Early life stress-induced alterations in the activity and morphology of ventral tegmental area neurons in female rats.

Childhood maltreatment, which can take the form of physical or psychological abuse, is experienced by more than a quarter of all children. Early life stress has substantial and long-term consequences, including an increased risk of drug abuse and psychiatric disorders in adolescence and adulthood, and this risk is higher in women than in men. The neuronal mechanisms underlying the influence of early life adversities on brain functioning remain poorly understood; therefore, in the current study, we used maternal separation (MS), a rodent model of early-life neglect, to verify its influence on the properties of neurons in the ventral tegmental area (VTA), a brain area critically involved in reward and motivation processing. Using whole-cell patch-clamp recordings in brain slices from adolescent female Sprague-Dawley rats, we found an MS-induced increase in the excitability of putative dopaminergic (DAergic) neurons selectively in the medial part of the VTA. We also showed an enhancement of excitatory synaptic transmission in VTA putative DAergic neurons. MS-induced alterations in electrophysiology were accompanied by an increase in the diameter of dendritic spine heads on lateral VTA DAergic neurons, although the overall dendritic spine density remained unchanged. Finally, we reported MS-related increases in basal plasma ACTH and corticosterone levels. These results show the long-term consequences of early life stress and indicate the possible neuronal mechanisms of behavioral disturbances in individuals who experience early life neglect.

Loss of mu and delta opioid receptors on neurons expressing dopamine receptor D1 has no effect on reward sensitivity.

Opioid signaling controls the activity of the brain's reward system. It is involved in signaling the hedonic effects of rewards and has essential roles in reinforcement and motivational processes. Here, we focused on opioid signaling through mu and delta receptors on dopaminoceptive neurons and evaluated the role these receptors play in reward-driven behaviors. We generated a genetically modified mouse with selective double knockdown of mu and delta opioid receptors in neurons expressing dopamine receptor D1. Selective expression of the transgene was confirmed using immunostaining. Knockdown was validated by measuring the effects of selective opioid receptor agonists on neuronal membrane currents using whole-cell patch clamp recordings. We found that in the nucleus accumbens of control mice, the majority of dopamine receptor D1-expressing neurons were sensitive to a mu or delta opioid agonist. In mutant mice, the response to the delta receptor agonist was blocked, while the effects of the mu agonist were strongly attenuated. Behaviorally, the mice had no obvious impairments. The mutation did not affect the sensitivity to the rewarding effects of morphine injections or social contact and had no effect on preference for sweet taste. Knockdown had a moderate effect on motor activity in some of the tests performed, but this effect did not reach statistical significance. Thus, we found that knocking down mu and delta receptors on dopamine receptor D1-expressing cells does not appreciably affect some of the reward-driven behaviors previously attributed to opioid signaling.

Repeated Neck Restraint Stress Bidirectionally Modulates Excitatory Transmission in the Dentate Gyrus and Performance in a Hippocampus-dependent Memory Task.

The consequences of stress depend on characteristics of the stressor, including the duration of exposure, severity, and predictability. Exposure of mice to repeated neck restraint has been shown to bidirectionally modulate the potential for long-term potentiation (LTP) in the dentate gyrus (DG) in a manner dependent on the number of restraint repetitions, but the influence of repeated brief neck restraint on electrophysiology of single DG neurons has not yet been investigated. Here, we aimed at finding the effects of 1, 3, 7, 14, or 21 daily neck restraint sessions lasting 10 min on electrophysiological characteristics of DG granule cells as well as excitatory and inhibitory synaptic inputs to these neurons. While the excitability of DG granule cells and inhibitory synaptic transmission were unchanged, neck restraint decreased the frequency of spontaneous excitatory currents after three repetitions but enhanced it after 14 and 21 repetitions. The consequences of repeated neck restraint on hippocampus-dependent memory were investigated using the object location test (OLT). Neck restraint stress impaired cognitive performance in the OLT after three repetitions but improved it after 14 and 21 repetitions. Mice subjected to three neck restraint sessions displayed an increase in the measures of depressive and anxiety-like behaviors, however, prolongation of the exposure to neck restraint resulted in a gradual decline in the intensity of these measures. These data indicate that stress imposed by an increasing number of repeated neck restraint episodes bidirectionally modulates both excitatory synaptic transmission in the DG and cognitive performance in the object location memory task.

Influence of chronic stress on brain corticosteroid receptors and HPA axis activity.

BACKGROUND: Disruption of the glucocorticoid negative feedback system evoked in animals by chronic stress can be induced by downregulation of glucocorticoid receptors (GRs) in several brain regions. In the present study, the dynamics of the changes in GRs, in brain structures involved in stress reactions, prefrontal cortex, hippocampus and hypothalamus was compared with the peripheral hypothalamo-pituitary-adrenocortical (HPA) axis hormones response to chronic stress. METHODS: Rats were exposed to 10 min restraint or restrained twice a day for 3, 7 or 14 days, and 24 h after the last stress session exposed to homotypic stress for 10 min. Control rats were not restrained. After rapid decapitation at 0, 1, 2, and 3 h after stress termination, trunk blood for plasma adrenocorticotropic hormone (ACTH) and corticosterone determinations was collected and prefrontal cortex, hippocampus and hypothalamus were excised and frozen. Plasma hormones were determined using commercially available kits and glucocorticoids and mineralocorticoids protein levels in brain structure samples were determined by western blot procedure. RESULTS: Restraint stress alone significantly decreased glucocorticoid receptor (GR) level in prefrontal cortex and hippocampus, and increased mineralocorticoid receptor (MR) level in hypothalamus. Prior repeated stress for 3 days significantly increased GR protein level in hippocampus and diminished that level in hypothalamus in 7 days stressed rats. Acute stress-induced strong increase in plasma ACTH and corticosterone levels decreased to control level after 1 or 2 h, respectively. Prior repeated stress for 3 days markedly diminished the fall in plasma ACTH level and repeated stress for 7 days moderately deepened this decrease. Plasma ACTH level induced by homotypic stress in rats exposed to restraint for 3, 7, and 14 days did not markedly differ from its control level, whereas plasma corticosterone response was significantly diminished. The fast decrease of stress-induced high plasma ACTH and corticosterone levels was accompanied by a parallel decline of GR level only in prefrontal cortex but not in the hippocampus or hypothalamus. CONCLUSIONS: Comparison of the dynamics of changes in plasma ACTH and corticosterone level with respective alterations in GR and MR in brain structures suggests that the buffering effect of repeated stress depends on the period of habituation to stress and the brain structure involved in regulation of these stress response.

Effect of repeated restraint on homotypic stress-induced nitric oxide synthases expression in brain structures regulating HPA axis.

BACKGROUND: Restraint stress (RS) markedly increases interleukin 1-ß (IL-1ß) generation in brain structures involved in hypothalamic-pituitary adrenocortical (HPA) axis regulation. The IL-1ß-induced transient stimulation of HPA axis activity was parallel in time and magnitude to respective changes in regulation of HPA activity. In the present experiment the expression of neuron al and inducible nitric oxide synthase (nNOS and iNOS) were investigated in prefrontal cortex, hippocampus and hypothalamus in response to acute restraint stress in control and prior repeatedly restrained rats. METHODS: Experiments were performed on male Wistar rats which were exposed to 10 min restraint stress or restrained twice a day for 3 days, and 24 h after the last stress period exposed to homotypic stress for 10 min. After rapid decapitation at 0, 1, 2 and 3 h after cessation of stress, trunk blood was collected and prefrontal cortex, hippocampus and hypothalamus were excised and frozen. Interleukin-1ß, adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels were determined in plasma using commercially available kits and neuronal nitric oxide synthase (nNOS) and inducible nitric oxide synthase (iNOS) in brain structure samples were analyzed by western blot procedure. RESULTS: Prior repeated restraint stress enhanced the acute restraint stress induced increase in IL-1ß levels in all three structures examined. Restraint stress for 10 min moderately decreased nNOS level in prefrontal cortex in control rats, augmented this level in hippocampus and markedly increased nNOS level in hypothalamus. Restraint itself significantly decreased iNOS level in prefrontal cortex, while it enhanced iNOS level in hippocampus and hypothalamus. Prior restraint stress for 3 days enhanced the nNOS level in prefrontal cortex and hippocampus and did not substantially affect nNOS levels response in hypothalamus. Repeated restraint stress considerably augmented the iNOS levels in both prefrontal cortex, hippocampus and hypothalamus induced by followed homotypic stress. CONCLUSION: These results indicate that during restraint stress nNOS regulate formation of low amount of NO and the high-output generation of NO is effected by inducible isoform of nitric oxide synthase. Prior repeated stress significantly enhances the homotypic stress-induced nNOS and iNOS responses.

Brain nitric oxide synthases in the interleukin-1ß-induced activation of hypothalamic-pituitary-adrenal axis.

BACKGROUND: Interleukin-1ß (IL-1ß), the major cytokine involved in activation of hypothalamic-pituitary-adrenal (HPA) axis modulates both central and peripheral components regulating HPA activity. The role of nitric oxide (NO) generated by neuronal nitric oxide synthase (nNOS) and inducible nitric oxide synthase (iNOS) in brain structures involved in HPA axis regulation has not been elucidated. The aim of the study was to assess the receptor selectivity of IL-1ß stimulatory action on HPA axis and to determine the involvement of nNOS and iNOS in this stimulation. METHODS: Experiments were performed on male Wistar rats which were injected intraperitoneally (ip) with IL-1ß (5 µg/kg) or IL-1 receptor antagonist (IL-1ra) (50 µg/kg or 100 µg/kg) 15 min before IL-1ß. Rats were sacrificed by rapid decapitation 1, 2 or 3 h after IL-1ß administration. Trunk blood for ACTH, corticosterone and IL-1ß determinations was collected and prefrontal cortex, hippocampus and hypothalamus were excised and snap frozen. Western blot analyses were performed and IL-1ß, nNOS and iNOS protein were determined in brain structures samples. RESULTS: IL-1ß significantly increased plasma ACTH, corticosterone and IL-1ß levels during 2 h after ip administration. IL-1 receptor antagonist was able to abolish the stimulatory effect of IL-1ß on plasma ACTH and corticosterone levels and significantly, but not totally, reduced plasma IL-1ß level. The role of NO in prefrontal cortex, hippocampus and hypothalamus in the IL-1ß-induced HPA axis activity alterations was determined by measuring the changes in nNOS and iNOS levels. The highest level of both izoenzymes 1 h following IL-1ß administration decreased in a regular, parallel manner 2 and 3 h later, approaching control values. These changes were almost totally prevented by pretreatment with IL-1 receptor antagonist. In the hypothalamus the IL-1ß-induced initial significant increase of nNOS regularly decreased in a modest rate and remained at significant higher level compared to control values. By contrast, iNOS level gradually increased 2 and 3 h after IL-1ß administration in a significant time-dependent manner. The changes in both NOS izoenzyme levels in hypothalamus were suppressed by pretreatment with IL-1 receptor antagonist. Results also show that a regular and parallel decrease of nNOS in the hypothalamus and prefrontal cortex are parallel in time and magnitude to respective fall in plasma IL-1ß and ACTH levels. CONCLUSION: The present study suggests that the IL-1ß-induced transient stimulation of HPA axis activity is parallel in time and magnitude to the respective changes of nNOS in hypothalamus and prefrontal cortex, the brain structures involved in regulation of HPA axis activity.

Brief neck restraint stress enhances long-term potentiation and suppresses long-term depression in the dentate gyrus of the mouse.

We studied the effects of brief (10 min) neck restraint on long-term potentiation (LTP) and long-term depression (LTD) in mouse dentate gyrus (DG) slices. Brain slices were prepared immediately after neck restraint and LTP/LTD induction was attempted 3.5h later. LTP enhancement and LTD suppression was observed in slices prepared from stressed animals. The corticosterone plasma concentration was elevated approx. fourfold just after the neck restraint session. To examine the role of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) activation in mediating the effects of neck restraint on LTP and LTD, the animals were pretreated with either the specific GR antagonist RU38486, the specific MR antagonist spironolactone, the corticosterone synthesis inhibitor metyrapone, or vehicle Tween 80. Injection with Tween 80 and subsequent neck restraint did not influence LTP enhancement or LTD suppression, but the effects of neck restraint were prevented by RU38486, spironolactone and metyrapone pretreatment. These data indicate that acute stress caused by brief neck restraint enhances LTP and impairs LTD in the dentate gyrus and that these effects are mediated by activation of GRs and MRs.

Effect of prior stress on interleukin-1ß and HPA axis responses to acute stress.

Interleukin-1ß (IL-1ß) level is modulated during multiple stress reactions both in brain structures involved in hypothalamic-pituitary-adrenal (HPA) axis regulation and peripheral systems. Multiple distinct stressors induce different IL-1ß and HPA axis responses. The purpose of the present study was to determine if the effect of prior repeated restraint stress on IL-1ß levels in prefrontal cortex, hippocampus, hypothalamus and plasma may have an impact on alterations induced in HPA axis responses. Experiments were performed on male Wistar rats which were exposed to 10 min restraint stress twice a day for 3 days. Twenty four hours after the last stress period rats were restrained for 10 min and decapitated at 0, 1, 2 or 3 h after cessation of stress. Control rats were injected ip with saline and some of experimental groups with IL-1ß receptor antagonist (IL-1ra). After rapid decapitation, trunk blood was collected and prefrontal cortex, hippocampus and hypothalamus were excised and frozen. Interleukin-1ß, adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels were determined in plasma using commercially available kits and IL-1ß levels in brain structures samples were analyzed by western blot procedure. Repeated restraint for 3 days alone did not alter resting plasma levels of IL-1ß, and moderately augmented plasma ACTH and CORT levels and IL-1ß content in brain structures 24 h after the last restraint. IL-1ß antagonist abolished the increase in plasma levels of IL-1ß, ACTH and CORT as well as IL-1ß in brain structures in response to repeated stress and also reduced these changes induced by 10 min stress. This suggests the selectivity of IL-1ß receptors in central and peripheral mechanisms modulating the stress-induced HPA axis responses. These results indicate that repeated stress markedly increases IL-1ß production in brain structures involved in HPA axis regulation. The present results support the role of brain and peripheral IL-1ß in adaptation of HPA response during prolonged stress.

Repeated restraint-induced modulation of long-term potentiation in the dentate gyrus of the mouse.

We studied the potential for induction of long-term potentiation (LTP) in slices of the dentate gyrus of C57BL/6 mouse, prepared 24h after 1, 3, 7, 14 or 21 successive daily neck restraint sessions lasting for 10min. In slices from control animals and mice restrained only once, LTP (155+/-2% of baseline) was evident for at least 2h, but LTP was impaired after 3 sessions of neck restraint. This effect could be blocked by administration of the glucocorticoid receptor (GR) antagonist RU38486 before each restraint. One day after 3 restraint sessions the levels of plasma corticosterone, GR and mineralocorticoid receptor (MR) proteins remained elevated. After 7 neck restraint sessions the LTP level returned to normal; after 14 and 21 restraint sessions LTP was enhanced. LTP enhancement after 14 daily restraint sessions was not blocked by the GR antagonist RU38486 administered before each restraint; the MR antagonist RU28318 blocked LTP induction completely. After 14 daily restraint sessions the level of GR and MR proteins did not differ from the control and the plasma corticosterone level remained slightly elevated. These data indicate that repeated stress imposed by brief neck restraint bidirectionally modulates the potential for LTP in the dentate gyrus in a manner dependent on the number of daily restraint sessions.