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1. In the avian brain, a high concentration of dopamine was found in a sharply contoured region of the nucleus basalis which may or may not have included the nucleus entopeduncularis, and therefore lay within the palaeostriatum of the nomenclature of Crosby and Huber. This was thus the only region which may be considered biochemically homologous to the mammalian corpus striatum. For purposes of macroscopic identification only, the region is described here as the `anterior part of the nucleus basalis'. The concentration of dopamine was 3 µg/g in the pigeon, about the same in the duck and chicken, and 7·5 µg/g in the finch. In the pigeon this region also contained some noradrenaline; the quantity of 5-hydroxytryptamine (1·4 µg/g) and 5-hydroxyindolylacetic acid (0·6 µg/g) was larger than in any other part of the brain.
2. In the brain of the pigeon and the chicken, the highest concentrations of noradrenaline (1·5 and 1·4 µg/g) were found in the hypothalamus.
3. The concentration of adrenaline was higher in the avian than in the mammalian brain. In the hypothalamus, it ranged from 0·4 µg/g in the pigeon to 1 µg/g in the chicken.
4. Fluorescence microscopy, using the formaldehyde condensation method, showed, in the anterior part of the nucleus basalis, a large area of diffuse green-yellow fluorescence, similar in appearance to the fluorescence of the striatum of the rat. In addition this part of the brain contained a small region of fluorescent fibres and varicosities. It is suggested that the diffuse fluorescence was produced by dopamine. It was absent from brains of reserpine-treated pigeons.
5. In the pigeon, reserpine, tetrabenazine and prenylamine produced a decrease in the concentration of brain monoamines, an effect which was comparable to that seen in mammals. Yet, none of these drugs raised the concentration of homovanillic acid, but they increased that of 5-hydroxyindolylacetic acid; these drugs raise the concentration of both acids in mammalian brain.
6. In the pigeon 
-tetrahydronaphthylamine decreased the concentration of all monoamines and their metabolites, an action quite different from that produced in the mammalian brain.
7. The main effect of morphine and of M 99 (6,14-endoetheno-7-(2-hydroxy-2-pentyl)-tetrahydro-oripavine hydrochloride) was a lowering of the noradrenaline concentration.
8. As in mammals, chlorpromazine affected only the dopamine metabolism.
9. In the guinea-pig and the pigeon, the administration of 
-methyl-DOPA led to a substitution of much of the cerebral noradrenaline by -methyl-noradrenaline, sometimes in excess of the lost noradrenaline. However, although the loss of dopamine was severe in both pigeon and guinea-pig, only little -methyl-dopamine accumulated in the pigeon brain, so that it did not consitute a replacement for the lost dopamine; in the guinea-pig, -methyl-dopamine was found in quantities similar to, or exceeding those, of the lost dopamine.
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