Noradrenergic synaptic transmission in the superior cervical ganglion.

Morphology of dendrites of principal neurons in the rat superior cervical ganglion (SCG) was re-examined by an intracellular staining method followed by electron microscopic analysis. They exhibit a varying complexity in their morphology and arborization. Some dendrites show specializations such as a glomerular plexus, where extensively branched dendritic collaterals form synaptic connections comprising not only axodendritic synapses between preganglionic axons and principal cell dendrites, but also dendrodendritic synapses between principal cell dendrites. Most presynaptic elements of adrenergic synapses observed by conventional methods appear to represent these specialized dendritic collaterals of principal neurons. In denervated SCG, focal stimulation revoked an inhibitory postsynaptic potential in some principal neurons. The response was completely blocked by yohimbine, an alpha 2-adrenoceptor antagonist. The inhibitory synaptic connection between principal neurons via dendrodendritic synapses may be an important addition to the conventional scheme of intraganglionic synaptic transmission. Sympathetic ganglia may thus function as more than a simple relay station, with specialized neuronal circuitry that may be involved in the modulation of cholinergic transmission.