This burning sensation indicates that a great deal of energy is flowing there, and that a blockage is being 'burned away'.

Apply the golden rule to this...keep doing the exercise, but take a break whenever it gets too strong. This should ease with time and practice, as your energy body develops.

The circuit you speak of also includes raising your eyes to look at the middle of your brow.

Holding the tongue and/or eyes in this position is uncomfortable and takes practice.

However, if kundalini is activated enough to fire up the central channel, you'll find the tongue and eyes will automatically switch to these positions effortlessly. What this tells us is that holding these positions is a way of trying to encouraging this level of activation.

robert