<unk>  is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this phenethylamine functions as a neurotransmitter, activating the five types of<unk>  receptors—D1, D2, D3, D4, and D5—and their variants.<unk>  is produced in several areas of the brain, including the substantia nigra and the ventral tegmental area.<unk>  is also a neurohormone released by the hypothalamus. Its main function as a hormone is to inhibit the release of prolactin from the anterior lobe of the pituitary.<unk>  is available as an intravenous medication acting on the sympathetic nervous system, producing effects such as increased heart rate and blood pressure. However, because<unk>  cannot cross the blood-brain barrier,<unk>  given as a drug does not directly affect the central nervous system. To increase the amount of<unk>  in the brains of patients with diseases such as Parkinson's disease and<unk> a-responsive dystonia, L-DOPA, which is the precursor of<unk> , can be given because it can cross the blood-brain barrier.
Dopamine
Octopamine
Norepinephrine
Synephrine
Growth hormone-releasing hormone
Acetylcholine
Somatropin
Cosyntropin