Phenylethanolamine N-methyltransferase-containing terminals synapse directly on sympathetic preganglionic neurons in the rat

The ultrastructural morphology as well as neuronal and glial associations of phenylethanolamine N-methyltransferase (PNMT)-containing terminals in the intermediolateral cell column (IML) of the thoracic spinal cord were examined in the rat utilizing the peroxidase-antiperoxidase (PAP) method. The PNMT-immunoreactive terminals were 0.5-1.4 μm in diameter and contained a few mitochondria, a large population of small clear vesicles and from 1 to 6 large dense-core vesicles. The terminals formed synapses primarily with dendrites. The type of axodendritic association (i.e. symmetric or asymmetric) varied with the size of the dendrite, such that the majority of synapses on large dendrites were symmetric and those on smaller dendrites and dendritic spines were asymmetric. Moreover, most of the synaptic associations of PNMT-containing terminals were with the smaller dendritic processes. Many of the PNMT-labeled terminals, as well as their postsynaptic targets, were closely invested with, or apposed to fibrous astrocytic processes. In a subsequent set of experiments, we combined immunoautoradiographic labeling for PNMT with horseradish peroxidase (HRP) retrograde identification of sympathetic preganglionic neurons (SPNs) in the IML to determine whether or not SPNs receive direct synaptic input from the adrenergic terminals. In these sections, PNMT-containing terminals directly synapsed on the HRP-containing (i.e. retrogradely labeled SPNs) perikarya and dendrites. The axosomatic synapses observed between PNMT-labeled terminals and SPN perikarya were exclusively symmetric: whereas the type of axodendritic association varied depending upon the size of the dendrite such that the majority were asymmetric. The findings provide ultrastructurral evidence that in the rat IML, adrenergic (i.e. PNMT-containing) terminals (1) may be either excitatory (asymmetric) or inhibitory (symmetric) depending on their site of termination and (2) can influence sympathetic nerve discharge through a direct effect on the SPN cell membrane.