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J. Neurochem. (2011) 116, 966–974. Abstract Dopamine (DA) receptors generate many cellular signals and play various roles in locomotion, motivation, hormone production, and drug abuse. According to the location and expression types of the receptors in the brain, DA signals act in either stimulatory or inhibitory manners. Although DA autoreceptors in the substantia nigra pars compacta are known to regulate firing activity, the exact expression patterns and roles of DA autoreceptor types on the firing activity are highly debated. Therefore, we performed individual correlation studies between firing activity and receptor expression patterns using acutely isolated rat substantia nigra pars compacta DA neurons. When we performed single-cell RT-PCR experiments, D1, D2S, D2L, D3, and D5 receptor mRNA were heterogeneously expressed in the order of D2L > D2S > D3 > D5 > D1. Stimulation of D2 receptors with quinpirole suppressed spontaneous firing similarly among all neurons expressing mRNA solely for D2S, D2L, or D3 receptors. However, quinpirole most strongly suppressed spontaneous firing in the neurons expressing mRNA for both D2 and D3 receptors. These data suggest that D2S, D2L, and D3 receptors are able to equally suppress firing activity, but that D2 and D3 receptors synergistically suppress firing. This diversity in DA autoreceptors could explain the various actions of DA in the brain.

Dopamine (DA) receptors generate many cellular signals and play various roles in locomotion, motivation, hormone production, and drug abuse. According to the location and expression types of the receptors in the brain, DA signals act in either stimulatory or inhibitory manners. Although DA autoreceptors in the substantia nigra pars compacta are known to regulate firing activity, the exact expression patterns and roles of DA autoreceptor types on the firing activity are highly debated. Therefore, we performed individual correlation studies between firing activity and receptor expression patterns using acutely isolated rat substantia nigra pars compacta DA neurons. When we performed single-cell RT-PCR experiments, D(1), D(2)S, D(2)L, D(3), and D(5) receptor mRNA were heterogeneously expressed in the order of D(2)LD(2)SD(3)D(5)D(1). Stimulation of D(2) receptors with quinpirole suppressed spontaneous firing similarly among all neurons expressing mRNA solely for D(2)S, D(2)L, or D(3) receptors. However, quinpirole most strongly suppressed spontaneous firing in the neurons expressing mRNA for both D(2) and D(3) receptors. These data suggest that D(2) S, D(2)L, and D(3) receptors are able to equally suppress firing activity, but that D(2) and D(3) receptors synergistically suppress firing. This diversity in DA autoreceptors could explain the various actions of DA in the brain.

The excitatory neurotransmitter, glutamate, generates a characteristic burst-pause type of firing in midbrain dopamine neurons in association with the reward behavior, but the cellular mechanism by which glutamate generates these bursts is unknown. Here, we show that the bursts in spontaneously firing dopamine neurons can be generated by the combinative actions of the brief stimulatory and the subsequent Ca(2+)-dependent inhibitory signals in response to glutamate stimulation. The two Ca(2+)-dependent firing-extinction signals are activated by different glutamate receptors. Although the activation of metabotropic glutamate receptors rapidly stopped the enhanced firing through the Ca(2+) release from intracellular stores, the activation of NMDA and AMPA/kainate receptors abolished the firing immediately after termination of the stimulation due to the Ca(2+) accumulation in the cell. These two Ca(2+)-dependent inhibitory mechanisms appear to participate in the generation of characteristic bursts in dopamine neurons by controlling the maximum firing number of single bursts and the duration of post-firing pauses.