PRL production is controlled by hormones produced in the hypothalamus. In response to either the presence or absence of a specific stimulus, a signal is sent to the brain that triggers the hypothalamus to secrete either a releasing hormone or an inhibitory hormone. The flowchart be shows the pathways for PRL production and regulation.

Here, the stimulus which is discussed in the question is the Suckling stimulus which is also known as Suckling reflex.  

Suckling by a child stimulates the suckling reflex system. The stimulus (information) reaches to the Hypothalamus. Hypothalamus triggers the TRH (thyrotropin-releasing hormone) which further triggers the releases of the Prolactin (PRL) hormone by the Anterior Pituitary.

Finally, when Prolactin reaches the blood, it stimulates the Mammary glands which being stimulated, start to produce milk.

Explanation:

The main function of Prolactin is to produce milk. Prolactin production is mainly controlled by Hypothalamus. Prolactin hormone is produced in Pituitary, uterus, etc. The prolactin hormone production is also controlled by the estrogen.

sorry, but you do not have posted the diagram.