An uncrossed tectopontine pathway mediates ipsiversive circling.

Stimulation of the superior colliculus (SC) of rodents, following knife cuts to the predorsal bundle decussation, evokes ipsiversive circling along with "cringing" or avoidance responses. A major uncut SC output is the uncrossed tectopontine pathway that projects heavily to the ventrolateral pons (VLP). Stimulation of this pathway in the VLP also evokes ipsiversive circling, but the circling is smoother, lacks the avoidance components, and begins with a shorter latency than SC circling. To determine whether continuous tectopontine axons mediate ipsiversive circling in both sites, the collision method of Shizgal et al. was used. Pairs of stimulating pulses were presented to the two sites, conditioning (C) pulses to one site and testing (T) pulses to the other site. Collision was evidenced when the frequencies required to evoke circling were higher at short conditioning-testing (C-T) intervals than at long C-T intervals. Between SC and VLP, collision varied from 25 to 64%. Refractory periods ranged from 0.4 to 1.0 ms in most VLP sites, and from 0.45 to roughly 3 ms in SC sites. Conduction velocities ranged from 1.2 to 19 m/s, but most were concentrated in two ranges, 1.2 to 2.7 m/s and 10 to 19 m/s. The contribution of the slower population was higher in electrode pairs where the percent collision was higher. Therefore, continuous axons from colliculus to ventrolateral pons mediate most of the ipsiversive circling produced by collicular stimulation. Slight asymmetries in the collision were observed between 3 pairs with high threshold colliculus electrodes, suggesting transsynaptic collisions across colliculus synapses transmitting from dorsal to ventral.