A bad experiment specification: it only tests that brains don’t work so well after you damage them. (That is, more detail is absolutely necessary in this case.)
From the link (pretty much the whole content unless you have access)
Dopamine-deficient (DD) mice have selective inactivation of the tyrosine hydroxylase
gene in dopaminergic neurons, and they die of starvation and dehydration at 3-4
weeks of age. Daily injections of L-DOPA (50 mg/kg, i.p.) starting approximately 2
weeks after birth allow these animals to eat and drink enough for survival and growth.
They are hyperactive for 6-9 h after receiving L-DOPA and become hypoactive
thereafter. Because these animals can be tested in the presence or absence of DA,
they were used to determine whether DA is necessary for learning to occur. DD mice > were tested for learning to swim to an escape platform in a straight alley in the
presence (30 min after an L-DOPA injection) or absence (22-24 h after an L-DOPA
injection) of dopamine. The groups were split 24 h later and retested 30 min or 22-24 h after their last L-DOPA injection. In the initial test, DD mice without dopamine
showed no evidence of learning, whereas those with dopamine had a learning curve
similar in slope to controls but significantly slower. A retest after 24 h showed that DD
mice can learn and remember in the absence of dopamine, leading to the inference
that the lack of dopamine results in a performance/motivational decrement that masks
their learning competence for this relatively simple task.
That is: The mice were made so they couldn’t manufacture dopamine naturally due to inability to make a precursor. Some were then given a dopamine precursor suplement they couldn’t make that enabled them to make dopamine. These learnt almost as well as mice that could manufacture dopamine by themselves. So they showed that they could make the mice learn almost as well as those without damage by replacing something that was lost.
If this doesn’t narrow down things enough, what more do you want?
A bad experiment specification: it only tests that brains don’t work so well after you damage them. (That is, more detail is absolutely necessary in this case.)
From the link (pretty much the whole content unless you have access)
That is: The mice were made so they couldn’t manufacture dopamine naturally due to inability to make a precursor. Some were then given a dopamine precursor suplement they couldn’t make that enabled them to make dopamine. These learnt almost as well as mice that could manufacture dopamine by themselves. So they showed that they could make the mice learn almost as well as those without damage by replacing something that was lost.
If this doesn’t narrow down things enough, what more do you want?
The dopamine and opiate mechanisms are rather tangled together in practice:
The following study tests the hypothesis that dopamine is an essential mediator of various opiate-induced responses:
http://www.nature.com/nature/journal/v438/n7069/full/nature04172.html