Nitric oxide facilitates GABAergic neurotransmission in the cat oculomotor system: a physiological mechanism in eye movement control

Abstract

Nitric oxide (NO) synthesis by prepositus hypoglossi (PH) neurons is necessary for the normal performance of horizontal eye movements. We have previously shown that unilateral injections of NO synthase (NOS) inhibitors into the PH nucleus of alert cats produce velocity imbalance without alteration of the eye position control, both during spontaneous eye movements and the vestibulo‐ocular reflex (VOR). This NO effect is exerted on the dorsal PH neuropil, whose fibres increase their cGMP content when stimulated by NO. In an attempt to determine whether NO acts by modulation of a specific neurotransmission system, we have now compared the oculomotor effects of NOS inhibition with those produced by local blockade of glutamatergic, GABAergic or glycinergic receptors in the PH nucleus of alert cats. Both glutamatergic antagonists used, 2‐amino‐5‐phosphonovaleric acid (APV) and 2,3‐dihydro‐6‐nitro‐7‐sulphamoyl‐benzo quinoxaline (NBQX), induced a nystagmus contralateral to that observed upon NOS inhibition, and caused exponential eye position drift. In contrast, bicuculline and strychnine induced eye velocity alterations similar to those produced by NOS inhibitors, suggesting that NO oculomotor effects were due to facilitation of some inhibitory input to the PH nucleus. To investigate the anatomical location of the putative NO target neurons, the retrograde tracer Fast Blue was injected in one PH nucleus, and the brainstem sections containing Fast Blue‐positive neurons were stained with double immunohistochemistry for NO‐sensitive cGMP and glutamic acid decarboxylase. GABAergic neurons projecting to the PH nucleus and containing NO‐sensitive cGMP were found almost exclusively in the ipsilateral medial vestibular nucleus and marginal zone. The results suggest that the nitrergic PH neurons control their own firing rate by a NO‐mediated facilitation of GABAergic afferents from the ipsilateral medial vestibular nucleus. This self‐control mechanism could play an important role in the maintenance of the vestibular balance necessary to generate a stable and adequate eye position signal.

A Cholinergic Synaptically Triggered Event Participates in the Generation of Persistent Activity Necessary for Eye Fixation

“…In contrast, results obtained with ACh antagonists (i.e., curved slow phases in darkness and postsaccadic centripetal drifts in light) were indicative of a loss of eye position in the horizontal plane. These findings, similar to those obtained by the blockade of glutamatergic receptors in the PH nucleus of alert cats (Moreno-Ló pez et al, 2002), suggest that activation of M1 receptors by ACh is necessary to generate correct eye position signals after an eye saccade in the activating direction. The absence of nystagmus in light suggests that the visual input is sufficient to override any effect induced by a higher concentration of ACh agonists.…”

“…These results were exclusive to horizontal eye movements, indicating that there was no diffusion of the applied drugs to the neighboring vestibular nuclei. The pharmacological effects of ACh agonists were in agreement with the effects induced by blockade of the GABA and glycine receptors, using the same animal preparation (Moreno-Ló pez et al, 2002), and are in accordance with the depolarization of the PH neurons induced by muscarinic activation in vitro. The linearity of the slow phases of the nystagmus in darkness and the correct maintenance of the eye position in light are both indicative of a correct gaze-holding system.…”

Cooperative Glutamatergic and Cholinergic Mechanisms Generate Short-Term Modifications of Synaptic Effectiveness in Prepositus Hypoglossi Neurons

“…Indeed, transient pharmacological inactivations or permanent electrolytic lesions of the PH nucleus indicate that prepositus neural circuits are necessary for the generation of eye position signals subsequent to on-and off-directed saccades (Cheron and Godaux, 1987;Arnold et al, 1999;Moreno-Ló pez et al, 2002). PH neurons receive eye velocity signals from excitatory burst neurons (EBN) located in the paramedian pontine reticular formation (PPRF) , and they project monosynaptically on extraocular motoneurons and other brainstem and cerebellar structures related to eye movements (McCrea and Baker, 1985).…”

Anatomical and pharmacological relationship between acetylcholine and nitric oxide in the prepositus hypoglossi nucleus of the cat: Functional implications for eye‐movement control

“…Moreover, as the cholinergic neurons must receive velocity signals, and NO has been proposed as controlling the firing rate of neurons receiving velocity signals (Moreno-López et al, 2002), we also examined the possible colocalization of the two neurotransmitter systems. Finally, the effects of cholinergic agonists and antagonists on spontaneous and vestibularly induced eye movements were compared with those obtained for the pharmacology of NO (Moreno-López et al, 1996.…”

“…Retrograde tracer studies, in combination with immunocytochemistry using antibodies against cGMP and GABA, show that GABAergic terminals from the ipsilateral medial vestibular nucleus are targets for the NO released in the PH nucleus (MorenoLópez et al, 2001). Local microinjections of NOS inhibitors in the PH nucleus of alert cats induce oculomotor deficits that mimic those produced by GABAergic and glycinergic receptor antagonists in the same animal preparation (Moreno-López et al, 2002). Taking together the special properties of NO as retrograde messenger and these immunohistological and pharmacological results, it could follow that NO might act as a mechanism of selfcontrol of excitability for PH neurons, helping to stabilize the eye-position generation process.…”