Anatomical and functional study of interleukin-2 in the brain : possible neuromodulatory significance

Interleukin-2 (IL-2), an immunomodulatory cytokine first isolated from the immune system, plays an essential role in the maturation of T lymphocytes. This thesis focusses on the neuroanatomical features of IL-2 and IL-2 receptors in the central nervous system, the neuromodulatory role of IL-2 on hippocampal acetylcholine release, and possible intracellular signalling mechanisms following IL-2 receptor activation in the brain.Using immunoautoradiography, IL-2-like immunoreactivities were observed in a selected pattern in the central nervous system, with particularly high densities seen in the hippocampal formation and the hypothalamus of the rodent brain. The cellular localisation of the immunostaining using immunohistochemical approaches reveals that this staining was seen most evidently on cell perikarya especially in areas of high labelling density. The distribution of IL-2 receptor binding sites using both in vitro receptor autoradiography and immunoautoradiography (anti-TAC antibody against the IL-2 receptor a chain) shows that IL-2 receptors are selectively distributed in the rodent brain, with the highest densities observed in the hippocampal formation and the hypothalamus, in accordance with the localisation of IL-2 peptide immunostaining. The postulated neuromodulatory role of IL-2 and IL-2 receptors in the hippocampus was investigated next focusing on cholinergic parameters (acetylcholine release) on the basis of previous results from our laboratory (Araujo et al., 1989).The neuromodulatory effects of IL-2 on acetylcholine (ACh) release was investigated using in vitro rat brain slices superfusion. IL-2 exerted potent effects on hippocampal ACh release, acting as a potentiating agent at low (pM) concentrations, while inhibiting release at higher (low nM) concentrations. An inhibitory effect (10 nM IL-2) on ACh release was also observed in the frontal cortex, but not in the parietal cortex or the striatum. This action was not shared by other interleukins such as IL-6. Both the stimulatory and inhibitory effects of IL-2 in hippocampal ACh release were blocked by an anti-IL-2 receptor antibody (TAC), suggesting the requirement of a genuine IL-2 receptor for both effects. The potentiating, in contrast to the inhibitory effect, was insensitive to tetrodotoxin, suggesting a direct action (or in close proximity) of IL-2 on cholinergic terminals to stimulate hippocampal ACh release. The inhibitory effect of IL-2 on ACh release was abolished by both bicuculline and phaclofen, suggesting the involvement of GABA acting on both GABA$ sb{ rm A}$ and GABA$ sb{ rm B}$ receptors present in the rat hippocampal formation.The signalling mechanisms of the IL-2 receptor in the rat brain was studied next in vitro by measuring the effects of IL-2 on cytidine-diphosphate diacylglycerol (CDP-DAG) turnover in rat brain slices. IL-2 potently inhibited basal CDP-DAG turnover in the frontal cortex and hypothalamus, but not in the hippocampus or striatum. However, IL-2 inhibited carbachol-stimulated CDP-DAG turnover in the hippocampus. This decrease was in parallel to an increase in choline production, suggesting a role for phospholipase D in brain IL-2 receptor signalling. (Abstract shortened by UMI.