BEHAVIORAL ASSOCIATIONS OF NEURONAL-ACTIVITY IN THE VENTRAL TEGMENTALAREA OF THE RAT

The ventral tegmental area (VTA) is a central element in a system that mediates the reinforcing properties of natural stimuli (such as food) , brain stimulation, and drugs of abuse. Although considerable effort has been applied to understanding how drugs of abuse influence this sy stem, relatively little work has examined its function during conditio ned reinforcement tasks in awake, behaving animals. In the present stu dies, bundles of four to eight microwire electrodes were chronically i mplanted in the VTA or prefrontal cortex (PFC) of male Wistar rats. Fo llowing recovery from surgery, simultaneous recordings from multiple s ingle neurons and unit clusters were obtained in rats pressing a lever for a sucrose solution on a fixed-ratio schedule of reinforcement. Co nsistent with the hypothesis that these neurons encode information rel ated to motivation, most of the neurons in both VTA and PFC showed sig nificant modulation of firing rate associated with one or more events occurring within the response/reinforcement cycle. These events includ ed lever presses, onset and end of a tone signaling sucrose delivery, and onset and end of sucrose consumption. Significant decreases in fir ing rate were observed, coincident with onset of the tone and sucrose delivery, or with consumption. These decreases were sustained through the end of sucrose consumption. A number of neurons also discharged bu rsts of activity associated with individual lever presses. These findi ngs provide a clear demonstration that VTA neuronal activity is modula ted during motivated behavior. Similar information about events within the ongoing response/reinforcement cycle appears to be distributed th rough many neurons within the VTA, and may be mirrored in target struc tures such as PFC.