Bimodal Locomotion Elicited by Electrical Stimulation of the Midbrain in the SalamanderNotophthalmus viridescens

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Abstract
The present experiments were designed to identify the mesencephalic locomotor region (MLR) in the salamander. Anin vitrosemi-intact preparation from a decerebrate adult salamander (Notophthalmusviridescens) was developed in which the locomotor activities were monitored from electromyographic and video recordings. The results show that the two locomotor modes exhibited by salamanders (i.e., stepping and swimming) were evoked by electrical microstimulation (5–15 Hz; 0.1–10 μA; 2 msec pulses) of a circumscribed region in the caudal mesencephalon. At threshold current strength (0.5–3.5 μA at 15 Hz), rhythmic limb movements and intersegmental coordination, such as during stepping, were induced. As the stimulation strength was subsequently increased, the frequency of stepping became more rapid, and, at 2.0–5.5 μA, the limbs were held back against the body wall and swimming movements of the trunk were induced. An additional increase of the stimulation strength induced an increase of the frequency and amplitude of the swimming movements. Anatomical studies conducted in parallel revealed the presence of choline acetyltransferase immunoreactive cells in the functionally identified MLR region. Together, the present results indicate that the MLR is present in salamanders and that its level of activation determines the mode of locomotion. Walking is induced at low activation levels, and swimming, which constitutes a faster mode of locomotion, requires stronger stimulation of the MLR. Furthermore, as in other vertebrates, the MLR contains cholinergic cells.