Neurobehavioural activation during peripheral immunosuppression.

Like other physiological responses, immune functions are the subject of behavioural conditioning. Conditioned immunosuppression can be induced by contingently pairing a novel taste with an injection of the immunosuppressant cyclosporine A (CsA) in an associative learning paradigm. This learned immunosuppression is centrally mediated by the insular cortex and the amygdala. However, the afferent mechanisms by which the brain detects CsA are not understood. In this study we analysed whether CsA is sensed via the chemosensitive vagus nerve or whether CsA directly acts on the brain. Our experiments revealed that a single peripheral administration of CsA increases neuronal activity in the insular cortex and the amygdala as evident from increased electric activity, c-Fos expression and amygdaloid noradrenaline release. However, this increased neuronal activity was not affected by prior vagal deafferentation but rather seems to partially be induced by direct action of CsA on cortico-amygdaloid structures and the chemosensitive brainstem regions area postrema and nucleus of the solitary tract. Together, these data indicate that CsA as an unconditioned stimulus may directly act on the brain by a still unknown transduction mechanism.