The effect of prolactin on dopamine release from rat striatum and medial basal hypothalamus

Sex differences in the relationship between prolactin levels and impaired processing speed in early psychosis

INTRODUCTION

Hyperprolactinaemia is commonly observed in people with psychotic disorders due to D2 receptor blockade by antipsychotic drugs, although it may also exist in drug-naïve patients with first-episode psychosis. Recent studies suggest that hyperprolactinaemia may have a negative impact on cognitive function in people with early psychosis. We aimed to explore whether there are sex differences in the association between prolactin levels and cognitive performance in early psychosis patients.

METHODS

We studied 60 young patients with early psychosis (aged 18-35 years, 35% females) and a sex- and age-matched control group of 50 healthy subjects. Cognitive assessment was performed with the MATRICS Consensus Cognitive Battery. Prolactin, total cortisol, follicular-stimulating hormone, luteal hormone and sex steroids (testosterone in men, oestradiol and progesterone in women) were measured in plasma. Salivary cortisol was measured at different sampling times (awakening response, 10:00 and 23:00). Psychopathological status was assessed, and antipsychotic treatment was registered. Multiple linear regression analyses were used to explore the relationship between prolactin and cognitive tasks while adjusting for covariates.

RESULTS

Prolactin levels were associated with impaired processing speed in men, and this association was independent of cortisol and testosterone. In women, prolactin levels were not associated with processing speed tasks, although we observed a negative effect of prolactin on verbal learning and spatial working memory in female healthy subjects. The male-dependent effect maintained its significance after adjusting for education status, antipsychotic treatment and negative symptoms.

CONCLUSION

Our study demonstrates that the previously reported association between high prolactin levels and impaired cognitive processes in early psychosis is restricted to men.

Postpartum and depression status are associated with lower [[¹¹C]raclopride BP(ND) in reproductive-age women

The early postpartum period is associated with increased risk for affective and psychotic disorders. Because maternal dopaminergic reward system function is altered with perinatal status, dopaminergic system dysregulation may be an important mechanism of postpartum psychiatric disorders. Subjects included were non-postpartum healthy (n=13), postpartum healthy (n=13), non-postpartum unipolar depressed (n=10), non-postpartum bipolar depressed (n=7), postpartum unipolar (n=13), and postpartum bipolar depressed (n=7) women. Subjects underwent 60 min of [¹¹C]raclopride-positron emission tomography imaging to determine the nondisplaceable striatal D₂/₃ receptor binding potential (BP(ND)). Postpartum status and unipolar depression were associated with lower striatal D₂/₃ receptor BP(ND) in the whole striatum (p=0.05 and p=0.02, respectively) that reached a maximum of 7-8% in anteroventral striatum for postpartum status (p=0.02). Unipolar depression showed a nonsignificant trend toward being associated with 5% lower BP(ND) in dorsal striatum (p=0.06). D₂/₃ receptor BP(ND) did not differ significantly between unipolar depressed and healthy postpartum women or between bipolar and healthy subjects; however, D₂/₃ receptor BP(ND) was higher in dorsal striatal regions in bipolar relative to unipolar depressives (p=0.02). In conclusion, lower striatal D₂/₃ receptor BP(ND) in postpartum and unipolar depressed women, primarily in ventral striatum, and higher dorsal striatal D₂/₃ receptor BP(ND) in bipolar relative to unipolar depressives reveal a potential role for the dopamine (DA) system in the physiology of these states. Further studies delineating the mechanisms underlying these differences in D₂/₃ receptor BP(ND), including study of DA system responsivity to rewarding stimuli, and increasing power to assess unipolar vs bipolar-related differences, are needed to better understand the affective role of the DA system in postpartum and depressed women.

Hormones, genes and the structure of sexual arousal

Despite the inherent difficulty of connecting individual genes with integrated mammalian behaviors, it has been determined that a series of genes are turned on by estrogenic hormones acting in forebrain. Their products are, in turn, facilitatory for female reproductive behaviors such as lordosis. The causal routes by which two genes contribute to the control of lordosis behavior, the classical estrogen receptor gene (ER-alpha) and a thyroid hormone (TH) receptor gene (TR-beta), have been delineated. Beyond the mechanisms underlying the expression of concrete, specific natural behaviors, lies the question of sexual motivation. Required as an intervening variable to explain fluctuations in natural behaviors in the face of constant stimuli, motivational states have both general and specific features. Most theoretical and experimental approaches toward the general aspects of motivation have depended heavily on concepts of 'arousal.' Sexual arousal is likely to depend both on very general, broadly distributed neuronal influences and on specific affiliative and sexual tendencies. Is 'general arousal' a monolithic, undifferentiated process? In no way can a review at this time settle such issues, but the reasons behind six new experimental approaches to these questions are described.

Effects of prolactin on expression of the mRNAs encoding the immediate early genes zif/268 (NGF1-A), nur/77 (NGF1-B), c-fos and c-jun in the hypothalamus

Prolactin (PRL) exerts a short-loop negative feedback effect on hypothalamic neurons which control its secretion from the anterior pituitary gland. The purpose of this study was to identify the location of hypothalamic neurons which respond to acute PRL exposure. Increasing evidence indicates that excitation of neurons often results in the rapid transcription of immediate early genes (IEGs). In the present study, quantitative in situ hybridization histochemistry (ISHH) was used to visualize the induction of mRNAs for four different IEGs: zif/268 (NGF1-A), nur/77 (NGF1-B), c-fos and c-jun. Three groups of male rats were compared: unmanipulated controls, rats injected s.c. with 2.4 mg ovine PRL (oPRL) suspended in polyvinylpyrrolidone (PVP), and PVP-injected controls. Animals were decapitated 0, 0.5, 1, 2, 3 or 4 h following injection. In all rats, the four probes labeled cells within the cortex, particularly the cingulate and piriform cortices, the hippocampus and the striatum. In the arcuate nucleus, there was a modest increase in the average number of cells/animal which expressed zif/268 mRNA following the injection of PVP and oPRL at all times studied. The average area of grains/cell representing zif/268 message also increased following the injection stimulus. The number of neurons expressing nur/77 mRNA was greater in PRL-treated rats compared with PVP-treated controls 0.5 and 1 h following injection. Nur/77-labeled neurons were co-extensive with the tuberoinfundibular dopaminergic (TIDA) neurons. The data suggest that cells located within the arcuate nucleus are involved in mediating PRL autofeedback on the brain.

Characterisation of the DA-ergic system in the mediobasal hypothalamus: a new approach to simultaneously monitor the release of DA from the TIDA neurons and the PRL secretion from the adenohypophysis in awake rats

The TIDA neurons, which constitute part of the arcuate nucleus-ME complex, play an important inhibitory role in the regulation of the PRL secretion from the adenohypophysis. To simultaneously study the release of DA from the TIDA neurons and the PRL secretion from the adenohypophysis in awake rats, a microdialysis probe was implanted into the MBH together with a permanent heartcannula in male rats. The extracellular levels of DA in the MBH as measured by microdialysis decreased to 25% of basal values after local infusion of TTX (1 mumol/l), indicating that the released DA was directly derived from neuronal activity. DOPAC levels were not affected. This local infusion of TTX into the MBH induced parallel to the immediate decrease in DA levels, a profound increase in PRL concentration in the blood (from 10 to 55 ng/PRL-RP-2/ml) directly after infusion. Thus, the area in which the dialysis probe was inserted indeed included the DA-ergic neurons that regulate the PRL secretion. Evidence for a functional re-uptake system in the MBH was obtained by local infusion of the re-uptake inhibitor nomifensine (5 mumol/l) which induced an increase in DA release to 350% of basal values, without affecting the DOPAC levels. In spite of this increase in DA levels, the PRL concentration in the blood was not affected. In pseudopregnant female rats, relatively high levels of extracellular DA in the MBH were obtained during the interphase during which the PRL levels are low, while lower DA levels were apparent during the phase the spontaneous nocturnal PRL surge normally appears. Taken together, the approach presented in this study, i.e. the simultaneous measurements of DA in the MBH and PRL in the blood, establishes an advanced method enabling studies on the DA-PRL interactions in awake animals.

Evidence for protein kinase C involvement in the short-term activation by prolactin of tyrosine hydroxylase in tuberoinfundibular dopaminergic neurons

The mechanism of the short-term activation by prolactin (PRL) of tyrosine hydroxylase (TH) in tuberoinfundibular dopaminergic neurons was examined in vitro on hypothalamic slices from ovariectomized rats. TH activity (determined by 3,4-dihydroxyphenylalanine accumulation in the median eminence after blockade of decarboxylase with NSD 1055) showed a dose-dependent increase within 2 h of incubation of the hypothalamic slices with PRL. To determine whether a phosphorylation process was involved in this increase in TH activity, we studied the sensitivity of the enzyme to dopamine (DA) feedback inhibition. In control median eminences, two kinetically different forms of TH coexisted, one exhibiting a Ki(DA) value of 29.92 +/- 0.49 microM, the other being approximately 15-fold more sensitive to DA inhibition with a Ki(DA) of 1.96 +/- 0.09 microM, likely corresponding to a phosphorylated and active form and to a nonphosphorylated and less active form, respectively. After PRL treatment, the TH form of low Ki(DA) remained unaffected, whereas the Ki(DA) of the purported active form of TH increased to 62.6 +/- 0.8 microM, suggesting an increase in the enzyme phosphorylation. This increase in the Ki(DA) of TH was selectively prevented by GF 109203X, a potent and selective inhibitor of protein kinase C, but not by a specific inhibitor of protein kinase A or calmodulin. Finally, this action of PRL could be mimicked by 12-O-tetradecanoylphorbol 13-acetate (a direct activator of protein kinase C). These results suggest that PRL, at the median eminence level, activates TH by increasing the enzyme phosphorylation and that this action may involve an activation of protein kinase C.

Modifications of mesolimbic and nigrostriatal dopaminergic activities after intracerebroventricular administration of prolactin

In the present study we examined the effects of intracerebroventricular (i.c.v.) injections of prolactin (PRL) on the presynaptic activity and post-synaptic sensitivity of mesolimbic and nigrostriatal dopaminergic neurons. In addition, the effects of PRL on in vitro release of dopamine (DA) from perifused striatal fragments were examined. Tyrosine hydroxylase (TH) activity and D2 receptor density in the striatum decreased after i.c.v. PRL administration; this was accompanied by an increase in D2 receptor affinity. These effects occurred after i.c.v. administration of PRL to normoprolactinemic rats, although normally they did not appear after administration to animals with pituitary grafting-induced hyperprolactinemia. Thus, in these animals, i.c.v. PRL failed to decrease TH activity and D1 and D2 receptor densities to a significant extent. In the case of D2 receptors, this was probably due to the fact that pituitary grafting-induced hyperprolactinemia itself was able to reduce the density of this receptor. No changes were observed in DA or L-3, 4-dihydroxyphenylacetic acid (DOPAC) contents after i.c.v. administration of PRL to both normo-and hyperprolactinemic animals. Basal and K(+)-evoked DA release in vitro from perifused striatal fragments of normoprolactinemic rats were not affected by the addition of PRL, whereas this hormone enhanced K(+)-evoked DA release when added to perifused striatal fragments from hyperprolactinemic animals. In the limbic forebrain, i.c.v. administration of PRL to normoprolactinemic animals produced a decrease in DA and DOPAC contents and D1 receptor density. Interestingly, none of these effects appeared when PRL was injected to hyperprolactinemic animals. In summary, our results suggest a possible inhibitory role of PRL on the activity of both the nigrostriatal and mesolimbic dopaminergic neuronal systems. These inhibitory effects were reflected in the decreases elicited in a set of neurochemical parameters, indicating either presynaptic activity or postsynaptic sensitivity, after i.c.v.-administered PRL. This observation supports the hypothesis of a possible neuromodulatory role for an extrapituitary PRL on the activity of these neurons, although the fact that most of these effects did not appear when i.c.v. administration was performed in hyperprolactinemic rats also suggests that they are influenced by peripheral PRL levels.

Effect of human prolactin administration on gonadotropin and thyrotropin secretion in normal men

To test the hypothesis that PRL is able to feedback negatively on its own secretion (short-loop feedback) in humans via augmentation of the turnover of tuberoinfundibular dopamine (TIDA), the effects of the administration of purified hPRL on endogenous LH, FSH and TSH were assessed. Purified hPRL, given in an i.v. loading dose of 90 micrograms followed by a continuous infusion of 1.39 micrograms/min to 4 normal male volunteers resulted in a tripling of PRL levels (10.5 +/- 1.9 micrograms/L increasing to 30.9 +/- 3.6 micrograms/L) at the end of 90 min. There were no changes in LH, FSH or TSH levels, however, during or following the infusion. Purified hPRL was also given in 1 and 8 micrograms/kg doses IM to 5 normal male volunteers. Although PRL levels did not rise significantly with the 1 microgram/kg dose, levels almost doubled with the 8 micrograms/kg dose (9.5 +/- 2.2 micrograms/L increasing to 17.4 +/- 1.5 micrograms/L). Again, LH, FSH and TSH levels did not change significantly over the three hour period of sampling with either dose. In conclusion, in this study we found that a 2-3 fold increase of circulating PRL levels maintained for 1.5-3 h exerted no apparent effects on the secretion of endogenous LH, FSH and TSH. This study provides direct evidence against the existence of a short-loop feedback occurring via TIDA activation in humans over this time interval but does not rule out the possibility that such feedback may occur with more prolonged states of hyperprolactinemia or via other mechanisms or the possibility of an effect on the hypothalamic pulse generator.

Effects of thyrotropin-releasing hormone and phorbol ester on dopamine release from dispersed rat tuberoinfundibular dopaminergic neurons

We have investigated the intracellular mechanisms underlying thyrotropin-releasing hormone (TRH)-mediated [3H]dopamine ([3H]DA) release from dispersed rat tuberoinfundibular dopaminergic (TIDA) neurons. The specific binding of [3H]Me-TRH to these cells is characterized by a single, high-affinity binding site (Kd = 1.2 nM) with a Bmax value of 178 fmol/mg protein. Thyrotropin-releasing hormone markedly increased [3H]DA release and intracellular free calcium concentration ([Ca2+]i) in TIDA neurons, and its effect was abolished by treatment with EGTA (5 mM) or chlordiazepoxide, a specific TRH receptor antagonist (10 microM). Furthermore, to examine the involvement of protein kinase C on [3H]DA release, we investigated the effect of phorbol myristate acetate (PMA), which is known to activate protein kinase C directly. Phorbol myristate acetate induced a significant increase in [3H]DA release in a concentration-dependent manner. Treatment with TRH (1 microM) plus PMA (100 nM) resulted in an additive increase in [3H]DA release. Thyrotropin-releasing hormone (1 microM) still increased [3H]DA release even after preincubation with PMA (500 nM) for 24 h, but PMA (100 nM) did not under the same conditions. These results suggest that TRH may induce DA release in dispersed rat TIDA cells by increasing calcium influx and activating the protein kinase C system.

Prolactin stimulates dopamine release from the rat corpus striatum in the absence of extra-cellular calcium

Prolactin (PRL) increased basal dopamine (DA) release and attenuated amphetamine (AMPH)-stimulated DA release in vitro from rat corpus striatum in a concentration-dependent manner with 10(-5) M PRL being the most effective. The effects of PRL on DA release were enhanced in the absence of extracellular calcium. PRL at 10(-5) M did not alter the DA post-superfusion content of the striatal tissue. These results indicate that the stimulatory effect of PRL on basal DA release does not require extra-cellular calcium and the inhibitory effect on AMPH-stimulated DA release is not due to depletion of DA stores.

Lack of effect of prolactin on the dopaminergic receptor sensitivity in striatal and limbic areas after experimentally-induced alterations in its peripheral levels

Although it had been suggested that prolactin (PRL) modulates the dopaminergic receptor sensitivity in extrahypothalamic areas, recent studies have questioned this role. We studied the effects of PRL on the receptor sensitivity in the striatum and the limbic forebrain, analyzing the number of D1 and D2 receptors and the amount of their second messenger, cyclic-adenosine monophosphate (cAMP). Tyrosine hydroxylase (TH) activity and dopamine (DA) and L-3,4-dihydroxyphenylacetic acid (DOPAC) content were also measured as indices of presynaptic activity. The study was carried out in male rats submitted to either acute (PRL injection) or chronic (pituitary grafts or diethylstilbestrol (DES)-induced pituitary tumors) rises of plasma PRL levels. The results showed a common lack of effect of PRL on the dopaminergic receptor sensitivity in both brain areas and, only some few effects on presynaptic activity in the striatum. Thus, grafted rats showed a slight decrease in DA content in the striatum, but neither D1 and D2 receptor number and cAMP content nor DOPAC content and TH activity, were modified, whereas DES animals exhibited no changes in all the parameters studied. A single injection of ovine PRL caused a decrease in DOPAC content and an increase in TH activity in the striatum. In the case of the limbic area, both chronic and acute hyperprolactinemia failed to alter any of the indices studied. In summary, we cannot support the view that PRL plays a role as modulator of dopaminergic receptor sensitivity. The only effects were always produced at the presynaptic level on the striatum, and after acute treatment, which supports the possible development of tolerance after chronic changes in peripheral PRL levels.

In vivo changes in responsiveness of the caudate nucleus to L-dopa infusion as a function of the estrous cycle

In the present experiment we measured changes in the responsiveness of the nigrostriatal dopaminergic system to L-DOPA as a function of the estrous cycle in the freely behaving animal with push-pull perfusion. Intact female rats were implanted with a push-pull cannula directed at the caudate nucleus and perfused repetitively on each day of their estrous cycle. During the perfusion, the response of the caudate nucleus was tested with two successive increasing doses of L-DOPA (1.0 and 10 microM) infused directly through the push side of the cannula. In response to 1.0 microM L-DOPA, significantly greater dopamine release was obtained for female rats perfused on proestrus, with all 6 females showing maximal responsiveness at this estrous cycle stage. There was an overall 3-4 fold greater increase in dopamine release to the 10 microM L-DOPA infusion, regardless of estrous cycle day. However, with the use of this 10 microM L-DOPA dose, the significant effect at proestrus was maintained. These results demonstrate that the responsiveness of the caudate nucleus to L-DOPA is maximal at proestrus. These data together with previous work from our laboratory indicate that the natural physiological changes which occur in hormone levels, in particular changes in progesterone during proestrus, exert a substantial influence upon this extra-hypothalamic central nervous system site.

Systemic administration of thyrotropin-releasing hormone enhances striatal dopamine release in vivo

We examined the effects of systemically administered thyrotropin-releasing hormone (TRH) on the release of dopamine (DA), as assessed by brain microdialysis within the corpus striatum of anesthetized rats. A single dose (10 micrograms i.v.) elevated DA levels in brain extracellular fluid (ECF) by 240% above baseline levels after 150 min. Systemic tyrosine ([TME] 20 mg/kg i.v.) also increased DA release (by 190% after 150 min), while combined treatment with both agents was associated with significant potentiation of the DA response (to 640% after 150 min). None of the treatments significantly altered striatal tissue levels of DA or its metabolites. A large dose of TRH (50 micrograms i.v.) significantly increased DA release (by 1150%) whether or not animals had received an active or denatured prolactin (PRL) antiserum prior to the experiment, suggesting that the TRH effect is not mediated by PRL. Although TRH is rapidly metabolized in plasma and penetrates the blood-brain barrier only poorly, our results suggest that even relatively small doses of the hormone can affect striatal dopaminergic neurotransmission.

Effect of age at the time of pituitary grafting on behavioral recovery in dwarf mice

The effects of ectopic pituitary isograft implanted in 13- or 21-day-old Snell dwarf mice on their freely expressed behavior were examined over a 14- or 31-day period. This mutant strain has severe anterohypophyseal deficiency which causes high levels of inactivity and abnormally low exploratory activity. The graft totally normalized these behaviors. Results showed that the time necessary for inducing behavioral changes is independent of age of the animals at the time of grafting, but the age at which implantation occurs influences the speed of full behavioral normalization. The possible involvement of prolactin and growth hormone in such behavioral improvement is discussed.

Evidence for a short feedback of prolactin on the tubero-infundibular endings: differential effect on the release of [ h]gamma-aminobutyric Acid and [ h]dopamine from superfused median eminence

Abstract Release experiments were undertaken on the tubero-infundibular terminals of the hypothalamic median eminence. Isolated median eminences were preloaded with [(3) H]y-aminobutyric acid (GABA) or [(3) H]dopamine in such conditions that non-specific uptake was pharmacologically impaired. Each median eminence was continuously superfused with Krebs bicarbonate medium and tritium releases were evoked with high potassium solutions. The evoked release was estimated by measuring the total radioactivity of which actual dopamine and GABA represented more than 60% and 80%, respectively. The experiments were done by applying two successive 5-min pulses of potassium, the second in the presence of prolactin. These stimulations were submaximal: for each, a KCl concentration was chosen that gave about half the maximal response (45 mM for GABA and 30 mM for dopamine); the responses were suppressed by calcium-free medium. Rat prolactin induced a significant increase of dopamine-evoked release in a dose-dependent manner with a maximum (30%) at 25 mug prolactin/ml of medium. Over the same range of concentration a significant but still weak (10%) increase of evoked GABA was observed only with the highest prolactin concentration (25 mug prolactin/ml). At the same concentrations, ovine prolactin induced a markedly reduced effect on dopamine release and had no effect on GABA release. Spontaneous releases of dopamine and GABA were not affected by prolactin. These results show that prolactin enhances acutely and directly, but differently, the evoked release of GABA and dopamine by the median eminence. This differential effect may reflect either a differential regulation or a greater heterogeneity of GABAergic endings. Our data give support for a short feedback exerted by prolactin directly on the tubero-infundibular neurons.

The importance of dopamine in the pathogenesis of experimental prolactinomas

The factors responsible for the production of prolactin-secreting tumors are obscure. One hypothesis, that chronic loss of dopamine control from the hypothalamus may be associated with prolactinoma formation, was tested. Female adult Fischer 344 rats were subjected to ovariectomy and were then given subcutaneous implants of diethylstilbestrol (DES) Silastic capsules to produce lactotrophic hyperplasia. Sequential studies assessed the neuronal activity of the tuberoinfundibular dopaminergic neurons of the arcuate nucleus of the hypothalamus (A12) during and after this estrogen-induced pituitary growth. Immunocytochemical staining for tyrosine hydroxylase was used as a marker for dopamine synthesis, plasma radioimmunoassay provided plasma prolactin levels, and magnetic resonance imaging and histological studies were performed to examine the structural changes occurring in the pituitary gland. Animals were sacrificed from 3 to 67 days after DES implantation. To determine the reversibility of the estrogen-induced changes, rats were also sacrificed at different time intervals after the removal of 30-, 40-, or 60-day DES implants. After 30 days of DES treatment, plasma prolactin levels increased 40-fold and pituitary weight increased more than threefold. Tyrosine hydroxylase immunoreactivity diminished gradually and was almost completely depleted at 30 days. Pituitary histology revealed marked prolactin cell hyperplasia. These changes were completely reversible; removal of the capsule after 30 days resulted in eventual normalization of plasma prolactin levels and pituitary size and in restoration of tyrosine hydroxylase immunoreactivity in the A12 region. Sixty days of DES treatment produced large hemorrhagic tumors with sustained high plasma prolactin levels and an irreversibly distorted A12 area. These observations suggest that in these animals loss of dopamine regulation secondary to estrogen stimulation initially produces prolactin hyperplasia but that prolonged loss leads to adenoma formation.

Yawning behavior in male rats is associated with decreases in in vivo DOPAC efflux from the caudate nucleus

Young adult male rats were implanted with a push-pull cannula aimed at the dorsal and rostral areas of the caudate nucleus. Perfusate samples were collected at two-minute intervals for approximately one hour and assayed for DOPAC concentrations. Simultaneously, yawning, penile erections and grooming behavior were recorded. Yawns were induced by systemic prolactin or apomorphine injections. While mean DOPAC efflux was elevated following prolactin (PRL) and apomorphine decreased mean DOPAC efflux as expected, yawns and penile erections induced by both compounds were associated with rapid momentary decreases in DOPAC efflux in these living animals. Although yawning was associated with significant decreases in DOPAC output, not every momentary DOPAC decrease was associated with a yawn, suggesting that the 'yawning generator' most likely requires additional inputs for the expression of a yawn.

Effects of TRH and prolactin in the behavioral despair (swim) model of depression in rats

The neuropeptides thyrotropin releasing hormone (TRH) and prolactin (PRL), which affect various behaviors in animals, showed "antidepressant" properties in an experimental model of depression. Subcutaneous administration of TRH reduced the total immobility time of rats tested in the despair (constrained swim) test and potentiated the anti-immobility effect of intraperitoneally administered desimipramine (DMI). This effect was not mimicked by the peripheral injection of TSH, T3 or T4. Hyperprolactinemia induced by pituitary homografts under the kidney capsule and the intracerebroventricular injection of PRL also potentiated the DMI-induced reduction of total immobility time of rats in the despair test and exerted "antidepressant" effects in aged rats.

Extra-hypothalamic dopamine is not involved in the effects of hyperprolactinemia on male copulatory behavior

Experiments were performed to determine if the inhibition of copulatory behavior observed in male rats with chronically elevated serum prolactin levels (hyperprolactinemia) is associated with changes in central dopaminergic function in the nigrostriatal and mesolimbic systems. Chronic hyperprolactinemia, induced by ectopic pituitary grafts, inhibited sexual activity but was not associated with changes in locomotor activity, serotyped behavior in response to various doses of apomorphine, or 3H-spiroperidol binding to striatal homogenates. However, open-field defecation was reduced in the pituitary grafted animals. The results of the present study show that changes in nigrostriatal dopamine receptor sensitivity do not contribute to the inhibition of sexual behavior in hyperprolactinemic male rats. In addition, these results also demonstrate that the effects of hyperprolactinemia are relatively specific to copulatory behavior and appear not to involve general behavioral suppression.

Hyperprolactinemia-induced modifications in vasoactive intestinal peptide binding site densities in the rat central nervous system and pituitary gland: evidence for an interaction between estradiol-17 beta and prolactin effects

Hyperprolactinemia was induced in ovariectomized rats by implanting estradiol-17 beta pellets, grafting extrapituitaries, or by a combination of both treatments. Subsequently, the effect of increasing plasma prolactin levels on both central and pituitary receptors for vasoactive intestinal peptide (VIP), a neuropeptide known to stimulate prolactin release was investigated. The results obtained by quantitative autoradiography show that the density of VIP binding sites is modified in restricted areas of the central nervous system (striatum, several cortical, thalamic and limbic structures) and in the pituitary in hyperprolactinemic animals. The present results suggest that changes in plasma prolactin levels may control VIP receptor site density in both brain and pituitary. Moreover, direct effects of estradiol-17 beta and possible interactions between estradiol-17 beta and prolactin are observed on both brain and pituitary VIP binding sites.

Comparison of the effect of prolactin on dopamine release from the rat dorsal and ventral striatum and from the mediobasal hypothalamus superfused in vitro

In this paper, by means of a superfusion technique, we have examined the effect of ovine prolactin on dopaminergic neurons innervating the dorsal striatum, ventral striatum and the mediobasal hypothalamus of male rats. Fragments from dorsal striatum were superfused with ovine prolactin dissolved in normal medium (Krebs-Ringer phosphate) or medium containing tetrodotoxin (TTX, 10(-6) M). Ovine prolactin stimulated the in vitro release of dopamine from dorsal and ventral striatal fragments. In dorsal striatal fragments a linear dose-dependent dopamine release was observed only when fragments were superfused with Krebs-Ringer phosphate-TTX medium. In addition, [Leu5]enkephalin (10(-6) and 10(-5) M) decreased the prolactin-induced in vitro dopamine release from dorsal striatal fragments superfused with Krebs-Ringer phosphate-TTX medium. Ovine prolactin (10(-9)-10(-5) M) did not elicit changes in dopamine, 3,4-dihydroxyphenylacetic acid or 5-hydroxyindoleacetic acid outputs from mediobasal hypothalamic fragments superfused with Krebs-Ringer phosphate medium containing TTX. The possible regulatory mechanisms of ovine prolactin on dopaminergic neurons are discussed.

Antagonism of behavioral effects of bromocriptine by prolactin in female cats

The effects of the dopaminergic agonist bromocriptine (BC) and exogenously administered prolactin (PRL) on the spontaneous behavior of female cats were investigated. The objective was to test whether BC-induced behavioral effects may be antagonized by PRL. BC (6 mg/kg ip) administration induced abnormal behaviors such as limb flicks, abortive grooms, head/body shakes, and hallucinatory-like behavior/escape as well as excessive grooming. PRL (5 mg/kg ip) administration induced biphasic changes in grooming. The first change was an increase in grooming frequency averaging 256% of baseline control values and lasting for 1 h. This change was followed by reductions in grooming of 75 and 82.5% below baseline during Hours 2 and 3 postinjection, respectively. Combined BC and PRL treatment antagonized the frequency of BC-induced motor effects such as limb flicks, abortive grooms, and head/body shakes. Limb flicks occurred nine times more often 2 h after BC alone than after BC and PRL. The combined treatment also antagonized the excessive grooming observed after separate administrations of BC and PRL. The observed interactions between PRL and BC behavioral effects support the notion that PRL may be an important modulator of dopamine-dependent motor behavior in female cats.

Influence of aging on catecholamine levels, accumulation, and release in F-344 rats

Neuronal function in selected brain areas has been evaluated in Fischer 344 rats aged 2-4 months, 11-14 months and 21-26 months. In vitro release of 3H-norepinephrine from hypothalamus and occipital cortex and 3H-dopamine from striatum has been evaluated using potassium, amphetamine, and field-stimulation. In vitro uptake of 3H-catecholamines has been evaluated in the same tissues. Catecholamine levels were measured in six brain areas: hypothalamus, striatum, cortex, cerebellum, brainstem and midbrain. Significant age-related decreases of NE levels, uptake, and release to high frequency stimulation were seen in the hypothalamus. The decreases in 3H-NE uptake and NE levels in the hypothalamus were apparent at 12 months, whereas the decrease in 3H-NE release after high frequency stimulation was seen in the senile rats.

Hyperprolactinaemia does not alter specific striatal 3H-spiperone binding in the rat

We have investigated the effect of single injections (1 mg/animal i.v.) of prolactin or vehicle, and repeated depot (0.125-1.0 mg/animal/day) or bolus (1.0 mg/animal/day) administration of prolactin or vehicle for 6 days to adult male rats. The density (Bmax) and affinity (Kd) of specific striatal [3H]spiperone binding was not changed by any of the prolactin treatment schedules used. Allogeneic transplants of anterior pituitary glands resulted in an increased concentration of circulating prolactin but did not alter the density or affinity of specific striatal [3H]spiperone binding in male rats, measured 2 weeks following the operation. Prolactin did not displace specific striatal [3H]spiperone binding when incorporated in vitro. Any effect of prolactin on striatal dopamine receptor function appears to be exerted only by high, non-physiological concentrations, and such effects are difficult to reproduce.

Reduced tuberoinfundibular dopaminergic neuronal function in rats after long-term withdrawal of estrogen treatment

Hypothalamic fragments from female rats treated repeatedly with estradiol valerate (EV) and bearing prolactin (PRL)-secreting tumors contained, seven months after the last EV injection, lower concentrations of dopamine (DA) than age-matched controls. Depolarizing concentrations of K+ (35 mM) and amphetamine (50 microM) evoked in PRL-secreting tumor bearing rats an endogenous DA release significantly lower than in controls.

ACTH-(1-24) and alpha-MSH antagonize dopamine receptor-mediated inhibition of striatal dopamine and acetylcholine release

The effects of ACTH-(1-24), alpha-MSH and ACTH-(4-10) were studied on the electrically evoked release of 3H-dopamine and 14C-acetylcholine from striatal slices in the absence and presence of the dopamine receptor agonist TL-99. None of the peptides affected transmitter release when TL-99 was not present. ACTH-(1-24) and alpha-MSH concentration-dependently antagonized the inhibition of striatal transmitter release induced by dopamine receptor stimulation due to the presence of TL-99. ACTH-(1-24), 10(-7)M, reduced the TL-99-induced inhibition of the release of both dopamine and acetylcholine by approximately 50%, and 5 X 10(-6) M ACTH-(1-24) restored the release fully to control values. alpha-MSH was less effective by a factor 20-30 in counteracting the release-inhibiting effect of TL-99. ACTH-(4-10) had no effect at any of the concentrations tested. These results show that ACTH/MSH-like neuropeptides may act by modulating dopamine receptor functions in rat striatum.

Avoidance behavior and plasma prolactin levels in lergotrile mesylate treated rats

We have previously shown that psychological factors play a major role in control of prolactin (PRL) secretion, and that PRL increases during shock-motivated avoidance conditioning. In the present studies, we examined whether we could attenuate acquisition performance by suppressing the PRL increase during avoidance testing. Rats were tested daily in a shuttle box. They were presented with a light stimulation followed by an electric footshock. During each trial, the rats were given the opportunity to escape the footshock by moving to a safe side of the box. Movement to the appropriate location after the warning signal (light) begins, but before the onset of the footshock, constitutes a conditioned avoidance response (CAR). Experimental rats were fitted with an intraperitoneal osmotic minipump which delivered lergotrile mesylate (LM), 0.69 mg/kg/day. Blood samples were collected from an indwelling cannula and analyzed by radioimmunoassay. Administration of LM blocked the PRL increase that occurred during early avoidance testing, but did not alter the acquisition of a CAR. These data do not support the idea that PRL acts to facilitate acquisition of avoidance behavior.

Effects of estrogen on the basal ganglia

Recent research suggests that estrogen regulates the activity of dopamine-containing fibers originating in the midbrain and terminating in the basal ganglia, and/or dopamine-sensitive cells in the basal ganglia. The mechanism by which estrogen acts is not clear, since cells in neither of these regions concentrate estrogens. Nevertheless, estrogens clearly affect behaviors mediated by the basal ganglia, as illustrated in human patients suffering from extrapyramidal disorders. Both biochemical and behavioral research in animals has confirmed that estrogen modulates basal ganglia function, but there has not been agreement concerning either the locus, the direction, or the mechanism of its action. These topics are the focus of this review. The effects of estrogen on behaviors mediated by DA in the basal ganglia depend on the dose of estrogen administered, the time interval between estrogen treatment and testing, the behavior measured, and the part of the basal ganglia from which the behavior is elicited. A high dose of estrogen results in an initial suppression and later enhancement of DA-related behaviors elicited from the striatum. However, no later enhancement of these behaviors occurs if a low dose of estrogen is given. Even after low doses of estrogen, the latency to behavioral suppression varies depending upon the behavior measured. These varying latencies suggest that more than one mechanism is involved in the effects of estrogen on basal ganglia output. In addition, estrogen may also act on some regions in the mesolimbic DA system. While estrogen may act indirectly via the catechol estrogens and prolactin, it has been demonstrated that estrogen can act directly on the striatum. These findings are related to the effects of estrogen on human extrapyramidal disorders.

In vitro dopamine biosynthesis in the median eminence of rat hypothalamic slices: involvement of voltage-dependent Ca2+ channels

It was investigated whether Ca2+ is involved in the regulation of basal depolarization-induced dopamine biosynthesis in tuberoinfundibular dopaminergic neurons. The rate of dopamine biosynthesis was estimated by in vitro dihydroxyphenylalanine (DOPA) synthesis in the median eminence following incubation of rat hypothalamic slices with a DOPA decarboxylase inhibitor. Depolarizing agents such as K+ and veratridine increased the synthesis rate of DOPA in the median eminence in a dose-dependent manner with a maximal synthesis rate obtained at concentrations of 50 mM and 50 microM, respectively. Removal of Ca2+ and addition of EGTA (1 mM) into the medium did not influence basal DOPA synthesis in the median eminence but blocked the K+- and veratridine-induced DOPA synthesis. The Ca2+ channel blockers verapamil (100 microM) and Co2+ (4 mM) were effective in reducing the depolarization-induced DOPA synthesis. A23187 (10 microM), a Ca2+ ionophore, stimulated basal DOPA synthesis in the median eminence. On the other hand, tetrodotoxin (2 microM), a Na+ channel blocker, did not change the basal and K+-induced DOPA synthesis in the median eminence whereas it completely inhibited the veratridine-induced DOPA synthesis. These results suggest that depolarization-induced synthesis of dopamine in tuberoinfundibular neurons requires Ca2+ influx through voltage-dependent Ca2+ channels.

Effects of endogenous hyperprolactinaemia on opiate-induced behavioral changes in rats

Endogenous hyperprolactinaemia, as induced by pituitary homografts under the kidney capsule, was followed by an inhibition of heroin self-administration in rats. This effect was mimicked by intracerebroventricular (i.c.v.) injection of rat prolactin (PRL) or peripheral administration of a hyperprolactinaemia-inducing drug, domperidone. I.c.v. injection of anti-PRL serum totally abolished the effect of hyperprolactinaemia on heroin self-administration behavior and facilitated it in rats with normal plasma PRL levels. The development of tolerance to morphine was facilitated in hyperprolactinaemic animals or in rats injected i.c.v. with rat PRL. This effect was abolished in hyperprolactinaemic animals by i.c.v. injection of anti-PRL serum. Furthermore, rats with normal plasma PRL levels showed an inhibited development of tolerance to morphine after i.c.v. injection of anti-PRL serum. Morphine-induced analgesia appeared to be potentiated in hyperprolactinaemic rats tested in the hot-plate test. It is concluded that endogenous hyperprolactinaemia can affect opiate-induced behavioral changes in rats, possibly through central mechanisms involving opioid transmission.

Genetic effects and sexual dimorphism in tyrosine hydroxylase activity in two mouse strains and their reciprocal F1 hybrids

This study further analyzed the environmental and genetic mechanisms underlying the previously reported strain differences in tyrosine hydroxylase (TH) activity in the nigrostriatal and hypothalamic dopamine system of the BALB/cJ (B) and CBA/J (C) inbred mouse strains and related behavioral processes using parental and reciprocal F1 hybrid generations. Significant strain differences were found in both sexes in all the measured characters. Comparing males and females, sexual dimorphisms were found in TH activity of substantia nigra (SN), corpus striatum (CS) and hypothalamus (H), and in exploratory behavior (IE). Presence of sexual dimorphism was genotype dependent, with the exception of TH activity in H. Major components of strain differences, maternal effects and additive gene effects, were separated by biometrical genetic methods. The analysis indicated that significant maternal effects were present in TH activity of TH and CS with a trend towards this phenomenon in the SN. Additive gene effects were significant in all characters and various degrees of dominance were expressed in the hybrids in TH activity of SN and CS, as well as in behavioral traits, IE and spontaneous locomotion (SL). All the biochemical and behavioral parameters were expressed at lower levels in CBA/J than in BALB/cJ mice and reciprocal F1 hybrids took intermediate positions between the two parental strains for all phenotypes examined, with the exception of IE, where complete dominance was found in (CXB)F1 females. These results are consistent with the hypothesis that some of the genes affecting TH activity in brain dopamine systems contribute to the expression of dopamine mediated behaviors. Our analysis also indicates the possibility that the maternal effects on TH activity in CS and SN are the consequences of X-chromosome linked gene effects. We suggest that the influence of the X-chromosome linked gene(s) is dependent upon the action of gonadal steroids during the critical period of ontogenesis, and X-chromosome linked gene(s) play a major role in the genotype dependent expression of sexual dimorphism in TH activity.

Prolactin secretion and biological activity in females with galactorrhoea and normal circulating prolactin concentrations at rest

Prolactin secretion and biological activity have been investigated in 20 females with persistent idiopathic galactorrhoea who had normal resting serum prolactin levels at presentation. Results were compared with those in 34 normal controls. Hyperprolactinaemia, which was persistent in one and intermittent in the other, developed in two patients over an observation period of 1.5 to 8.5 years. Resting prolactin levels stayed normal in the remaining eighteen who were further investigated. Menstruation was disordered in only six of the 18, while ovulation occurred (serum progesterone greater than 20 nmol/l) in all seven patients who were studied over a 5 week period. Serum prolactin concentrations over 24 h were similar in patients and controls (24 h mean +/- SEM prolactin, 288 +/- 36 mU/l, patients, n = 7; 291 +/- 21 mU/l, controls, n = 9) as were prolactin levels estimated twice weekly for 5 weeks. Prolactin responses to thyrotrophin-releasing hormone, 200 micrograms (at 20 min, 2417 +/- 658 mU/l, patients, n = 7; 2113 +/- 424 mU/l, controls, n = 8), the dopamine antagonist, domperidone, 10 mg (at 30 min, 5949 +/- 536 mU/l, patients, n = 7; 5858 +/- 460 mU/l, controls, n = 8) and insulin-induced hypoglycaemia (at 60 min, 1441 +/- 551 mU/l, patients, n = 7; 1298 +/- 183 mU/l, controls, n = 7) were similar in patients and controls. Two different radioimmunoassays using two different antisera gave similar estimates of serum prolactin levels and prolactin bioactivity in serum was normal in an in-vitro bioassay based on the ability of prolactin to stimulate proliferation of Nb2 node rat lymphoma cells (basal bioassayable prolactin, patients 355 +/- 43 mU/l, n = 10; controls 348 +/- 64 mU/l, n = 7). Metabolic abnormalities similar to those previously noted in hyperprolactinaemia were observed in the patients' 24 h profiles. These included mild hyperglycaemia (24 h mean +/- SEM glucose, 5.47 +/- 0.08 mmol/l, patients; 5.05 +/- 0.14 mmol/l, controls; P less than 0.05) and elevations in circulating lactate, pyruvate and alanine. Blood glycerol was decreased (24 h mean +/- SEM, 0.044 +/- 0.004 versus 0.058 +/- 0.004 mmol/l, P less than 0.05). In the majority of patients with idiopathic galactorrhoea, prolactin concentrations, regulation of secretion and bioactivity in vitro are normal. The galactorrhoea and metabolic abnormalities suggest increased tissue sensitivity to the lactogenic and metabolic actions of prolactin, while ovarian cyclical function is relatively spared.

Effects on growth hormone secretion following intravenous and subcutaneous injections of growth hormone-releasing factor (hGRF-44 NH2): comparison of immunoreactive plasma GRF levels

The effects of subcutaneous administration of three doses of human growth hormone-releasing factor (hGRF-44 NH2 or hGRF) at doses of 100, 300 and 600 micrograms were studied in six normal young men. GH responses obtained with 100 and 300 micrograms were negligible. In contrast, the 600 micrograms dose gave a profile of response comparable in timing and magnitude to that obtained with i.v. hGRF at maximal effect doses (20, 80, 100 micrograms). Plasma immunoreactive hGRF levels (IR-hGRF) were compared after s.c. and i.v. hGRF. Mean maximal plasma concentrations were comparable with s.c. 600 micrograms and i.v. 20 micrograms. Peaks occurred earlier with i.v. hGRF (5 min as opposed to 15 min): however, return to undetectable values was obtained between 90 and 120 min after s.c. or i.v. injections. These data suggest a great loss of the peptide between the subcutaneous space and blood, without delayed absorption. High variability in plasma IR-hGRF concentrations between the subjects after the same s.c. doses was observed.

Further evidence for the presence of specific binding sites for prolactin in the rabbit brain. Preferential distribution in the hypothalamus and substantia nigra

In the present research we have extended our work on the presence of binding sites for prolactin in the rabbit brain focusing our attention on the brain areas with high dopamine cell bodies density. Among these areas the hypothalamus showed the highest specific binding of labeled ovine prolactin (oPRL). Clearly detectible specific binding was observed also in substantia nigra, whereas in other brain regions the specific binding was very small, except for the striatum where a low but not negligible binding was found in female rabbits. The binding of 125I-oPRL showed a hormonal specificity and Scatchard analysis of the binding showed no clear difference in dissociation constant (Kd) between hypothalamus, nigra and striatum.

Prolactin and sexual behavior: a review

Despite the large body of evidence showing that prolactin (PRL) can suppress sexual behavior in humans and rodents, it is still unclear how this hormone affects sexual capacity of male subjects. Few studies have been performed on the effects of PRL on female sexual behavior. Short-term hyperprolactinaemia seems to facilitate some elements of sexual behavior in male rats. Furthermore, contrasting finding exist on the effects of drug-induced hyperprolactinaemia on sexual capacity of male animals. The possible mechanisms of action (on peripheral organs, endocrine, central) or PRL on male behavior are discussed in details.

Chronic hyperprolactinemia causes progressive changes in hypothalamic dopaminergic and noradrenergic neurons

Studies were undertaken to evaluate the effects of chronic hyperprolactinemia (HYP) on catecholamine concentrations and turnover rates in brain regions of the female rat. HYP was induced by inoculation of tissue derived from the prolactin secreting MtTW15 tumor. When serum prolactin (PRL) levels were moderately elevated, medial basal hypothalamus (MBH) dopamine (DA) turnover was enhanced and DA concentrations were moderately reduced. Later, as serum PRL levels increased to greater than 10 micrograms/ml, DA concentrations were further reduced and DA turnover was concomitantly reduced to below pre-tumor levels. In the preoptic area-anterior hypothalamus (POA-AH), DA concentrations were reduced as PRL levels increased and this was associated with a reduction in DA turnover. Between 5 and 8 weeks of tumor growth, DA turnover remained low, but DA concentrations increased. In the neurointermediate lobe of the pituitary (NIL) the tumor reduced DA turnover at 5 weeks only. Norepinephrine (NE) turnover, but not concentration, was reduced in both the POA-AH and MBH. Surgical removal of the tumor at 5 weeks of growth reduced serum PRL levels to near normal, but MBH DA concentrations and turnover remained depressed while POA-AH and NIL DA levels and turnover increased. Despite removal of the tumor, NE turnover remained depressed in both the MBH and POA-AH. These studies indicate that severe chronic HYP causes progressive alterations in hypothalamic catecholamine neurons which are not reversed by normalization of serum PRL levels. These results suggest that chronic HYP can cause long-lasting effects on some DA and NE neuronal systems.

Effects of mastectomy and vagotomy on grooming behavior of the rat: possible involvement of prolactin

Surgical interventions such as unilateral mastectomy or vagotomy affect plasma prolactin (PRL) levels. Right-side mastectomized rats exhibiting high levels of plasma PRL showed increased grooming behavior. Left-side mastectomized rats with low levels of plasma PRL performed poor grooming activity. Bilateral mastectomy that caused a slight increase in plasma PRL levels was followed by enhanced grooming behavior. Both left and right vagotomy resulted in a significant increase in plasma PRL levels. However, only left-side vagotomized rats exhibited increased levels of grooming behavior, while no difference between right-side vagotomized rats and controls occurred. It is possible that changes in plasma PRL levels induced by surgical interventions affect grooming activity of the rat. However, the hypothesis that the integrity of peripheral organs is important for the display of grooming behavior cannot be ruled out.

Effect of cholinomimetics on the release and uptake of L-[3H] glutamic acid in rat neostriatum

The effects of cholinomimetics on the release and uptake of L-glutamic acid have been studied by means of local superfusion of rat neostriatum (in vivo) and using striatum slices (in vitro). Cholinomimetics regulate the release of L-glutamic acid through both muscarinic (m-) and nicotinic (n-) choline receptors according to the mechanism of negative feedback. Cholinomimetics regulate release through presynaptic receptors. The effect of n-cholinomimetics is mediated by neostriatum interneurons.

Enhancement of hypophysectomy-induced dopamine receptor hypersensitivity in male rats by chronic haloperidol administration

It has been reported that hypophysectomy (HYPOX) would antagonize the development of a neuroleptic-induced dopamine receptor hypersensitivity, and suggested that the neuroleptic-induced dopamine receptor hypersensitivity may be mediated by the neuroleptic-induced hyperprolactinemia. Conversely, we and others have reported on the ability of HYPOX animals to develop a neuroleptic-induced dopamine receptor hypersensitivity. The present study was undertaken to define the possible role(s) of prolactin in the modulation of striatal dopamine receptor sensitivity. The data from these studies indicate: that HYPOX alone will result in the development of a striatal dopamine receptor hypersensitivity; that the HYPOX-induced dopamine receptor hypersensitivity could be increased by the chronic administration and withdrawal of haloperidol; that administration of prolactin to HYPOX rats would partially antagonize the development of the neuroleptic-induced dopamine receptor hypersensitivity; and that the administration of prolactin alone had minimal effects on the apomorphine-induced behavior or neurochemistry of the HYPOX animals. These results suggest that the neuroleptics do not require the presence of a pituitary secretion (specifically, prolactin) to induce a striatal dopamine receptor hypersensitivity; however, they do indicate that a pituitary secretion, perhaps prolactin, may have the ability to modulate striatal dopamine sensitivity.

Influence of sex steroids and prolactin on haloperidol-induced catalepsy

The effects of sex steroids and prolactin on haloperidol-induced catalepsy were investigated in male rats. Repeated administration with estradiol benzoate (5 micrograms/rat, twice daily for 10 days) significantly potentiated catalepsy induced by 0.25 or 0.5 mg/kg haloperidol, but no effect was observed 10 min or 1 h after a single injection of estradiol benzoate (5 or 50 micrograms/rat). Conversely, a single administration with the catecholestrogen 2-hydroxyestradiol (50 micrograms/rat) significantly increased haloperidol-induced catalepsy, suggesting that catecholestrogens may directly interfere with nigrostriatal dopaminergic transmission. Haloperidol-induced catalepsy has been found to be attenuated in conditions of hyperprolactinaemia resulting from anterior pituitary isograft underneath the kidney capsule. This is consistent with the hypothesis that prolactin may stimulate nigro-striatal dopaminergic function. Results obtained also indicate that medroxy-acetate progesterone, a progesterone derivative, may influence haloperidol-induced catalepsy. Specifically, a single administration with medroxy-acetate progesterone (5 mg/kg, i.p.) enhanced catalepsy but opposite effects were observed after repeated administration of medroxy-acetate progesterone (5 mg/kg, i.p., once a day for 7 days).

Uptake and release of [3H]dopamine by the median eminence: evidence for presynaptic dopaminergic receptors and for dopaminergic feedback inhibition

The accumulation and release of [3H]dopamine by the median eminence in vitro was studied after treatments with different pharmacological agents, to determine whether such a procedure would be useful for measuring neuronal activity in the tuberoinfundibular dopaminergic system. The accumulation of [3H]dopamine was temperature, time, and sodium dependent, and reduced by unlabelled dopamine and by a potent dopamine uptake blocker, nomifensine. The outflow of tritium was studied after blocking the oxidative deamination of dopamine by nialamide. The outflow of tritium was elicited consistently by biphasic square wave electrical pulses and by high molarity potassium ions. The response to electrical stimulation was dependent largely on calcium and partially on sodium. The response to high molarity potassium ions was reduced in the absence of calcium ions. The response to electrical stimulation was increased by nomifensine and by a dopaminergic antagonist, haloperidol, and was reduced by dopamine and by a dopaminergic agonist, piribedil. The inhibitory action of dopamine was antagonized by haloperidol. These results indicate the existence of uptake and release mechanisms in the tuberoinfundibular dopamine neurons, and suggest that dopamine may inhibit its own release via dopaminergic receptors. This in vitro method may be useful for measuring dopamine uptake and release by tuberoinfundibular dopaminergic neurons.

The effect of opioid drugs on the release of dopamine and 5-hydroxytryptamine from rat striatum following activation of nicotinic-cholinergic receptors

The effect of (Met5)enkephalin, (D-Ala2,D-Met5)enkephalin, (Leu5)enkephalin, (D-Ala2,D-Met5)enkephalin and morphine on the release of [3H]dopamine, endogenous dopamine and [3H]5-hydroxytryptamine produced by the nicotinic-cholinergic agonist, dimethylphenyl piperazinium iodide (DMPP), was examined in rat striatal slices. The DMPP-induced release of [3H]dopamine and endogenous dopamine was reduced by the presence of (Met5)enkephalin, (D-Ala2,D-Met5)enkephalin (1-10 microM) or morphine (10 microM) but not by (Leu5)enkephalin or (D-Ala2,D-Leu5)enkephalin. The DMPP-induced release of [3H]5-hydroxytryptamine was reduced by (Leu5)enkephalin, (D-Ala2,D-Leu5)enkephalin, (Met5)enkephalin, (D-Ala2,D-Leu5)enkephalin (1-10 microM), and morphine (10 microM). All three opioids failed to alter the release of [3H]dopamine induced by field stimulation or potassium depolarization (30 microM). The inhibitory effects of opioid peptides and morphine demonstrated in the present study appear to be due to an initial interaction with nicotinic-cholinergic receptors in the striatum.

Role of prolactin-opiate interactions in the central regulation of pain threshold

Endogenous hyperprolactinaemia induced by anterior pituitary transplantation under the kidney capsule has been found to reduce the behavioural responsiveness to electrical footshock and to increase morphine-induced analgesia. The apparent analgesic effect of prolactin has been related to the stimulation of nigro-striatal dopaminergic transmission, as suggested by the increase in striatal dopamine turnover observed in hyperprolactinaemic rats. It seems likely that central opiate system is involved in the behavioural effects of prolactin. Thus, naloxone prevents the effects of hyperprolactinaemia on footshock responsiveness and heroin self-administration is decreased in hyperprolactinaemic rats.

Tumors producing chronic hyperprolactinemia do not alter [3H]spiperone binding and dopamine turnover in the corpus striatum of the female rat

We studied the effect of prolactin on the dopamine (DA) receptor density and DA turnover of the rat corpus striatum. Chronic hyperprolactinemia was produced in female rats by implanting prolactin-secreting pituitary tumors. Even in the presence of circulating prolactin concentrations that were two to three orders of magnitude greater than normal, there was no apparent change in the affinity or maximal site numbers of DA receptors in the corpus striatum. In concert with this observation, prolactin also failed to alter DA turnover in the corpus striatum as reflected in the DA/DOPAC ratio. Under the conditions of our experiments, there was no evidence to confirm previous observations of increased density of DA receptors of the corpus striatum in the hyperprolactinemic state.

Prolactin increases the activity of tuberoinfundibular and nigroneostriatal dopamine neurons: prolactin antiserum inhibits the haloperidol-induced increases in dopamine synthesis rates in median eminence and striatum of rats

The role of PRL in mediating the haloperidol-induced increase in tuberoinfundibular dopamine synthesis rate was assessed by studying the effects of administration of PRL antiserum. Antiserum to PRL generated in rabbits and not cross-reacting with other anterior pituitary hormones was administered IV to adult, male rats which received haloperidol 2.5 mg/kg or tartaric acid vehicle SC 22 hr and 12 hr before measurement of dopamine turnover. Comparable groups of haloperidol or vehicle-treated animals received normal rabbit serum as control. Dopamine synthesis or turnover rate was estimated by measurement of accumulation of L-dihydroxyphenylalanine following inhibition of L-aromatic amino acid decarboxylase with m-hydroxybenzylhydrazine. Haloperidol increased median eminence dopamine synthesis rate, and PRL antiserum completely prevented this effect, supporting the thesis that the haloperidol-induced increase in tuberoinfundibular dopamine turnover is mediated by PRL. PRL antiserum did not alter basal median eminence dopamine synthesis rate in male rats. In addition to its effect in median eminence, PRL antiserum blunted the haloperidol-induced increase in striatal dopamine synthesis rate, suggesting that the haloperidol-induced increase in nigroneostriatal dopamine turnover is mediated in part by PRL. Neither haloperidol nor PRL antiserum altered serotonin synthesis rate in mediobasal hypothalamus or striatum. The data provide further support for a mechanism by which PRL can regulate its own secretion. They also suggest that prolactin alters the activity not only of tuberoinfundibular but also of nigroneostriatal neurons.

Prolactin-enhanced grooming behavior: interaction with ACTH

The interaction between prolactin (PRL) and ACTH in enhancing grooming behavior after intracerebroventricular (i.c.v.) administration was investigated in intact and endogenously hyperprolactinaemic rats. In intact rats, 4 h after the i.c.v. injection of rat PRL or ACTH, a subsequent administration of ACTH or rat PRL did induce similar excessive grooming as observed after the first injections. In hyperprolactinaemic rats, which displayed excessive grooming 12 days after homografting adenopituitaries under the kidney capsule, i.c.v. injection of rat PRL failed to enhance further the grooming activity while this behavior was substantially enhanced by i.c.v. injection of ACTH1-24. Twenty-six days after surgery, when the grooming activity of hyperprolactinaemic rats was at the same level as control animals, the i.c.v. injection of rat PRL was effective in inducing excessive grooming in control but not in hyperprolactinaemic animals. In contrast, at this time the i.c.v. injection of ACTH1-24 induced again excessive grooming in both hyperprolactinaemic and control rats. Accordingly, there was no cross-tolerance between PRL and ACTH in inducing excessive grooming, but hyperprolactinaemic rats became hyposensitive to exogenous PRL. It is suggested that although PRL and ACTH may affect common neurotransmitter systems in the brain, the two hormones probably act on independent neural mechanisms in inducing excessive grooming in the rat.