Function of tuberoinfundibular dopamine neurons in pargyline- and reserpine-treated rats

The synthesis and release of dopamine within tuberoinfundibular neurons were studied in rats treated with reserpine and/or pargyline. The effect of reserpine to elevate the serum concentration of prolactin was accompanied by a 70-80% reduction in the concentration of dopamine in pituitary stalk plasma. When rats were given pargyline (75 mg/kg) prior to the administration of reserpine (2.5 mg/kg), the reserpine-induced alterations in serum prolactin and stalk plasma dopamine concentrations were completely prevented. Pargyline treatment alone resulted in a significant elevation of the concentration of dopamine in pituitary stalk plasma and a reduction in the serum concentration of prolactin. The effects of reserpine and pargyline on the synthesis of dopamine in the median eminence were found to be the opposite of their effects on the release of dopamine. Dopamine synthesis (as estimated by the accumulation of dihydroxyphenylalanine after inhibition of decarboxylase activity) was increased after reserpine and decreased after pargyline administration. Thus, these data serve to illustrate the point that, under certain conditions, the release of dopamine from tuberoinfundibular neurons can be dissociated from its rate of synthesis. More importantly, it appears the release of dopamine from these neurons is dependent upon intact dopamine storage function and monoamine oxidase activity, in addition to continued catecholamine synthesis.     

Responsiveness of tuberoinfundibular dopamine neurons in the aged female rat to the stimulatory actions of prolactin

The autoregulatory feedback control of prolactin, which is mediated by tuberoinfundibular dopamine (DA) neurons, is altered in the aged rat; this is evidenced by increased circulating concentrations of prolactin and decreased activity of these neurons. In the present study the action of prolactin on tuberoinfundibular DA neurons in young and aged female rats was estimated by measuring the rate of DA synthesis (dopa accumulation following the administration of a decarboxylase inhibitor) in the median eminence. The rate of dopa accumulation in the median eminence of the aged (26 months) rat was reduced to 50-60% of that in the young (3 months) rat. The acute systemic administration of haloperidol, a DA antagonist which increases serum concentrations of prolactin or intracerebroventricular infusions of prolactin increased the rate of dopa accumulation in the median eminence of both young and aged rats by the same relative amount. The administration of haloperidol and prolactin increased the rate of DA synthesis to a greater extent in young than in aged rats. The administration of bromocriptine, a DA agonist which reduces serum concentrations of prolactin, decreased the rate of dopa accumulation in the median eminence of both young and aged rats. In young animals the intracerebroventricular administration of prolactin reversed the bromocriptine-induced decrease in DA synthesis in the median eminence after 4 h and caused a further increase after 12 h. Qualitatively similar effects were seen in the aged rats; however, prolactin-treated young rats had much higher levels of DA synthesis than aged rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of extracellular calcium and arachidonate in [3H] dopamine release from rat tuberoinfundibular neurons

The mechanism of [3H]dopamine [( 3H]DA) release was investigated using primary cultures of dispersed cells from the rat tuberoinfundibular region, which contains tyrosine hydroxylase (TH)-like immunoreactive neurons. The calcium ionophore A23187 at 10 nM and above caused a significant and dose-dependent increase in [3H]DA release. In the presence of 50 microM A23187, [3H]DA release was detectable within 30 s and reached a plateau in 15 min. The induction of [3H]DA release by 50 microM A23187 was abolished by lowering the extracellular calcium concentration with 2 mM EDTA. Maitotoxin, another calcium-channel activator, also increased [3H]DA release at a concentration of 50 ng/ml. Exogenous additions of 100 mIU/ml phospholipase A2 and 10 microM arachidonate caused significant release of [3H]DA. Furthermore, A23187 stimulated [3H]arachidonate release from tuberoinfundibular dopaminergic (TIDA) neurons in a dose- and time-dependent manner. These results suggest that extracellular calcium and arachidonate are involved in the process of [3H]DA release from rat TIDA neurons.     

Short-term treatment with 17-beta estradiol enhances spontaneous [3H] dopamine release from cultured rat tuberoinfundibular neurons

The effects of 17-beta estradiol (E2) on spontaneous [3H] dopamine ([3H]DA) release was investigated using primary cultured cells from the tuberoinfundibular region of rat hypothalamus, which includes DA neurons. [3H] DA uptake by the neurons in the presence of E2 at 10(-8) mol/l was similar to that by control cells. Pretreatment with E2 at 10(-9) mol/l or more resulted in dose-dependent increase in spontaneous [3H] DA release from the cultured hypothalamic cells. The spontaneous [3H] DA release reached almost a plateau on pretreatment with E2 at 10(-9) mol/l for 6 hours. Pretreatment with 1 nM E2 also enhanced DA release induced by 10 microM ionophore A23187. These results indicate that estrogen stimulates tuberoinfundibular DA neuronal activity.     

Role of testosterone in the regulation of tuberoinfundibular dopaminergic neurons in the male rat

The effects of testosterone on the tuberoinfundibular dopamine (DA) neuronal activity was examined by determining the rate of DA synthesis-accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor and the concentration of a DA metabolite,--3,4-dihydroxyphenylacetic acid (DOPAC)--in the median eminence in the male rat. Within 1 week after orchidectomy, there was an increase in the accumulation of DOPA and the concentration of DOPAC in the median eminence, but there was no change in the concentration of DA. Conversely, 1 day after testosterone administration to orchidectomized rats, the elevated DOPAC concentrations in the median eminence returned to levels comparable to those in gonadally intact rats. Neither orchidectomy nor testosterone replacement had any effect on plasma prolactin concentrations, but inhibition of prolactin secretion following administration of the DA agonist bromocriptine blocked the increase in DOPA accumulation in the median eminence of orchidectomized rats; this latter effect was reversed by intracerebroventricular administration of prolactin. On the other hand, intracerebroventricular injection of prolactin caused a similar increase in the accumulation of DOPA in the median eminence of gonadally intact, orchidectomized, and testosterone-treated orchidectomized rats. Immobilization stress decreased the accumulation of DOPA and the concentration of DOPAC in the median eminence of orchidectomized rats, but had no effect in intact or testosterone-treated orchidectomized rats. These results indicate that testosterone inhibits the basal activity of tuberoinfundibular DA neurons and blocks the inhibitory effects of physical restraint on these neurons, but does not alter the ability of these neurons to respond to delayed activation by prolactin.     

Exposure to pheromones increases plasma corticosterone concentrations in a terrestrial salamander

Sensory cues involved in social interactions can influence plasma steroid hormone concentrations. Although pheromonal communication is common in amphibians, it is unknown whether pheromones can alter hormone levels in amphibians as they do in mammals. We tested whether courtship pheromones would alter steroid hormone concentrations in male and female terrestrial salamanders (Plethodon shermani). Plasma corticosterone concentrations were elevated in male salamanders exposed to mental gland courtship pheromones, as compared to males exposed to female skin secretions or a saline control. Chemosensory cues had no effect on testosterone levels in males or on corticosterone or estradiol levels in females. These results provide the first evidence that pheromones have priming effects on the endocrine system in amphibians.     

Dehydration selectively increases dopamine synthesis in tuberohypophyseal dopaminergic neurons

The concentration of dopamine (DA) and the accumulation of DOPA following the administration of a decarboxylase inhibitor were determined in the rat striatum, median eminence and posterior pituitary, regions containing terminals of nigrostriatal, tuberoinfundibular and tuberohypophyseal DA nerves, respectively. Severe dehydration (2 days water deprivation followed by 3 days of 2% NaCl substituted for drinking water) increased the concentration of DA in the posterior pituitary but not in the striatum or median eminence. Less severe dehydration caused by 2 or 3 days of water deprivation did not alter steady state concentrations of DA, but increased DOPA accumulation, an index of DA nerve activity, only in the posterior pituitary. Food deprivation for 3 days did not alter DOPA accumulation in the posterior pituitary. These results suggest that dehydration selectively activates the tuberohypophyseal DA neuronal system in the rat.     

Evidence that prolactin mediates the stimulatory effects of estrogen on tuberoinfundibular dopamine neurons in female rats.

The effects of ovariectomy and estrogen on prolactin secretion and/or the activity of tuberoinfundibular dopamine (TIDA) neurons were examined by either concurrently measuring concentrations of prolactin in plasma and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence of female rats or by determining the rate of DA synthesis (accumulation of 3,4-dihydroxyphenylalanine (DOPA) after the administration of a decarboxylase inhibitor) in the median eminence. For comparison, concentrations of alpha-melanocyte-stimulating hormone (alpha MSH) in plasma and DOPAC in the intermediate lobe of the pituitary (an index of the activity of tuberohypophysial DA neurons) were also determined. Ovariectomy produced a time-dependent decrease in the accumulation of DOPA and the concentrations of DOPAC in the median eminence and prolactin in plasma with maximal effects occurring by 7 days. Estrogen administration to ovariectomized rats increased plasma prolactin and median eminence DOPAC concentrations to levels comparable to those in diestrous controls. In contrast, neither ovariectomy nor estrogen replacement altered the concentrations of alpha MSH in plasma or DOPAC in the intermediate lobe. Administration of the DA agonist bromocriptine blocked the ability of estrogen to increase plasma prolactin and median eminence DOPAC concentrations. Also, administration of antiserum to rat prolactin blocked the stimulatory action of estrogen on median eminence DOPAC concentrations. Taken together, these results indicate that the stimulatory effect of estrogen on the activity of TIDA neurons is mediated by prolactin.     

Subfornical organ efferents influence the excitability of neurohypophyseal and tuberoinfundibular paraventricular nucleus neurons in the rat

Electrical stimulation in the subfornical organ (SFO) alters the excitability of antidromically identified paraventricular nucleus neurons. Extracellular recordings demonstrate that the dominant effect of single stimuli delivered to the SFO on neurohypophyseal oxytocin and vasopressin containing neurons is an increase in excitability. In 35% of cells tested, this excitation showed a long latency (44.3 +/- 3.4 ms) prolonged duration (208.7 +/- 23.5 ms), while in 16% of the neurons the excitation observed may be described as short latency (24.7 +/- 1.8 ms) short duration (11.6 +/- 1.4 ms). Of the remaining cells antidromically identified as projecting to the posterior pituitary, 12% showed initial decreases in excitability following SFO stimulation while the remaining 37% were unaffected. Evidence is presented demonstrating that stimulation in the region of the SFO results in short latency (27.9 +/- 2.4 ms) short duration (7.8 +/- 0.7 ms) increases in excitability in 22% of antidromically identified PVN tuberoinfundibular neurons tested. These data provide electrophysiological evidence in support of the proposed role of the subfornical organ in the control of posterior and anterior pituitary function.     

Age-related loss of the responsiveness of the tuberoinfundibular dopaminergic neurons to prolactin in the female rat

In the old female rat the previous findings of a sustained reduction of the secretory activity of the hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons associated with a persistent hyperprolactinemia as well as the observation of a failure of the prolactin (PRL) short-loop feedback mechanism have been suggestive of an age-related loss of the responsiveness of the TIDA neurons to the stimulatory action of PRL. Yet the existence of significant impairments in the capacity of the neurons to respond to PRL could not be demonstrated in an earlier study using multiparous old rats in constant estrus compared to young nulliparous estrous rats. In the present study we have readdressed the issue using nulliparous old rats (24-26 months) compared to virgin young rats (4-5 months); two sets of old rats were studied which displayed distinct senile reproductive states, namely persistent diestrus or repetitive pseudopregnancy, and they were compared to young rats in diestrus or in repetitive pseudopregnancy, respectively. The secretory activity of the TIDA neurons was evaluated by measurement of dopamine biosynthesis in the neurons (DOPA accumulation in the median eminence after decarboxylase inhibition) and dopamine release into hypophysial portal blood, and PRL influence on the activity of the TIDA neurons was studied after repeated s.c. administrations of ovine PRL (oPRL) or the solvent vehicle. A reduced activity of the TIDA neurons was observed in both groups of nulliparous aged rats compared to their respective young control group.(ABSTRACT TRUNCATED AT 250 WORDS)     

The anterior hypothalamus provides stimulatory input to tuberoinfundibular dopamine neurons which is not mediated by prolactin

Complete or retrochiasmatic deafferentations of the mediobasal hypothalamus were made in female rats 7 days prior to experimentation in order to determine the role played by putative afferent neuronal connections (1) in maintaining the basal neuronal activity of tuberoinfundibular dopaminergic (TIDA) neurons, and (2) in the stimulatory actions of prolactin on these neurons. The neuronal activity of TIDA neurons was estimated by measuring the rates of synthesis, turnover or metabolism of dopamine (DA) in the terminals of these neurons in the median eminence. Complete deafferentation of the mediobasal hypothalamus reduced the basal rate of DA synthesis, and retrochiasmatic deafferentation decreased the rates of synthesis, turnover and metabolism of DA in the median eminence. A knife cut 1 mm rostral to the retrochiasmatic cut failed to alter basal TIDA neuronal activity. These results suggest that afferent neuronal inputs originating in or coursing through the caudal portion of the anterior hypothalamus mediate a tonic stimulatory influence on TIDA neurons in the female rat. Intracerebroventricular administration of rat prolactin or systemic administration of haloperidol (which increases circulating levels of prolactin) increased DA synthesis in the median eminence of both sham-operated rats and retrochiasmatic-deafferentated rats. Thus, the stimulatory action of prolactin was not blocked by retrochiasmatic deafferentation. In addition, elimination of the basal stimulatory action of endogenous prolactin by pretreating animals with bromocriptine reduced the rate of DA synthesis in the median eminence of both sham- and retrochiasmatic-deafferentated rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Elevation of serum corticosterone in rats by dopamine agonists related in structure to pergolide

Two chemical analogs of pergolide were examined to test further the idea that pergolide elevates serum corticosterone concentration in rats by activation of brain dopaminergic receptors. LY116467, which contains an N-methyl substituent in place of the N-n-propyl substituent in pergolide, was less potent than pergolide in lowering brain levels of 3,4-dihydroxyphenylacetic acid (Dopac), the metabolite of dopamine. LY116467 increased serum corticosterone concentration in rats at a dose of 3 mg/kg (a higher dose than is required for pergolide), and the effect was prevented by spiperone pretreatment. LY141865, which has been reported to differ from pergolide in not activating dopamine-sensitive adenylate cyclase and which was found in this study to have much less affinity for serotonin receptors than does pergolide, increased serum corticosterone in much the same manner as pergolide, only slightly higher doses being required. The effect of LY141865 was prevented by pretreatment with haloperidol but not domperidone. Both haloperidol and domperidone increased serum prolactin concentration when given alone or in combination with LY141865, indicating they were both capable of blocking peripheral (pituitary) dopamine receptors. In contrast, haloperidol but not domperidone caused a marked elevation in brain levels of Dopac and of homovanillic acid and prevented the lowering of these brain dopamine metabolites by LY141865. The ability of LY141865 to increase serum corticosterone concentration was attenuated in rats that had received four daily injections of pergolide mesylate, indicating corss-tolerance had occurred. These results strengthen the hypothesis that activation of brain dopaminergic receptors leads to increased serum corticosterone concentration in rats.     

Orexin A activates locus coeruleus cell firing and increases arousal in the rat.

The localization of orexin neuropeptides in the lateral hypothalamus has focused interest on their role in ingestion. The orexigenic neurones in the lateral hypothalamus, however, project widely in the brain, and thus the physiological role of orexins is likely to be complex. Here we describe an investigation of the action of orexin A in modulating the arousal state of rats by using a combination of tissue localization and electrophysiological and behavioral techniques. We show that the brain region receiving the densest innervation from orexinergic nerves is the locus coeruleus, a key modulator of attentional state, where application of orexin A increases cell firing of intrinsic noradrenergic neurones. Orexin A increases arousal and locomotor activity and modulates neuroendocrine function. The data suggest that orexin A plays an important role in orchestrating the sleep-wake cycle.     

A trophoblast-specific factor(s) suppresses circulating prolactin levels and increases tyrosine hydroxylase activity in tuberoinfundibular dopaminergic neurons.

Rat choriocarcinoma (Rcho) cells, which are morphologically similar to trophoblast giant cells of the normal placenta and produce placental lactogen-I in vivo, were used to investigate placental feedback on PRL secretion and tuberoinfundibular dopaminergic neuronal activity. Rcho cells were injected into female rats either intracerebroventricularly 60-65 h before use or under the kidney capsule 10-14 days before use. The following endocrine conditions were used: 1) ovariectomized rats with or without bromocriptine treatment, 2) immature (40-44 days old) rats, 3) adult cycling (diestrous) rats, and 4) pregnant rats. Serum PRL levels in ovariectomized, diestrous, and immature female rats were suppressed to less than 20% of control levels by secretions from the Rcho cells. Tyrosine hydroxylase (TH) activity in the stalk-median eminence (SME) was increased 2-fold above control activity in Rcho-treated ovariectomized and immature female rats. When TH activity was reduced to 40% of control levels by 50 h of bromocriptine treatment, secretions from Rcho cells increased TH activity 3.5-fold to levels similar to those for Rcho alone. Even though Rcho treatment suppressed PRL levels, TH activity in the SME of cycling (diestrous) rats was not altered after either central (65 h) or peripheral (12 days) administration of cells. TH mRNA levels in the arcuate nuclei were unaltered by Rcho cells in immature female and adult cycling rats. TH mRNA levels in ovariectomized rats were markedly reduced 75% by 50 h of bromocriptine treatment and modestly reduced 33% 65 h after injection of Rcho cells. However, Rcho cells partially reversed the bromocriptine-induced decline in TH mRNA to levels seen for Rcho cells alone. On day 7 of pregnancy, secretions from Rcho cells abolished the nocturnal and diurnal PRL surges characteristic of early pregnancy and suppressed circulating PRL levels throughout the day to less than 20% of intersurge PRL levels. Rcho cells eliminated the semicircadian rhythm in TH activity in the SME, which was out of phase with the twice daily PRL surges of early pregnancy. TH activity was increased by Rcho factor(s) at 0330 h (nocturnal surge) and 1800 h (diurnal surge), but not at 1000 h (intersurge). MMQ cells, pituitary-derived clonal PRL-secreting cells, similarly terminated the biphasic rhythm of PRL release and tuberoinfundibular dopaminergic neuronal activity during early pregnancy.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prolonged exercise increases serum insulin-like growth factor-binding protein concentrations

We studied the response of serum 34K insulin-like growth factor-binding protein (IGF-BP) to two types of prolonged exercise. In the first study, 11 normal men performed cycle ergometer exercise, after an overnight fast, for 3 h at the intensity corresponding to 45-50% of their maximal aerobic power. After the exercise, the mean serum IGF-BP concentration was 4.9-fold higher than the baseline level (P less than 0.01), while the IGF-I concentration did not change. Resting for the same time period resulted in a 38% fall in the serum IGF-BP level (P less than 0.01). In the second study, 10 normal men and 8 type 1 diabetic men exercised, on the average, for 7.5 h in a 75-km cross-country ski race. The mean serum IGF-BP concentration increased 5.4-fold (P less than 0.01) in the normal men and 4.2-fold (P less than 0.01) in the diabetic men. The serum IGF-I concentration decreased by 19% and 21% in the normal and diabetic men, respectively (P less than 0.01). After completion of the ski race, the serum insulin and IGF-BP levels (r = -0.70; P less than 0.05), and serum IGF-I and IGF-BP levels (r = -0.72; P less than 0.05) were inversely correlated in the normal men. No such correlations were found in the diabetic men. We conclude that 1) long term exercise increases serum IGF-BP concentrations in both normal and type 1 diabetic men; and 2) the increases are inversely related to alterations in serum IGF-I and insulin concentrations in normal men. These data suggest that IGF-BP may be involved in regulation of the biological action of IGF-I during prolonged exercise.     

Responses of serum corticosterone and corticosteroid-binding globulin to acute and prolonged stress in the rat

Responses of serum corticosterone (B) and corticosteroid-binding globulin (CBG) to ether anesthesia (a “classic” acute stress) and to a number of stressors influencing metabolic homeostasis—fasting, physical exercise, cold exposure, and water deprivation—were studied in male and female rats. Metabolic stressors included placing in an ice bath, physical exercise (swimming), fasting for 2 d, swimming after fasting for 2 d, cold-room (4°C) exposure for 2 d, fasting in combination with cold-room exposure for 1 d, and water deprivation for 2 d. The study demonstrated clear differences between males and females in basal B levels and B responses to some stressors. Only ether anesthesia and fasting resulted in similar B levels in males and females whereas in control and all other groups serum B levels were higher in females. Serum CBG was considerably higher in females. In females, ether, swimming, swimming after fasting, fasting, and fasting during cold exposure resulted in a decrease in circulating CBG. Ice bathing and cold exposure did not influence CBG, and water deprivation elevated serum CBG. In males, animals subjected to fasting and fasting during cold exposure had CBG levels lower than control animals. Other groups did not differ from the control. Higher CBG levels in females counterbalanced higher total B in setting circulating free B: significant sex differences in free B were observed only after swimming or fasting during cold exposure. Stress-responsive changes in CBG levels seem to contribute little to changes in free B; the main contributing factor is the rise in total B. However, CBG may play a special role, independent of the functions of corticosteroids. It is proposed that the need for substantial mobilization of spare fuel (as it takes place during physical exercise or fasting) is critical in involving CBG in the stress response.     

GABAergic biochemical parameters of the tuberoinfundibular neurons following chronic hyperprolactinemia

The effect of chronic hyperprolactinemia was studied on (a) GABA concentration in the pituitary anterior lobe; (b) GABA biosynthesis enzyme, glutamate decarboxylase (GAD) activity in the hypothalamic median eminence, and (c) GABA degradation enzyme GABA-transaminase (GABA-T) activity at both levels. In male rats bearing the prolactin-secreting tumor MtTF4 for 1 month or treated for 5 days with estradiol benzoate, the plasma prolactin concentration was markedly increased (between 4- and 10-fold basal values). In both cases, GABA concentration was significantly increased (40-60%) in the anterior pituitary lobe. A slight reduction (20-30%) in GABA-T activity was observed in the anterior lobe while no change in GAD or GABA-T activity was measured in the median eminence. These results are discussed in relationship to a possible feedback input of prolactin on the tuberoinfundibular GABAergic system.