The corticotropin-releasing hormone neurons of the hypothalamic paraventricular nucleus are the final common pathway of the neuroendocrine adaptative response to a variety of stressors. To meet varied homeostatic needs, corticotropin-releasing hormone neurons exhibit a marked phenotypical plasticity, enabling them to rapidly modify their neuroendocrine output. In particular, they synthesize the neuropeptides vasopressin and neurotensin. Under many experimental circumstances, it is observed that corticotropin-releasing hormone and vasopressin are regulated in parallel, whereas the expression of neurotensin seems dissociated, in these neurons, evoking different transcriptional control over the co-existing neuropeptides depending on the adaptative response required. Using radioactive and dual-label in situ hybridization techniques, we have studied the respective expression of paraventricular corticotropin-releasing hormone, vasopressin and neurotensin messenger RNAs in the context of an immune challenge. A single intraperitoneal injection of the endotoxin lipopolysaccharide was administered to adult male rats that were killed 8 h later. Compared to control animals, lipopolysaccharide-injected rats showed elevated plasma corticosterone (614+/-65 vs 185+/-40 ng/ml in control) and increased expression of paraventricular corticotropin-releasing hormone messenger RNA (+200%); expression of neurotensin messenger RNA was induced in about one-third of corticotropin-releasing hormone neurons, whereas vasopressin messenger RNA expression remained unchanged. Therefore, in this experimental context and at the time-point examined, co-existing corticotropin-releasing hormone and vasopressin appeared differentially expressed, and an additional stimulus (inflammation) is demonstrated to result in neurotensin expression in neuroendocrine corticotropin-releasing hormone neurons. Neurotensin may be released in the pituitary portal blood to trigger pituitary response associated with mobilization of the immune system.