Oxytocin action at the lactotroph is required for prolactin surges in cervically stimulated ovariectomized rats

Abstract

Cervical stimulation induces two daily rhythmic prolactin surges, nocturnal and diurnal, which persist for several days. We have shown that a bolus injection of oxytocin initiates a similar prolactin rhythm, which persists despite low levels of oxytocin after injection. This suggests that oxytocin may trigger the cervical stimulation-induced rhythmic prolactin surges. To investigate this hypothesis, we infused an oxytocin antagonist that does not cross the blood-brain barrier for 24 h before and after cervical stimulation and measured serum prolactin. We also measured dopaminergic neuronal activity because mathematical modeling predicted that this activity would be low in the presence of the oxytocin antagonist. We thus tested this hypothesis by measuring dopaminergic neuronal activity in the tuberoinfundibular, periventricular hypophyseal, and tuberohypophyseal dopaminergic neurons. Infusion of oxytocin antagonist before cervical stimulation abolished prolactin surges, and infusion of oxytocin antagonist after cervical stimulation abolished the diurnal and significantly decreased the nocturnal surges of prolactin. The rhythmic prolactin surges returned after the clearance of the oxytocin antagonist. Hypothalamic dopaminergic activity was elevated in antiphase with prolactin surges, and the antiphase elevation was abolished by the oxytocin antagonist in the tuberoinfundibular and tuberohypophyseal dopaminergic neurons, consistent with the mathematical model. These findings suggest that oxytocin is a physiologically relevant prolactin-releasing factor. However, the cervical stimulation-induced prolactin surges are maintained even in the absence of oxytocin actions at the lactotroph, which strongly suggests the maintenance of prolactin surges are not dependent upon oxytocin actions at the pituitary gland.

Figure 1

Connections within the network model. A filled arrow represents a stimulatory influence, while an open arrow represents an inhibitory influence. The connection from lactotrophs to DAergic neurons is delayed by τ = 3 h. The dashed arrow represents an indirect inhibitory connection. VIP input to DAergic neurons is applied periodically in the morning of each day for 3 h. A version of this model was described in an earlier publication (25).

Figure 2

Model simulation. (A) Cervical stimulation on day 0 induces a circadian PRL rhythm. (B) DAergic neuronal activity peaks at noon (asterisks) and is out of phase with the PRL surges. VIP surges (dashed) occur every morning for 3 h and have an inhibitory action on DAergic neurons. The DAergic neuronal activity time course has been translated upward by one unit for clarity. (C) Cervical stimulation results in a surge in OT that triggers PRL surges. Cervical stimulation also activates OTergic neurons in the paraventricular nucleus, which provide stimulatory drive to the lactotrophs. The lactotrophs feedback onto and inhibit the OTergic neurons, producing an OT rhythm. (D-F) Same variables, but with an OT antagonist present through day 3 (simulated by setting ν _o = 0 ). Once the OT antagonist is removed, DAergic neurons interact with lactotrophs to produce a circadian rhythm in PRL and DA, with DAergic neuronal activity peaking at noon (asterisks).

Figure 3

Rhythmic secretion of PRL on Day 2 after 24h infusion of the OT antagonist (dotted line) or saline (solid line) in OVX rats before/during cervical stimulation. Infusion of the OT antagonist 24h before cervical stimulation prevented initiation of PRL surges. Values are expressed as mean ng/mL of PRL ± SE (n=3-10 serial samples/point). *Significantly lower PRL levels than corresponding times in saline infused animals (P < 0.05). There were no significant elevation in PRL secretion at any time in the OT antagonist-treated group. ^aStatistical difference from all other time points within the saline infused animals.

Figure 4

Rhythmic secretion of PRL when the OT antagonist infusion (dotted line) or saline infusion (solid line) began after cervical stimulation in OVX rats. The OT antagonist decreases the nocturnal and abolishes the diurnal surges of PRL when infused after cervical stimulation. Values are expressed as mean ng/ml of PRL ± SE (n=3-10 serial samples). *Significantly lower PRL levels than corresponding times in saline infused animals (P < 0.05). ^aStatistical difference from all other time points within the OT antagonist or saline infused animals.

Figure 5

Rhythmic secretion of PRL returns two days after cessation of the OT antagonist infusion. OVX-cervically stimulated rats were infused with the OT antagonist (dotted line) or saline (solid line) for 24 h as in Figure 3. Blood samples were obtained for 24 h beginning 30 h after cessation of infusion. Values are expressed as mean ng/ml of PRL ± SE (n=3-10 serial samples). There were no differences in serum concentrations of PRL within any time point between animals infused with saline or OT antagonist. ^aStatistical difference from all other time points within the saline or OT antagonist infused animals.

Figure 6

DOPAC:DA ratio in the median eminence, intermediate lobe, and neural lobe indicating DA neuronal activity of the TIDA, PHDA, THDA, respectively of OVX, OVX-cervically stimulated, and OVX-cervically stimulated/OT antagonist treated rats at 0900 h, 1200 h, and 1700 h. Cervical stimulation induces an increase in DA neuronal activity in the TIDA and THDA neurons at 1200 h (P < 0.05), in anti-phase with PRL surges. The presence of the OT antagonist disrupts the anti-phasic DA neuronal activity. Values are expressed as mean DOPAC:DA ± SE (n=3-6). Astericks represent significant differences within anatomical area at the corresponding times between OVX, OVX-cervically stimulated, and OVX-cervically stimulated/OT antagonist (*P<0.05, **P<0.01). Dissimilar letters represent statistical differences between times within each treatment group.