From hippocampus to dorsal horn: the pervasive impact of IL-1 on learning and memory spans the length of the neuroaxis

Novel aspects of glucocorticoid actions

Normal hypothalamic-pituitary-adrenal (HPA) axis activity leading to the rhythmic and episodic release of adrenal glucocorticoids (GCs) is essential for body homeostasis and survival during stress. Acting through specific intracellular receptors in the brain and periphery, GCs regulate behaviour, as well as metabolic, cardiovascular, immune and neuroendocrine activities. By contrast to chronic elevated levels, circadian and acute stress-induced increases in GCs are necessary for hippocampal neuronal survival and memory acquisition and consolidation, as a result of the inhibition of apoptosis, the facilitation of glutamatergic neurotransmission and the formation of excitatory synapses, and the induction of immediate early genes and dendritic spine formation. In addition to metabolic actions leading to increased energy availability, GCs have profound effects on feeding behaviour, mainly via the modulation of orexigenic and anorixegenic neuropeptides. Evidence is also emerging that, in addition to the recognised immune suppressive actions of GCs by counteracting adrenergic pro-inflammatory actions, circadian elevations have priming effects in the immune system, potentiating acute defensive responses. In addition, negative-feedback by GCs involves multiple mechanisms leading to limited HPA axis activation and prevention of the deleterious effects of excessive GC production. Adequate GC secretion to meet body demands is tightly regulated by a complex neural circuitry controlling hypothalamic corticotrophin-releasing hormone (CRH) and vasopressin secretion, which are the main regulators of pituitary adrenocorticotrophic hormone (ACTH). Rapid feedback mechanisms, likely involving nongenomic actions of GCs, mediate the immediate inhibition of hypothalamic CRH and ACTH secretion, whereas intermediate and delayed mechanisms mediated by genomic actions involve the modulation of limbic circuitry and peripheral metabolic messengers. Consistent with their key adaptive roles, HPA axis components are evolutionarily conserved, being present in the earliest vertebrates. An understanding of these basic mechanisms may lead to novel approaches for the development of diagnostic and therapeutic tools for disorders related to stress and alterations of GC secretion.

Nervous and immune systems signals and connections: cytokines in hippocampus physiology and pathology

Signaling through secretion of small molecules is a hallmark of both nervous and immune systems. The scope and influence of the intense message exchange between these two complex systems are only now becoming objects of scientific inquiry. Both neurotransmitters and cytokines affect their target cells through surface receptors and also by other molecular mechanisms. Cytokine receptors are present in neurons and glial cell populations in discrete brain regions. This review firstly focuses on the role of cytokines in hippocampal physiological processes, such as memory and learning, and secondly on the pathological involvement of cytokines in diseases like depression and epilepsy. Interleukin-1β is necessary for long-term potentiation (LTP) maintenance in the hippocampus. On the other hand, interleukin-6 has a negative regulatory role in long-term memory acquisition. Astrocyte-secreted tumor necrosis factor plays a role in synaptic strength by increasing surface translocation of glutamate AMPA receptors, and the chemokine CXCL12 can silence the tonic activity of Cajal-Retzius neurons in the hippocampus. Manifold increased concentrations of interleukin-10, interferon-γ, ICAM1, CCL2, and CCL4 are observed in the hippocampi of patients with temporal lobe epilepsy. A contemporary view of the role of cytokines as neuromodulators is emerging from studies in humans and manipulations of experimental animals. Despite the accumulating evidence of the role of cytokines on nervous system physiology and pathology, it is important not to exaggerate its relevance.

Central infusion of interleukin-1 receptor antagonist blocks the reduction in social behavior produced by prior stressor exposure

Pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) in the brain modulate sickness behavior in rodents, in which animals show complex changes in behavior such as the reduction of general activity, reduced social motivation, and fever response. The present studies examined the impact of lipopolysacharide (LPS) and stressor (footshock) exposure on the later expression of social behavior in Sprague-Dawley rats using two separate behavioral paradigms. In Experiment 1, a traditional test for social interaction in which animals were allowed to investigate a juvenile rat in their home cages was conducted at 4 different time points following LPS or footshock treatment. In Experiment 2, social investigation task which allowed the animals to sniff the hole connected to the other chamber where a stimulus animal was placed, but prevented physical contact, was used to measure social investigation at several time points following LPS or footshock treatment. Both systemic infusion of LPS (100 microg/kg) and 2 h footshock exposure (80 shocks, 1 mA, 5 s duration) elicited a time-dependent reduction of social interaction (Experiment 1) and investigation (Experiment 2); LPS-treated rats displayed a more profound reduction of social investigation from 2 h to 6 h after treatment, while rats exposed to footshock showed a reduction 6 h after the footshock exposure. In Experiment 3, the footshock-induced reduction of social investigation was blocked by pretreatment with IL-1 receptor antagonist (IL-1Ra; 100 microg icv) infusion. Together, these findings support a growing body of literature showing that stress-dependent changes in brain cytokines play a key role in mediating behavioral consequences of stressor exposure.