PLASMA CONCENTRATIONS OF PROLACTIN AND
THYROTROPHIN DURING SUCKLING: EFFECTS OF STIMULATION OF THE MEDIAN EMINENCE J. B. WAKERLEY AND M. B. TER HAAR Institute A.R.C. of Animal Physiology, Babraham, Cambridge, CB2 4AT
(Received 1 November 1977)
Thyrotrophin releasing hormone (TRH) can have a stimulatory effect on the release of both prolactin and thyrotrophin (TSH; Deis & Alonso, 1973), although in the rat, supraphysiological doses of TRH are required to affect the secretion of prolactin (Burnet & Wakerley, 1976). A more important factor in the control of the release of prolactin is considered to be prolactin release inhibiting factor (PIF), which is thought to act through the catecholamine, dopamine (MacLeod, 1976). Stimuli which cause the concomitant release of TSH and prolactin are thought to have a direct effect at the hypothalamic level such that neurones releasing TRH are excited, whereas those releasing PIF are inhibited. In the present work, we have tested this hypothesis using the suckling stimulus to elicit the simultaneous release of prolactin and TSH (Blake, 1974; Burnet & Wakerley, 1976). If this hypothesis holds, electrical stimulation of the median eminence (where the relevant neurones containing TRH and PIF terminate) should potentiate the release of TSH while suppressing the release of prolactin. Experiments were performed on lactating (day 10) Wistar rats which had been separated from their pups overnight. The animals were anaesthetized with urethane (1-1 g/kg body weight, i.p.) and prepared for the collection of blood samples and the recording of intra¬ mammary pressure as described previously (Burnet & Wakerley, 1976). For electrical stimulation of the median eminence, the rats were implanted with a monopolar electrode (diameter 0-75 mm) made of stainless steel. The stereotaxic frame served as the indifferent electrode. The stimulating electrode was positioned under stereotaxic control using a dorsal approach to the median eminence. After implantation, the electrode was fixed to the skull with screws and dental cement and the animal was removed from the stereotaxic apparatus for the suckling test. This test consisted of applying a litter of ten pups to the mother's nipples for 3 h. Blood samples were collected 20 and 10 min before and at 30 min intervals after the start of the suckling period (see Fig. 1) and the concentrations of pro¬ lactin and TSH were determined by radioimmunoassay (Brown-Grant & ter Haar, 1977). In one group of rats, biphasic square-wave pulses (7-5 Hz, 2 ms duration, 600 µ ) were appiled to the median eminence for the 3 h period during which the pups were suckled. In the other groups (controls) no stimulation was applied during suckling. At the end of the experiment, the rats were killed and the position of the stimulating electrode confirmed histologically. The mean changes in the concentrations of prolactin and TSH in the plasma during suckling in the six stimulated and eight unstimulated rats are shown in Fig. 1. In the absence of electrical stimulation, the level of prolactin rose by 100 ng/ml within 1 h of the start of suckling and remained high thereafter. Stimulation of the median eminence greatly attenuated this response; the level of prolactin did not begin to rise until 2-5 h after the onset *
Present address:
Elsevier/North-Holland Biomedicai Press, 14A Regent Street, Cambridge, CB2 1DB.
200
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THYROTROPHIN DURING SUCKLING: EFFECTS OF STIMULATION OF THE MEDIAN EMINENCE J. B. WAKERLEY AND M. B. TER HAAR Institute A.R.C. of Animal Physiology, Babraham, Cambridge, CB2 4AT
(Received 1 November 1977)
Thyrotrophin releasing hormone (TRH) can have a stimulatory effect on the release of both prolactin and thyrotrophin (TSH; Deis & Alonso, 1973), although in the rat, supraphysiological doses of TRH are required to affect the secretion of prolactin (Burnet & Wakerley, 1976). A more important factor in the control of the release of prolactin is considered to be prolactin release inhibiting factor (PIF), which is thought to act through the catecholamine, dopamine (MacLeod, 1976). Stimuli which cause the concomitant release of TSH and prolactin are thought to have a direct effect at the hypothalamic level such that neurones releasing TRH are excited, whereas those releasing PIF are inhibited. In the present work, we have tested this hypothesis using the suckling stimulus to elicit the simultaneous release of prolactin and TSH (Blake, 1974; Burnet & Wakerley, 1976). If this hypothesis holds, electrical stimulation of the median eminence (where the relevant neurones containing TRH and PIF terminate) should potentiate the release of TSH while suppressing the release of prolactin. Experiments were performed on lactating (day 10) Wistar rats which had been separated from their pups overnight. The animals were anaesthetized with urethane (1-1 g/kg body weight, i.p.) and prepared for the collection of blood samples and the recording of intra¬ mammary pressure as described previously (Burnet & Wakerley, 1976). For electrical stimulation of the median eminence, the rats were implanted with a monopolar electrode (diameter 0-75 mm) made of stainless steel. The stereotaxic frame served as the indifferent electrode. The stimulating electrode was positioned under stereotaxic control using a dorsal approach to the median eminence. After implantation, the electrode was fixed to the skull with screws and dental cement and the animal was removed from the stereotaxic apparatus for the suckling test. This test consisted of applying a litter of ten pups to the mother's nipples for 3 h. Blood samples were collected 20 and 10 min before and at 30 min intervals after the start of the suckling period (see Fig. 1) and the concentrations of pro¬ lactin and TSH were determined by radioimmunoassay (Brown-Grant & ter Haar, 1977). In one group of rats, biphasic square-wave pulses (7-5 Hz, 2 ms duration, 600 µ ) were appiled to the median eminence for the 3 h period during which the pups were suckled. In the other groups (controls) no stimulation was applied during suckling. At the end of the experiment, the rats were killed and the position of the stimulating electrode confirmed histologically. The mean changes in the concentrations of prolactin and TSH in the plasma during suckling in the six stimulated and eight unstimulated rats are shown in Fig. 1. In the absence of electrical stimulation, the level of prolactin rose by 100 ng/ml within 1 h of the start of suckling and remained high thereafter. Stimulation of the median eminence greatly attenuated this response; the level of prolactin did not begin to rise until 2-5 h after the onset *
Present address:
Elsevier/North-Holland Biomedicai Press, 14A Regent Street, Cambridge, CB2 1DB.
200
800h-
150
5 600 ·?
(
tt
«
100
»
«
o
—
.
-
.
o
ß: >i 400
.£ 50
c CJ
