Other tissues and cells of the immune system

A variety of effects of prolactin has been described on normal tissues and cells of the immune system, deriving from rats, mice and humans [21,22], The nature of the effects varies they can be either stimulatory or inhibitory, depending on the system studied and the concentration of prolactin. Thus, in the rat, hypophysectomy leads to impairment of the immune response which can be restored by injection of prolactin [118]. On the other hand, excessively high levels of prolactin can also give rise to an impaired immune response. 

Prolactin has been shown to enhance the stimulation of T lymphocyte proliferation induced by mitogens in vitro [119]. A possible role for cyclic GMP in the process has been suggested. The effect of the hormone depends on the amount of 

A variety of studies on the binding of labelled prolactin to cells or membrane preparations from immune tissues have been described, with rather variable results. In some cases, at least, substantial binding has been shown to membrane fractions from spleen and thymus and to human lymphocytes [21,120,121]. It has been reported that this binding can be enhanced or blocked by the immunosuppressive drug cyclosporin A [21,120-122], and a mechanism for this drug involving such inhibition of prolactin binding has been proposed. 

Cyclosporin A may also block some of the actions of prolactin, including effects on ornithine decarboxylase activity in vivo in spleen and thymus [21], providing further support for the idea that the actions of this immunosuppressant may be at least partly explained by interference with the normal function of prolactin. 

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