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TERESA PELÁEZ Y COLS. ANAL. REAL ACAD. NAC. FARM.

observed. It has been possible to demonstrate that melatonin effect

is to increase the rate of cell migration rather than mitosis. Moreover,

immunocytochemical staining of these cells demonstrate the

presence of MT2 melatonin receptors (5). So, till date both MT1 and
MT2 receptors are present in the corneal epithelium but it is not still
clear if both contribute to acelarate the rate of epithelial cell

migration or if they combine sequencially: one to facilitate cell

migration and other triggering mitosis to complete corneal re-

epithelialisation.

Melatonin and diadenosine polyphosphates two good
teammates

Very recently, it has been possible to investigate the role of
melatonin on the ocular surface and in particular on tear secretion.
Melatonin alone, topically applied to New Zealand rabbits, inhibits
tear production. This is an interesting aspect since melatonin is
elevated in the night and the rate of tear production during the nigh,
specially when sleeping is significantly reduced. So, it seems that the
general scheme fits well, when less tear is necessary melatonin is
increased therefore reducing tear production.

On the other hand the dinucleotide diadenosine tetraphoshate,
Ap4A, enhances tear production. Surprisingly, when melatonin and
Ap4A are both applied simultaneously, the amount of tear production
is enhanced 34 %, by means of a mechanism which is sensitive to
luzindole, therefore melatonin receptor dependent (10). The cross-
talk between melatonin and purinergic receptors may be the reason
for such an increase in tear production, although the detailed
mechanism is still under study.

OTHER RELEVANT ASPECTS OF MELATONIN IN THE EYE

The controversy: MT3 or QR2?

Some authors discuss on the possibility that the MT3 receptor is
not an independent melatonin receptor and it is the enzyme quinone
reductase 2. Uniquely, the MT3 has been identified as quinone

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