Short- and long-term effects of interleukin-2 on weight, food intake, and hedonic mechanisms in the rat

Depression: a repair response to stress-induced neuronal microdamage that can grade into a chronic neuroinflammatory condition?

Depression is a major contributor to the global burden of disease and disability, yet it is poorly understood. Here we review data supporting a novel theoretical model for the biology of depression. In this model, a stressful life event leads to microdamage in the brain. This damage triggers an injury repair response consisting of a neuroinflammatory phase to clear cellular debris and a spontaneous tissue regeneration phase involving neurotrophins and neurogenesis. During healing, released inflammatory mediators trigger sickness behavior and psychological pain via mechanisms similar to those that produce physical pain during wound healing. The depression remits if the neuronal injury repair process resolves successfully. Importantly, however, the acute psychological pain and neuroinflammation often transition to chronicity and develop into pathological depressive states. This hypothesis for depression explains substantially more data than alternative models, including why emerging data show that analgesic, anti-inflammatory, pro-neurogenic and pro-neurotrophic treatments have antidepressant effects. Thus, an acute depressive episode can be conceptualized as a normally self-limiting but highly error-prone process of recuperation from stress-triggered neuronal microdamage.

Investigating the hedonic effects of interferon-alpha on female rats using brain-stimulation reward

Interferon-alpha (IFN-alpha) is used as a front-line treatment for cancer and other diseases. Reports of depression as a consequence of IFN-alpha therapy scatter the literature, generating interest in the CNS disruptions elicited by this cytokine. In the present work, we investigated the short- and long-term effects of a single systemic injection of vehicle, 10, or 1000 units of IFN-alpha on temperature, body weight, food intake, sickness behaviours, locomotor activity, and brain stimulation reward (BSR) thresholds elicited from the ventral tegmental area in female Long-Evans rats. Pioneered for studying motivational processes, BSR has been exploited as a tool for tracking hedonic status in animal models of depression. In this study, the main findings were that IFN-alpha did not induce anhedonia as defined by no increase in frequency thresholds. However, the analyses of sickness behaviours unveiled a significant increase in piloerection in all sham control animals that received an IFN-alpha injection while the BSR animal scores remained relatively unchanged between pre- and post-injection days. This pattern was also evident in the overall total sickness behaviour scores. Our data suggest that a single exposure to IFN-alpha treatment in female rats elicits long-term somatic effects, without altering hedonic status.

Behavioral and physiological effects of a single injection of rat interferon-alpha on male Sprague-Dawley rats: a long-term evaluation

Interferon-alpha (IFN-alpha) is a cytokine used as a first line of defense against diseases such as cancer and hepatitis C. However, reports indicate that its effectiveness as a treatment is countered by central nervous system (CNS) disruptions in patients. Our work explored the possibility that it may also cause long-term behavioral disruptions by chronicling the behavioral and physiological disturbances associated with a single injection of vehicle, 10, 100, or 1,000 units of IFN-alpha in male Sprague-Dawley rats (n = 5/dose). Following 1 day of locomotor baseline collection, we monitored sickness behaviors (ptosis, piloerection, lethargy, and sleep), food and water intake, body weight, temperature, and motor activity. Observations were recorded 4 days prior to and 4 days following the IFN-alpha injection. Temperature and sickness behaviors were recorded three times daily at 9:00, 15:00, and 21:00 h, and all other indices, once daily. On the injection day, temperature values were highest in the animals receiving the 10-unit IFN-alpha dose 15 min and 13 h post-injection. In the case of sickness behaviors, a significant increase was observed in piloerection in all IFN-alpha groups at each time point measured, while the scores of the rats in the vehicle condition remained unchanged between pre- and post-injection days. Analyses of overall sickness behaviors during morning and night observation periods indicated increased scores in all IFN-alpha groups following injection. Cumulatively, these data suggest that a single IFN-alpha exposure may elicit long-term behavioral disruptions and that its consequences should be thoroughly investigated for its use in clinical populations.

Cytokine mRNA levels in brain and peripheral tissues of the rat: relationships with plus-maze behavior

There is evidence that interleukin (IL)-2 may be related to anxiety as measured in the elevated plus-maze. Recently, we showed that normal adult male Wistar rats can differ systematically in this test of avoidance behavior, that is, time spent on the open arms of the elevated plus-maze. Rats with low open arm time had higher striatal levels of IL-2 mRNA than those with high open arm time, but did not differ significantly in expression of other striatal cytokine mRNA. Here, we investigated whether these expression effects are anatomically specific to the striatum. Therefore, we asked in this double-blind study whether elevated plus-maze behavior may also be related to endogenous levels of cytokine mRNA in other brain regions, which play a role for anxiety, namely the amygdala, hippocampus, and the prefrontal cortex. Additionally, and as peripheral controls, immuno-neuro-endocrine relevant tissues (adrenal glands, spleen) were analyzed. Based on open arm time in the elevated plus-maze, male Wistar rats were divided into sub-groups with either low or high open arm time behavior. Then, IL-1beta, IL-2, IL-6, and tumor necrosis factor (TNF)-alpha cDNA levels were measured post-mortem using semi-quantitative, competitive, reverse transcription polymerase chain reaction. First, we found that cytokine expressions differed considerably between and within these central and peripheral tissues. Secondly, rats with high compared to low open arm time behavior showed higher IL-2 mRNA levels in the prefrontal cortex, which is an inverse pattern to what we recently found in the striatum. These results provide new evidence indicating that cytokine mRNA in the brain can be related to elevated plus-maze behavior and that this relationship is site (prefrontal cortex, striatum)- and cytokine mRNA-specific (IL-2).