Pedunculopontine tegmental nucleus controls conditioned responses of midbrain dopamine neurons in behaving rats

Midbrain dopamine (DA) neurons respond to sensory cues that predict reward. We tested the hypothesis that projections from the pedunculopontine tegmental nucleus (PPTg) are involved in driving this DA cell activity. First, the activity of PPTg and DA neurons was compared in a cued-reward associative learning paradigm. The majority of PPTg neurons showed phasic responses to the onset of sensory cues, at significantly shorter latency than DA cells, consistent with a PPTg-to-DA transmission of information. However, unlike DA cells, PPTg responses were almost entirely independent of whether signals were associated with rewards. Second, DA neuron responses to the cues were recorded in free-moving rats during reversible inactivation of the PPTg by microinfusion of local anesthetic. The results showed clear suppression of conditioned sensory responses of DA neurons after PPTg inactivation that was not seen after saline infusion or in non-DA cells. We propose that the PPTg relays information about the precise timing of attended sensory events, which is integrated with information about reward context by DA neurons.