Prolactin is one of the hormones produced by the anterior pituitary gland. It has a variety of roles but is particularly important in breast development and production of breast milk in females.
This article will discuss prolactin, how its secretion is regulated and relevant clinical conditions to the dysfunction of this hormone axis.
Prolactin (PRL) is produced by the lactotroph cells of the anterior pituitary gland. There is no stimulatory hormone produced by the hypothalamus.
Functions of Prolactin
Prolactin has well known physiological functions in females only. Gradual increases in PRL during pregnancy, along with high oestrogen and progesterone levels result in full breast development.
The main actions of prolactin are listed below:
- Initiate milk production by alveolar cells (lactogenic)
- Maintain milk production once it has been established (galactopoietic)
- Proliferation of alveolar and duct cells (mammogenic)
Prolactin has primarily lactogenic and galactopoietic effects.
Regulation of Prolactin Secretion
Prolactin release is inhibited by dopamine (DA), known in this context as a PRL-inhibitory factor (PIF).
Suckling is the most powerful stimulus for PRL release. Stimulation of the nipple sends signals via afferent neural pathways up through the spinal cord. This inhibits dopaminergic neurons in the hypothalamus. Thus, there is inhibition of an inhibitory neurotransmitter. This is known as disinhibition.
Conversely, thyrotropin releasing hormone (TRH), also from the hypothalamus, stimulates the release of prolactin. Furthermore, oestrogens increase the sensitivity of lactotrophs to TRH, as well as decreasing their sensitivity to dopamine inhibition.
Note: the prolactin axis does not technically have a negative feedback system. Without inhibition by dopamine, prolactin would be secreted indefinitely.
Clinical Relevance – Breastfeeding
Lactation generally inhibits normal ovarian cycles. It decreases the release of GnRH by neurons in the hypothalamus. This causes decreased FSH and LH release, which are responsible for normal ovarian function.
Nursing the infant will not indefinitely cease ovarian cycles, and eventually they will resume regardless of whether the infant is nursed or not. If the mother does not nurse the infant, ovulatory cycles will generally resume 8-10 weeks after delivery.
Clinical Relevance – Hyperprolactinaemia
Hyperprolactinaemia is a state of raised prolactin levels. It can cause:
- Menstrual disturbances – amenorrhoea or oligomenorrhoea
- Galactorrhoea (leaking of milk through the nipples)
- hypogonadism causing infertility and erectile dysfunction, as well as osteoporosis
The causes can be remembered as the 5 Ps:
- Physiological – breastfeeding, stress, acute rises post-intercourse
- Pharmacological e.g. dopamine antagonists, antipsychotics (due to dopamine antagonist effects)
- Prolactinoma (a prolactin producing tumour in the pituitary)
- Polycystic Ovarian Syndrome (PCOS). The mechanism is not exactly known but is thought to be due to raised oestrogen levels having a stimulatory effect on prolactin production
Diagnosis is based on finding the underlying cause and treatment will be tailored accordingly.