The connectivity of the area postrema in the ferret

The area postrema (AP) is the chemosensitive trigger zone for the emetic reflex. We have investigated the connectivity of the AP and adjacent solitary complex (SC) to identify possible sites of the motor emetic center. The AP and SC were infused with HRP or WGA-HRP in 30 ferrets that were perfused transcardially after 24-72 h. A block from the pons to upper cervical spinal cord, and one with hypothalamus and basal forebrain, was cut at 50 microns, reacted, and mounted. Data support the conclusion, at variance with those from other preparations, that in ferrets the AP has reciprocal connections only with the SC, which serves as a relay in both ascending and descending pathways between AP and higher levels of the neuraxis. Connectivity of the SC with brain stem and forebrain structures including the rostral ventrolateral medulla, parabrachial nuclei, paraventricular nucleus, and amygdala was demonstrated. At least in ferrets, our results suggest that the motor emetic center must be located within the SC. While this may not apply to all species, it is also possible that some reports of AP projections elsewhere were results of label within the SC. Alternatively, the somewhat different pattern of emesis in the ferret as compared to the dog (greater role for vagal inputs in response to radiation and cytotoxic drugs, lesser role for humoral inputs) may reflect differences in AP connectivity.