These axons (likely, but dendrites have not been ruled out) have collaterals that innervate neurons in the nucleus accumbens, a brain structure where oxytocin receptors are expressed. The endocrine effects of hormonal oxytocin and the cognitive or behavioral effects of oxytocin neuropeptides are thought to be coordinated through its common release through these collaterals. Oxytocin is also produced by some neurons in the paraventricular nucleus that project to other parts of the brain and to the spinal cord. Depending on the species, oxytocin receptor-expressing cells are located in other areas, including the amygdala and bed nucleus of the stria terminalis. In the pituitary gland, oxytocin is packaged in large, dense-core vesicles, where it is bound to neurophysin I as shown in the inset of the figure; neurophysin is a large peptide fragment of the larger precursor protein molecule from which oxytocin is derived by enzymatic cleavage. Secretion of oxytocin from the neurosecretory nerve endings is regulated by the electrical activity of the oxytocin cells in the hypothalamus. These cells generate action potentials that propagate down axons to the nerve endings in the pituitary; the endings contain large numbers of oxytocin-containing vesicles, which are released by exocytosis when the nerve terminals are depolarised.
Principal ideal domain, an algebraic structure
Process identifier, a number used by many operating systems to identify a process

Organisations
Police Intelligence Department, a staff department of the Singapore Police Force
Political Intelligence Department (disambiguation), multiple organisations
Politieke Inlichtingen Dienst (Political Intelligence Department), was the main security agency for the Dutch East Indies
Prague Integrated Transport (Pražská integrovaná doprava – PID in Czech), an integrated public transport system in Prague

Other uses
Pathways into Darkness, a 1993 video game by Bungie
"Paul is dead", a 1960s urban legend that Paul McCartney was dead
Pid (video game), a 2012 video game by Might and Delight
Project Initiation Documentation, in project management
Party identification, the political party with which an individual identifies
Everett Pid Purdy (1904–1951), American athlete who played in both Major League Baseball and the National Football League

See also
PIDS (disambiguation)