DIFFERENTIAL EFFECT OF INJECTIONS OF KAINIC ACID INTO THE PREPOSITUS AND THE VESTIBULAR NUCLEI OF THE CAT

1. In order adequately to control eye movements, oculomotoneurones hav e to be supplied with both an eye-velocity signal and an eye-position signal. However, all the command signals of the oculomotor system are velocity signals. Nowadays, there is general agreement about the exist ence of a brainstem network that would convert velocity command-signal s into an eye-position signal. This circuit, because of its function, is called the oculomotor neural integrator. The most obvious symptom o f its eventual failure is a gaze-holding deficit: in this case, saccad es are followed by a centripetal post-saccadic drift. Although the ocu lomotor neural integrator is central in oculomotor theory, its precise location is still a matter for debate. 2. Previously, microinjections of kainic acid (KA) into the region of the nucleus prepositus hypoglo ssi (NPH) and of the medial vestibular nucleus (MVN) were found to ind uce a horizontal gaze-holding failure both in the cat and in the monke y. However, the relatively large volumes (1-3 mul) and concentrations (2-4 mug mul-1) used in these injections made it difficult to know if the observed deficit was due to a disturbance of the NPH or of the nea rby MVN. These considerations led us to inject very small amounts of k ainic acid (50 nl, 0.1 mug mul-1) either into the rostral part of the MVN or into different sites along the NPH of the cat. 3. The search co il technique was used to record (1) spontaneous eye movements (2) the vestibulo-ocular reflex (VOR) induced by a constant-velocity rotation (50 deg s-1 for 40 s) and the optokinetic nystagmus (OKN) elicited by rotating an optokinetic drum at 30 deg s-1 for 40 s. 4. In each inject ion experiment, the location of the abducens nucleus of the alert cat was mapped out by recording the antidromic field potentials evoked by the stimulation of the abducens nerve. Two micropipettes were then glu ed together in such a way that when the tip of the recording micropipe tte was in the centre of the abducens nucleus the tip of the injection micropipette was in a target area. The twin pipettes were then lowere d in the brainstem until the recording micropipette reached the centre of the abducens nucleus. Kainic acid was then injected into the brain stem of the alert cat through the injection micropipette by an air pre ssure system. 5. Carried out according to such a protocol, KA injectio ns into the NPH or the rostral part of the MVN consistently led to spe cific eye-movement changes. A unilateral injection into the NPH caused a bilateral horizontal gaze-holding failure without any spontaneous n ystagmus or any significant deviation of the null-position of the gaze . A unilateral injection into the rostral MVN, for its part, induced a nystagmus whose slow phases were linear and directed to the contralat eral side. No gaze-holding deficit accompanied this imbalance. 6. Take n together with the data from the literature showing that the principa l signal carried by the neurones of the NPH is an eye-position signal, our results demonstrate that the NPH is a major component of the ocul omotor neural integrator.