The distribution of glutamic acid decarboxylase immunoreactivity in the diencephalon of the opossum and rabbit

Abstract

We have examined the distribution of neurons and terminals immunoreactive for glutamic acid decarboxylase (GAD) in the thalamus and adjacent structures of the opossum (Didelphis virginiana) and the rabbit and have compared this distribution with the distributions we described previously for the cat and bushbaby (Galago senegalensis). The significance of these experiments depends, first, on the fact that GAD is the synthetic enzyme for GABA, and therefore that GAD immunoreactivity is a marker for GABAergic inhibitory neurons, and second, on previous findings that suggest that GABAergic neurons in the dorsal thalamus are local circuit neurons. In both cat and Galago, GAD-immunoreactive neurons are distributed essentially throughout the entire thalamus. In the opossum, GAD neurons are chiefly confined to the dorsal lateral geniculate nucleus and the lateral extremity of the lateral posterior nucleus. The distribution of GAD neurons in the rabbit is intermediate between that found in the opossum on the one hand and cat and Galago on the other. Like opossum, about 25% of the neurons in the lateral geniculate nucleus of rabbit are GAD immunoreactive. Unlike opossum, however, as many as 18% of the cells in the ventral posterior nucleus of the rabbit are GAD immunoreactive, and scattered cells are also labeled in other thalamic areas, such as the medial geniculate and the lateral group. Aside from the findings in the dorsal thalamus, the chief observation is that GAD-immunoreactive neurons and/or terminals densely fill all principal targets of the optic tract, including the ventral lateral geniculate nucleus; the superficial gray layer of the superior colliculus; the anterior, posterior, and olivary pretectal nuclei; the nucleus of the optic tract; and the medial and lateral terminal nuclei of the accessory optic tract. These results support the idea first put forward by Cajal that local circuit neurons increase in number during the course of the evolution of complex mammalian brains. If we can assume that the conservative opossum retains characteristics reflecting an early stage of mammalian evolution, the results suggest that thalamic local circuit neurons arose first in the visual system and only later in evolution spread throughout the thalamus.

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

“…GABA is the major inhibitory neurotransmitter in the brain and plays a prominent role in the NOT and AOS pathways mediating optokinetic nystagmus and visual-vestibular interaction as GABAergic neurons are numerous in somata and axon terminals of the AOS nuclei (e.g., Ottersen and Storm-Mathisen, 1984;Penny et al, 1984;Giolli et al, 1985b;Magnaini et al, 1985) Double-label retrograde and GABA immunocytochemistry in rat and rabbit show that these nuclei are extensively interconnected by GABAergic projection neurons forming the NOT-MTN and NOT-MTN pathways. Similar pathways have been demonstrated in non-human primate in which the main interconnections are between the NOT and LTN (Mustari et al, 1994;Blanks et al, 1995).…”

Intracellular Recordings in Response to Monaural and Binaural Stimulation of Neurons in the Inferior Colliculus of the Cat

“…These inputs can be inhibitory or excitatory (e.g., Irvine, 1986;Oliver and Heurta, 1992). For example, excitation from stimulation of one ear could come from the contralateral cochlear nucleus, contralateral lateral superior olive (L SO), and ipsilateral medial superior olive (MSO), whereas inhibition could arise from the ventral nuclei of the lateral lemniscus (VNLL) and the dorsal nuclei of the lateral lemniscus (DN L L) (e.g., Glendenning and Masterton, 1983;Adams and Mugnaini, 1984;Penney et al, 1984). E xcitation from the stimulation of the other ear could come from the ipsilateral L SO and MSO, whereas inhibition could arise from the ipsilateral L SO and the DN L L of both sides (Saint Marie et al, 1989;Glendenning et al, 1992).…”

“…The most unusual form of ITD sensitivity was displayed by the neuron in Figure 14, in which inhibitory mechanisms appear to play a dominant role. Such ITD sensitivity might be attributable to inhibitory inputs from ITD-sensitive neurons in the DNLL (Brugge et al, 1970;Adams and Mugnaini, 1984;Penney et al, 1984).…”

Evolution of GABAergic circuitry in the mammalian medial geniculate body.

“…In contrast, the ventrobasal complex (Vb), like the MGB, has no GABAergic cells in lizards (49), rats (50), and the mustache bat (Fig. 1A: ventral posterior lateral and medial nuclei), a few in rabbits (51), and many more in cats (52) and monkeys (53). The …”