Phenytoin blocks the reversal of a classically conditioned discriminative eyeblink response in rabbits.
OBJECTIVE
Cognitive deficits associated with chronic treatment with phenytoin (PHT) have been reported. PHT blocks transfer from a signaled appetitive bar press to an active avoidance response in rats. We investigated the effects of PHT and the prodrug fosphenytoin in rabbits required to learn a discrimination and reversal of a classical eyeblink conditioning paradigm.


METHODS
Before drug treatment was started, rabbits were trained to produce a discriminated eyeblink response. PHT (n = 7) was administered centrally or the prodrug fosphenytoin (n = 2) was given systemically. Control animals were similarly treated centrally with either saline (n = 3) or no drug treatment (n = 13). Rabbits were then challenged with a stimulus reversal while being maintained on the respective drug.


RESULTS
On the first day of reversal training, control animals typically displayed high response rates to both tones, followed by a reduction in responsiveness to the new conditioned stimulus (CS-) in the ensuing days. In contrast, PHT-treated animals failed to suppress responsiveness to the new CS-.


CONCLUSIONS
The response patterns observed are similar to those observed in rabbits with hippocampal ablations, leading us to suggest that the adverse effects of phenytoin may be due to actions in the hippocampus.
Churchill
J D
JD
Department of Psychology, Indiana University, Bloomington 47405, USA.
Voss
S E
SE
Miller
D P
DP
Steinmetz
J E
JE
Garraghty
P E
PE
D000161:Acoustic Stimulation; D000818:Animals; D001307:Auditory Perception / Q000187:drug effects; D001767:Blinking / Q000187:drug effects*; D003214:Conditioning, Classical / Q000187:drug effects*; D003216:Conditioning, Operant / Q000187:drug effects; D004193:Discrimination Learning / Q000187:drug effects*; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology; D008297:Male; D010672:Phenytoin / Q000031:analogs & derivatives / Q000494:pharmacology*; D011355:Prodrugs / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D012193:Reversal Learning / Q000187:drug effects / Q000502:physiology
D011355:Prodrugs; D010672:Phenytoin; C043114:fosphenytoin