regions have low densities of these neurons. The PFR and RMTg contain a low density of tyrosine hydroxylase (TH)-positive cell bodies that are small in size and lightly stain; the RMTg is composed mostly of GABAergic cells. On the other hand, the PN and PBP consist mainly of medium to large sized TH-positive cell bodies that stain moderately. Inputs Almost all areas receiving projections from the VTA project back to it. Thus, the ventral tegmental area is reciprocally connected with a wide range of structures throughout the brain suggesting that it has a role in the control of function in the phylogenetically newer and highly developed neocortex, as well as that of the phylogenetically older limbic areas. The VTA is a heterogeneous region consisting of a variety of neurons that are characterized by different neurochemical and neurophysiological properties. Therefore, glutamatergic and GABAergic inputs are not exclusively inhibitory nor exclusively excitatory. The VTA receives glutamatergic afferents from the prefrontal cortex, pedunculopontine tegmental nucleus (PPTg), laterodorsal tegmental nucleus, subthalamic nucleus, bed nucleus of the stria terminalis, superior colliculus, periaqueductal gray, lateral habenula, dorsal raphe nucleus, and lateral hypothalamic and preoptic areas. These glutamatergic afferents play a key role in regulating VTA cell firing. When the glutamatergic neurons are activated, the firing rates of the dopamine neurons increase in the VTA and induce burst firing. Studies have shown that these glutamatergic actions in the VTA are critical to the effects of drugs of abuse. In contrast, the tail of the ventral tegmental area (tVTA, the RMTg) projects to the VTA with GABAergic afferents, functioning as a "master brake" for the VTA dopamine pathways. GABAergic inputs to the VTA also include the nucleus accumbens, ventral pallidum, dorsal raphe nucleus, lateral hypothalamus, periaqueductal gray, bed nucleus of the stria terminalis, and rostromedial tegmental nucleus (RMTg). The lateral habenula can also exert an inhibitory effect on dopaminergic neurons in the VTA by exciting RMTg GABAergic neurons, which is thought to play an important role in reward prediction errors. Subpallidal afferents into the VTA are mainly GABAergic and, thus, inhibitory. There is a substantial pathway from the subpallidal area to the VTA. When this pathway is disinhibited, an increase in the dopamine release in the mesolimbic pathway amplifies locomotor activity. There are also cholinergic inputs to the VTA, although less studied than the glutamatergic and GABAergic inputs. Optogenetic studies in mice looking at cholinergic inputs from the