The bidirectionality of motor learning in the vestibulo-ocular reflex is a function of cerebellar mGluR1 receptors

Bidirectional changes in synaptic transmission have the potential to optimize the control of movement. However, it can be difficult to establish a causal relationship between the bidirectionality of synaptic plasticity and bidirectional changes in the speed of actual movements. We asked whether metabotropic glutamate receptor 1 (mGluR1) receptors, which participate in cerebellar long-term depression (LTD), are necessary for bidirectional motor learning in the vestibulo-ocular reflex (VOR). Cerebellar LTD and long-term potentiation (LTP) are thought to cause increases and decreases, respectively, in the gain of the VOR; the direction of learning depends on the behavioral protocol. We injected either the mGluR1 agonist (S)-DHPG or the antagonist YM 298198 bilaterally into the flocculus of alert cats, and then induced motor learning. In the presence of YM 298198, the VOR gain decreased in gain-up, as well as in gain-down protocols. (S)-DHPG augmented gain-up learning. Gain-down learning was not significantly affected by either drug. These results supported the hypothesis that gain-up learning relies on cerebellar LTD, but gain-down learning relies on a different mechanism. In the absence of mGluR1 activity, cerebellar LTD may be replaced with LTP, permitting learning in only one direction.

Type B GABA receptors contribute to the restoration of balance during vestibular compensation in mice

Following unilateral vestibular damage (UVD), vestibular compensation restores both static and dynamic vestibular reflexes. The cerebellar cortex provides powerful GABAergic inhibitory input to the vestibular nuclei which is necessary for compensation. Metabotropic GABA type B (GABA(B)) receptors in the vestibular nuclei are thought to be involved. However, the contribution of GABA(B) receptors may differ between static and dynamic compensation. We tested static and dynamic postural reflexes and gait in young mice, while they compensated for UVD caused by injection of air into the vestibular labyrinth. The effects of an agonist (baclofen), an antagonist (CGP56433A) and a positive allosteric modulator (CGP7930) of the GABA(B) receptor were evaluated during compensation. Static postural reflexes recovered very rapidly in our model, and baclofen slightly accelerated recovery. However, CGP56433A significantly impaired static compensation. Dynamic reflexes were evaluated by balance-beam performance and by gait; both showed significant decrements following UVD and performance improved over the next 2 days. Both CGP56433A and baclofen temporarily impaired the ability to walk on a balance beam after UVD. Two days later, there were no longer any significant effects of drug treatments on balance-beam performance. Baclofen slightly accelerated the recovery of stride length on a flat surface, but CGP7930 worsened the gait impairment following UVD. Using immunohistochemistry, we confirmed that GABA(B) receptors are abundantly expressed on the vestibulospinal neurons of Deiters in mice. Our results suggest that GABA(B) receptors contribute to the compensation of static vestibular reflexes following unilateral peripheral damage. We also conclude that impairment of the first stage of compensation, static recovery, does not necessarily result in an impairment of dynamic recovery in the long term.

Rapid consolidation of motor memory in the vestibuloocular reflex

Motor memory is relatively labile immediately after learning but can become more stable through consolidation. We investigated consolidation of motor memory in the vestibuloocular reflex (VOR). Cats viewed the world through telescopic lenses during 60 min of passive rotation. Learned decreases (gain-down learning) and increases in the VOR gain (gain-up learning) were measured during sinusoidal rotation at 2 Hz. We found that if rotation in darkness immediately followed learning, the gain of the VOR reverted toward its prelearning value, indicating that expression of the memory was disrupted. If after gain-down learning the cat spent another 60 min stationary without form vision, subsequent disruption did not occur, suggesting that memory had consolidated. Consolidation was less robust for gain-up learning. We conclude that memory in the VOR is initially labile but consolidates rapidly and consistently after gain-down learning.

Adaptive rescaling of central sensorimotor signals is preserved after unilateral vestibular damage

Adaptive rescaling is a widespread phenomenon that dynamically adjusts the input-output relationship of a sensory system in response to changes in the ambient stimulus conditions. Rescaling has been described in the central vestibular neurons of normal cats. After recovery from unilateral vestibular damage, the vestibulo-ocular reflex (VOR) remains nonlinear for rotation toward the damaged side. Therefore, rescaling in the VOR pathway may be especially important after damage. Here, we demonstrate that central vestibular neurons adjust their input-output relationships depending on the input velocity range, suggesting that adaptive rescaling is preserved after vestibular damage and can contribute to the performance of the VOR. We recorded from isolated vestibular neurons in alert cats that had recovered from unilateral vestibular damage. The peak velocity of 1-Hz sinusoidal rotation was varied from 10 to 120 degrees/s and the sensitivities and dynamic ranges of vestibular neurons were measured. Most neuronal responses showed significant nonlinearities even at the lowest peak velocity that we tested. Significant rescaling was seen in the responses of neurons both ipsilateral and contralateral to chronic unilateral damage. On the average, when the peak rotational velocity increased by a factor of 8, the average sensitivity to rotation decreased by roughly a factor of 2. Rescaling did not depend on eye movement signals. Our results suggest that the dynamic ranges of central neurons are extended by rescaling and that, after vestibular damage, adaptive rescaling may act to reduce nonlinearities in the response of the VOR to rotation at high speeds.