Urocortin 2 increases c-Fos expression in serotonergic neurons projecting to the ventricular/periventricular system

Serotonin plays an important role in the regulation of anxiety states and physiological responses to aversive stimuli. Intracerebroventricular (i.c.v.) injection of the stress- and anxiety-related neuropeptide urocortin 2 (Ucn 2) increases c-Fos expression in serotonergic neurons in the dorsal (DRD) and caudal (DRC) parts of the dorsal raphe nucleus. These regions contain a subset of serotonergic neurons that projects via the dorsal raphe periventricular tract to periventricular structures, including the subfornical organ and ependymal layer, and to the ventricular system. To determine if Ucn 2 activates ventricle/periventricular-projecting serotonergic neurons in the midbrain raphe complex, we made i.c.v. injections of the retrograde tracer Fluoro-Gold into the lateral ventricle, followed 7 days later by i.c.v. injection of Ucn 2. The DRD at -8.18 mm and the DRC at -8.54 mm and -9.16 mm bregma were analyzed using a combined bright field and immunofluorescence technique. Approximately 40% of the ventricle/periventricular-projecting neurons in the subdivisions sampled were serotonergic. Urocortin 2 increased c-Fos expression in ventricle/periventricular-projecting serotonergic neurons in the DRC and in non-ventricle/periventricular-projecting serotonergic neurons in the DRD and DRC. Of the total population of ventricle/periventricular-projecting serotonergic neurons in the DRC at -8.54 and -9.16 mm bregma, 35% expressed c-Fos following Ucn 2 injections. These data are consistent with previous studies showing that i.c.v. injection of Ucn 2 activates subpopulations of serotonergic neurons restricted to the mid-rostrocaudal DRD and DRC and further demonstrate that these include both subsets of serotonergic neurons that do and do not project to the ventricle/periventricular system.