Selective serotonin receptor stimulation of the ventral tegmentum differentially affects appetitive motivation for sugar on a progressive ratio schedule of reinforcement.

Serotonin signaling influences satiety and motivation through known actions in the hindbrain and hypothalamus. Recently, we reported that some classes of serotonin receptors also modulate food intake through actions in the ventral tegmentum and the nucleus accumbens. In the current experiments, we examined whether activation or blockade of individual serotonin receptor subtypes in the ventral tegmentum might also affect appetitive motivation for sugar pellets as assessed in a progressive ratio (PR) task. Separate groups of rats were tested following stimulation or blockade of ventral tegmental serotonin 1A, 1B, 2A, 2B, 2C, or 3 receptors. Rats within each group received multiple doses of a single drug across days; each test was separated by 72 h. Progressive ratio break point was significantly affected by stimulation of ventral tegmental serotonin 1A receptors with 8-OH-DPAT (0, 2, 4, 8 μg/side) or stimulation of serotonin 3 receptors with mCPBG (0, 10, & 20 μg/side). High doses of both agents tended to decrease break point. Additionally, stimulation of serotonin 2C receptors with RO60-0175 (at 0, 2, and 5 μg/side) reduced total lever presses and demonstrated a trend towards reducing break point. There were no effects of stimulating ventral tegmental serotonin 1B, 2A, or 2B receptors on break point; neither did antagonism of any of the serotonin receptor subtypes significantly affect performance. These data provide additional evidence that serotonergic signaling in the mesolimbic pathway affects motivated behavior, and demonstrate that a subset of serotonin receptors impact not only food consumption, but appetitive food-seeking as well.
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