How did Underdog get his powers in the 1960s cartoon?

The 2007 Underdog film was amusing, but not the most logical or plausible superhero [parody] movie. In it, a normal canine is granted various Superman-like powers after being doused in various, unspecified serums and vials of diverse animal DNA. It brought to light the question:

How did Underdog in the original series get his powers?

In almost every episode, the hero Underdog would be at full, Mighty-Mouse comparable power levels before his strength began to fade and he’d need to “recharge” with an Underdog Super Energy pill, which he kept in a ring. Nothing further was ever explained. Not where he got the pills from… Not why his powers would wear down… Not how he got them originally.

Note: This was a 1960s “spoof” show that purposefully didn’t hold too close to logic; anthropomorphic animals coincided with humans, some folks had green skin, and clouds from another planet wanted Earth’s silver to make linings for their coats.

I checked the first episode, and no details were provided there either. In the movie, it was Dr. Simon Bar Sinister who accidentally gave the regular dog his powers, but he was a mere supervillian in the cartoon.

Is there any real back story or origin for the character?