Central compensation of vestibular deficits. I. Response characteristics of lateral vestibular neurons to roll tilt after ipsilateral labyrinth deafferentation

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Abstract
The dynamic control exerted by the macular labyrinth receptors on neurons located in the lateral vestibular nucleus (LVN) of Deiters has been investigated in precollicular decerebrate cats submitted to ipsilateral acute (aVN) or chronic vestibular neurectomy (cVN); the results obtained were then compared with those elicited in decerebrate cats with intact labyrinths. LVN neurons (170) recorded after aVN and LVN neurons (189) recorded after cVN were tested during slow sinusoidal tilt of the animal at the standard parameters (0.026 Hz, 10.degree. peak displacement). The neurons were histologically located either in the rostroventral (rvLVN) or in the dorsocaudal parts (dcLVN) of Deiters'' nucleus, which are known to project mainly to the cervical and the lumbosacral cord, respectively. After aVN, the proportion of responsive units decreased with respect to control experiments in rvLVN (from 91.2 to 72.2%), but not in dcLVN (from 67.6 to 69.9%). The mean discharge rate of the responsive LVN neurons decreased from 43.6 .+-. 54.5 (SD) impulses/s in control experiments to 23.4 .+-. 20.7 (SD) impulses/s. The average sensitivity, and to a lesser extent the gain, of responses of rvLVN neurons to the labyrinth input was almost twice that of the dcLVN units in preparations with the vestibular nerves intact; these regional differences disappeared after aVN, particularly due to an increase in gain and sensitivity of responses of dcLVN neurons. The proportion of LVN neurons, which were maximally excited by animal position, increased from 74% in the control experiments to 86%. While in control experiments the proportion of units excited during side-down tilt was twice that of the units excited by side-down tilt, in acute preparations the proportion of units excited by side-up tilt increased, particularly in dcLVN, due to a reduction in the number of units showing intermediate phase angle of responses. The average phase lead of responses relative to the extreme animal displacement decreased from +12.3.degree. in control experiments to +3.3.degree.. After cVN, the the proportion of responsive units was higher in rvLVN (76.7%) than in dcLVN (61.6%), thus approaching the values obtained in control experiments with intact labyrinths. The mean discharge rate of the responsive LVN neurons (20.9 .+-. 17.1 (SD) inpulses/d) did not change with respect to the value obtained after a VN, suggesting that compensation of the postural deficits elicited by the vestibular neurectomy may result from a redistribution of the excitatory drive within different populations of LVN neurons. The average gain and sensitivity of responses of the LVN neurons to the labyrinth input, which were particularly increased in the dcLVN after aVN, decreased after cVN, approaching the values obtained in preparations with the intact labyrinths. After cVN, the proportion of LVN neuron excited by the extreme animal position corresponded to 94.8%, while the number of units excited by side-up tilt was comparable to that of the units excited by side-down tilt; these findings were similar to those obtained in acute preparations. The average phase lead of the responses relative to the extreme animal displacement increased from +3.3.degree. after aVN to +16.2 after cVN, reaching the value obtained in control experiments. The responses of LVN neurons to increasing amplitudes and frequencies of tilt were recorded after aVN and cVN and the corresponding results were compared with those obtained in control experiments. The functional significance of these findings is discussed.