Dopamine

Dopamine is believed to be involved in motivation, reward, and reinforcement.^{[Purves et al., 2008]} Dopamine also plays an essential role in the control and coordination of movement.^{[Purves et al., 2008]}
Dopamine receptors all influence the activity of the second messenger, cyclic AMP, which is involved in many biochemical processes within a neurone.^{[Kandel et al., 2000]} Dopamine receptor subtypes D1 and D5 increase the levels of cyclic AMP, whereas D2, D3, and D4 subtypes decrease the levels of cyclic AMP.^{[Kandel et al., 2000]} Thus, depending on which receptor subtype the neurotransmitter binds with on the postsynaptic neurone, dopamine can increase or decrease cellular activity.^{[Kandel et al., 2000]} Dopamine, and its actions within the CNS, is critical to the underlying disease state of many conditions, including Parkinson’s disease, which is characterised by a dearth of dopamine, and schizophrenia, which appears to be caused by an overactivity of dopamine within certain brain regions.^{[Stahl, 2013]} Furthermore, many addictive substances work by altering the effect of dopamine within the brain.^{[Purves et al., 2008]} For instance, the drug cocaine appears to act by inhibiting the reuptake of dopamine, which leads to an increased concentration of dopamine in the synapse, prolonging its action.^{[Purves et al., 2008]}

Delusion – a disturbance in thought leading to false beliefs. Hallucination – a false sensory perception, such as hearing voices or seeing things that are not there. Prolactin – a hormone, produced and released into the bloodstream by the pituitary gland, that is involved in the secretion of milk and breast growth; unusually high amounts are responsible for impotence and loss of libido in both men and women.

References:
Kandel ER, Schwartz JH, Jessell TM (eds). Principles of Neural Science. 4th edition. McGraw-Hill, 2000.

Purves D, Augustine GJ, Fitzpatrick D, et al. (eds). Neuroscience. 4th edition. Sinauer Associates, 2008. Stahl SM.

Stahl’s Essential Psychopharmacology. Neuroscientific Basis and Practical Applications. 4th edition. © Cambridge University Press, 2013.