An anatomical substrate for the spatiotemporal transformation.

The purpose of the present experiments was to test the hypothesis that the metrics of saccades caused by the activation of distinct collicular sites depend on the strength of their projections onto the burst generators. This study of morphofunctional correlations was limited to the horizontal components of saccades. We evoked saccades by stimulation of the deeper layers of the superior colliculus (SC) in alert, head-fixed cats. We used standard stimulus trains of 350 msec duration, 200 Hz pulse rate, and intensity set at two times saccade threshold in all experiments. Evoked saccades were analyzed quantitatively to determine the amplitude of the horizontal component of their "characteristic vectors". This parameter is independent of eye position and was used as the physiological, saccade-related metric of the stimulation sites. Anatomical connections arising from these sites were visualized after anterograde transport of biocytin injected through a micropipette adjoining the stimulation electrode. The stimulation and injection sites were, therefore, practically identical. We counted boutons deployed in regions of the paramedian pontine reticular formation reported to contain long-lead and medium-lead burst neurons of the horizontal burst generator. Regression analysis of the normalized bouton counts revealed a significant positive correlation with the size of the horizontal component of the characteristic vectors. This data supports a frequent modelling assumption that the spatiotemporal transformation in the saccadic system relies on the graded strength of anatomical projections of distinct SC sites onto the burst generators.

Stimulatory effect of gonadotropin-releasing hormone (GnRH) on in vitro prolactin secretion and presence of GnRH specific receptors in a subset of human prolactinomas.

The purpose of this study was to determine whether gonadotropin-releasing hormone (GnRH) may exert a direct action on human prolactinomas. On a series of 17 adenomas, we studied the effect of GnRH on the in vitro prolactin (PRL) secretion of dispersed and perifused cells of 10 cases and the [125I]GnRH agonist binding on frozen sections of three out of the adenomas studied in perifusion and on the membrane preparations of seven other cases. Two 20-min pulses of GnRH (10(-7) mol/l) stimulated the in vitro PRL secretion of three adenomas out of 10 (increase of 200, 444 and 205%, respectively, above basal levels). The GnRH receptors of three adenomas bound GnRH agonist (Des-Gly10-(D-Ala6)-GnRH ethylamide). The binding was specific, with a high affinity (Kd = 0.60, 0.48 and 0.40 nmol/l) similar to that of two human anterior pituitaries obtained post-mortem (Kd = 0.70 and 0.40 nmol/l). Indirect immunoperoxidase revealed that the majority of the cells (60-90%) in all the adenomas studied contained immunoreactive PRL. Four of them also contained cells immunoreactive to the alpha-subunit of the glycoprotein hormones. In none of the prolactinomas were cells immunoreactive to antiserum of anti-beta-luteinizing hormone, anti-beta-follicle-stimulating hormone or anti-beta-thyrotropin. All the prolactinomas that were responsive to GnRH in perifusion experiments and/or bound specifically to [125I]GnRH agonist were also immunoreactive for alpha-subunit. These results show that GnRH, via GnRH specific receptors, exerts a stimulation on in vitro PRL secretion in a subset of prolactinomas characterized by the presence of alpha-subunit.

Thyrotropin-releasing hormone (TRH) binding sites and thyrotropin response to TRH are regulated by thyroid hormones in human thyrotropic adenomas.

In order to see whether, in thyrotropic adenomas with thyrotoxicosis, plasma thyroid hormones regulate the thyrotropin-releasing hormone (TRH) binding sites and the thyrotropin (TSH) response to TRH, we investigated: the presence of TRH binding sites in two cases of thyrotropic adenomas associated with hyperthyroidism and in one case of thyrotropic adenoma secondary to thyroid failure: and the in vitro effect, in a perifusion system, of triiodothyronine (T3) on the response of TSH to TRH in three cases of TSH-secreting adenomas associated with hyperthyroidism. The TRH binding sites were absent in the adenomas associated with high levels of circulating thyroid hormones, whereas they were present in the adenoma secondary to primary thyroid failure (Kd = 47 nmol/l, Bmax = 40 nmol/kg membrane proteins). In vitro, the three adenomas spontaneously released TSH in the perifusion medium (1.49 +/- 0.06 (mean +/- SEM), 7.25 +/- 0.12 and 16.73 +/- 0.36 mIU.l-1 x 10(6) cells-1 x 2 min-1) and exhibited an ample TSH response to 10(-7) mol/l TRH pulses. In two cases, tumoral secretion of fragments was compared with those of fragments maintained since the time of surgical removal in the presence of 10(-8) mol/lT3. The TSH responses to TRH were abolished in the presence of T3 in these two cases. We conclude that thyrotropic adenomas associated with hyperthyroidism are still controlled in vivo by T3. In particular, T3 regulates the TSH response to TRH, probably via a down-regulation of the TRH binding sites.

Neuropeptides of anterior pituitary origin. Autocrine or paracrine functions?

Several neuropeptides classically associated with the hypothalamus have been found in the anterior pituitary. The question arises whether they are locally synthesized and if they play a paracrine or autocrine role on pituitary cell functions. Using normal and tumoral human pituitaries we found neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries, while stimulatory hormones (TRH and GHRH) were predominantly found in tumoral tissue, implying an imbalance of pathophysiological importance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. The in situ synthesis was demonstrated for SRIH by the evidence of SRIH mRNA, the detection of SRIH immunoreactivity in peculiar cells and the presence of SRIH precursor. The possible role of these pituitary neuropeptides was suggested for instance by the negative correlation found in vitro between SRIH and GH secretions. Moreover, neuropeptides could interact with each other. Indeed DA stimulated TRH release while PRL secretion decrease at the same time. Pulses of TRH had differential effects on SRIH release according to the nature of the tissue as TRH inhibited SRIH release from adenoma while it stimulated SRIH release from normal pituitary. Concerning the effects of SRIH and GHRH on GH secretion, there was an endogenous regulatory pattern comparable to that observed in rat portal blood vessels. Pulses of GHRH induced GH secretion only when endogenous SRIH release was not stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal and adenomatous human pituitaries secrete thyrotropin-releasing hormone in vitro: modulation by dopamine, haloperidol, and somatostatin.

TRH is present in human normal pituitaries and in pituitary adenomas. In this study we demonstrated that the same tissues can release TRH in vitro. Fragments from seven normal pituitaries (10-15 mg/syringe) and dispersed cells from eight prolactinomas, four GH-secreting and two nonsecreting adenomas (1-3 x 10(6) cells/syringe) were perifused using a Krebs-Ringer culture medium. After 1 h of equilibration the perifusion medium was collected every 2 min (1 mL/fraction) for 3 h. TRH, PRL, and GH were measured by RIA under basal conditions and in the presence of 10(-10) to 10(-6) mol/L dopamine (DA), alone or concomitant with haloperidol, or in the presence of 10(-10) or 10(-6) mol/L somatostatin. Both normal pituitary fragments and pituitary adenomatous cells (from all types of adenomas studied) spontaneously released TRH in vitro. TRH was detected in the perifusion medium either immediately after the end of the equilibration period or 30-60 min later. The molecular identity of TRH was assessed by high pressure liquid chromatography. There was no difference in the profile and the rate of TRH secretion between normal and tumoral tissues, and no correlation was found between the level of TRH release and that of PRL or GH secretion. DA stimulated TRH release from normal pituitaries and from PRL- and GH-secreting adenomas at doses as low as 10(-10) mol/L. A concomitant decrease in PRL and GH release was observed from adenomatous cells and in one case of normal tissue. Haloperidol (10(-7) mol/L) antagonized the effect of 10(-8) mol/L DA on both TRH and PRL secretion in normal pituitary and in prolactinomas. DA had no effect on TRH release from two nonsecreting tumors. The amounts of TRH released during 1 h of perifusion were 60-1640 pg/2 mg wet wt tissue in normal pituitaries and 54-2174 pg/10(6) cells in adenomas; these values were very high compared to those precedently reported within the tissues. These results indicate that pituitary cells can release TRH in vitro and suggest that TRH might be synthesized in situ. We suggest that TRH could act on pituitary hormone secretion and/or cell proliferation via a paracrine and/or an autocrine mechanism.

Changes of prolactin response to dopamine during the rat estrous cycle.

The effects of dopamine (DA) on prolactin (PRL) secretion in anterior pituitary tissue from rats selected during various stages of the estrous cycle were analyzed under in vitro conditions. The results were examined in relation to plasma PRL, estradiol and progesterone levels. During the estrous cycle there was a marked variation in the responsiveness of the lactotrophs to the inhibitory action of DA. Rapid changes occurred during proestrus: pituitary lactotrophs were not sensitive to the inhibitory action of 10 nM DA at 15.00 and 17.00 h and were less sensitive to 1 microM DA compared to 09.00 h (p less than 0.01), 12.00 h (p less than 0.05) and 19.00 h (p less than 0.05). This lowest PRL response to DA was associated with the preovulatory PRL surge. The recovery of a higher PRL response at 19.00 h coincided with the decrease of plasma PRL levels. During the remainder of the cycle, plasma PRL levels remained low in estrus, diestrus 1 and diestrus 2. Lactotrophs were sensitive to 1 microM and 10 nM DA during estrus and diestrus 2 but a significant lower PRL response to 1 microM DA (p less than 0.05) and no PRL response to 10 nM DA was observed in diestrus 1. These data show that alterations in the sensitivity of the lactotrophs' responsiveness to DA occur in the anterior pituitary gland during the rat estrous cycle and can lead to the removal of DA inhibition in the presence of physiological DA concentrations. This phenomenon is consistent with the fact that DA could be involved in the preovulatory PRL surge during the estrous cycle.

[Neuropeptides of anterior pituitary origin]. / Neuropeptides d'origine antéhypophysaire.

Several neuropeptides classically associated with the hypothalamus have been found in the anterior pituitary. The question arises whether they are locally synthesized and if they play a paracrine or autocrine role on pituitary hormone secretion. Using normal and tumoral human pituitaries we found neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries, while stimulatory hormones (TRH and GHRH) were predominantly found in tumoral tissue, implying an imbalance of pathophysiological importance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. The in situ synthesis was demonstrated for SRIH by the evidence of SRIH mRNA, the detection of SRIH immunoreactivity in peculiar cells and the presence of SRIH precursor. The possible role of these pituitary neuropeptides was suggested for instance by the negative correlation found in vitro between SRIH and GH secretions. Moreover neuropeptides could interact on each other. Indeed DA stimulated TRH release while PRL secretion decreased at the same time. Pulses of TRH had differential effects on SRIH release according to the nature of the tissue as TRH inhibited SRIH release from adenoma while it stimulated SRIH release from normal pituitary. Concerning the effects of SRIH and GHRH on GH secretion, there was an endogenous regulatory pattern comparable to that described in rat portal blood vessels. Pulses of GHRH induced GH secretion only when endogenous SRIH release was not stimulated.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of oestradiol on dopamine receptors and protein kinase C activity in the rat pituitary: binding of oestradiol to pituitary membranes.

Oestradiol exerts an important modulatory influence on the release of prolactin which is accomplished partly through disruption of the inhibitory influence of dopamine. We have focused on the status of the anterior pituitary D2 dopamine receptor in female rats treated chronically with oestradiol or progesterone. A direct membrane effect of these steroids on the dopamine system was also investigated in vitro. Both steroids affected the status of the D2 receptor, oestradiol decreasing the number of sites in vitro and progesterone increasing it both in vitro and in vivo. The in vitro studies demonstrated that these steroids exert a direct membrane effect on the D2 receptor. These results correlated with an in vitro short-term physiological effect of oestradiol and progesterone on the dopaminergic inhibition of prolactin release, oestradiol decreasing it while progesterone had the opposite effect. Binding studies with [3H] oestradiol on pituitary membranes revealed a site for oestradiol of high affinity and low capacity, indicating that oestradiol's membrane effects could be mediated by a specific receptor. In vivo treatment with oestradiol also induces proliferation of prolactin-secreting cells (lactotrophs). We focused on the effect of oestradiol on protein kinase C activity, which is involved in both secretion and proliferation. In female rats treated with oestradiol total protein kinase C activity was increased by 74% (particulate 90%, soluble 71%) in comparison with controls. This effect was reversed by concomitant treatment with a dopamine agonist. Thus in the pituitary oestradiol and progesterone affect the characteristics of membrane components that are implicated in the physiological control of the cell. Whether these effects are post-transcriptional only or are also mediated through direct membrane mechanisms needs further investigation.

[Neurohormones coming from the normal and tumoral human anterior pituitary. Secretion and regulation in vitro]. / Neurohormones provenant de l'antéhypophyse humaine normale et tumorale. Sécrétion et régulation in vitro.

Several neuropeptides, classically associated with the hypothalamus have been found in the anterior pituitary and their local synthesis has been hypothesized. Using normal and tumoral human pituitaries we found in the tissue itself different neuropeptides (TRH, SRIH, GHRH) and dopamine in variable quantities according to the nature of the tissue. They were all present in normal pituitaries while only stimulatory neurohormones like TRH and GHRH were found in tumoral tissue implying an imbalance between the stimulatory and inhibitory control of hypophyseal hormones (PRL and GH) in pituitary adenomas. Fragments from normal pituitaries and dispersed cells from GH, PRL and nonsecreting adenomas, were perifused for 4 hours in a Krebs-Ringer medium collected every 2 min and GH, PRL, TRH, GHRH and SRIH were measured by RIA under basal conditions and in the presence of 10(-6) mol/L DA, TRH or SRIH. Neuropeptides and DA were characterized by HPLC. Both normal and tumoral pituitaries released TRH, SRIH and GHRH in large amounts suggesting their local synthesis. There was an in situ regulation between SRIH and GH as their secretion profile was negatively correlated, GH secretion decreasing while SRIH secretion was increasing. Moreover the release of TRH was stimulated 5 to 20 folds by DA, while PRL decreased at the same time. Pulses of TRH and SRIH had differential effects on GHRH and SRIH release according to the nature of the tissue as TRH stimulated SRIH release from normal pituitary while it inhibited SRIH release from adenoma. These results indicate that anterior pituitary cells can release neuropeptides which are probably endogenously synthesized and have a local regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal and growth hormone (GH)-secreting adenomatous human pituitaries release somatostatin and GH-releasing hormone.

Neuropeptides such as vasoactive intestinal peptide, LHRH, or TRH have been found in rat pituitary tissue and could act via paracrine or autocrine actions in this tissue. In this study we investigated whether normal human pituitary tissue and GH-secreting human pituitary adenomas could release somatostatin (SRIH) and GHRH. Fragments from three human pituitaries and dispersed cells from six GH-secreting adenomas (four adenomas were studied for GHRH release and five for SRIH release) were perifused using a Krebs-Ringer culture medium, and the perifusion medium was collected every 2 min (1 mL/fraction for 5 h). GH, GHRH, and SRIH were measured by RIA under basal conditions and in the presence of 10(-6) mol/L TRH or SRIH. Both normal pituitaries and GH-secreting pituitary adenomas released SRIH and GHRH. SRIH release commenced 90-180 min after initiation of the perifusion, at which time GH secretion had decreased significantly. TRH stimulated SRIH release from normal pituitary tissue and inhibited SRIH release from adenoma tissue. GHRH was present at the start of the perifusion, but rapidly disappeared. However, SRIH stimulated GHRH release from normal pituitary tissue, but not from adenoma tissue. Significant amounts of GHRH and SRIH were released during the experiments, suggesting their local synthesis. These results indicate that pituitary cells can release hypothalamic peptides. The liberation of these neuropeptides is regulated, and moreover, their regulation differs between normal and adenomatous pituitaries.

Receptors and neurohormones in human pituitary adenomas.

In order to go further into the pathogenesis of human pituitary adenomas, we studied receptors for neurohormones (thyroliberin, TRH; dopamine, DA; somatostatin, SRIH), for estradiol and epidermal growth factor (EGF) thought to influence hormone secretion and/or cell growth. The following results were obtained: (1) the receptors listed above, with the exception of EGF receptors in the adenomas, are present in normal pituitary tissue and in prolactin (PRL)- and growth hormone (GH)-secreting adenomas; (2) they are functional and their affinities are not different in normal or tumoral tissues; (3) their density is variable and depends on the type of secreting adenoma (GH or PRL), the size of the tumor and the plasma level of the hormone which is secreted, and (4) in nonsecreting adenomas, only TRH receptors are found with characteristics identical to those observed in secreting adenomas. We also showed that TRH is contained in normal and tumoral pituitary tissues. TRH and SRIH are released in vitro from adenomatous cells in large amounts, suggesting their possible synthesis by the pituitary. In both cases a local regulation is observed. TRH release is stimulated in the presence of DA while SRIH is inhibited in the presence of TRH. This neuropeptide release may be implicated in the pituitary hormone regulation through a paracrine or an autocrine mechanism. Thus, the neurohormone receptors found in pituitary adenomas should be dependent on a more complex regulation than it has been envisaged till now.

[In vitro secretion of somatostatin (SRIH) by human adenomatous somatotropic cells. Relation with somatotropic hormone (GH) release and modulation by thyroliberin (TRH)]. / Sécrétion in vitro de somatostatine (SRIH) par les cellules somatotropes adénomateuses humaines. Relation avec la libération d'hormone somatotrope (GH) et modulation par la thyrolibérine (TRH).

SRIH and GH secretions by GH-secreting adenomatous human pituitary cells were analyzed in vitro in a perifusion system. Of the 13 adenomas studied, 7 secreted SRIH, in variable amounts (50 to 700 pg/ml/2 min., corresponding to 600 10,700 pg for the total experiment. SRIH secretion increased during the perifusion, the highest levels being observed at the end of the perifusion. GH secretion also varied from one adenoma to the other (6 to 500 ng/ml). In most cases, the secretion profiles were negatively correlated, GH secretion decreasing while SRIH secretion was increasing. In the presence of 10(-7) M TRH, GH secretion increased while that of SRIH decreased. The hypothesis of a paracrine and/or an autocrine role for SRIH as well as its possible in situ synthesis are discussed.

[Release of thyrotropin releasing hormone (TRH) from human prolactin-secreting pituitary adenoma cells. Modulation by dopamine]. / Libération de thyrolibérine (TRH) par les cellules adénomateuses hypophysaires humaines à prolactine. Modulation par la dopamine.

We found, by radioimmunoassay, that thyrotropin-releasing-hormone (TRH) was present in human prolactin (PRL)-secreting adenomas (mean: 89 +/- 45 (SEM) fmol/mg proteins) and was released by perifused adenomatous cells at levels varying from 5 to 60 fmol/10(6) cell/2 min. TRH release was increased in the presence of dopamine (DA) 10(-6) M but was not modified by the presence of somatostatin (SRIH) 10(-6) M.

Binding of (+)-PN 200-110 to rat pituitaries and to normal and adenomatous human pituitaries.

Endocrine cells possess voltage-sensitive Ca2+ channels involved in the modulation of hormonal secretion. Using the dihydropyridine, (+)-PN 200-110, we have investigated the binding characteristics of this ligand to pituitary membrane Ca2+ channels from normal rat, normal and adenomatous human pituitaries. [3H]PN 200-110 binds specifically to rat pituitary membranes to one class of sites (Kd = 0.41 +/- 0.10 mM; Bmax = 39 +/- 1.3 fmol/mg protein). At 37 degrees C, equilibrium is reached in 45 min and half-life of the binding is 13 min. No significant changes were observed for either the Kd or Bmax values between normal rat and human pituitaries or between the different types of adenomas (GH- and PRL-secreting and non-secreting). As the secretory activity of the pituitary adenomas, involving Ca2+ mobilization, varies from one adenoma to another, our results could indicate that, if there is a modified regulation of Ca2+ entry in the adenomas, it may not be related to a varying number of calcium channels, at least the channels labeled by the dihydropyridine (+)-PN 200-110.

Correlative studies between the presence of thyrotropin-releasing hormone (TRH) receptors and the in vitro stimulation of growth-hormone (GH) secretion in human GH-secreting adenomas.

Specific receptors for TRH were characterized on cellular membranes of 6 out of 13 somatotrophic adenomas obtained from acromegalic patients. These receptors had the same dissociation constant (Kd: 62 +/- 10 nM) as those found in human PRL-secreting adenomas, but their maximal number of binding sites (Bmax: 76 +/- 24 fmol/mg of protein) was six fold smaller. A good correlation was found between the presence of TRH receptors and the in vitro TRH-induced stimulation of GH secretion. The increase in GH release varied from 25 to 200%. It was thus concluded that these receptors are functional. However, why only some of the human somatotrophic adenomas possess TRH receptors and respond to TRH in vitro needs further investigations.

In vitro and in vivo antagonistic regulation by estradiol and progesterone of the rat pituitary domperidone binding sites: correlation with ovarian steroid regulation of the dopaminergic inhibition of prolactin secretion in vitro.

The influences of in vivo and in vitro estradiol (E2) and progesterone (P) treatments on the characteristics of [3H]domperidone binding to intact and ovariectomized (OVX) rat pituitary membranes were analyzed and compared to the modulation by these steroids of dopamine (DA) inhibition of PRL secretion in vitro from intact and OVX rat pituitaries. Using intact rat pituitaries, high and low affinity binding sites for domperidone were detected; the dose-dependent DA inhibition curve of PRL secretion was biphasic (range, 10(-13) - 10(-10) M DA, IC50 = 6 X 10(-12) M; range, 10(-10) - 10(-6) M DA, IC50 = 2 X 10(-8) M). Using OVX rat pituitaries, only the high affinity sites for domperidone were detected, and the dose-dependent DA inhibition curve of PRL secretion was monophasic (range, 10(-10) - 10(-6) M DA, IC50 = 10(-8) M). E2 and P did not modify the characteristics of the high affinity sites either after in vivo treatment or when directly added to the in vitro binding assay. However, using in vivo and in vitro tests, a modulation of the low affinity sites by E2 and P was demonstrated. When E2 is in excess and P levels are low or undetectable, these sites are not detectable, and P is able to restore there presence. A parallelism has been established between this antagonistic E2 and P regulation and the modulation of DA inhibition of PRL secretion (range, 10(-13) - 10(-10) M DA). When intact rat pituitaries are perifused in the presence of 10(-8) M E2, the biphasic dose-dependent inhibition curve of the control is changed into the monophasic curve of the OVX rat pituitaries. Conversely, when OVX rat pituitaries are perifused in the presence of 10(-6) M P, the monophasic curve of the control is changed into the biphasic curve of the intact rat pituitaries. Thus, the DA inhibition in the range 10(-13) - 10(-10) M might result from an interaction between DA and the low affinity site for domperidone. In summary, the biological regulation of PRL by DA at the pituitary level may be mediated by two different DA sites, one being submitted to an antagonistic E2 and P regulation directly at the membrane level. The consequence of this regulation is that, whereas E2 decreases the sensitivity of the cell to DA, P is necessary for a normal DA response of the lactotroph.

Correlation between the number of thyroliberin binding sites, the tumour size and the plasma prolactin level in human prolactin-secreting adenomas.

Basal plasma prolactin (Prl) level, tumour size and [3H]thyroliberin (TRH) binding to tumour membranes were studied in 18 patients bearing Prl-secreting adenomas. Big tumours (grade III) were associated with high plasma Prl levels (median value: 1929 (range: 207-3570) ng/ml) and possessed numerous membrane TRH receptors (median value: 508 (range: 0-1200) fmol/mg of protein). By contrast, smaller tumours (grade II and I) were associated with lower plasma Prl levels (median values: 1085 (range: 40-1890) and 295 (range: 98-788) ng/ml, respectively) and possessed fewer membrane TRH receptors (median values 122 (range: 11-328) and 13 (range: 0-52) fmol/mg of protein, respectively). A direct positive correlation was demonstrated between the plasma Prl level and the number of [3H]TRH binding sites (rho: 0.729 P less than 0.001). That the higher number of TRH receptors is associated with the largest tumours may be of importance in hyperprolactinaemia and should be taken in account when speculating on the pathogenesis of human Prl-secreting adenomas.

Interaction between somatostatin and TRH on growth hormone secretion in perifused human growth hormone tumor cells.

The interaction between somatostatin (SRIF) and thyrotropin-releasing hormone (TRH) on growth hormone (GH) release has been studied on dispersed somatotrophic tumor cells obtained from 7 acromegalic patients. TRH increased GH secretion in 4 cases and SRIF decreased GH secretion in 5 cases. When TRH and SRIF were concomitantly perifused, SRIF, when active by itself, prevented and reversed the TRH-induced stimulation of GH release, while TRH never antagonized the inhibitory effect of SRIF. We conclude that, in these adenomatous cells, the physiological inhibitory factor (SRIF) overcomes the nonphysiological stimulatory factor (TRH) in the control of GH secretion.

Modifications of the high and low affinity pituitary domperidone-binding sites in chronic estrogenized rats.

The effect of chronic estrogen treatment on the anterior pituitary domperidone-binding sites was studied in female rats. The rats were implanted from 1-6 months with a Silastic capsule containing 17 beta-estradiol. The Feldman analysis of [3H]domperidone binding to anterior pituitary membranes of control or estrogenized rats revealed the presence of two sites. The binding characteristics of the higher affinity site were identical for both groups (Kd of the high affinity site, 0.30-0.45 nM; maximum number of binding sites of the high affinity site, 74-95 fmol/mg protein); however, those of the lower affinity site were affected by the estrogen treatment: the Kd of the low affinity site increased from 17.4 +/- 3.2 to 41.5 +/- 9 (+/- SE) nM, and the maximum number of binding sites of the low affinity site increased from 214 +/- 22 to 343 +/- 35 fmol/mg protein. Thus, in chronic estrogenized rats, the total number of binding sites was increased by 54%. These changes, induced by chronic estrogenization, were reversible, since 2 weeks after removal of the 17 beta-estradiol pellet, the binding characteristics were no longer different from those observed in control rats. In contrast to chronic estrogen treatment, ovariectomy reduced markedly the total number of [3H]domperidone-binding sites in anterior pituitary membranes (-70%). Feldman analysis revealed that this reduction resulted from the complete disappearance of the low affinity sites in those membranes. No significant change in the binding characteristics of the high affinity site was detected in ovariectomized rats. Since estradiol induces a decrease in the anterior pituitary content of dopamine, a denervation supersensitivity-like mechanism might be responsible for the increase in pituitary domperidone-binding sites in estrogenized rats. Conversely, a hyposensitivity mechanism could be implicated in the decrease in the total number of the pituitary domperidone-binding sites in ovariectomized rats, since pituitary dopamine levels are increased in those animals. Whether the antidopaminergic properties of estrogen are also involved in this modulation after chronic estradiol treatment requires further investigation.