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The Journal of Neuroscience, June 27, 2007, 27(26):6923-6930; doi:10.1523/JNEUROSCI.0958-07.2007

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Behavioral/Systems/Cognitive
Lateral Habenula Stimulation Inhibits Rat Midbrain Dopamine Neurons through a GABA_A Receptor-Mediated Mechanism

Huifang Ji and Paul D. Shepard

Maryland Psychiatric Research Center and Department of Psychiatry, University of Maryland School of Medicine, Baltimore, Maryland 21228

Correspondence should be addressed to Dr. Paul D. Shepard, Maryland Psychiatric Research Center, P.O. Box 21247, Baltimore, MD 21228. Email: pshepard{at}mprc.umaryland.edu

Transient changes in the activity of midbrain dopamine neurons encode an error signal that contributes to associative learning. Although considerable attention has been devoted to the mechanisms contributing to phasic increases in dopamine activity, less is known about the origin of the transient cessation in firing accompanying the unexpected loss of a predicted reward. Recent studies suggesting that the lateral habenula (LHb) may contribute to this type of signaling in humans prompted us to evaluate the effects of LHb stimulation on the activity of dopamine and non-dopamine neurons of the anesthetized rat. Single-pulse stimulation of the LHb (0.5 mA, 100 µs) transiently suppressed the activity of 97% of the dopamine neurons recorded in the substantia nigra and ventral tegmental area. The duration of the cessation averaged 85 ms and did not differ between the two regions. Identical stimuli transiently excited 52% of the non-dopamine neurons in the ventral midbrain. Electrolytic lesions of the fasciculus retroflexus blocked the effects of LHb stimulation on dopamine neurons. Local application of bicuculline but not the SK-channel blocker apamin attenuated the effects of LHb stimulation on dopamine cells, indicating that the response is mediated by GABA_A receptors. These data suggest that LHb-induced suppression of dopamine cell activity is mediated indirectly by orthodromic activation of putative GABAergic neurons in the ventral midbrain. The habenulomesencephalic pathway, which is capable of transiently suppressing the activity of dopamine neurons at a population level, may represent an important component of the circuitry involved in encoding reward expectancy.

Key words: associative learning; apamin; bicuculline; GABAergic; habenula; substantia nigra; ventral tegmental area

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Received March 2, 2007; revised May 10, 2007; accepted May 24, 2007.

Correspondence should be addressed to Dr. Paul D. Shepard, Maryland Psychiatric Research Center, P.O. Box 21247, Baltimore, MD 21228. Email: pshepard{at}mprc.umaryland.edu

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