A final common pathway for depression? Progress toward a general conceptual framework

Postsynaptic alpha-2 adrenergic receptors are critical for the antidepressant-like effects of desipramine on behavior

The antidepressant desipramine inhibits the reuptake of norepinephrine (NE), leading to activation of both pre- and postsynaptic adrenergic receptors, including alpha-1, alpha-2, beta-1, and beta-2 subtypes. However, it is not clear which adrenergic receptors are involved in mediating its antidepressant effects. Treatment of mice with desipramine (20 mg/kg, i.p.) produced an antidepressant-like effect, as evidenced by decreased immobility in the forced-swim test; this was antagonized by pretreatment with the alpha-2 adrenergic antagonist idazoxan (0.1-2.5 mg/kg, i.p.). Similarly, idazoxan, administered peripherally (0.5-2.5 mg/kg, i.p.) or centrally (1-10 microg, i.c.v.), antagonized the antidepressant-like effect of desipramine in rats responding under a differential-reinforcement-of-low-rate (DRL) 72-s schedule, ie, decreased response rate and increased reinforcement rate. By contrast, pretreatment with the beta-adrenergic antagonists propranolol and CGP-12177 or the alpha-1 adrenergic antagonist prazosin did not alter the antidepressant-like effect of desipramine on DRL behavior. The lack of involvement of beta-adrenergic receptors in mediating the behavioral effects of desipramine was confirmed using knockout lines. In the forced-swim test, the desipramine-induced decrease in immobility was not altered in mice deficient in beta-1, beta-2, or both beta-1 and beta-2 adrenergic receptors. In addition, desipramine (3-30 mg/kg) produced an antidepressant-like effect on behavior under a DRL 36-s schedule in mice deficient in both beta-1 and beta-2 adrenergic receptors. As antagonism of presynaptic alpha-2 adrenergic receptors facilitates NE release, which potentiates the effects of desipramine, the present results suggest that postsynaptic alpha-2 adrenergic receptors play an important role in its antidepressant effects.

Enhanced synchronization of thalamic theta band local field potentials in patients with essential tremor

Local field potentials (LFPs) were recorded in 13 patients from pairs of microelectrodes driven through thalamus during functional localization prior to implantation of a thalamic deep brain stimulation electrode for treatment of tremor or pain. Six patients had a history of essential tremor (ET), 3 of multiple sclerosis, and the remaining 4 had symptoms of chronic pain. Specific to the ET group was the observation that oscillatory field potentials recorded from the two microelectrodes in the motor thalamus (ventralis intermedius--Vim, ventralis oralis posterior--Vop) were highly coherent at frequencies characteristic of pathological tremor (4-7 Hz). This stands in contrast to the significantly more desynchronized state observed in the somatosensory thalamus (ventralis caudalis--Vc) for that frequency band. In addition, higher frequency coherent oscillations typically associated with physiological tremor (8-12 Hz) were observed in the ET patients in motor thalamus and Vc and in motor thalamus of pain patients. An examination of the inter-frequency correlation of the LFPs in Vim and Vop showed that the low frequency theta waves correlated with high frequency oscillations in the beta and gamma ranges. These findings are consistent with and extend those of other studies suggesting that alterations in thalamic oscillatory activity are involved in the pathophysiology of ET and furthermore suggest that increased synchronization in the 4-7 Hz range is related to the occurrence of tremor in the ET patient group. Furthermore, they support the idea that therapies such as lesions and high frequency stimulation of the motor thalamus are effective in reducing tremor symptoms since they destroy the abnormal low frequency synchronization in motor thalamus.

Rhythmicity in mice selected for extremes in stress reactivity: behavioural, endocrine and sleep changes resembling endophenotypes of major depression

BACKGROUND

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, including hyper- or hypo-activity of the stress hormone system, plays a critical role in the pathophysiology of mood disorders such as major depression (MD). Further biological hallmarks of MD are disturbances in circadian rhythms and sleep architecture. Applying a translational approach, an animal model has recently been developed, focusing on the deviation in sensitivity to stressful encounters. This so-called 'stress reactivity' (SR) mouse model consists of three separate breeding lines selected for either high (HR), intermediate (IR), or low (LR) corticosterone increase in response to stressors.

METHODOLOGY/PRINCIPLE FINDINGS

In order to contribute to the validation of the SR mouse model, our study combined the analysis of behavioural and HPA axis rhythmicity with sleep-EEG recordings in the HR/IR/LR mouse lines. We found that hyper-responsiveness to stressors was associated with psychomotor alterations (increased locomotor activity and exploration towards the end of the resting period), resembling symptoms like restlessness, sleep continuity disturbances and early awakenings that are commonly observed in melancholic depression. Additionally, HR mice also showed neuroendocrine abnormalities similar to symptoms of MD patients such as reduced amplitude of the circadian glucocorticoid rhythm and elevated trough levels. The sleep-EEG analyses, furthermore, revealed changes in rapid eye movement (REM) and non-REM sleep as well as slow wave activity, indicative of reduced sleep efficacy and REM sleep disinhibition in HR mice.

CONCLUSION/SIGNIFICANCE

Thus, we could show that by selectively breeding mice for extremes in stress reactivity, clinically relevant endophenotypes of MD can be modelled. Given the importance of rhythmicity and sleep disturbances as biomarkers of MD, both animal and clinical studies on the interaction of behavioural, neuroendocrine and sleep parameters may reveal molecular pathways that ultimately lead to the discovery of new targets for antidepressant drugs tailored to match specific pathologies within MD.

Evaluation of the repeated open-space swim model of depression in the mouse

The present study evaluated the extension of a new rat model of depression, repeated open-space swimming, which overcomes drawbacks of existing models, to mice. Mice were swum for 15 min daily in a large tank of tepid water for 4 days and thereafter at 4 day intervals for a period of 3 weeks. Some of the animals were provided with an active coping (escape) response. Variables measured included time floating, distance swum, immobility on a subsequent tail-suspension test, sucrose preference and brain cell proliferation (Ki67 immunohistochemistry) as well as responses to 2 antidepressant drugs, desmethylimipramine and fluoxetine, and 2 non-antidepressant drugs, haloperidol and diazepam. The repeated swims were found to increase time floating and tail-suspension immobility and to decrease distance swum, sucrose preference and brain cell proliferation. Both chronic antidepressant drugs as well as the active coping response attenuated the increased time floating while neither of the non-antidepressant drugs had this effect. The distance swum measure was found to be more variable. Chronic fluoxetine also reversed the increased tail-suspension immobility, reduced sucrose preference and reduced brain cell proliferation caused by the model. It is concluded that repeated open-space swim represents a useful new model of depression in the mouse.