Physiological stimulation enhances HRP marking of salivary neurons in rats

Neurons of the superior salivary nucleus in rats have been labeled by retrograde transport of HRP injected into the lingual nerve. The nucleus is formed by a band of small, multipolar cells diagonally oriented in the lateral reticular formation of the caudal pons, just caudal to the root of the facial nerve. As time for transport of HRP was increased from 14 1 2 to 24 hr, the number of labeled cells increased exponentially along the curve, Y = 0.001 exp(0.51T), r = 0.995, to a maximum of 272 cells. Stimulating neuronal activity immediately after HRP injection, by swabbing the rat's mouth with 1% acetic acid, increased the number of labeled cells at 14 1 2 (p < 0.05), 16, 18 and 20 (p < 0.01) hr transport time. Counts of labeled cells in stimulated animals fell along a cumulative, normal distribution curve (ogive) with a mean at 18.2 hrs, S.D. 1.6 hr. Enhanced labeling of activated cells could be due to more rapid transport rates or to increased amounts of HRP transported.