Effects of Melatonin Implants on Structures and Behaviors of the House Finch (Carpodacus mexicanus) Eye

Abstract

This study aimed to determine the extraretinal effects of melatonin upon the eyes of an avian species, the House Finch (Carpodacus mexicanus). Twelve birds (full-grown, second-year males) each received a Silastic tubing intraperitoneal implant, six containing melatonin (average release = 24 micrograms/d/bird; = M birds) and six being empty (= C birds). Microscopic study of pupillary and palpebral behaviors during the final week demonstrated lesser pupillary diameters and interpalpebral distances in M birds under all test conditions. These effects could have diminished mean light levels reaching parts of the retina. Characteristics of the relative miosis and ptosis of M birds resemble signs in some CNS disorders, such as altered inhibition of the Edinger-Westphal nucleus, and especially lesions in, or lowered activity of, higher sympathetic centers (a subtype of Horner's syndrome). Weights of eyes and their parts were the same in M and C birds, contrasting with previously reported results from male Golden Hamsters, possibly due to species differences and/or preexperimental attainment of full growth in the finches. Effects of melatonin on pupillary and palpebral behaviors, demonstrated here for the first time, foster caveats for simplistic experimental designs and interpretations with melatonin when sensory-neural-behavioral interactions are affected. Quantitative changes in pupillary and palpebral behaviors may, nevertheless, provide a window for monitoring central actions of melatonin in living test subjects in chronic studies.

Horner's syndrome in an eastern screech owl (Megascops asio)

“…Ptosis is the most consistent finding in both spontaneous and experimentally induced HS in birds (Isomura 1973, Quay 1986, Williams and Cooper 1994, Gancz and others 2005. This is consistent with the existence of dense adrenergic innervation to the smooth muscle in avian eyelids (Bennett andMalmfors 1970, Isomura 1973).…”

“…In the African spotted eagle owl with HS, ptosis and mild miosis were the only clinical findings (Williams and Cooper 1994), while in the red-bellied parrot, ptosis, but not miosis, was seen, as well as asymmetry in the position and movement of feathers of the head and neck (Gancz and others 2005). Experimentally induced HS (by excision of the CCG) in domestic fowl did not cause miosis or a change in pupillary light reflex (Isomura 1973), while central HS, induced in house finches (Carpodacus mexicanus) by intraperitoneal administration of melatonin, did result in miosis (Quay 1986). It has been suggested that the lesser degree of miosis in avian cases of HS compared with HS in mammals is due to the dominance of striated muscle fibres in the avian dilator muscle of the pupil (King andMcLelland 1984, Williams andCooper 1994).…”

“…In domestic mammals, HS is most frequently diagnosed in the dog and cat (Kern andothers 1989, Morgan andZanotti 1989), but it has also been described in horses and cattle (Firth 1978, Guard and others 1984, Pace and others 1997. HS has also been induced experimentally in both mammalian and avian species (Bistner and others 1970, Isomura 1973, Firth 1978, Skarf and Czarnecki 1982, Quay 1986. Naturally occurring HS appears to be rare and/or underdiagnosed in birds; to the authors' knowledge there are only two previous reports of HS in birds (Williams andCooper 1994, Gancz andothers 2005).…”