FREE AMINO ACIDS IN NERVOUS TISSUE 473 
the brain may, indeed, be accurately reflected by this emphasis on its metabolic 
role. In vertebrates, the concentration of free glutamic acid is higher in brain than 
in the other tissues, and in brain its concentration is greater than that of any other 
amino acid!®, 165; in invertebrate nerve the most abundant amino acid is aspartic 
acid#6, 47, 96, 109 or taurine*®. Glutamic acid and glutamine have been found in high 
concentrations in the brain of all species examined; in addition to the species listed 
in Table IA, the two compounds have also been found in alligator and opposum 
brains!*°. 
Coincident with the development of the brain from the fetal or neonatal state to 
that of the adult, there is an increase in the concentration of glutamic acid and gluta- 
mine. This has been demonstrated in the mouse, rat79, 171, calf!) rabbit??, 
sheep, and in man®, 127, Marked variations (greater than 2 : 1) in the occurrence 
TABLE IB 
GLUTAMIC AND ASPARTIC ACIDS AND RELATED COMPOUNDS IN NERVE 
Concentrations in wmoles/g. Reference numbers indicated between ( ). 


Species Nerve ee as Glutamine yee Meet Hig sae ee 
Rabbit Spinal cord 2.9 (72) | 5.8 (72) 
Rabbit Sciatic nerve 2272) ease 4.2 
| (72) 
Rat Sciatic nerve 2eau72 | QED ae 7.2) 
Hen Spinal cord 6.9 (136) |e ate (135) bal: 8.61 (53 6) il ae O mee 
| | (16, 136) 
Hen Sciatic nerve 17 1(£36) | 0.50 (136) | 1.5 (136) 1.1 (136) 
Crab Leg nerve 35 (109) | | 138 (109) 
Lobster Leg nerve 25 (109) | 112 (109) 
Cuttlefish | Axon 39 (109) | 82 (109) 
Squid (Loligo pealiz) | Giant nerve TEE ab yuan ey? | 62.5, 79.1 
axoplasm (46, 96) | (46, 96) 
Squid (Dosidicus gigas) | Giant nerve 27.6 (46) | 111.3 (46) 
axoplasm | 





of glutamic acid and glutamine in the various parts of the central nervous system 
are not found?’, 72, 80, 100, 127, 171, though the concentration in nerve (hen sciatic) is 
much lower than in brain*, Large amounts of glutamic acid are found in retina, 
The occurrence of glutamic acid in peptide form is discussed below. Lipid-bound 
glutamic acid has also been reported’. 
y-Aminobutyric acid (Tables LA and B). This compound (reviewed in detail by ELLIOT 
AND JASPER® and the subject of a recent book!!) has attracted considerable atten- 
tion because of its unique occurrence in nervous tissue: )%, 142, 169 and because it is 
a component of extracts of mammalian brain which have inhibitory effects on 
nerve preparations, particularly the crayfish stretch receptor?*: 24, 6, 68,110. However, 
although y-aminobutyric acid does exhibit marked pharmacological activity, there 
has grown up a considerable body of evidence demonstrating that the active principle 
of the extract (termed Factor I) is not solely y-aminobutyric acid, and, indeed, 
preparations of Factor I may be obtained in which no y-aminobutyric acid is detect- 
References p. 482/485 
