DISTRIBUTION OF GABA-IMMUNOREACTIVE NERVE-FIBERS AND CELLS IN THE CERVICAL AND THORACIC PARAVERTEBRAL SYMPATHETIC TRUNK OF ADULT-RAT - EVIDENCE FOR AN ASCENDING FEEDFORWARD INHIBITION SYSTEM

Neurochemical and immunohistochemical evidence suggests that the super ior cervical ganglion (SCG) contains all components of a gamma-aminobu tyric acid (GABA)ergic transmission system, which includes GABAergic a xons of unknown origin. The number of nerve fibers with and without GA BA-like immunoreactivity was determined in interganglionic connectives at all cervical and thoracic levels of the paravertebral sympathetic trunk. In addition, the distribution of GABA-immunoreactive (IR) neuro ns was established within the ganglion chain and compared with the rel ative frequency of principal neurons richly innervated by GABA-IR axon terminals. The following results were obtained: 1) the total number o f nerve fibers in cross sections did not significantly vary between th e cervical levels, but it increased steadily from upper to lower thora cic segments; 2) in contrast, the number of GABA-IR fibers decreased f rom the cervical sympathetic trunk below the SCG (approximately 300 fi bers) down to the seventh to tenth thoracic ganglion, below which no s uch fiber was seen; 3) GABA-IR nerve fibers originate from a subclass of GABA-IR cells; these are small, bipolar neurons with predominantly ascending, unmyelinated axon-like processes; 4) the number of principa l neurons richly innervated by GABA-IR nerve fibers decreased from the SCG to the upper thoracic ganglia, and was very small below; and 5) a part from basket-like innervation, GABA-IR axons also formed diffuse n etworks around GABA-negative principal neurons predominantly in cervic al and upper thoracic ganglia. These data suggest that the GABAergic i nnervation of paravertebral sympathetic ganglia is more complex than p reviously suspected. What appears as preganglionic afferents from seve ral spinal segments (C8-Th7) innervate GABAergic neurons in the sympat hetic trunk which have ascending axons and focus their inhibitory effe cts on the cervical sympathetic ganglia, predominantly the SCG. These data suggest that GABAergic small interganglionic neurons form a feed- forward inhibition system, which may be driven by multisegmental spina l input in the paravertebral sympathetic ganglion chain. (C) 1993 WILE Y-LISS, INC.