Effects on the vestibulo- and opto-oculomotor system in rats by lesions of the commissural vestibular fibres

Resolving the active versus passive conundrum for head direction cells

Head direction (HD) cells have been identified in a number of limbic system structures. These cells encode the animal's perceived directional heading in the horizontal plane and are dependent on an intact vestibular system. Previous studies have reported that the responses of vestibular neurons within the vestibular nuclei are markedly attenuated when an animal makes a volitional head turn compared to passive rotation. This finding presents a conundrum in that if vestibular responses are suppressed during an active head turn how is a vestibular signal propagated forward to drive and update the HD signal? This review identifies and discusses four possible mechanisms that could resolve this problem. These mechanisms are: (1) the ascending vestibular signal is generated by more than just vestibular-only neurons, (2) not all vestibular-only neurons contributing to the HD pathway have firing rates that are attenuated by active head turns, (3) the ascending pathway may be spared from the affects of the attenuation in that the HD system receives information from other vestibular brainstem sites that do not include vestibular-only cells, and (4) the ascending signal is affected by the inhibited vestibular signal during an active head turn, but the HD circuit compensates and uses the altered signal to accurately update the current HD. Future studies will be needed to decipher which of these possibilities is correct.

The normal distribution and projections of constitutive NADPH-d/NOS neurons in the brainstem vestibular complex of the rat

The vestibular system is a highly conserved sensory system in vertebrates that is largely responsible for maintenance of one's orientation in space, posture, and balance and for visual fixation of objects during motion. In light of the considerable literature indicating an involvement of nitric oxide (NO) in sensory systems, it is important to determine whether NO is associated with vestibular pathways. To study the relationship of NO to vestibular pathways, we first examined the normal distribution of constitutive NADPH-diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), in the vestibular complex (VC) and then examined its association with selected vestibular projection neurons. Survey of the four major vestibular nuclei revealed that only the medial vestibular nucleus contained significant numbers of perikarya stained for NADPH-d/NOS. By contrast, all the vestibular nuclei contained a network of fine processes that stained positive for NADPH-d, although the density of this network varied among the individual nuclei. To determine whether NADPH-d/NOS neurons project to vestibular efferent targets, injections of the retrograde tracer Fluoro-Gold were made into known targets of second-order vestibular neurons. Vestibular neurons containing constitutive NADPH-d/NOS were found to project predominantly to the oculomotor nucleus. A small number of neurons also participate in vestibulothalamic and intrinsic vestibular connections. These results indicate that NADPH-d/NOS neurons are prevalent in the MVN and that a subpopulation of these neurons project to the oculomotor complex. Nitric oxide is probably released locally from axons located throughout the vestibular complex but may play a particularly important role in vestibulo-ocular pathways.

Frequency-dependent effects of glutamate antagonists on the vestibulo-ocular reflex of the cat

In the central nervous system, sensory and motor signals at different frequencies are transmitted most effectively by neural elements that have different dynamic characteristics. Dynamic differences may be due, in part, to the dynamics of neurotransmitter receptors. For example, N-methyl-D-aspartate (NMDA) receptors are thought to be a component of the "neural integrator" of the vestibulo-ocular reflex (VOR), which generates a signal proportional to eye position. We measured the effects of blockade of NMDA and AMPA/kainate receptors on the gain and phase of the VOR at frequencies between 0.1 and 8 Hz in alert cats. The competitive NMDA antagonist, APV, and the non-competitive antagonists, MK-801 and ketamine, all caused a pronounced reduction in VOR gain. Gain was more strongly attenuated at low frequencies (0.1-1 Hz) than at higher frequencies (2-8 Hz). The phase lead of the eye with respect to the head was increased up to 30 degrees. In contrast, the reduction in gain associated with drowsiness or surgical anesthesia was not frequency-dependent. Blockade of AMPA/kainate receptors by the competitive antagonists, CNQX and NBQX, reduced the gain of the VOR at all frequencies tested. We evaluated our results using a control systems model. Our data are consistent with participation of NMDA receptors in neural integration, but suggest that NMDA receptors also participate in transmission by other components of the VOR pathway, and that neural integration also employs other receptors. One possibility is that between 0.1 and 10 Hz, higher-frequency signals are transmitted primarily by AMPA/kainate receptors, and lower frequencies by NMDA receptors. This arrangement would provide a biological substrate for selective motor learning within a small frequency range.