psychomotor effects. compulsive drug-taking behaviors are a result of the permanent functional changes in the mesolimbic dopamine system arising from repetitive dopamine stimulation. molecular and cellular adaptations are responsible for a sensitized dopamine activity in the vta and along the mesolimbic dopamine projection in response to drug abuse. in the vta of addicted individuals, the activity of the dopamine-synthesizing enzyme tyrosine hydroxylase increases, as does the ability of these neurons to respond to excitatory inputs. the latter effect is secondary to increases in the activity of the transcription factor creb and the up regulation of glur1, an important subunit of ampa receptors for glutamate. these alterations in neural processing could account for the waning influence of adaptive emotional signals in the operation of decision making faculties as drug-seeking and drug-taking behaviors become habitual and compulsive. experiments in rats have shown that they learn to press a lever for the administration of stimulant drugs into the posterior vta more readily than into the anterior vta. other studies have shown that microinjections of dopaminergic drugs into the nucleus accumbens shell increase locomotor activity and exploratory behaviors, conditioned approach responses, and anticipatory sexual behaviors. the withdrawal phenomenon occurs because the deficit in reward functioning initiates a distress cycle wherein the drugs become necessary to restore the normal homeostatic state. recent research has shown that even after the final stages of withdrawal have been passed, drug-seeking behavior can be restored if exposed to the drug or drug-related stimuli. comparative anatomy and evolution all studies since 1964 have emphasized the impressive general similarity between the vta of all mammals from rodents to humans. these studies have focused their efforts on rats, rabbits, dogs, cats, opossum, non-human primates, and humans. there have been slight differences noted, such as changes in the dorsal extent of the a10 cells. to be specific, the dorsal peak of a10 cells is more extensive in primates when compared to other mammals. furthermore, the number of dopaminergic cells in the vta increases with phylogenetic progression; for instance, the vta of the mouse contains approximately 25,000 neurons, while the vta of a 33-year-old man contains around 450,000 cell bodies.