Impaired reward learning and intact motivation after serotonin depletion in rats.
Aside from the well-known influence of serotonin (5-hydroxytryptamine, 5-HT) on emotional regulation, more recent investigations have revealed the importance of this monoamine in modulating cognition. Parachlorophenylalanine (PCPA) depletes 5-HT by inhibiting tryptophan hydroxylase, the enzyme required for 5-HT synthesis and, if administered at sufficiently high doses, can result in a depletion of at least 90% of the brain's 5-HT levels. The present study assessed the long-lasting effects of widespread 5-HT depletions on two tasks of cognitive flexibility in Long Evans rats: effort discounting and reversal learning. We assessed performance on these tasks after administration of either 250 or 500 mg/kg PCPA or saline (SAL) on two consecutive days. Consistent with a previous report investigating the role of 5-HT on effort discounting, pretreatment with either dose of PCPA resulted in normal effortful choice: All rats continued to climb tall barriers to obtain large rewards and were not work-averse. Additionally, rats receiving the lower dose of PCPA displayed normal reversal learning. However, despite intact motivation to work for food rewards, rats receiving the largest dose of PCPA were unexpectedly impaired relative to SAL rats on the pretraining stages leading up to reversal learning, ultimately failing to approach and respond to the stimuli associated with reward. High performance liquid chromatography (HPLC) with electrochemical detection confirmed 5-HT, and not dopamine, levels in the ventromedial frontal cortex were correlated with this measure of associative reward learning.
Laboratory of Cognitive Neuroscience, Department of Psychology, California State University, Los Angeles, CA 90032, USA. aizquie@calstatela.edu
Alicia
A
Izquierdo
Kathleen
K
Carlos
Serena
S
Ostrander
Danilo
D
Rodriguez
Aaron
A
McCall-Craddolph
Gargey
G
Yagnik
Feimeng
F
Zhou
D000704:Analysis of Variance; D000818:Animals; D002851:Chromatography, High Pressure Liquid; D004192:Discrimination, Psychological / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005106:Exploratory Behavior / Q000187:drug effects; D005215:Fasting / Q000502:physiology; D010134:Fenclonine / Q000633:toxicity; D007859:Learning Disabilities / Q000139:chemically induced / Q000378:metabolism*; D008297:Male; D018782:Maze Learning / Q000187:drug effects; D009042:Motivation / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D020318:Rats, Long-Evans; D012193:Reversal Learning / Q000187:drug effects; D012201:Reward; D012701:Serotonin / Q000172:deficiency*; D012702:Serotonin Antagonists / Q000633:toxicity
D012702:Serotonin Antagonists; D012701:Serotonin; D010134:Fenclonine