Labelling of neurons in the rat superior cervical ganglion after injection of wheat-germ agglutinin-horseradish peroxidase into the contralateral ganglion: evidence of transneuronal labelling.

Recent studies have shown that injection of the tracer wheat-germ agglutinin-horseradish peroxidase (WGA-HRP) into the superior cervical ganglion (SCG) of one side results in labelling of neurons in the contralateral SCG and the stellate ganglion. This study was designed to verify whether or not bilateral projections from the superior cervical ganglion to the midline structures, particularly to the pineal gland, play a role in the transport of WGA-HRP to the contralateral SCG. One group of rats received WGA-HRP injection into the right SCG (group I). Four groups of rats underwent the following operations prior to the injection of WGA-HRP into the right superior cervical ganglion: transection of the external carotid nerve (group II), transection of the internal carotid nerve (group III), transection of the external carotid nerve combined with pinealectomy (group IV), transection of both the internal and the external carotid nerves (group V). The mean number of labelled neurons in the left SCG of each group were found as follows: group I, 1516 +/- 221 (mean +/- S.D.); group II, 861 +/- 122; group III, 543 +/- 99; group IV, 562 +/- 144; group V, 220 +/- 52. The results of this study suggest that the contralateral labelling depends on the transneuronal transport of WGA-HRP through the terminal fields of innervation of the midline structures that receive bilateral projections from both SCGs.