Mini Review Mathews Journal of Veterinary
Prolac n in Female Domes c Dogs: A Mini-Review
Gu érrez Rufo J1, Gazzano A1, Mari C1
1Dip.to Veterinary Sciences – University of Pisa, Viale delle Piagge 2, 56124 Pisa, Italy.
Corresponding Author: Mari C, Dip.to Veterinary Sciences – University of Pisa, Viale delle Piagge 2, 56124 Pisa, Italy, Tel:
0039-050-2216837; Email: chiara.mari @unipi.it Received Date: 14 Dec 2016
Accepted Date: 21 Dec 2016 Published Date: 26 Dec 2016
Copyright © 2016 Mari C
Cita on: Rufo JG, Gazzano A and Mari C. (2016). Prolac n In Fe-
male Domes c Dogs: A Mini-Review. M J Vetr. 1(1): 006.
Prolac n is a luteotrophic agent, ac ng from mid-luteal phase in both pregnant and non-pregnant animals. Its role in s mu- la ng and maintaing the corpus luteum, allowing the gesta on period to be maintained, as well as the role in preparing and maintaining lacta on (in concert with estrogens, progesterone, cor costeroids, somatropin and insulin) in the bitch has been recognized. Prolac n seems to be also involved in ensuring maternal behaviour, including the prepara ons for delivery and the care of the li er therea er.
Bitch; Dog; Estrus; Pregnancy; Prolac n.
Among domes c animals, the dog (Canis familiaris) is a mono- estric species, with a long oestrous cycle (12 months in most non-domes cated species; 7 ± 1.5 months in the dog) and with a period of about 4 months of anestrus, in which there is no ovarian ac vity . Reproduc on and pregnancy in dogs are regulated by many hormones, among which prolac n.
Prolac n (PRL) is a single-chain pep de hormone, composed of almost 200 amino acids, produced by the lactotrotropic cells of the anterior pituitary . Prolac n secre on is pul- sa le and this pulse rate is probably a result of the combined e ect of di erent hypothalamic factors. Dopamine ac vates prolac n synthesis. Also thyrotropin-releasing factor (TRH), serotonin, vasoac ve intes nal pep de (VIP), and estrogen substances ac vate prolac n synthesis, but about 100 mes less than dopamine does. Gamma-aminobutyric acid (GABA) inhibits the synthesis of prolac n with an inhibitory e ect 100-fold lower than the ac va ng e ect of prolac n.
In dogs, prolac n is secreted in a pulsa le fashion  and most prolac n is released during the second half of the luteal phase. Prolac n is an essen al luteotropin in the dog from mid-luteal phase in both pregnant and non-pregnant animals. It appears to act by sustaining corpus luteum lifespan and func on rather than by direct s mulatory e ects on proges- terone secre on . Indeed, prolac n s mulates and main- tains the corpus luteum of the ovary, which is the source of
progesterone, allowing the gesta on period to be maintained from the second half of the pregnancy. However, prolac n does not act alone; during pregnancy, prolac n, estrogen and progesterone s mulate the development of the mammary gland.
Originally, the observa on in the bitch that PRL blood values rise during the second half of pregnancy, while progesterone values were simultaneously declining, led to the conclusion that PRL might have an an -luteotropic e ect . The use of PRL inhibi ng drugs, i.e. dopamine agonists like bromocrip – ne and cabergoline during the second half of pregnancy have shown that PRL must have a luteotrophic e ect as their use results in an immediate sharp drop in the blood PRL concen- tra ons followed by a drop in levels of blood progesterone . In addi on, using the same PRL-inhibitors in the treat- ment of dogs with clinical anoestrus (abnormally prolonged anoestrous period) reliably induces a fer le oestrus .
In pregnant bitches, the plasma prolac n concentra on starts to rise about 1 month a er ovula on, which is when the plasma progesterone concentra on begins to decline. Also in healthy cyclic bitches, most prolac n is released during the second half of the luteal phase. The changes in growth hor- mone (GH) and prolac n release during the luteal phase may promote the physiological prolifera on and di eren a on of mammary gland ssue in the bitch. In the early part of the luteal phase progesterone-induced mammary GH ini ates
Cita on: Rufo JG, Gazzano A and Mari C. (2016). Prolac n In Female Domes c Dogs: A Mini-Review. M J Vetr. 1(1): 006. 1
prolifera on of the mammary epithelium, whereas in the late luteal phase, when progesterone concentra ons decrease, prolac n release increases and promotes lobulo-alveolar dif- feren a on . Hence, the declining plasma progesterone concentra ons during the second half of the luteal phase ap- pear to in uence prolac n secre on.
In monogastric animals, such as dogs, cats, primates and women, prolac n is also fundamental during the prepara on and maintenance of lacta on a er birth. As a ma er of fact, a er delivery and during lacta on, prolac n levels con nue to be elevated .
Prolac n allows the growth and di eren a on of the mam- mary gland, maintains segrega on of breast milk, reduces fer lity during the lacta on period, facilitates immunologi- cal regula on in the female, as well as an exchange of water and electrolytes during pregnancy. The tac le s mula on of the nipple or breast in the mother a er birth inhibits the release of dopamine into the hypothalamus, increasing the concentra on of prolac n in blood. On the other hand, high concentra ons of prolac n inhibit the secre on of gonado- tropin-releasing hormone (LH, luteinizing hormone and FSH, follicle-s mula ng hormone), which prevents gonadotropins to act on the gonads. Thanks to this mechanism, fer lity levels are reduced during lacta on, preven ng females to become pregnant while feeding their o spring .
In pregnant bitches, plasma prolac n levels exceed 100 ng/ ml . Weaning, which can be de ned as the phase of mater- nal care during which lacta on decreases most rapidly , causes a decrease in prolac n blood values.
The use of potent prolac n inhibitors, mostly dopamine ago- nists like bromocrip ne, metergoline and cabergoline, has con rmed that prolac n is the luteotropic hormone from day 30 of pregnancy onward and that prolac n is essen al for the prepara on, commencement and maintenance lacta on, as well as for the ac va on maternal of maternal and sexual be- havior . Prolac n seems to be involved in ensuring mater- nal behaviour, including the prepara ons for delivery and the care of the li er therea er, although it is not yet clear how it shares these e ects with oxytocin .
Prolac n secre on is under the tonic inhibitory control of hy- pothalamic dopaminergic neurons and the s mulatory ac on of estrogens, with a number of other hypothalamic hormones playing a modulatory role in the control of prolac n secre- on . In most vertebrates, neuroendocrine func ons are organized in regular cycles of di erent periodicity. Hormone secre on pa erns are mainly regulated endogenously, al- though they are also under environmental in uence . For
instance, seasonal varia ons in temperature and photoperiod have an in uence on circannual rhythms of reproduc ve sea- sonal species . Also dogs show a circannual rhythmicity of prolac n , whose func on is unknown; it may be inherited from wolves func onally altered through domes ca on. Male and female dogs and wolves show almost iden cal seasonal changes in prolac n blood concentra ons, with peak levels before mid-year and the nadir just before the year’s end .
The rise in prolac n produc on during spring and towards the summer peak causes social tensions to subside and as- sures care for the o spring by females and males alike. This seasonality in canine prolac n produc on is maintained fol- lowing pinealectomy . This indicates the existence of a strong gene c base for prolac n’s annual rhythm which might be in uenced by melatonin but is neither triggered nor con- trolled by pineal hormones. This explains the persistence of the physiological pseudopregnancy in the bitch, although her reproduc ve cycle has shortened and no longer coincides with the seasons .
Informa on regarding the presence of a circadian rhythm of prolac n secre on in bitches is scarce, but Gobello et al.  have reported that in most anoestrous bitches there appears to be no circadian rhythm of prolac n secre on. Furthermore, Corrada et al.  concluded that there are ultradian di er- ences between male and female dogs.
In certain cases, it is possible to nd hyperprolac nemia, which is the presence of high levels of prolac n in blood. In- creased prolac n secre on may be caused by damage to the hypophysial stem that disrupts the ow of dopamine from the hypothalamus or even triggers tumours. There may also be increased prolac n due to physiological disorders, high blood pressure or aches . In some species which require prolac- n in their reproduc ve processes, like dogs, as well as in hu- mans, where the need for prolac n as a hormone for repro- duc on is not established, hyperprolac nemia inhibits male and female reproduc on .
In conclusion, the role of prolac n as a luteotrophic agent, as well as an essen al hormone for preparing and maintaining lacta on (in concert with estrogens, progesterone, cor co- steroids, somatropin and insulin) in the bitch has been recog- nized. Prolac n seems to be involved in ensuring maternal be- haviour, including the prepara ons for delivery and the care of the li er therea er, although it is not yet clear how it shares these e ects with oxytocin.
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Cita on: Rufo JG, Gazzano A and Mari C. (2016). Prolac n In Female
Domes c Dogs: A Mini-Review. M J Vetr. 1(1): 006. 3