Putative neurotransmitters of the avian visual pathway

Abstract

The ability of homogenates of the chick optic lobe to accumulate a series of possible neurotransmitters has been studied. High affinity uptake of several possible neurotransmitters was examined in optic lobes of 21-day-old embryos that had a single eye removed on the third day of incubation and in 23-day-old chicks that had an eye removed at hatch. Embryonic enucleation resulted in severe reduction of development of the ability of the contralateral optic lobe to take up tritiated GABA, dopamine, choline, serotonin and glutamate from solutions around 10(-8)M. Unilateral eye removal of new-hatched chicks caused failure of the denervated optic lobe to grow, but only the uptake capacity for glutamate was significantly recuced. This deficit was apparent as early as 4 days after enucleation. The transport of other compounds was unimpaired. The uptake of glutamate by homogenates of the optic tract was 43% of that or the optic lobe. This was a much greater fraction than the corresponding value for other postulated neurotransmitters. These data suggest that glutamate may be the primary neurotransmitter of the fibers of the optic tract originating in the retinal ganglion cells.

The nature of the excitatory transmitter mediating X and Y cell inputs to the cat dorsal lateral geniculate nucleus

“…The primary candidates are obviously L-glutamate and L-aspartate (Curtis & Johnston, 1974;Johnson, 1979) and if it can be considered of relevance, the evidence from the avian and amphibian brain would favour L-glutamate (Roberts & Yates, 1976, Henke et al 1976, Bondy & Purdy 1977, Wang et al 1978. It is obviously important to consider whether the present data can provide any hints on this matter.…”

“…However, accepting the possibility that the action of 5-HT may be due to block of the release or synthesis of the optic nerve transmitter (Curtis & Davis, 1962), the very limited documentation of the effect of HA-966 on optic nerve responses (Tebecis, 1973) leaves the matter at best unresolved. Indeed in the avian and amphibian brain there is good evidence favouring glutamate as the optic nerve transmitter (Roberts & Yates, 1976; Henke, Schenker & Cuenod, 1976;Bondy & Purdy, 1977;Wang, Felix & Frangi, 1978) and this taken together with the general acceptance of glutamate and aspartate as excitatory transmitters in the mammalian central nervous system (Curtis & Johnston, 1974; Johnson, 1979) suggests that a re-evaluation of the situation with respect to the optic nerve is needed.…”

Biochemical Maturation of the Non-Innervated Chick Optic Lobe

“…Although the number of neurons in the experimental lobe is reduced, the levels of several nerve-specific enzymes and proteins and uptake mechanisms are much above levels found in the optic lobes of the 10-dayold embryo [7,IO,35]. Therefore, the biochemical maturation of many surviving neurons may be relatively normal.…”

“…Thus, the cell population lost in the experimental lobes may have been predominantly neuronal rather than glial. This is substantiated by the reduced velocity of high affinity uptake of putative neurotransmitters or their precursors by the nonafferented experimental lobes [7]. Several uptake mechanisms are largely confined to the presynaptic area, especially those of choline, dopamine and serotonin [22] and so their reduced levels imply extensive failure of synaptogenesis.…”

Effects of prevention of afferentation of the development of the chick optic lobe

“…Thus, parameters of neuronal development (e.g., increased levels of acetylcholinesterase, choline acetyltransferase and the appearance of high affinity transport mechanisms) of several putative neurotransmitters are more dramatically impaired than are indices of glial maturation (e.g., levels of nonspecific cholinesterase) [2,7,8,15].…”