Anhedonic and anxiogenic effects of cytokine exposure

Cytokine-induced sickness behavior: where do we stand?

Sickness behavior refers to the coordinated set of behavioral changes that develop in sick individuals during the course of an infection. At the molecular level, these changes are due to the effects of proinflammatory cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNFalpha), in the brain. Peripherally released cytokines act on the brain via a fast transmission pathway involving primary afferent nerves innervating the body site of inflammation and a slow transmission pathway involving cytokines originating from the choroid plexus and circumventricular organs and diffusing into the brain parenchyma by volume transmission. At the behavioral level, sickness behavior appears to be the expression of a central motivational state that reorganizes the organism's priorities to cope with infectious pathogens. There is clinical and experimental evidence that activation of the brain cytokine system is associated with depression, although the exact relationship between sickness behavior and depression is still elusive.

Understanding cytokines. Part II: Implications for nursing research and practice

Cytokines are small signaling proteins released from a variety of cells that influence virtually every aspect of growth and development and every host response to infection, injury, and inflammation. Because of their widespread and potent effects across the life span, cytokines without a doubt influence nursing research and practice. From physiological and adaptive effects of cytokines to cytokine-induced diseases, nurses and nursing care are involved. Part II of this review highlights a few of the many examples of cytokines functioning in response to infection and inflammation, during the processes of reproduction, and in a variety of pathophysiological states. Implications for nursing research and practice are emphasized.

Dysthymia: a review of pharmacological and behavioral factors

Although dysthymia, a chronic, low-grade form of depression, has a morbidity rate as high as that of major depression, and increases the risk for major depressive disorder, limited information is available concerning the etiology of this illness. In the present report we review literature concerning the biological and characterological features of dysthymia, the effectiveness of antidepressant treatments, the influence of stressors in the precipitation and maintenance of the disorder, and both quality of life and psychosocial correlates of the illness. We also provisionally suggest that dysthymia may stem from disturbances of neuroendocrine and neurotransmitter functioning (eg, corticotropin releasing hormone and arginine vasopressin within the hypothalamus, or alternatively monoamine variations within several extrahypothalamic sites), and may also involve cytokine activation. The central disturbances may reflect phenotypic variations of neuroendocrine processes or sensitization of such mechanisms. It is suggested that chronic stressor experiences or stressors encountered early in life lead to the phenotypic neurochemical alterations, which then favor the development of the dysthymic state. Owing to the persistence of the neurochemical disturbances, vulnerability to double depression is increased, and in this instance treatment with antidepressants may attenuate the symptoms of major depression but not those of the basal dysthymic state. Moreover, the residual features of depression following treatment may be indicative of underlying neurochemical disturbances, and may also serve to increase the probability of illness recurrence or relapse.

Behavioral aspects of experimental autoimmune encephalomyelitis

Acute inflammation is known to induce a depressive-like sickness behavior syndrome in humans and in experimental animals. In the present study, we sought to determine whether a chronic neuroautoimmune inflammation is also associated with a similar behavioral syndrome. Experimental autoimmune encephalomyelitis (EAE) was induced in SJL/J female mice by adoptive transfer of lymph node cells, and sickness behavior symptoms, including anorexia, loss of body weight, reduced social exploration, and decreased preference for sucrose solution were measured. We report that these components of sickness behavior were induced during the acute phase of the disease, and recovered in later phases. Moreover, the onset and recovery of the behavioral symptoms preceded the onset and recovery of the neurological signs, respectively. Since EAE is considered a model for multiple sclerosis (MS), it is suggested that EAF-induced behavioral changes may serve as a model for the depressive symptomatology that characterizes most MS patients.

Delayed effects of stress and immune activation

Stress responses play a crucial adaptive role but impose potentially subversive demands on the organism. The same holds for the symptoms of illness as seen after immune activation by pathogens or tissue damage. The responses to immune stimuli and stressors show remarkable similarities and rely on similar control mechanisms in the brain: i.e. they involve neuropeptides of the corticotropin releasing factor (CRF) family. Immune and non-immune challenges lead to responses that normally show a temporal relationship with the duration and intensity of the stimulus and the (re)activity of the stress-responsive systems return to their pre-challenged state within hours or days. However, exposure of animals or man to specific stimuli can induce delayed and long-lasting (weeks, months) alternation in stress responsive systems, resulting in a prolonged period of increased stress vulnerability. Immune stimuli are particularly powerful in eliciting such a stress vulnerable state. Various adaptive changes in the (neuro)biological substrate as seen during this stress vulnerable state also occur in depression, and may be causally related to the depressive symptoms that are often associated with infectious and inflammatory diseases.

Behavioral and neurochemical consequences of lipopolysaccharide in mice: anxiogenic-like effects

Systemic administration of lipopolysaccharide (LPS) induces sickness behaviors, as well as alterations of hypothalamic-pituitary-adrenal functioning commonly associated with stressors. In the present investigation, it was demonstrated that systemic LPS treatment induced a sickness-like behavioral profile (reduced active behaviors, soporific effects, piloerection, ptosis), which appeared to be dependent upon the novelty of the environmental context in which animals were tested. As well, LPS induced anxiogenic-like responses, including decreased time spent in the illuminated portion of a light-dark box, reduced open-arm entries in a plus-maze test, and decreased contact with a novel stimulus object in an open-field situation. The behavioral changes were accompanied by increased plasma ACTH and corticosterone levels. As well, LPS induced increased turnover of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in the paraventricular nucleus (PVN), median eminence plus arcuate nucleus, hippocampus, as well as NE turnover within the locus coeruleus and DA turnover within the nucleus accumbens. Although these neurochemical variations were reminiscent of those elicited by stressors, LPS was not particularly effective in modifying DA activity within the prefrontal cortex or NE within the amygdala, variations readily induced by stressors. Whether the LPS-induced anxiogenic-like responses were secondary to the illness engendered by the endotoxin remains to be determined. Nevertheless, it ought to be considered that bacterial endotoxin challenge, and the ensuing cytokine changes, may contribute to emotionality and perhaps even anxiety-related behavioral disturbances.