and 5-hydroxytryptophan as reserpine antagonists," Nature, vol.
Transmitter-like basal and [K.sup.+]-evoked release of 3,4-dihydroxyphenylalanine
from the striatum in conscious rats studied by microdialysis.
On the other hand, tyrosine is degraded by UV light in the presence of air to 3,4-dihydroxyphenylalanine
(DOPA), ammonia, and a yellow-brown pigment.
Tyrosinase plays a crucial role in the initial step of melanin synthesis by catalyzing the oxidation of L-tyrosine (L-Tyr) to 3,4-dihydroxyphenylalanine
(DOPA) and the oxidation of DOPA to dopaquinone [9,13-15].
These include neuroactive substances such as [gamma]-aminobutyric acid (GABA), 3,4-dihydroxyphenylalanine
(DOPA), catecholamines, and choline derivatives, organic and inorganic compounds, and the inorganic ions [Mg.sup.2+], [Li.sup.+], N[H.sub.4.sup.+], [Cs.sup.+], and [K.sup.+] (Pearce and Scheibling, 1994; Avila et al., 1996; Fleck, 1998; Kawaii et al., 1999; Carpizo-Ituarte et al., 2002).
In the body, the rate of 3,4-dihydroxyphenylalanine
(DOPA) formation by autooxidation of tyrosine was found to be negligible compared with the rate of formation by enzymatic catalysis.
The enzyme converts tyrosine to 3,4-dihydroxyphenylalanine
(L-DOPA) and oxidizes L-DOPA to form dopaquinone, which plays a prominent part in melanin biosynthesis [6, 7] (Figure 1).
The responses of the larvae to the neuroactive compounds 3,4-dihydroxyphenylalanine
(DOPA, 1-100 [[micro]molar]), epinephrine (EP, 1 [[micro]molar]), [Gamma]-aminobutyric acid (GABA, 0.1-20 mM), and hydrogen peroxide (50 and 100 [[micro]molar]) were then examined.