27 - Vasopressin Receptors

*The cellular effects of vasopressin (ADH) are mediated mainly by interactions of the hormone with the three types of receptors, V_1a, V_1b, and V₂.

*The V_1a receptor is the most widespread subtype of Vasopressin receptor; it is found in vascular smooth muscle, the adrenal gland, myometrium, the bladder, adipocytes, hepatocytes, platelets, renal medullary interstitial cells, vasa recta in the renal microcirculation, epithelial cells in the renal cortical collecting-duct, spleen, testis, and many CNS structures.

*V_1b receptors have a more limited distribution and are found in the anterior pituitary, several brain regions, the pancreas, and the adrenal medulla.

*V₂ receptors are located predominantly in principal cells of the renal collecting-duct system but also are present on epithelial cells in the thick ascending limb and on vascular endothelial cells.

*Although originally defined by pharmacological criteria, vasopressin receptors now are defined by their primary amino acid sequences.

*The cloned vasopressin receptors are typical heptahelical G protein–coupled receptors.

*Manning and coworkers (1999) have synthesized novel hypotensive vasopressin peptide agonists that do not interact with V_1a, V_1b, or V₂ receptors and may stimulate a putative vasopressin vasodilatory receptor. Finally, two additional putative receptors for vasopressin have been cloned.

*A vasopressin-activated Ca²⁺-mobilizing receptor with one transmembrane domain binds vasopressin and increases intracellular Ca²⁺. A dual angiotensin II–vasopressin heptahelical receptor activates adenylyl cyclase in response to both angiotensin II and vasopressin. The physiological roles of these putative vasopressin receptors are unclear.

26 - Nocturnal Penile Tumescence ( NPT )

Nocturnal penile tumescence (NPT) is the spontaneous occurrence of an erection of the penis during sleep. All men without physiological erectile dysfunction experience this phenomenon, usually three to five times during the night. It typically happens during REM sleep. NPT has been given numerous colloquial names which are typically related to the first time the erection is discovered, namely upon waking in the morning — such an erection is colloquially referred to as morning wood in North America, 'dawn horn' the United Kingdom, whilst in other areas morning glory is used. In the United Kingdom the colloquialism 'piss-proud' is often used to imply a causal relationship between a full bladder and an erection.


The existence and predictability of nocturnal tumescence is used by sexual health practitioners to ascertain whether a given case of erectile dysfunction (E.D.) is psychological or physiological in origin. A patient presenting with E.D. is fitted with an elastic device to wear around his penis during sleep; the device detects changes in girth and relays the information to a computer for later analysis. If nocturnal tumescence is detected, then the E.D. is presumed to be due to a psychosomatic illness such as sexual anxiety; if not, then it is presumed to be due to a physiological cause.


The cause of NPT is not known with certainty. Bancroft (2005) hypothesizes that the noradrenergic neurons of the locus ceruleus are inhibitory to penile erection, and that the cessation of their discharge that occurs during REM sleep may allow testosterone-related excitatory actions to manifest as NPT.


Colloquial naming and anecdotal evidence supporting the possibility that a full bladder can stimulate an erection has existed for some time and is characterised as a 'reflex erection'. The nerves that control a man’s ability to have a reflex erection are located in the sacral nerves (S2-S4) of the spinal cord. A full bladder is known to mildly stimulate nerves in the same region. This mild stimulus which during the day is normally suppressed in adult males by competing stimuli and other distractions, could during sleep with the absence of such factors instigate a reflex erection.


The possibility of a full bladder causing an erection, especially during sleep, is perhaps further supported by the beneficial physiological effect of an erection inhibiting urination, thereby helping to avoid nocturnal enuresis.