Adrenergic neurons in the rostral ventrolateral medulla: ultrastructure and synaptic relations with other...

The first part of this chapter demonstrates that the C1 adrenergic neurons have high mitochondrial content and a close proximity to capillaries and glia suggestive of a high metabolic activity and a possible chemosensory function. Adrenergic terminals arising primarily from these neurons (1) can influence sympathetic nerve discharge through direct contacts on sympathetic preganglionic neurons in the IML of the spinal cord; and (2) are one of the more prevalent synaptic inputs to the principally noradrenergic neurons in the locus coeruleus. In both the IML and locus coeruleus, adrenergic terminals may be either excitatory (asymmetric synapses) or inhibitory (symmetric synapses) depending on their distribution on the post-synaptic target. The second part of this chapter shows that C1 adrenergic neurons in the RVL are modulated by synaptic associations with a variety of transmitter systems (see schematic Fig. 8). Specifically, C1 adrenergic neurons receive (1) major inhibitory input (symmetric synapses) from GABA-ergic and opioid terminals as well as from unidentified (unlabelled) transmitter-containing terminals; (2) major excitatory input (asymmetric synapses) from terminals containing substance P as well as other unidentified terminals and (3) minor inputs from cholinergic, adrenergic and noradrenergic pathways. Moreover, cholinergic terminals in the RVL form symmetric synapses mainly on unidentified transmitter-containing neurons rather than the C1 neurons suggesting that the reported cardiovascular effects of cholinergic agents in the RVL are most likely mediated via inhibitory interneurons. Within the RVL, adrenergic and noradrenergic terminals innervate cholinergic and opioid neurons. Thus, these results not only provide direct evidence that a number of transmitters modulate the activity of C1 adrenergic neurons, but also suggest new directions for studies of functional interactions involving catecholaminergic regulation of other transmitter-containing neurons within the RVL.