I thought pica was the ingesting of inedible items? And the buprenorphine injectable pain medication causes this effect in rats if used in high doses.
from:
http://ratguide.com/meds/analgesics/buprenorphine_hydrochloride.php
Important note to veterinarians: It has been reported that rats may develop pica after receiving this drug.1,2,4
Be sure to instruct pet owners to monitor for this behavior if an opioid medication is given, and to contact vet immediately. Also have the pet owner remove the rat from any bedding source that might be chewed and ingested until medication has passed through the system. In severe cases it may be necessary to reduce or eliminate the drug from the rat