Salsolinol is a putative endogenous neuro-intermediate lobe prolactin-releasing factor

Patterns of prolactin secretion

Prolactin is pleotropic in nature affecting multiple tissues throughout the body. As a consequence of the broad range of functions, regulation of anterior pituitary prolactin secretion is complex and atypical as compared to other pituitary hormones. Many studies have provided insight into the complex hypothalamic-pituitary networks controlling prolactin secretion patterns in different species using a range of techniques. Here, we review prolactin secretion in both males and females; and consider the different patterns of prolactin secretion across the reproductive cycle in representative female mammals with short versus long luteal phases and in seasonal breeders. Additionally, we highlight changes in the pattern of secretion during pregnancy and lactation, and discuss the wide range of adaptive functions that prolactin may have in these important physiological states.

Stimulatory effect of dopamine derivative, salsolinol, on pulsatile luteinizing hormone secretion in seasonally anestrous sheep: Focus on dopamine, kisspeptin and gonadotropin-releasing hormone

In the present study, there was testing of the hypothesis that a centrally administered dopamine (DA) derivative, salsolinol, could affect pulsatile luteinizing hormone (LH) secretion in seasonally anestrous sheep by affecting the neuronal components of the estradiol (E2) negative feedback. In two experiments performed during early spring (increasing day length - March/April), salsolinol or Ringer-Locke solution (control) were administered into the third brain ventricle (IIIv): 1) in several injections for three consecutive days; and 2) in several hour-long infusions. In addition to determining the LH concentration (in both experiments), the abundances of gonadotropin-releasing hormone (GnRH) and kisspeptin mRNA were examined in the hypothalamus and LHβ subunit mRNA in the pituitary (Experiment 1). In Experiment 2, concentrations of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) were determined in perfusates collected from the infundibular nucleus/median eminence (IN/ME) by the push-pull method. In both experiments, salsolinol increased both LH pulse frequency (P < 0.05) and plasma LH concentration (P < 0.001) compared to controls. The injected salsolinol also increased (P < 0.05) the abundance of GnRH mRNA in the mediobasal hypothalamus and kisspeptin mRNA in the arcuate nucleus. The two doses of infused salsolinol decreased DA to undetectable concentrations and DOPAC concentration by 60% in perfusates collected from the IN/ME. In conclusion, exogenous salsolinol functioning centrally stimulates pulsatile LH secretion in sheep during seasonal anestrus. It is suggested that salsolinol may have this effect by reducing the activity of the hypothalamic neuroendocrine dopaminergic system, which results in an increase in both kisspeptin and GnRH neurons activity.

Adaptive Modifications of Maternal Hypothalamic-Pituitary-Adrenal Axis Activity during Lactation and Salsolinol as a New Player in this Phenomenon

Both basal and stress-induced secretory activities of the hypothalamic-pituitary-adrenal (HPA) axis are distinctly modified in lactating females. On the one hand, it aims to meet the physiological demands of the mother, and on the other hand, the appropriate and stable plasma cortisol level is one of the essential factors for the proper offspring development. Specific adaptations of HPA axis activity to lactation have been extensively studied in several animal species and humans, providing interesting data on the HPA axis plasticity mechanism. However, most of the data related to this phenomenon are derived from studies in rats. The purpose of this review is to highlight these adaptations, with a particular emphasis on stress reaction and differences that occur between species. Existing data on breastfeeding women are also included in several aspects. Finally, data from the experiments in sheep are presented, indicating a new regulatory factor of the HPA axis-salsolinol-which typical role was revealed in lactation. It is suggested that this dopamine derivative is involved in both maintaining basal and suppressing stress-induced HPA axis activities in lactating dams.

Salsolinol: an Unintelligible and Double-Faced Molecule-Lessons Learned from In Vivo and In Vitro Experiments

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is a tetrahydroisoquinoline derivative whose presence in humans was first detected in the urine of Parkinsonian patients on l-DOPA (l-dihydroxyphenylalanine) medication. Thus far, multiple hypotheses regarding its physiological/pathophysiological roles have been proposed, especially related to Parkinson’s disease or alcohol addiction. The aim of this review was to outline studies related to salsolinol, with special focus on in vivo and in vitro experimental models. To begin with, the chemical structure of salsolinol together with its biochemical implications and the role in neurotransmission are discussed. Numerous experimental studies are summarized in tables and the most relevant ones are stressed. Finally, the ability of salsolinol to cross the blood–brain barrier and its possible double-faced neurobiological potential are reviewed.

Involvement of salsolinol in the suckling-induced oxytocin surge in sheep

During lactation, the main surge of oxytocin is induced by a suckling stimulus. Previous studies have shown that salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), a dopamine-derived compound, stimulates both the synthesis and the release of oxytocin in lactating sheep. The objective of the present study was to verify the hypothesis that salsolinol is involved in the mechanism that generates the oxytocin surge that occurs during suckling. Thus, a structural analogue of salsolinol, 1-methyl-3,4-dihydroisoquinoline (1MeDIQ), known to antagonize some of its actions, was infused into the third ventricle of the brain of lactating sheep nursing their offspring. Serial 30-min infusion of 1MeDIQ (4 × 60 μg/60 μL) or vehicle were administered at 30-min interval from 10 AM to 2 PM. The experimental period in every ewe consisted of a nonsuckling period (10 AM-12 PM) and a suckling period (12 PM-2 PM). Blood samples were collected every 10 min, to measure plasma oxytocin concentration by RIA. In control sheep, oxytocin surges of high amplitude were observed during the suckling period. The oxytocin surges induced by suckling were significantly (P < 0.01) diminished in sheep receiving 1MeDIQ infusions as compared to those that received control infusions. However, no significant effect of 1MeDIQ was observed on basal oxytocin release, before suckling. Furthermore, oxytocin release, as measured by the area under the hormone response curve (AUC), was significantly decreased by the administration of 1MeDIQ during the suckling period. This study shows that elimination of the effect of salsolinol within the central nervous system of lactating sheep attenuates the oxytocin surge induced by suckling. Therefore, salsolinol may be an important factor in the oxytocin-stimulating pathway in lactating mammals.

Racemic Salsolinol and its Enantiomers Act as Agonists of the μ-Opioid Receptor by Activating the Gi Protein-Adenylate Cyclase Pathway

Background: Several studies have shown that the ethanol-derived metabolite salsolinol (SAL) can activate the mesolimbic system, suggesting that SAL is the active molecule mediating the rewarding effects of ethanol. In vitro and in vivo studies suggest that SAL exerts its action on neuron excitability through a mechanism involving opioid neurotransmission. However, there is no direct pharmacologic evidence showing that SAL activates opioid receptors.

Methods: The ability of racemic (R/S)-SAL, and its stereoisomers (R)-SAL and (S)-SAL, to activate the μ-opioid receptor was tested in cell-based (light-emitting) receptor assays. To further characterizing the interaction of SAL stereoisomers with the μ-opioid receptor, a molecular docking study was performed using the crystal structure of the μ-opioid receptor.

Results: This study shows that SAL activates the μ-opioid receptor by the classical G protein-adenylate cyclase pathway with an half-maximal effective concentration (EC₅₀) of 2 × 10⁻⁵ M. The agonist action of SAL was fully blocked by the μ-opioid antagonist naltrexone. The EC₅₀ for the purified stereoisomers (R)-SAL and (S)-SAL were 6 × 10⁻⁴ M and 9 × 10⁻⁶ M respectively. It was found that the action of racemic SAL on the μ-opioid receptor did not promote the recruitment of β-arrestin. Molecular docking studies showed that the interaction of (R)- and (S)-SAL with the μ-opioid receptor is similar to that predicted for the agonist morphine.

Conclusions: It is shown that (R)-SAL and (S)-SAL are agonists of the μ-opioid receptor. (S)-SAL is a more potent agonist than the (R)-SAL stereoisomer. In silico analysis predicts a morphine-like interaction between (R)- and (S)-SAL with the μ-opioid receptor. These results suggest that an opioid action of SAL or its enantiomers is involved in the rewarding effects of ethanol.

(R)-Salsolinol, a product of ethanol metabolism, stereospecifically induces behavioral sensitization and leads to excessive alcohol intake

Ethanol is oxidized in the brain to acetaldehyde, which can condense with dopamine to generate (R/S)-salsolinol [(RS)-SAL]. Racemic salsolinol [(RS)-SAL] is self-infused by rats into the posterior ventral tegmental area (VTA) at significantly lower concentrations than those of acetaldehyde, suggesting that (RS)-SAL is a most active product of ethanol metabolism. Early studies showed that repeated intraperitoneal or intra-VTA administration of (RS)-SAL (10 mg/kg) induced conditioned place preference, led to locomotor sensitization and increased voluntary ethanol consumption. In the present study, we separated the (R)- and (S)-enantiomers from a commercial (RS)-SAL using a high-performance liquid chromatography with electrochemical detection system fitted with a β-cyclodextrin-modified column. We injected (R)-SAL or (S)-SAL (30 pmol/1.0 μl) into the VTA of naïve UChB rats bred as alcohol drinkers to study whether one or both SAL enantiomers are responsible for the motivated behavioral effects, sensitization and increase in voluntary ethanol intake. The present results show that repeated administration of (R)-SAL leads to (1) conditioned place preference; (2) locomotor sensitization; and (3) marked increases in binge-like ethanol intake. Conversely, (S)-SAL did not influence any of these parameters. Overall, data indicate that (R)-SAL stereospecifically induces motivational effects, behavioral sensitization and increases ethanol intake.

Salsolinol Up-Regulates Oxytocin Expression and Release During Lactation in Sheep

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is a dopamine-derived compound present in the central nervous system and pituitary gland. Several previous studies on lactating sheep and rats have reported that salsolinol plays a crucial role in the regulation of prolactin secretion. The present study investigated the effects of salsolinol, which was infused into the third ventricle of the brain, on oxytocin expression and release in lactating sheep, 48 h after weaning of 8-week-old lambs. Serial 30-min infusions of salsolinol and vehicle were performed at 30-min intervals from 10.00 to 15.00 h. Blood samples were collected every 10 min. The supraoptic nucleus (SON), paraventricular nucleus (PVN) and posterior pituitary were collected immediately after the experiment. Expression levels of mRNAs for oxytocin and peptidylglycine α-amidating monooxygenase (PAM), the terminal enzyme in the oxytocin synthesis pathway, were measured using a real-time polymerase chain reaction. Oxytocin peptide content in the posterior pituitary was measured by an enzyme-linked immunosorbent assay, and plasma oxytocin concentration was measured by radioimmunoassay. Salsolinol treatment significantly up-regulated oxytocin and PAM gene expression in the SON (P < 0.01 and P < 0.05, respectively), PVN (P < 0.01 and P < 0.05, respectively) and posterior pituitary (P < 0.05 and P < 0.05, respectively). Oxytocin peptide content in the posterior pituitary and the area under the response curve of plasma oxytocin were significantly (P < 0.05 and P < 0.01, respectively) higher in salsolinol-treated sheep than in control animals. The present study shows for the first time that salsolinol stimulates oxytocin secretion during lactation in sheep.

Salsolinol: a potential modulator of the activity of the hypothalamic-pituitary-adrenal axis in nursing and postweaning sheep

The most well-known physiological action of salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is the stimulation of prolactin secretion, especially during lactation. In addition, our recent work demonstrated that salsolinol inhibits the stress-induced activity of the hypothalamic-pituitary-adrenal (HPA) axis in lactating sheep. Here, we investigated whether salsolinol regulates the basal activity of the HPA axis in lactating sheep and whether its inhibitory action on the stress-induced activity of the HPA axis is present during the postweaning period. The first experiment was performed during the fifth week of lactation, in which unstressed sheep received an intracerebroventricular infusion of an antagonistic analogue of salsolinol, 1-MeDIQ (1-methyl-3,4-dihydroisoquinoline). Simultaneously, the infundibular nucleus and/or median eminence was perfused using the push-pull method. Sheep that received 1-MeDIQ infusion showed significantly higher concentration of plasma ACTH during the second, third, and fourth hour (P < 0.001, P < 0.01, and P < 0.001, respectively) and cortisol during the third and fourth hour (P < 0.001 and P < 0.01, respectively) than did sheep that received control infusion. There was no significant difference in the mean perfusate corticotropin-releasing hormone concentration between the 1-MeDIQ and control treatments. In the second experiment, sheep received an intracerebroventricular infusion of salsolinol during the ninth week of lactation and 48 h after lamb weaning. A comparison between the control groups in the first and second experiments revealed that sheep after weaning (ninth week of lactation) had significantly higher mean ACTH (P < 0.001) and cortisol (P < 0.001) concentrations during the first 2 h of the experiment than the nursing females (fifth week of lactation) had. Salsolinol significantly reduced the increased concentrations of ACTH and cortisol (P < 0.01) in sheep after lamb weaning. However, there was no difference in the expression of proopiomelanocortin messenger RNA within the anterior pituitary between the control and salsolinol-treated groups. In conclusion, salsolinol regulates the basal activity of the HPA axis in lactating sheep. In addition, the HPA axis of postweaning females is more sensitive to stressors associated with the experimental procedures, and salsolinol attenuates ACTH and cortisol release in this phenomenon.

Neuroendocrine regulation of lactation and milk production

Prolactin (PRL) released from lactotrophs of the anterior pituitary gland in response to the suckling by the offspring is the major hormonal signal responsible for stimulation of milk synthesis in the mammary glands. PRL secretion is under chronic inhibition exerted by dopamine (DA), which is released from neurons of the arcuate nucleus of the hypothalamus into the hypophyseal portal vasculature. Suckling by the young activates ascending systems that decrease the release of DA from this system, resulting in enhanced responsiveness to one or more PRL-releasing hormones, such as thyrotropin-releasing hormone. The neuropeptide oxytocin (OT), synthesized in magnocellular neurons of the hypothalamic supraoptic, paraventricular, and several accessory nuclei, is responsible for contracting the myoepithelial cells of the mammary gland to produce milk ejection. Electrophysiological recordings demonstrate that shortly before each milk ejection, the entire neurosecretory OT population fires a synchronized burst of action potentials (the milk ejection burst), resulting in release of OT from nerve terminals in the neurohypophysis. Both of these neuroendocrine systems undergo alterations in late gestation that prepare them for the secretory demands of lactation, and that reduce their responsiveness to stimuli other than suckling, especially physical stressors. The demands of milk synthesis and release produce a condition of negative energy balance in the suckled mother, and, in laboratory rodents, are accompanied by a dramatic hyperphagia. The reduction in secretion of the adipocyte hormone, leptin, a hallmark of negative energy balance, may be an important endocrine signal to hypothalamic systems that integrate lactation-associated food intake with neuroendocrine systems.

Salsolinol, an endogenous compound triggers a two-phase opposing action in the central nervous system

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), an endogenous compound present in the brain, was suspected of participation in the etiopathogenesis of Parkinson’s disease, the most common serious movement disorder worldwide. In this study, we evaluated the effect of different (50, 100, and 500 µM) concentrations of salsolinol on markers of glutamate-induced apoptotic and neurotoxic cell damage, such as caspase-3 activity, lactate dehydrogenase (LDH) release, and the loss of mitochondrial membrane potential. Biochemical data were complemented with the cellular analysis, including Hoechst 33342 and calcein AM staining, to visualize apoptotic DNA-fragmentation and to assess cell survival, respectively. The assessment of all investigated parameters was performed in primary cultures of rat or mouse hippocampal and striatum cells. Our study showed that salsolinol had biphasic effects, namely, at lower concentrations (50 and 100 µM), it demonstrated a distinct neuroprotective activity, whereas in the highest one (500 µM) caused neurotoxic effect. Salsolinol in concentrations of 50 and 100 µM significantly antagonized the pro-apoptotic and neurotoxic effects caused by 1 mM glutamate. Salsolinol diminished the number of bright fragmented nuclei with condensed chromatin and increased cell survival in Hoechst 33342 and calcein AM staining in hippocampal cultures. Additionally, in the low 50 µM concentration, it produced a significant inhibition of glutamate-induced loss of membrane mitochondrial potential. Only the highest concentration of salsolinol (500 µM) enhanced the glutamate excitotoxicity. Ex vivo studies indicated that both acute and chronic administration of salsolinol did not affect the dopamine metabolism, its striatal concentration or α-synuclein and tyrosine hydroxylase protein level in the rat substantia nigra and striatum. Summarizing, the present studies exclude possibility that salsolinol under physiological conditions could be an endogenous factor involved in the neurogenerative processes; conversely, it can exert a protective action on nerve cells in the brain. These findings may have important implications for the development of the new strategies to treat or prevent neural degeneration.

Suckling and salsolinol attenuate responsiveness of the hypothalamic-pituitary-adrenal axis to stress: focus on catecholamines, corticotrophin-releasing hormone, adrenocorticotrophic hormone, cortisol and prolactin secretion in lactating sheep

In mammals, the responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to stress is reduced during lactation and this mainly results from suckling by the offspring. The suckling stimulus causes a release of the hypothalamic 1-metyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) (a derivative of dopamine), one of the prolactin-releasing factors. To investigate the involvement of salsolinol in the mechanism suppressing stress-induced HPA axis activity, we conducted a series of experiments on lactating sheep, in which they were treated with two kinds of isolation stress (isolation from the flock with lamb present or absent), combined with suckling and/or i.c.v infusion of salsolinol and 1-methyl-3,4-dihydro-isoqinoline (1-MeDIQ; an antagonistic analogue of salsolinol). Additionally, a push-pull perfusion of the infundibular nucleus/median eminence (IN/ME) and blood sample collection with 10-min intervals were performed during the experiments. Concentrations of perfusate corticotrophin-releasing hormone (CRH) and catecholamines (noradrenaline, dopamine and salsolinol), as well as concentrations of plasma adenocorticotrophic hormone (ACTH), cortisol and prolactin, were assayed. A significant increase in perfusate noradrenaline, plasma ACTH and cortisol occurred in response to both kinds of isolation stress. Suckling and salsolinol reduced the stress-induced increase in plasma ACTH and cortisol concentrations. Salsolinol also significantly reduced the stress-induced noradrenaline and dopamine release within the IN/ME. Treatment with 1-MeDIQ under the stress conditions significantly diminished the salsolinol concentration and increased CRH and cortisol concentrations. Stress and salsolinol did not increase the plasma prolactin concentration, in contrast to the suckling stimulus. In conclusion, salsolinol released in nursing sheep may have a suppressing effect on stress-induced HPA axis activity and peripheral prolactin does not appear to participate in its action.

Effects of hypothalamic dopamine (DA) on salsolinol (SAL)-induced prolactin (PRL) secretion in male goats

The aim of the present study was to clarify the effects of hypothalamic dopamine (DA) on salsolinol (SAL)-induced prolactin (PRL) release in goats. The PRL-releasing response to an intravenous (i.v.) injection of SAL was examined after treatment with augmentation of central DA using carbidopa (carbi) and L-dopa in male goats under 8-h (8 h light, 16 h dark) or 16-h (16 h light, 8 h dark) photoperiod conditions. The carbi and L-dopa treatments reduced basal PRL concentrations in the 16-h photoperiod group (P < 0.05), while a reduction was not observed in the 8-h photoperiod group. The mean basal plasma PRL concentration in the control group for the 8-h photoperiod was lower than that for the 16-h photoperiod (P < 0.05). SAL significantly stimulated the release of PRL promptly after the injection in both the 8- and 16-h photoperiod groups (P < 0.05). PRL-releasing responses for the 16-h photoperiod were greater than those for the 8-h photoperiod (P < 0.05). The carbi and L-dopa treatments blunted SAL-induced PRL release in both the 8- and 16-h photoperiods (P < 0.05). These results indicate that hypothalamic DA blunts the SAL-induced release of PRL in male goats, regardless of the photoperiod, which suggests that both SAL and DA are involved in regulating the secretion of PRL in goats.

The possible involvement of salsolinol and hypothalamic prolactin in the central regulatory processes in ewes during lactation

Salsolinol, a dopamine-related compound and prolactin-producing cells were found in the ovine hypothalamus. This study was designed to test the hypothesis that salsolinol, acting from the CNS level, is able to stimulate pituitary prolactin release as well as prolactin mRNA expression in the anterior pituitary cells (AP) and in the mediobasal hypothalamus (MBH) in lactating ewes. The intracerebroventricular infusions of salsolinol in two doses, total of 50 ng or 5 μg, were performed in a series of five 10-min infusions at 20-min intervals. All infusions were made from 12:30 to 15:00 and the pre-infusion period was from 10:00 to 12.30 h. The prolactin concentration in plasma samples, collected every 10 min, was determined by radioimmunoassay; prolactin mRNA expression in AP and MBH tissues was determined by real-time PCR. The obtained results showed that salsolinol infused at the higher dose significantly (p < 0.001) increased plasma prolactin concentration in lactating ewes, when compared with the concentration noted before the infusion and with that in lactating controls. In lactating ewes, the relative levels of prolactin mRNA expression in the AP and MBH were up to twofold and fivefold higher respectively than in non-lactating ewes (p < 0.05). In our experimental design, salsolinol did not significantly affect the ongoing process of prolactin gene expression in these tissues. We conclude that in ewes, salsolinol may be involved, at least, in the process of stimulation of prolactin release during lactation and that hypothalamic prolactin plays an important role in the central mechanisms of adaptation to lactation.

Revisiting the controversial role of salsolinol in the neurobiological effects of ethanol: old and new vistas

The possible involvement of salsolinol (Sal), an endogenous condensation product of ACD (the first metabolite of ethanol) and dopamine, in the neurochemical basis underlying ethanol action has been repeatedly suggested although it has not been unequivocally established, still being a controversial matter of debate. The main goal of this review is to evaluate the presumed contribution of Sal to ethanol effects summarizing the reported data since the discovery in the 1970s of Sal formation in vitro during ethanol metabolism until the more recent studies characterizing its behavioral and neurochemical effects. Towards this end, we first analyze the production and detection of Sal, in different brain areas, in basal conditions and after alcohol consumption, highlighting its presence in regions especially relevant in regulating ethanol-drinking behaviour and the importance of the newly developed methods to differentiate both enantiomers of Sal which could help to explain some previous negative findings. Afterwards, we review the behavioral and neurochemical studies. Finally, we present and discuss the previous and current enunciated mechanisms of action of Sal in the CNS.

Bromocriptine inhibits salsolinol-induced prolactin release in male goats

The secretion of prolactin (PRL) is under the dominant and tonic inhibitory control of dopamine (DA); however, we have recently found that salsolinol (SAL), an endogenous DA-derived compound, strongly stimulated the release of PRL in ruminants. The aim of the present study was to clarify the inhibitory effect of DA on the SAL-induced release of PRL in ruminants. The experiments were performed from late June to early July. Male goats were given a single intravenous (i.v.) injection of SAL (5mg/kg body weight (BW)), a DA receptor antagonist (sulpiride, 0.1mg/kg BW), or thyrotropin-releasing hormone (TRH, 1µg/kg BW) before and after treatment with a DA receptor agonist (bromocriptine), and the effect of DA on SAL-induced PRL release was compared to that on sulpiride- or TRH-induced release. Bromocriptine completely inhibited the SAL-induced release of PRL (P<0.05), and the area under the response curve (AUC) for a 120-min period after the treatment with bromocriptine was 1/28 of that for before the treatment (P<0.05). Bromocriptine also completely inhibited the sulpiride-induced release (P<0.05). The AUC post-treatment was 1/17 that of pre-treatment with bromocriptine (P<0.05). Bromocriptine also inhibited the TRH-induced release (P<0.05), though not completely. The AUC post-treatment was 1/3.8 that of pre-treatment (P<0.05). These results indicate that DA inhibits the SAL-induced release of PRL in male goats, and suggest that SAL and DA are involved in regulating the secretion of PRL. They also suggest that in terms of the regulatory process for the secretion of PRL, SAL resembles sulpiride but differs from TRH.

Role of salsolinol in the regulation of pituitary prolactin and peripheral dopamine release

(R)-Salsolinol (SAL), a dopamine (DA)-related tetrahydroisoquinoline, has been found in extracts of the neuro-intermediate lobes (NIL) of pituitary glands and in the median eminence of the hypothalamus obtained from intact male rats and from ovariectomized and lactating female rats. Moreover, analysis of SAL concentrations in NIL revealed parallel increases with plasma prolactin (PRL) in lactating rats exposed to a brief (10 min) suckling stimulus after 4-h separation. SAL is sufficiently potent in vivo to account for the massive discharge of PRL that occurs after physiological stimuli (i.e. suckling). At the same time, it was without effect on the secretion of other pituitary hormones. It has been also shown that another isoquinoline derivative, 1-methyldihydroisoquinoline (1MeDIQ), which is a structural analogue of SAL, can dose-dependently inhibit the in-vivo PRL-releasing effect of SAL. Moreover, 1MeDIQ can inhibit the elevation of plasma PRL induced by physiological stimuli, for example suckling, or in different stressful situations also. 1MeDIQ also has a psycho-stimulant action, which is fairly similar to the effect of amphetamine, i.e. it induces an increase in plasma catecholamine concentrations. It is clear from these data that this newly discovered endogenous compound could be involved in regulation of pituitary PRL secretion. It has also been observed that SAL is present in peripheral, sympathetically innervated organs, for example the atrium, spleen, liver, ovaries, vas deferens, and salivary gland. Furthermore, SAL treatment of rats results in dose-dependent and time-dependent depletion of the DA content of the organs listed above without having any effect on the concentration of norepinephrine. More importantly, this effect of SAL can be completely prevented by amphetamine and by 1MeDIQ pretreatment. It is clear there is a mutual interaction between SAL, 1MeDIQ, and amphetamine or alcohol, not only on PRL release; their interaction with catecholamine "synthesis/metabolism" of sympathetic nerve terminals is also obvious.

Differential action of two prolactin isoforms on ischemia and re-perfusion-induced arrhythmias in rats in vivo

BACKGROUND

The different influences of one of the PRL isoforms (PRL I) on the cardiovascular system have been described in the past.

AIM

Our goal was to establish an appropriate iv dose of 2 PRL isoforms (PRL I and PRL II) in intact rats. After establishing this dose, PRL I (0.01 mg/kg) or PRL II (0.001 mg/kg) was administered in bolus 10 min before left anterior descending coronary artery occlusion (7 min) followed by re-perfusion (15 min). We then aimed to study and compare the effects of these isoforms on ischemia- and re-perfusion-induced arrhythmias in the ischemia and re-perfusion-induced arrhythmias model in rats.

MATERIALS AND METHODS

Mortality index, ventricular fibrillation and tachycardia (VF, VT) incidence and duration, systolic, diastolic, and mean arterial blood pressure, heart rate and myocardial index of oxygen consumption [pressure rate product (PRP)] were measured and calculated.

RESULTS

Both PRL isoforms reduced animal mortality (from 50 to 18.75 and 25%, respectively). PRL II significantly reduced VF incidence (to 25%) as well as VT duration (18.21 ± 3.09) and these effects were markedly different from PRL I and from the control group (p<0.05). Both PRL reduced PRP in the recovery phase (p<0.05).

CONCLUSIONS

We proved that supraphysiological doses of PRL isoforms administered in bolus could protect against sudden cardiac death as well as severe arrhythmias episodes during re-perfusion. Because of PRL's positive influence on the cardiovascular system and as an endogenous, well-tolerated substance, it might be of potential clinical use.

Plasma prolactin concentrations and copulatory behavior after salsolinol injection in male rats

Purpose

The dopamine-derived endogenous compound, R-salsolinol (SAL), was recently identified as a putative endogenous prolactin (PRL)-releasing factor. However, how SAL influences copulatory behavior is unknown. In this study, we examined the relationship between SAL and copulatory behavior in male rats.

Methods

Male Sprague-Dawley rats administered SAL were exposed to female rats in estrus, the plasma PRL concentration was measured, and the behavioral frequency and time during copulatory behavior were noted.

Results

In the control and SAL groups, plasma PRL concentrations at 15 min before exposure to the female were 7.3 ± 2.0 and 8.0 ± 1.5 ng/ml, respectively. Moreover, plasma PRL concentrations in males immediately after exposure to the female were 7.4 ± 1.2 and 68.0 ± 5.9 ng/ml, respectively (P < 0.05). All (8/8) of the control animals ejaculated in the presence of the female, whereas only 33% (2/6) of the SAL group ejaculated. An increasing tendency for mount latency and intromission latency and a decreasing tendency for intromission frequency were observed in the SAL group.

Conclusions

Copulatory behavior was inhibited in male rats after SAL injection, suggesting that SAL is a copulatory behavior inhibiting factor.

Characteristics of prolactin-releasing response to salsolinol in vivo in cattle

The aims of the present study were to clarify the effect of salsolinol (SAL), a dopamine (DA)-derived endogenous compound, on the secretion of prolactin (PRL) in cattle. The experiments were performed from April to June using calves and cows. A single intravenous (i.v.) injection of SAL (5mg/kg body weight [BW]) or sulpiride (a DA receptor antagonist, 0.1mg/kg BW) significantly stimulated the release of PRL in male and female calves (P<0.05), though the response to SAL was smaller than that to sulpiride. The secretory pattern of PRL in response to SAL or sulpiride in female calves resembled that in male calves. A single i.v. injection of SAL or sulpiride significantly stimulated the release of PRL in cows (P<0.05). There was no significant difference in the PRL-releasing response between the SAL- and sulpiride-injected groups in cows. A single intracerebroventricular injection of SAL (10mg/head) also significantly stimulated the release of PRL in castrated calves (P<0.05). These results show that SAL is involved in the regulatory process for the secretion of PRL, not only in male and female calves, but also in cows. The results also suggest that the potency of the PRL-releasing response to SAL differs with the physiological status of cattle.

Characteristics of prolactin-releasing response to salsolinol (SAL) and thyrotropin-releasing hormone (TRH) in ruminants

The secretion of prolactin (PRL) is stimulated by thyrotropin-releasing hormone (TRH), and inhibited by dopamine (DA). However, we have recently demonstrated that salsolinol (SAL), a DA-derived endogenous compound, is able to stimulate the release of PRL in ruminants. The aims of the present study were to compare the characteristics of the PRL-releasing response to SAL and TRH, and examine the relation between the effects that SAL and DA exert on the secretion of PRL in ruminants in vivo and in vitro. Three consecutive intravenous (i.v.) injections of SAL (5mg/kg body weight (b.w.): 19.2micromol/kgb.w.) or TRH (1microg/kgb.w.: 2.8nmol/kgb.w.) at 2-h intervals increased plasma PRL levels after each injection in goats (P<0.05); however, the responses to SAL were different from those to TRH. There were no significant differences in each peak value between the groups. The rate of decrease in PRL levels following the peak was attenuated in SAL-treated compare to TRH-treated animals (P<0.05). PRL-releasing responses to SAL were similar to those to sulpiride (a DA receptor antagonist, 0.1mg/kgb.w.: 293.3nmol/kgb.w.). In cultured bovine anterior pituitary (AP) cells, TRH (10(-8)M) significantly increased the release of PRL following both 15- and 30-min incubation periods (P<0.05), but SAL (10(-6)M) did not increase the release during the same periods. DA (10(-6)M) completely blocked the TRH-induced release of PRL for a 2-h incubation period in the AP cells (P<0.05). Sulpiride (10(-6)M) reversed this inhibitory effect but SAL (10(-6)M) did not have any influence on the action of DA. These results show that the mechanism(s) by which SAL releases PRL is different from the mechanism of action of TRH. Furthermore, they also show that the secretion of PRL is under the inhibitory control of DA, and SAL does not antagonize the DA receptor's action.

Regional distribution of tetrahydroisoquinoline derivatives in rodent, human, and Parkinson's disease brain

Several members of the tetrahydroisoquinoline (TIQ) family of monoamine alkaloids can be formed from dopamine or its oxidized metabolites and may be involved in the pathogenesis of monoaminergic cell death in Parkinson's disease (PD). Using enantiomeric-selective high-performance liquid chromatography with electrochemical detection and liquid chromatography with tandem mass spectroscopy, the regional concentrations of several TIQ derivatives, including salsolinols, were determined in mouse, rat, normal human, and PD brain. TIQ derivatives were detected in all regions subjected to analysis. In general, salsolinols were present at higher concentrations than TIQ and its benzyl and methyl derivatives, especially in human brain. Moreover, salsolinols were concentrated in areas with increased dopamine synthesis and turnover such as the ventral midbrain and striatum, respectively. A possible consequence of nigrostriatal dopaminergic cell death, significantly lower levels of (R)salsolinol, (S)salsolinol, N-methyl-(R)salsolinol and N-methyl-(S)salsolinol were found in the caudate nuclei of PD in comparison with normal human brain. Our data support the hypothesis of endogenous synthesis of salsolinols and provide evidence for their accumulation in catecholaminergic neurons.

Prolactin: a pleiotropic neuroendocrine hormone

The neuroendocrine control of prolactin secretion is unlike that of any other pituitary hormone. It is predominantly inhibited by the hypothalamus and, in the absence of a regulatory feedback hormone, it acts directly in the brain to suppress its own secretion. In addition to this short-loop feedback action in the brain, prolactin has been reported to influence a wide range of other brain functions. There have been few attempts to rationalise why a single hormone might exert such a range of distinct and seemingly unrelated neuroendocrine functions. In this review, we highlight some of the original studies that first characterised the unusual features of prolactin neuroendocrinology, and then attempt to identify areas of new progress and/or controversy. Finally, we discuss a hypothesis that provides a unifying explanation for the pleiotrophic actions of prolactin in the brain.

Interaction between salsolinol (SAL) and thyrotropin-releasing hormone (TRH) or dopamine (DA) on the secretion of prolactin in ruminants

We have recently demonstrated that salsolinol (SAL), a dopamine (DA)-derived compound, is present in the posterior pituitary gland and is able to stimulate the release of prolactin (PRL) in ruminants. The aim of the present study was to clarify the effect that the interaction of SAL with thyrotropin-releasing hormone (TRH) or DA has on the secretion of PRL in ruminants. A single intravenous (i.v.) injection of SAL (5mg/kg body weight (b.w.)), TRH (1microg/kg b.w.), and SAL plus TRH significantly stimulated the release of PRL in goats (P<0.05). The cumulative response curve (area under the curve: AUC) during 120min was 1.53 and 1.47 times greater after the injection of SAL plus TRH than either SAL or TRH alone, respectively (P<0.05). A single i.v. injection of sulpiride (a DA receptor antagonist, 0.1mg/kg b.w.), sulpiride plus SAL (5mg/kg b.w.), and sulpiride plus TRH (1microg/kg b.w.) significantly stimulated the release of PRL in goats (P<0.05). The AUC of PRL during 120min was 2.12 and 1.78 times greater after the injection of sulpiride plus TRH than either sulpiride alone or sulpiride plus SAL, respectively (P<0.05). In cultured bovine anterior pituitary (AP) cells, SAL (10(-6)M), TRH (10(-8)M), and SAL plus TRH significantly increased the release of PRL (P<0.05), but the additive effect of SAL and TRH detected in vivo was not observed in vitro. In contrast, DA (10(-6)M) inhibited the TRH-, as well as SAL-induced PRL release in vitro. All together, these results clearly show that SAL can stimulate the release of PRL in ruminants. Furthermore, they also demonstrate that the additive effect of SAL and TRH on the release of PRL detected in vivo may not be mediated at the level of the AP, but that DA can overcome their releasing activity both in vivo and in vitro, confirming the dominant role of DA in the inhibitory regulation of PRL secretion in ruminants.

Salsolinol is present in the bovine posterior pituitary gland and stimulates the release of prolactin both in vivo and in vitro in ruminants

The aims of the present study were to determine whether salsolinol (SAL), a dopamine-related compound, is present in the bovine posterior pituitary (PP) gland, and to clarify the effect of SAL on the secretion of prolactin (PRL) in ruminants. SAL was detected in extract of bovine PP gland using high-pressure liquid chromatography with electrochemical detection (HPLC-EC). A single intravenous (i.v.) injection of SAL (5 and 10mg/kg body weight) significantly and dose-dependently stimulated the release of PRL in goats (P<0.05). Plasma PRL levels reached a peak 10min after the injection, then gradually returned to basal values in 60-80min. The PRL-releasing pattern was similar to that in response to sulpiride (a dopamine receptor antagonist). The intracerebroventricular (i.c.v.) injection of 1mg of SAL had no significant effect on the release of PRL in calves, however, 5mg significantly stimulated the release (P<0.05) with peak values reached 30-40min after the injection. Moreover, SAL significantly stimulated the release of PRL from cultured bovine anterior pituitary cells at doses of 10(-6) and 10(-5)M, compared to control cells (P<0.05). Taken together, our data clearly show that SAL is present in extract of the PP gland of ruminants, and has PRL-releasing activity both in vivo and in vitro. Therefore, this endogenous compound is a strong candidate for the factor having PRL-releasing activity that has been previously detected in extract of the bovine PP gland.

Simultaneous determination of 5-hydroxyindoleacetic acid and 5-hydroxytryptamine in urine samples from patients with acute appendicitis by liquid chromatography using poly(bromophenol blue) film modified electrode

The fabrication and application of a novel electrochemical detection (ED) system with a poly(bromophenol blue) (PBPB) film chemically modified electrode (CME) for high performance liquid chromatography (HPLC) were described. The electrochemical behaviors of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) at this CME were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the PBPB CME efficiently exhibited electrocatalytic effect on the current responses of 5-HT and 5-HIAA with relatively high sensitivity, stability and long life of activity. In HPLC-ED, the two analytes had good and stable current responses at the CME and their linear ranges were over four orders of magnitude (R> or =0.9992) with the detection limits being 0.25 nmol L(-1) for 5-HT and 0.50 nmol L(-1) for 5-HIAA. The application of this method for the determination of 5-HT and 5-HIAA in urine samples from patients with acute appendicitis (AA) was satisfactory.

Prolactin response to formalin is related to the acute nociceptive response and it is attenuated by combined application of different stressors

Subcutaneous injection of diluted formalin (0.2 ml of 4% solution/100 g BW) can influence the increase of plasma epinephrine levels in rats exposed to exteroceptive (handling, immobilization), as well as to interoceptive stressors (insulin-induced hypoglycemia), without having any effect on norepinephrine release. In the present studies, the effect of the above-mentioned stressors has been investigated on formalin-induced prolactin (PRL) and corticosterone secretion. Administrations of formalin via chronically implanted subcutaneous cannula into the hind limb without handling induce an immediate increase in both plasma PRL and corticosterone levels. While PRL concentration reaches its peak value within 5 min then returns to the basal level by the end of the 30th min, corticosterone level also starts to rise immediately after formalin administration reaching its highest concentration within 15-30 min, but it remains at this high level during the next 60 min, then it declines and returns to the pre-injection level. Application of formalin to animals exposed to different heterotypic stressors (like handling or insulin-induced hypoglycemia) produces an attenuated PRL response, while plasma corticosterone levels induced by the same nociceptive component remained unchanged. Combinations of formalin injection with immobilization also show an attenuated PRL response. The present data indicate that plasma PRL response to formalin is related to its acute nociceptive phase, and application of different stressors prior to formalin injection significantly attenuate plasma PRL levels, while it does not influence corticosterone responses.

Determination of isatin and monoamine neurotransmitters in rat brain with liquid chromatography using palladium hexacyanoferrate modified electrode

The fabrication and application of a novel electrochemical detector (ED) with palladium hexacyanoferrate (PdHCF) chemically modified electrode (CME) for liquid chromatography (LC) were described. The electrochemical behaviors of isatin, monoamine neurotransmitters and their metabolites at this CME were investigated by cyclic voltammetry. It was found that the CME exhibited efficiently electrocatalytic of isatin and showed high sensitivity and stability for determination of monoamine neurotransmitters. The linear ranges were over three orders of magnitude and the detection limits were 2.5 x 10(-8) mol L(-1) for isatin, 2.5 x 10(-10) mol L(-1) for norepinephrine (NE), 2.5 x 10(-10) mol L(-1) for 5-hydroxyindoleacetic acid (5-HIAA), 5.0 x 10(-10) mol L(-1) for dopamine (DA), 1.0 x 10(-9)mol L(-1) for 3,4-dihydroxyphenylacetic acid (DOPAC), 1.2 x 10(-10) mol L(-1) for 5-hydroxytryptamine (5-HT) and 2.5 x 10(-9)mol L(-1) for homovanillic acid (HVA). Combined with microdialysis, the method was successfully applied to study the effect of isatin on the levels of monoamine neurotransmitters in experimental Parkinsonian rats. The results showed that isatin could significantly increase striatal monoamine neurotransmitters release to the basal level.

Non-NMDA Glutamate Receptor Antagonist Injected into the Hypothalamic Paraventricular Nucleus Inhibits the Prolactin Response to Formalin Stress of Male Rats

The aim of the present investigations was to test the involvement of the glutamatergic innervation of the hypothalamic paraventricular nucleus in the prolactin response to stress. A non-NMDA (6-cyano-7-nitroquinoxaline-2,3-dione disodium, CNQX) or an NMDA glutamate receptor antagonist (dizocilpine hydrogen malate, MK-801) was injected bilaterally into the paraventricular nucleus of freely moving male rats and 15 min later the animals were exposed to formalin stress. Blood samples for prolactin and corticosterone were taken at different time points before and after administration of formalin. CNQX, when injected into the paraventricular nucleus, inhibited the formalin-induced rise in plasma prolactin and not significantly the increase in corticosterone. A similar effect was not observed if MK-801 was administered into the paraventricular nuclei or CNQX was injected outside the cell group. The findings indicate that the glutamatergic innervation of the paraventricular nucleus is involved in the mediation of the formalin-induced prolactin release.

Control of prolactin secretion by excitatory amino acids

This article provides an overview of the increasing number of observations indicating that excitatory amino acids are involved in the control of prolactin secretion. The information available suggests that these amino acids exert a stimulatory action on hypophysial prolactin. Administration of a glutamate receptor agonist induces significant increase in prolactin release in rats, monkeys, and rams. In contrast, noncompetitive antagonists of N-methyl-D-aspartate receptors decrease plasma levels and attenuate the preovulatory surge of prolactin. It appears that the endogenous glutamatergic system participates not only in the regulation of basal secretion of prolactin, but also in the control of physiological prolactin responses induced by the suckling stimulus or by stress. Recent findings suggest that the glutamatergic innervation of the hypothalamic paraventricular nucleus is involved in the mediation of the neural signal of the suckling stimulus-induced prolactin release as well as in the mediation of the stress-induced release of prolactin.

Neuroendocrine regulation of prolactin secretion during late pregnancy: easing the transition into lactation

Prolactin is an anterior pituitary hormone critical for maintaining pregnancy and lactation. Under normal conditions, prolactin secretion is tightly regulated by inhibitory dopaminergic neuronal systems within the mediobasal hypothalamus in a process known as short-loop negative feedback. This review focuses on neuroendocrine adaptations to prolactin negative feedback during late pregnancy. It is suggested that, in terms of prolactin regulation, late pregnancy is a transition period into lactation because many of the neuroendocrine adaptations promoting hyperprolactinemia in lactation develop during late pregnancy. As a consequence, the maternal brain is geared to provide unrestrained prolactin release critical for milk production, maternal care and thus survival of the offspring before parturition. The mechanisms responsible for these changes are discussed.

Non-NMDA glutamate receptor antagonist injected into the hypothalamic paraventricular nucleus blocks the suckling stimulus-induced release of prolactin

The aim of the present investigations was to test the involvement of the glutamatergic innervation of the hypothalamic paraventricular nucleus in the prolactin response to the suckling stimulus. A non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-dione disodium (CNQX), or an NMDA receptor antagonist, dizocipine hydrogen malate (MK-801), was injected bilaterally into the hypothalamic paraventricular nucleus of lactating freely moving rats before the end of a 4-h separation of the dams from their pups. The litters were then returned. Blood samples for prolactin were taken at different time points. The effect of the non-NMDA receptor antagonist was also tested in animals receiving the drug bilaterally into the dorsomedial nucleus area or the arcuate nucleus. Bilateral injection of CNQX into the paraventricular nucleus blocked the elevation in plasma prolactin concentration induced by the suckling stimulus. In contrast, bilateral administration of the NMDA receptor antagonist MK-801 into the paraventricular nucleus or bilateral injection of CNQX into the dorsomedial nucleus area or the arcuate nucleus did not interfere with the prolactin response to the suckling stimulus. The findings indicate that the glutamatergic innervation of the paraventricular nucleus is involved in the mediation of the neural signal of the suckling stimulus inducing prolactin release.

Salsolinol, an antagonist of prolactoliberine, induces an increase in plasma catecholamine levels in the rat

It has been recently observed that salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), a putative endogenous prolactin-releasing factor is a potent inhibitor of stress-induced release of epinephrine and norepinephrine. The prolactin release caused by salsolinol was inhibited by 1-methyl-3,4-dihydroisoquinoline (1MeDIQ). Therefore, the aim of our present studies was to investigate the effect of 1MeDIQ on plasma catecholamine levels. It has been found that 1MeDIQ is able to induce a massive increase in plasma catecholamine levels. Pretreatment of the animals with a ganglionic blocker, chlorisondamine, could completely abolish the effect of 1MeDIQ on plasma norepinephrine, and plasma epinephrine levels were only significantly attenuated. Spinal cord transection between cervical and thoracic segments eliminated 1MeDIQ induced increase in epinephrine, whereas increase in plasma norepinephrine was not affected. Hence, this effect of 1MeDIQ on sympathoadrenal system activity is most probably mediated through the level of sympathetic ganglia or partially at more centrally located sites of the nervous system. These results suggest that elevation of plasma catecholamines is involved in the mechanism of action of 1MeDIQ inhibiting the biological effect of salsolinol.

Comparison of column performances in direct high-performance liquid chromatographic enantioseparation of 1- or 3-methyl-substituted tetrahydroisoquinoline analogs. Application of direct and indirect methods

The enantioseparability of 1- or 3-methyl-substituted tetrahydroisoquinolines was investigated by direct and indirect high-performance liquid chromatography. Alpha- and beta-cyclodextrin-, macrocyclic glycopeptide- and cellulose-based chiral columns and isothiocyanate- and Sanger-type chiral derivatizing agents (CDAs) were applied to attain satisfactory enantioseparation. Of the chiral columns, beta-cyclodextrin-, vancomycin- and teicoplanin-containing macrocyclic glycopeptide-based columns appeared to be most suitable; of the CDAs, (1S,2R)-1-acetoxy-1-phenyl-2-propyl isothiocyanate was most favorable. The differences between the selectivities of the columns and CDAs were compared and optimized to yield the best resolution.

Salsolinol induces a decrease in cyclic AMP at the median eminence and an increase at the adenohypophysis in lactating rats

Investigating the cellular events in the pituitary gland, the intracellular cyclic AMP (cAMP) of the median eminence (ME), neuro-intermediate lobe (NIL) and the anterior lobe (AL) have been measured following 15-min of intravenous injection of salsolinol (SAL). Parallel to the elevation of plasma prolactin (PRL), SAL induced a significant decrease of cAMP concentration in the ME. In contrast, SAL injection resulted in a significant increase of cAMP at the level of the AL. Changes in cAMP of the NIL as well as in the plasma level of vasopressin (VP) could not be detected. The observed changes in the level of cAMP following the acute treatment of SAL in the ME and the AL seems to be related to interacting neuroendocrine signals delivered from the ME to the AL through the long portal vessels to release PRL.

Immobilization stress-induced increase in plasma catecholamine levels is inhibited by a prolactoliberin (salsolinol) administration

Catecholamines (CAs) are significantly involved in the regulation of homeostasis of the organism at rest and especially during stressful situations. Stress induces increases in plasma CA (epinephrine and norepinephrine) levels and prolactin (PRL) release from the adenopituitary. We have recently observed that salsolinol, which is produced by the neuro-intermediate lobe of the pituitary gland and by the hypothalamus, can selectively release PRL. Salsolinol is therefore considered to be a putative endogenous PRL-releasing factor. Based on the similarity of CA and PRL responses to stressors, we investigated whether salsolinol plays a role in the regulation of plasma CA levels at rest and of CA release induced by immobilization stress (IMO). Salsolinol did not affect CA baseline levels; however, it did inhibit IMO-induced CA release. Thus, the present study shows for the first time that salsolinol is not only a PRL-releasing factor but is also a potent inhibitor of stress-induced release of epinephrine and norepinephrine.

Pivotal Role of an Endogenous Tetrahydroisoquinoline, Salsolinol, in Stress- and Suckling-Induced Release of Prolactin

In mammals, the role of a prolactin-releasing factor (PRF) in the acute changes of prolactin (PRL) secretion that usually occur after challenges (e.g., suckling stimulus or stress) of homeostasis has been suspected for a long time. We have recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, salsolinol (SAL), produced by the hypothalamus and the neuro-intermediate lobe (NIL) of the pituitary gland, can selectively release PRL from the anterior lobe (AL). Moreover, binding sites for SAL have been detected in areas like median eminence, NIL, and AL. It has been proposed that SAL is a putative endogenous PRF. We have also found that a structural analogue of SAL, 1-methyl-3,4-dihydroisoquinoline (1MeDIQ), is able to block dose-dependently SAL-, suckling-, and immobilization (IMO) stress-induced release of PRL without having any influence on alpha-methyl-p-tyrosine (alphaMpT)-induced PRL responses. Neither SAL nor 1MeDIQ has any effect on alpha-melanocyte-stimulating hormone (alphaMSH), adrenocorticotrophic hormone (ACTH), beta-endorphin (beta-END) and arginine-vasopressin (AVP) secretion. Moreover, SAL-induced PRL response was attenuated in male rats pretreated with dexamethasone (DEX). These results strongly suggest that SAL has an important role in the regulation of PRL release induced by physiologic and environmental stimuli; therefore, it can be considered as the strongest candidate for being the PRF in the hypothalamo-hypophysial system. Our findings also indicate that the adrenal steroids may play an inhibitory feedback role in SAL-mediated PRL response.