Adult male Syrian hamsters bred from birth in a 14L:10D photoperiod (long day) and then were placed in a 10L:14 D photoperiod (short day). The table below shows the treatment (which began immediately after being placed in the SD photoperiod) and testes sizes of these hamsters at the end of 12 weeks of treatment:
Group Treatment Testes size
Group 1: once every 7 days with an inert drug vehicle 5h after dark onset 0.5g (small)
Group 2: MK-801 dissolved in the vehicle 5h after dark onset 0.5g (small)
Group 3: the vehicle followed 1 hour later by a 5 minute pulse of light 3.0g (large)
Group 4: MK-801 followed 1 hour later with a 5 minute light pulse 0.5g (small)
Devise hypotheses to account for each of these 4 findings for each of the 4 groups.
Hints: MK-801 is a specific antagonist of the NMDA subclass of excitatory amino acid receptor. The light pulse was given 6 hours after the onset of darkness in the 10L:14D photoperiod.