Postembedding immunocytochemistry demonstrates directly that both retinal and cortical terminals in the cat superior colliculus are glutamate immunoreactive

Abstract

Although the excitatory neurotransmitter glutamate is known to be present in the cat superior colliculus (SC), the types of synapses that contain glutamate have not been examined. We, therefore, studied the ultrastructure of synaptic profiles labeled by a glutamate antibody by using electron microscopic postembedding immunocytochemistry. In addition, unilateral aspiration lesions of areas 17-18 were made at 5-28 days before death in order to determine whether degenerating terminals from visual cortex were glutamate immunoreactive (Glu-ir). Three types of axon terminal were glu-ir: 1) those containing large, round synaptic vesicles and pale mitochondria, characteristic of retinal terminals (RT profiles); 2) those containing small, round synaptic vesicles and dark mitochondria (RSD profiles); and 3) those containing large, round synaptic vesicles and dark mitochondria (RLD profiles). Measures of mean gold particle density revealed that RT, RSD, and RLD profiles had similar average grain densities (11.3-12.7 particles/unit area). Other labeled profile types included cell bodies, large-calibre dendrites, and myelinated axons. Axon terminals containing flattened synaptic vesicles and vesicle-containing presynaptic dendrites, both of which contain gamma-aminobutyric acid (GABA), had many fewer gold particles (3.6 and 4.8 mean particles/unit area, respectively). Following unilateral removal of visual cortex, normal RSD terminals were observed infrequently in the SC ipsilateral to the lesion. Synaptic terminals in the initial stages of degeneration were heavily labeled by the glutamate antibody, as were axon terminals and myelinated axons undergoing hypertrophied or neurofilamentous degeneration. These results show that both major sensory afferents to the superficial layers of cat SC contain glutamate--RT terminals from the retina and RSD terminals from visual cortex. The origin of RLD terminals is unknown.

Comparison of the ultrastructure of cortical and retinal terminals in the rat dorsal lateral geniculate and lateral posterior nuclei

“…To compare the ultrastructure and synaptic targets of RL corticothalamic terminals in the LPN to those of retinogeniculate terminals, in the same LGN tissue that contained labeled cortical terminals, we also examined large profiles that contained loosely packed round vesicles and distinctive pale mitochondria (RLP profiles). This ultrastructure has been found to be specifically associated with retinal terminals (Guillery, 1969; Lieberman and Webster, 1974; Robson and Mason, 1979; Nakamura et al, 1981; So et al, 1985; Hamos et al, 1987; Nunes Cardozo et al, 1991; Luth et al, 1993; Mize and Butler, 1996; Mize, 1996; Klooster and Vresnsen, 1997). Examples of RLP profiles are illustrated in Figure 10.…”

“…We identified retinal terminals in the rat LGN on the basis of their distinctive pale mitochondria. Multiple studies have suggested that pale mitochondria can be used to identify retinal terminals in the LGN, superior colliculus, and pretectum (Guillery, 1969; Lieberman and Webster, 1974; Robson and Mason, 1979; Nakamura et al, 1981; So et al, 1985; Hamos et al, 1987; Nunes Cardozo et al, 1991; Luth et al, 1993; Mize and Butler, 1996; Mize, 1996; Klooster and Vresnsen, 1997). Similar to RL corticothalamic terminals, we observed that retinogeniculate terminals are large profiles that contain round vesicles and are surrounded by a multilayer glial sheath.…”

Decreased metabolic response to visual stimulation in the superior colliculus of mice lacking the glial glutamate transporter GLT‐1

“…We have focused on the superior colliculus because this structure receives a dense glutamatergic projection of retinal axons (Mize & Butler, 1996) and because it displays the highest metabolic response to visual stimuli (Toga & Collins, 1981). Careful observation of the superior colliculus following 3‐D reconstruction revealed an anatomically restricted ‘activated’ volume located within the superficial grey layers.…”

Glutamate containing neurons in the cat superior colliculus revealed by immunocytochemistry

“…The methodology for the synthesis of the molecule and its specificity have been reported previously (Madl et al, 1986); (2) a commercially prepared polyclonal antibody raised in rabbit against protein-conjugated glutamate, hereafter abbreviated GLU (Chemicon, Inc., Temecula, CA). Although both antibodies recognize both neurotransmitter and metabolic pools of glutamate, recent studies suggest that moderate to dense labeling in immunocytochemistry experiments reflects the neurotransmitter pools of this amino acid (see Mize & Butler, 1996, for a discussion of this issue).…”