Neither acute nor chronic exposure to a naturalistic (predator) stressor influences the interleukin-1β system, tumor necrosis factor-α, transforming growth factor-β1, and neuropeptide mRNAs in specific brain regions

Abstract

Physical (neurogenic) stressors may influence immune functioning and interleukin-1β (IL-1β) mRNA levels within several brain regions. The present study assessed the effects of an acute or repeated naturalistic, psychogenic stressor (predator exposure) on brain cytokine and neuropeptide mRNAs. Acute predator (ferret) exposure induced stress-like behavioral effects, including elicitation of a startle response and reduced exploratory behaviors; these responses diminished after 30 sessions. Moreover, acute and repeated predator exposure, like acute restraint stress, increased plasma corticosterone levels measured 5 min later, but not 2 h after stressor exposure. In contrast, none of the stressors used influenced IL-1β, IL-1 receptor antagonist, IL-1 receptor type I, IL-1 receptor accessory proteins I and II, or tumor necrosis factor-α mRNA levels in the prefrontal cortex, amygdala, hippocampus, or hypothalamus. Likewise, there were no stressor effects on transforming growth factor-β1, neuropeptide Y, glycoprotein 130, or leptin receptor mRNAs in brain regions. Thus, the naturalistic/psychogenic stressor used does not affect any of the brain cytokine component mRNAs studied. It is suggested that this type of stressor activates homeostatic mechanisms (e.g., glucocorticoid release), which act to preclude brain cytokine alterations that would otherwise favor neuroinflammatory/neuroimmunological responses and the consequent increase of brain sensitivity to neurotoxic and neurodegenerative processes.

Introduction

Bioactivity and mRNA expression for various cytokines and their receptors have been demonstrated in brain 7, 8, 9, 11, 12, 13, 14, 27, 33. Challenges, including peripheral or brain (central) bacterial endotoxin administration, cerebral ischemia, seizures, and tumors all increase brain cytokine expression 10, 11, 14, 19, 24, 27, 30, 37, 38, 40. Thus, it has been proposed that cytokines play a role in the development and/or progression of pathological states, or alternatively that the increase of cytokines in response to disease processes or challenges may reflect their potential role in reparatory processes depending on the condition 31, 32.

Stressful events may also influence brain cytokine expression. For instance, it was reported that immobilization (physical restraint) increased interleukin-1β (IL-1β) mRNA expression in the hypothalamus, although no changes occurred in other brain regions [22]. Inescapable (tail) shock, on the other hand, increased IL-1β protein in various brain regions, including the hypothalamus and hippocampus in adrenalectomized rats [23]. Commensurate with the notion that cytokine changes contribute to other stressor-provoked neurochemical alterations, the central administration of IL-1 receptor antagonist (IL-1Ra) prevented the stressor-elicited in vivo release of hypothalamic norepinephrine (NE), dopamine (DA), and serotonin (5-HT), as well as the elevated plasma adrenocorticotrophic hormone (ACTH) 34, 35. Likewise, IL-1Ra was reported to prevent the development of behavioral disturbances ordinarily provoked by an uncontrollable stressor [20]. These data provisionally suggest that IL-1β may play an intermediate role in promoting behavioral and several neuroendocrine changes associated with stressors, particularly with respect to hypothalamic-pituitary-adrenal functioning.

Most commonly, the stressors used to examine effects on central cytokine activity have been neurogenic in nature (i.e., of physical origin), including immobilization stress, inescapable (tail shock) stress, or formalin injection 34, 35, 39. Because of the intensity of these stressors as well as the inconsistent findings reported, it is of interest to establish whether brain IL-1β variations would likewise be observed following a naturalistic, psychogenic stressor (i.e., of purely psychological origin), namely that of exposure to a predator. In addition, it is important to determine whether induction of brain cytokines in response to stressors is dependent on the local production of cytokines by brain cells. This can be assessed by determination of mRNA as indicator of local synthesis and modulation of transcriptional mechanisms in the brain. Since various neurochemical sequelae of an acute challenge diminish with repeated exposure to stressors [4], we also assessed the effects of repeated exposure to a naturalistic stressor.

Considering that the effects of stressors need not to be restricted to variations of IL-1β mRNA, we also determined changes of mRNA expression for IL-1Ra, IL-1 receptor type I (IL-1RI), IL-1 receptor accessory proteins I and II, tumor necrosis factor-α (TNF-α), transforming growth factor (TGF-β1), glycoprotein 130 (gp 130), leptin receptor, pro-opiomelanocortin (POMC), and neuropeptide Y (NPY). The simultaneous investigation of various components of the IL-1β system (i.e., the ligand, signaling receptor, receptor accessory proteins, and endogenous inhibitor) can provide information on the feedback regulation and contribution of each cytokine system component. Analysis of TNF-α (a pro-inflammatory cytokine) and TGF-β1 (an anti-inflammatory or inhibitory cytokine) also provides insights into cytokine–cytokine interactions with positive feedback (IL-1β and TNF-α interactions which can be synergistic) and negative modulation (IL-1Ra and TGF-β1 inhibit pro-inflammatory cytokine production and action). Gp 130 is a common signal transducer among receptors for members of the IL-6 subfamily that is homologous to the leptin receptor (OB receptor). Moreover, since cytokine-endogenous opioids [18] and cytokine–NPY interactions [25] occur in various in vivo models, we also examined POMC and NPY mRNAs. Cytokine- and neuropeptide-associated mRNAs were examined in the same brain region samples used to assess potential cytokine–cytokine and cytokine–neuropeptide interactions. Plasma corticosterone levels were also determined, because this hormone tends to be particularly responsive to stressors [3].