus (LC), which modulates many of the behavioral and physiological responses to stress and has been implicated in mood disorders. Using in situ hybridization, we demonstrate that neurotrophin 3 (NT-3) is expressed in noradrenergic neurons of the LC. Recurrent, but not acute, immobilization stress increased NT-3 mRNA levels in the LC. In contrast, chronic treatment with antidepressants decreased NT-3 mRNA levels. The effect occurred in response to antidepressants that blocked norepinephrine uptake, whereas serotonin-specific reuptake inhibitors did not alter NT-3 levels. Electroconvulsive seizures also decreased NT-3 expression in the LC as well as the hippocampus. Ntrk3 (neurotrophic tyrosine kinase receptor type 3; formerly TrkC), the receptor for NT-3, is expressed in the LC, but its mRNA levels did not change with stress or antidepressant treatments. Because, NT-3 is known to be trophic for LC neurons, our results raise the possibility that some of the effects of stress and antidepressants on LC function and plasticity could be mediated through NT-3. Moreover, the coexpression of NT-3 and its receptor in the LC suggests the potential for autocrine mechanisms of action.

Restraint stress in biomedical research: an update

Since the publication of our initial review of restraint stress in 1986, much work has continued with this technique, either as a tool for the investigation of other pharmacological, physiological, or pathologic phenomena or with restraint stress itself serving as the object of the study. As we noted in 1986, the major use of restraint has been for the induction of stress responses in animals and, more specifically, for the investigation of drug effects, particularly as they affect typical stress-related pathology--gastrointestinal, neuroendocrine, and immunological agents have been extensively studied. In compiling this update on restraint stress and its effects, we noted an increasing emphasis on central nervous system mechanisms in peripheral disease, especially gastrointestinal disease. In particular, many CNS-active agents have been tested for their effects on gastric and duodenal lesion formation and gastric secretion, including antidepressants, antipsychotics, anxiolytics, noradrenergic, serotonergic, dopaminergic, and peptidergic compounds. Some of these agents are especially active in the gastrointestinal tract even when administered centrally, further solidifying the concept of a brain-gut axis. The present update includes studies of: methods and procedures, pre-restraint manipulations, post-restraint/healing effects, and drug effects. In addition, a current bibliography of reports that have employed restraint is included.

Degeneration of locus coeruleus axons in stress-induced depression model

Antidepressants such as desipramine induce axonal regeneration of brain noradrenergic neurons. This novel action of antidepressants suggests the involvement of degeneration or retraction of brain noradrenergic axons in the pathophysiology of clinical depression. The present study was designed to further confirm this view in an animal model of stress-induced depression. The depression model was produced by exposing rats to prolonged forced walking stress. To see if axonal degeneration of noradrenergic neurons occurred in the depression model, the density of noradrenergic axons in the cerebral cortex was assessed by three different methods, antidromic stimulation technique, retrograde tracing with horseradish peroxidase and immunohistochemical staining with dopamine-beta-hydroxylase antiserum. These methods all assured of degenerative changes of noradrenergic axon terminals in the depression model. Furthermore, it was found that repeated treatments of the depression-model rats with imipramine could cause regeneration of cortical noradrenergic axons. These findings support the view that degeneration or retraction of noradrenergic axons is involved in the pathophysiology of depression.

Tianeptine attenuates stress-induced morphological changes in the hippocampus

Repeated 6-h daily restraint stress over 21 days reduces length and number of branch points of hippocampal CA3c pyramidal dendrites in the hippocampal formation of adult male rats. This effect is mimicked by daily injections of 40 mg/kg corticosterone. Daily treatment with tianeptine (15 mg/kg) prior to stress sessions or the corticosterone treatment prevented these effects of stress or corticosterone, respectively. Tianeptine treatment did not prevent the effects of stress to increase adrenal/body weight ratio, nor did it prevent the effects of stress to decrease body weight gain, indicating that its actions are not mediated solely by effects on stress-induced secretion of corticosterone. Because tianeptine is known to enhance neural uptake of serotonin, these results suggest that the serotonergic system may be involved in modulating stress and corticosterone effects on dendritic morphology.

Stress induces atrophy of apical dendrites of hippocampal CA3 pyramidal neurons

The hippocampus is vulnerable to the damaging actions of insults such as transient ischemia and repetitive stimulation, as well as repeated exposure to exogenous glucocorticoids. This study investigated effects of a repeated psychological stressor, restraint, on the CA3 pyramidal neurons which are vulnerable to damage by repetitive stimulation. Repeated daily restraint stress for 21 days caused apical dendrites of CA3 pyramidal neurons to atrophy, while basal CA3 dendrites did not change. Rats undergoing this treatment were healthy and showed some adaptation of the glucocorticoid stress response over 21 days; however, stress reduced body weight gain by 14% and increased adrenal weight relative to body weight by 20%. Results are discussed in relation to the possible role of adrenal steroids and excitatory amino acids.

Electrophysiological evidence for axonal degeneration of locus coeruleus neurons following long-term forced running stress

Using electrophysiological methods, a change in the density of axon terminals of locus coeruleus (LC) neurons in the cerebral cortex of rats following long-term forced running stress was examined. The stressed animals were classified into two groups based on spontaneous running activity (SRA) measured for 2 weeks after the stress treatment: 1) animals showing early restoration of SRA (poststress active rat) and 2) animals showing little or no SRA (poststress inactive rat). To quantify the density of LC axon terminals in the cerebral cortex, the percentage of LC neurons antidromically activated by cortical stimulation (projection index, P-index) was assessed. The P-indices for the cortex decreased in the poststress inactive rats. Since the threshold currents for antidromic activation were not altered by the stress treatment, the observed change was considered to reflect a change of the density of LC axon terminals rather than physiological consequences. Therefore, when animals receive a prolonged, severe stress, LC neurons in a certain group of the animals may cause axonal retraction or degeneration in the cerebral cortex.

Axonal sprouting of noradrenergic locus coeruleus neurons following repeated stress and antidepressant treatment

Plastic changes in axon terminals of NA LC neurons following repeated stress and antidepressant treatments were examined using electrophysiological or morphological methods. For stress treatment, rats restrained in a small cage were immersed up to the neck in warm water for 10 min daily. Electrophysiological experiments were performed under urethane anesthesia on the day following the termination of stress treatment. To quantify the density of cortical axon terminals arising in the LC, the percentage of LC neurons activated antidromically from the cerebral cortex was assessed. The percentage of LC neurons showing antidromic response to cortical stimulation was increased in the animals stressed for two weeks but not for one week. Since threshold currents for antidromic activation were not changed by the stress treatment, the observed changes were interpreted as morphological (axonal sprouting) rather than physiological consequences in NA axon terminals of LC neurons. To test the ability of antidepressants to induce the regeneration of central NA axons, local injections of 6-OHDA were made bilaterally into the symmetrical sites of the FC. Two weeks after the 6-OHDA injections, the same cortical site of one hemisphere was infused with the antidepressant MPL, DMI, or MIA, and the corresponding site of the other hemisphere with SAL. The density of glyoxylic acid-induced catecholamine fibers was greater in the cortical hemisphere infused with the antidepressants than that infused with SAL. These findings indicate that repeated mild stress and antidepressant treatments induce sprouting of NA LC axons in the cerebral cortex. Axonal sprouting of LC neurons can explain both the delayed onset of the clinical response to antidepressants and subsensitivity of beta-adrenoceptors following repeated stress and antidepressant treatments, and may be a common mechanism for the clinical efficacy of antidepressant drugs and electroconvulsive shock. Furthermore, the findings suggest the possibility that axonal retraction or degeneration of central NA neurons may be involved, at least in part, in the pathology of clinical depression.