IMMUNOHISTOCHEMICAL LOCALIZATION OF NEUROTRANSMITTERS UTILIZED BY NEURONS IN THE ROSTRAL INTERSTITIAL NUCLEUS OF THE MEDIAL LONGITUDINAL FASCICULUS (RIMLF) THAT PROJECT TO THE OCULOMOTOR AND TROCHLEAR NUCLEI IN THE CAT

The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) contains excitatory and inhibitory burst neurons that are rel ated to the control of vertical and torsional eye movements. In the pr esent study, light microscopic examination of the immunohistochemical localization of amino acid neurotransmitters demonstrated that the riM LF in the cat contains overlapping populations of neurons that are imm unoreactive to the putative inhibitory neurotransmitter gamma-aminobut yric acid (GABA) and the excitatory neurotransmitters glutamate and as partate. By using a double-labelling paradigm, GABA-, glutamate-, and aspartate-immunoreactive neurons in the riMLF were retrogradely labell ed by transport of horseradish peroxidase (HRP) from the oculomotor an d trochlear nuclei. Electron microscopy showed that the oculomotor and trochlear nuclei contain synaptic endings that are immunoreactive to GABA, glutamate, or aspartate. Each neurotransmitter-specific populati on of synaptic endings has distinctive ultrastructural and synaptic fe atures. Synaptic endings in the oculomotor and trochlear nuclei that a re anterogradely labelled by transport of biocytin from the riMLF are immunoreactive to GABA, glutamate, or aspartate. Taken together, the f indings from these complimentary retrograde and anterograde double-lab elling studies provide rather conclusive evidence that GABA is the inh ibitory neurotransmitter, and glutamate and aspartate are the excitato ry neurotransmitters, utilized by premotor neurons in the riMLF that a re related to the control of vertical saccadic eye movements. (C) 1996 Wiley-Liss, Inc.