Importance of new catecholamine pathways in control of blood pressure

The first comprehensive maps of central catecholamine pathways referred to both noradrenaline and dopamine neurons. They described the catecholamine neurons as having their cell bodies in the brainstem with spinal axons descending mainly from two medullary cell groups (A1 and A2) and with ascending axons arising mainly from more rostral groups (A3-A13). More recent work utilising immunohistochemistry has established the presence of adrenaline neurons in the brain, in two medullary cell groups (C1 and C2). While these were originally thought to lie within the rostral part of the A1 and A2 cell groups, work from this laboratory has now established that the adrenaline neurons are topographically distinct from the A1 and A2 cells, do not fluoresce with standard methods, and are collected into three groups, the third group (C3) lying in the midline of the rostral medulla. Work in this laboratory using a combination of histochemical fluorescence and retrograde transport of horseradish peroxidase has demonstrated that existence of a descending dopaminergic projection from the hypothalamus to the spinal cord, indicating that descending pathways can arise well above the medulla. Recent studies on the A1 neurons have established the presence of projections from the A1 cells to the median eminence of the hypothalamus and towards the nucleus tractus solitarius in the medulla. Other experiments have demonstrated that most of the descending catecholamine axons do not arise from the A1 and A2 cell groups in the caudal medulla, but higher up in the brainstem especially from the A5 and A7 cell groups. These new descriptions of central catecholamine neurons will necessarily modify the interpretation of many experiments on the central regulation of arterial pressure.