Atrial natriuretic factor-like immunoreactivity in the central nervous system of the frog

The distribution of atrial natriuretic factor-like immunoreactivity in the central nervous system of the frog Rana ridibunda was investigated by indirect immunofluorescence and the immunogold technique, using an antiserum generated in rabbits against synthetic atrial natriuretic factor (Arg 101-Tyr 126). A stereotaxic atlas of neurons containing atrial natriuretic factor-like material was prepared to show the widespread distribution of atrial natriuretic factor-positive cell bodies and fibres in the brain. Appreciable numbers of immunoreactive perikarya were observed in the dorsal and medial pallium, the medial septal nucleus, the anteroventral and ventrolateral areas of the thalamus, the lateral forebrain bundle, the posterocentral and posterolateral thalamic nuclei, the preoptic nucleus, the dorsal infundibular nucleus and the anteroventral tegmental nucleus of the mesencephalon. A heavy accumulation of atrial natriuretic factor-like immunoreactive cell bodies and very dense fibre bundles were noted in the interpeduncular nucleus of the mesencephalon. Fibres were generally seen where cell bodies were observed, particularly in all regions of the pallium and septum nuclei, in the ventral thalamus, the infundibular nucleus and the tegmental area. Moderate numbers of fibres were also noted in several regions where cell bodies were absent, mainly in the amygdala and the infundibular nucleus, the median eminence and most mesencephalic regions. At the electron microscopic level, the immunoreactivity was restricted to dense core vesicles and absent in clear vesicles. These results provide the first evidence for the presence of atrial natriuretic factor in the brain of a non-mammalian chordate. The localization of atrial natriuretic factor-positive material in the frog central nervous system suggests that this peptide may act as a neuromodulator or a neurotransmitter in amphibians.