Untreated mice showed only trace levels of 9-
cis retinal after 3 and 7 days of dark rearing (3 days, 2.86 ± 0.13 pmol/retina; 7 days, 2.23 ± 0.13 pmol/retina,
n = 6). Administration of 9-
cis retinal by IP injection resulted in a modest, but significant, increase of 9-
cis retinal in mice after dark rearing for 3 or 7 days compared with untreated mice (3 days, 15.94 ± 3.02 pmol/retina; 7 days, 10.54 ± 1.78 pmol/retina;
n = 6;
P < 0.05). However, administration of 9-
cis retinal in Matrigel showed a significant increase in 9-
cis retinal levels at both time points compared with IP injection (3 days, 49.12 ± 5.74 pmol/retina; 7 days, 73.11 ± 14.36 pmol/retina;
n = 6,
P < 0.05) and with untreated mice (
P < 0.01,
Fig. 4B). Further, there was a significant increase in 9-
cis retinal after 7 days of dark rearing compared with 3 days in the Matrigel-treated animals, showing that the retinoid was still being released. In the IP-injected animals, the levels were decreased at 7 days. This increase in 9-
cis retinal levels observed in the Matrigel-treated mice was confirmed by quantification of isorhodopsin pigment levels through absorption difference spectra, where isorhodopsin levels doubled (3 days, 13.83 ± 2.20 pmol/retina; 7 days, 27.44 ± 3.14 pmol/retina;
n = 6;
P < 0.01;
Fig. 4C).