Aldosterone-sensitive neurons of the nucleus of the solitary tract: multisynaptic pathway to the nucleus accumbens.

The nucleus accumbens (NAc) is part of a forebrain system implicated in reward, motivation, and learning. NAc neurons become activated during various ingestive activities, including salt intake. A subset of neurons within the nucleus tractus solitarius (NTS) shows c-Fos activation during prolonged sodium deprivation in rats. These neurons express mineralocorticoid receptors and the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), which makes them selectively sensitive to aldosterone-an adrenal hormone that modulates sodium appetite. Here we tested whether these neurons project multisynaptically to the core or shell subregions of the NAc. Pseudorabies virus (PRV)-a retrograde transneuronal tracer-was injected into the NAc in rats and after 3-4 days PRV-infected HSD2 neurons were identified. PRV injections into the NAc core yielded greater numbers of PRV-labeled HSD2 neurons than did comparable injections into the NAc shell. Transneuronal labeling was also found in brainstem sites that receive direct projections from HSD2 neurons, namely, lateral parabrachial and prelocus coeruleus nuclei. In other experiments a retrograde neural tracer (cholera toxin beta-subunit) was injected into the NAc. Extensive retrograde labeling was found in the midline thalamus and frontal cortical regions, but no cells were labeled in the NTS or parabrachial region. These findings indicate that the HSD2 neurons project via a multisynaptic pathway to the NAc, which may be relayed sequentially through two sites: the dorsolateral pons and the paraventricular thalamic nucleus. HSD2 neurons may be part of an ascending pathway involved in the salt-seeking behavior of sodium-depleted rats.