Brain Research 879 2000 139–147 www.elsevier.com locate bres
Research report
Ovarian steroids influence the activity of neuroendocrine dopaminergic neurons
Jamie E. DeMaria, John D. Livingstone, Marc E. Freeman
Program in Neuroscience , Department of Biological Science, Florida State University, Tallahassee, FL 32306-4340, USA
Accepted 25 July 2000
Abstract
The secretion of prolactin PRL from the anterior lobe AL of the pituitary gland is tonically inhibited by dopamine DA of hypothalamic origin. While ovarian steroids play a role in the regulation of the secretion of PRL, their effect on all three populations of
hypothalamic neuroendocrine dopaminergic neurons is not fully understood. In this study we describe the effects of ovarian steroids on regulation of the release of DA from tuberoinfundibular dopaminergic TIDA, tuberohypophyseal dopaminergic THDA and
periventricular-hypophyseal dopaminergic PHDA neurons. Adult female rats were bilaterally ovariectomized OVX and, 10 days following ovariectomy day 0, injected with corn oil vehicle, estrogen, or estrogen plus progesterone day 1. Animals were sacrificed
every 2 h from 09.00 to 21.00 h by rapid decapitation. Trunk blood was collected and the concentration of PRL in serum was determined by radioimmunoassay. The median eminence ME and the AL, intermediate IL and neural NL lobes of the pituitary gland were
dissected and the concentration of DA and DOPAC in each was measured by HPLC-EC. OVX rats presented small but significant increases in the secretion of PRL at 15.00 and 17.00 h. Replacement of estrogen or estrogen plus progesterone increased the basal
concentration of PRL. Moreover, injection of estrogen only, or estrogen plus progesterone increased the concentration of PRL in serum at 15.00 h through 19.00 h, respectively, followed by a decrease to baseline thereafter. The turnover of DA in the ME and NL of OVX rats
increased at 13.00 and returned to low levels. Turnover of DA in the IL of OVX rats increased in the morning by 11.00 h and remained elevated before decreasing by 17.00 h. The turnover of DA in the ME, IL and NL of OVX rats increased by 19.00 h. Injection of estrogen
advanced the increase of TIDA activity by 2 h in the ME compared to OVX rats. Moreover, administration of estrogen suppressed the activity of THDA and PHDA neurons in the afternoon compared to OVX rats. In estrogen plus progesterone-treated rats, the activity of
hypothalamic neuroendocrine dopaminergic neurons terminating in the ME, IL, and NL was inhibited prior to the increase in the secretion of PRL. The concentration of DA in the AL diminished prior to the estrogen-induced increase of PRL. Administration of progesterone, in
concert with estrogen, delayed the increase of PRL in serum and the decrease of DA in the AL, compared to estrogen-treated rats, by 4 h. These data suggest a major role for ovarian steroids in controlling increases in the secretion of PRL by not only stimulating PRL release
from lactotrophs, but also by inhibiting the activity of all three populations of hypothalamic neuroendocrine DAergic neurons.
2000
Elsevier Science B.V. All rights reserved.
Theme : Endocrine and autonomic regulation
Topic : Neuroendocrine: other
Keywords : Estrogen; Prolactin; Pituitary gland; Median eminence; Arcuate nucleus; Periventricular nucleus
1. Introduction gland is stimulated by hormone-specific releasing factors
originating within the hypothalamus [25,46–48]. While a With the exception of prolactin PRL, the secretion of
single PRL releasing factor, analogous to those found for hormones from the anterior lobe AL of the pituitary
other pituitary hormones, has yet to be identified, several candidates, including ovarian steroids, have been consid-
ered [41].
Corresponding author. Tel.: 11-850-644-3896; fax: 11-850-644-
The secretion of PRL is tonically inhibited by dopamine
4583. E-mail address
: freemanneuro.fsu.edu M.E. Freeman.
DA [4]. DA arrives at the AL through three distinct
0006-8993 00 – see front matter
2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 0 6 - 8 9 9 3 0 0 0 2 7 6 3 - 3
140 J
dopaminergic DAergic pathways: 1 Tuberoinfundibular libitum. Animal procedures were approved by the Florida
DAergic TIDA neurons arise from throughout the arcuate State University Animal Care and Use Committee. Rats
nucleus and terminate on the primary capillary loops of the were bilaterally ovariectomized OVX under Halothane
long portal vessels within the external zone of the median anesthesia and subsequently divided into three groups. Ten
eminence ME [22]. Long portal vessels transport DA days following OVX, rats in group one estrogen-treated
released from
TIDA neurons
to the
AL. 2
received an injection of estrogen 20 mg, i.p., Sigma, St Tuberohypophyseal DAergic THDA neuron cell bodies
Louis, MO at 10.00 h on day zero and corn oil at 12.50 h are located in the rostral portion of the arcuate nucleus and
on day one. Rats in group two received an injection of terminate in both the intermediate IL and neural NL
estrogen 20 mg, i.p., Sigma at 10.00 h on day zero and an lobes of the pituitary gland [6]. III Periventricular-hypo-
injection of progesterone 5 mg, i.p., Sigma at 12.50 h on physeal DAergic PHDA neurons originate in the periven-
day one. Rats in group three received an injection of corn tricular nucleus of the hypothalamus and terminate exclu-
oil 200 ml, i.p., vehicle at 10.00 h on day zero and 12.50 sively in the IL [23,24]. DA released from THDA and
h on day one. Five rats from each treatment group were PHDA neurons is transported to the AL through short
sacrificed by rapid decapitation every 2 h from 09.00 to portal vessels.
21.00 h on day one. Rats decapitated at 13.00 h on day 1 The increasing concentration of estradiol in serum on
had received corn oil or progesterone 10 min before the afternoon of diestrus-2 of the estrous cycle of the rat
decapitation. The ME, AL, IL, and NL from each animal stimulates the proestrous increase of PRL in serum [40].
were dissected and individually stored in cryogenic vials Immunoneutralization of endogenous estrogen during the
with 250 ml of homogenization buffer [0.2 N perchloric afternoon of diestrus-2 prevents the increase of PRL on the
acid, 26 mM EGTA, 700 pM dihydroxybenzylamine afternoon of proestrus [20,40]. In addition to stimulating
DHBA, internal standard] at 2408C until day of assay. the secretion of PRL [41], estrogen diminishes and or
Trunk blood was collected, allowed to clot, centrifuged, reverses the inhibitory response of lactotrophs to DA
and serum was removed and stored at 2408C until the [8,12,19,34], recruits cells within the pituitary gland to
concentration of PRL was measured by RIA. produce [31] and secrete [7,18,26] PRL and increases
transcription of the PRL gene [30–32,36,38,50–53]. Un- like estrogen, the role of progesterone in the regulation of
2.2. HPLC-EC the secretion of PRL is not as thoroughly defined. How-
ever, when administered in concert with estrogen, proges- ME, AL, IL, and NL samples were thawed, homogen-
terone is a potent stimulator of the secretion of PRL [10]. ized, and sonicated. Twenty microliters of homogenate
It has previously been shown that the activity of were removed for protein assay. The remaining sample
dopaminergic neurons terminating in the ME [42] and the was centrifuged 10 min 10003g. The supernatant was
concentration of DA in portal vessels is decreased [5] prior filtered through a 0.2 mm nylon microfiltration unit Os-
to the peak of PRL on the afternoon of proestrus. In monics, Livermore, CA, and then placed into autosampler
addition, we previously reported that the concentration of vials.
DA and DOPAC decreases in the AL and IL, but not the The concentration of DA and DOPAC in each sample
NL prior to increased secretion of PRL on the afternoon of was measured using high performance liquid chromatog-
proestrus [16]. Moreover, it has been shown that ovarian raphy coupled to electrochemical detection HPLC-EC, as
steroids regulate biosynthesis [2,3,28,49,55] and release previously described [15–17,39]. Twenty microliters of
[13,14] of DA from TIDA neurons. each sample was injected by an autosampler WISP 710
In this study we have described the effects of ovarian Autosampler, Waters Corp., Milford, MA. Mobile phase
steroids on the activity of all three populations of hypo- consisting of 75 mM sodium dihydrogen phosphate mono-
thalamic neuroendocrine dopaminergic neurons. Changes hydrate EM Science, Gibbstown, NJ, 1.7 mM 1-octane
in the concentrations of DA and DOPAC in the ME, AL, sulfonic acid Fisher Scientific, 100 ml l triethylamine
IL, and NL and the concentration of PRL in serum were Aldrich, Milwaukee, WI, 25 mM EDTA Fisher Sci-
monitored following administration of estrogen and es- entific, 6 acetonitrile EMScience, titrated to pH 3.0
trogen plus progesterone in ovariectomized rats. with phosphoric acid Fisher Scientific, was delivered by
a dual piston pump Kratos Analytical Instruments, Ram- sey, NJ at 600 ml min. Water was purified on a Milli-Q
2. Materials and methods system Millipore, Bedford, MA to 18 MV and polished
