Oh, I forgot the part referring to the Olds and Milner research on the septal nucleus of the hypothalamus -- the "pleasure center" in the brain:

Just a horny rat pressing on the bar;
It gives him a delightful jar!

I don't remember whether there was more to that verse or whether I just ended it with
Just a little rat, just a little rat,
What have they done to the rat?
Genie