Catecholamine and serotonin colocalization in projection neurons of the area postrema

A large population of the rat area postrema (AP) neurons which project to the parabrachial area (PBA) are serotonergic. Many AP neurons contain tyrosine hydroxylase (TH) and are presumed to be noradrenergic. However, it was not previously known whether TH-containing AP neurons also have projections to the PBA. TH-containing and serotonin (5-HT)-containing neurons have wide and overlapping distributions within the AP, and it was therefore possible that TH and 5-HT may be contained within the same AP neurons. In the present study immunohistochemical and retrograde axonal transport techniques were combined to determine whether TH-containing AP neurons project to the PBA and whether TH and 5-HT coexist in AP neurons. Adult male rats were given bilateral injections of the retrograde transport tracer, True blue (TB), into the PBA. After a 4-day survival period, vibratome sections of the caudal brainstem were processed for both TH and 5-HT immunohistochemistry. Examination of the sections revealed that over 25% of the 5-HT and over 30% of the TH-containing AP neurons were retrogradely labelled with TB. More surprising were our findings that many AP neurons displayed both TH and 5-HT immunoreactivity and that almost 40% of these double-labelled cells project to the PBA. Our results indicate that serotonin and noradrenaline coexist in a substantial proportion of the neurons of a major ascending viscerosensory pathway from the AP to the PBA.