Central antidopaminergic properties of 2-bromolisuride, an analogue of the ergot dopamine agonist lisuride

2-Bromolisuride (2-Br-LIS), a derivative of the ergot dopamine (DA) agonist lisuride, was investigated in rodents in comparison with the DA antagonist haloperidol with regard to its influence on DA related behaviour, cerebral DA metabolism and prolactin (PRL) secretion. 2-Br-LIS produced catalepsy in mice (ED50 3.3 mg/kg i.p.), antagonized apomorphine-induced stereotypies in mice (ED50 0.4 mg/kg i.p.), antagonized DA agonist-induced stereotypies in rats (0.1-1.56 mg/kg i.p.), inhibited locomotor activity in rats (0.025-6.25 mg/kg i.p.), antagonized the hyperactivity produced by various DA agonists in rats (0.025-6.25 mg/kg i.p.) and inhibited the apomorphine-induced hypothermia in mice (0.05-0.78 mg/kg i.p.). 2-Br-LIS (0.03-10 mg/kg i.p.) stimulated DA biosynthesis and DOPAC formation in the striatum and DA rich limbic system of rats, but had no effect on serotonin turnover. In striatum and limbic forebrain of gamma-butyrolactone-pretreated rats 2-Br-LIS reversed the apomorphine-induced inhibition of DOPA accumulation. 2-Br-LIS (0.03 - 3 mg/kg) enhanced PRL secretion in intact male rats. These findings indicate DA antagonistic properties of 2-Br-LIS presumably due to blockade of central pre- and postsynaptic DA receptors being of approximately the same order of potency as haloperidol. 2-Br-LIS is the first ergot compound with definite antidopaminergic properties suggesting its potential usefulness as a neuroleptic.     

Novel 8 alpha-ergolines with inhibitory and stimulatory effects on prolactin secretion in rats

Four derivatives of the ergot dopamine (DA) agonist lisuride (LIS), namely 6-n-propyl-lisuride (6-n-propyl-LIS), transdihydrolisuride (TDHL), 6-n-propyl-transdi-hydrolisuride (6-n-propyl-TDHL) and 2-bromolisuride (2-Br-LIS) were investigated in female rats with regard to their influence on hyperprolactinaemia induced by pretreatment with reserpine (2 mg/kg i.p., 24 h) at various intervals following their subcutaneous or oral administration (0.05 mg/kg). Two hours after administration, LIS, 6-n-propyl-LIS, and 6-n-propyl-TDHL caused a statistically significant inhibition of reserpine-induced hyperprolactinaemia of about the same extent. Eight hours after administration 6-n-propyl-LIS and 6-n-propyl-TDHL were as active as after 2 h in inhibiting prolactin (PRL) secretion whereas LIS was almost ineffective in this respect. TDHL caused a statistically significant inhibition of PRL secretion at 2 and 8 h after oral administration; this effect was less pronounced after s.c. administration. In contrast to the aforementioned derivatives 2-Br-LIS further increased the reserpine-induced hyperprolactinaemia. In normal male rats pretreatment with 2-Br-LIS (0.025-6.25 mg/kg s.c., 2 h) dose-dependently stimulated PRL secretion. The present data support the assumption of the longlasting DA agonistic action of 6-n-propyl-LIS and 6-n-propyl-TDHL and of the antidopaminergic properties of 2-Br-LIS recently derived from behavioural studies.     

Activation of 5-HT(1A) but not 5-HT(1B) receptors attenuates an increase in extracellular dopamine derived from exogenously administered L-DOPA in the striatum with nigrostriatal denervation

In order to determine whether L-DOPA-derived extracellular dopamine (DA) in the striatum with dopaminergic denervation is affected by activation of serotonin autoreceptors (5-HT(1A) and 5-HT(1B) receptors), we applied in vivo brain microdialysis technique to 6-hydroxydopamine-lesioned rats and examined the effects of the selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the selective 5-HT(1B) receptor agonist CGS-12066 A on L-DOPA-derived extracellular DA levels. Single L-DOPA injection (50 mg/kg i.p.) caused a rapid increase and a following decrease of extracellular DA, with a peak value at 100 min after L-DOPA injection. Pretreatment with both 0.3 mg/kg and 1 mg/kg 8-OH-DPAT (i.p.) significantly attenuated an increase in L-DOPA-derived extracellular DA and the times of peak DA levels were prolonged to 150 min and 225 min after L-DOPA injection, respectively. These 8-OH-DPAT-induced changes in L-DOPA-derived extracellular DA were antagonized by further pretreatment with WAY-100635, a selective 5-HT(1A) antagonist. In contrast, intrastriatal perfusion with the 5-HT(1B) agonist CGS-12066 A (10 nM and 100 nM) did not induce any changes in L-DOPA-derived extracellular DA. Thus, stimulation of 5-HT(1A) but not 5-HT(1B) receptors attenuated an increase in extracellular DA derived from exogenous L-DOPA. These results support the hypothesis that serotonergic neurons are primarily responsible for the storage and release of DA derived from exogenous L-DOPA in the absence of dopaminergic neurons.     

Selective stimulation of serotonin2c receptors blocks the enhancement of striatal and accumbal dopamine release induced by nicotine administration

The effects of acute and repeated nicotine administration on the extracellular levels of dopamine (DA) in the corpus striatum and the nucleus accumbens were studied in conscious, freely moving rats by in vivo microdialysis. Acute intraperitoneal (i.p.) injection of nicotine (1 mg/kg) increased DA outflow both in the corpus striatum and the nucleus accumbens. Repeated daily injection of nicotine (1 mg/kg, i.p.) for 10 consecutive days caused a significant increase in basal DA outflow both in the corpus striatum and the nucleus accumbens. Acute challenge with nicotine (1 mg/kg, i.p.) in animals treated repeatedly with this drug enhanced DA extracellular levels in both brain areas. However, the effect of nicotine was potentiated in the nucleus accumbens, but not in the corpus striatum. To test the hypothesis that stimulation of 5-HT (5-hydroxytryptamine, serotonin)(2C) receptors could affect nicotine-induced DA release, the selective 5-HT(2C) receptor agonist RO 60-0175 was used. Pretreatment with RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently prevented the enhancement in DA release elicited by acute nicotine in the corpus striatum, but was devoid of any significant effect in the nucleus accumbens. RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently reduced the stimulatory effect on striatal and accumbal DA release induced by an acute challenge with nicotine (1 mg/kg, i.p.) in rats treated repeatedly with this alkaloid. However, only the effect of 3 mg/kg RO 60-0175 reached statistical significance. The inhibitory effect of RO 60-0175 on DA release induced by nicotine in the corpus striatum and the nucleus accumbens was completely prevented by SB 242084 (0.5 mg/kg, i.p.) and SB 243213 (0.5 mg/kg, i.p.), two selective antagonists of 5-HT(2C) receptors. It is concluded that selective activation of 5-HT(2C) receptors can block the stimulatory action of nicotine on central DA function, an effect that might be relevant for the reported antiaddictive properties of RO 60-0175.     

Acute effects of the cannabinoid receptor agonist WIN55212-2 on dopamine release in rat: an in vivo electrochemical study

The aim of this study was to investigate the effects of the cannabinoid receptor agonist, WIN55212-2, and the cannabinoid receptor antagonist, SR141716A, on dopamine (DA) release evoked by KC1 (120 mM) microinjected into the striatum. The cannabinoid agonist WIN55212-2 (1 and 5 mg/kg, i.p.) dose-dependently attenuated DA release in the striatum, whereas the cannabinoid receptor antagonist SR141716A (3 mg/kg, i.p.) produced the opposite effect. SR141716A (3 mg/kg, i.p.) blocked the effects on DA release by WIN55212-2 (5 mg/kg, i.p.). Vehicle alone did not change DA release. These results suggest that cannabinoids modulate DA release in the striatum.     

Regionally and functionally distinct serotonin3 receptors control in vivo dopamine outflow in the rat nucleus accumbens

Central serotonin(3) (5-HT(3)) receptors control the mesoaccumbens dopamine (DA) pathway. This control is thought to be conditional and might involve regionally distinct subpopulations of 5-HT(3) receptors. Here, using in vivo microdialysis in rats, we assessed the relative contribution of nucleus accumbens (Nacc) 5-HT(3) receptors to the overall influence exerted by 5-HT(3) receptors on accumbal DA release induced by different drugs or treatments. In freely moving rats, pre-treatment with 5-HT(3) antagonists (0.1 mg/kg ondansetron and/or 0.03 mg/kg MDL 72222, s.c.) reduced DA efflux enhanced by morphine (1-10 mg/kg, s.c.) and haloperidol (0.01 mg/kg, s.c.), but not amphetamine (1-2.5 mg/kg, i.p.) or cocaine (10-20 mg/kg, i.p.), the latter two drugs do not trigger depolarization-stimulated DA exocytosis. Intra-Nacc administration of ondansetron (1 microm) in freely moving rats reduced the DA effects elicited by 10 mg/kg morphine, but not 1 mg/kg morphine or haloperidol. The 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg, s.c.), known to decrease central 5-HT tone, reduced 10 but not 1 mg/kg morphine-stimulated DA outflow in freely moving rats. In halothane-anaesthetized rats, intra-Nacc ondansetron (1 microm) application reduced dorsal raphe nucleus electrical stimulation (20Hz)-induced DA outflow. Our results show that regionally distinct populations of 5-HT(3) receptors control the depolarization-dependent exocytosis of DA and suggest that the involvement of Nacc 5-HT(3) receptors occurs only when central DA and 5-HT tones are concomitantly increased.     

Partial reversal of behavioral action of the agonist D-Trp-6-LH-RH by naloxone in mice

The effects of the superactive agonist analog D-Trp-6-LH-RH were investigated in several neuropharmacological tests: inhibition of picrotoxin-induced seizures, open-field behavior, hot-plate and tail-flick tests, assessment of catalepsy and apomorphine-induced cage-climbing. In most tests, D-Trp-6-LH-RH was administered subcutaneously (sc.) at the dose of 100 micrograms/kg. The opiate involvement in the peptide action was checked by using naloxone HCl (NX) in a dose of 1 mg/kg intraperitoneally (ip.), with the exception of the analgesic tests where the dose was 0.5 mg/kg. The analog significantly suppressed the open-field parameters of ambulation, rearing and grooming; except for grooming, these actions were fully antagonized by NX. Similarly, NX pretreatment restored to the control levels the latencies of seizure parameters increased by D-Trp-6-LH-RH. The hot-plate latencies did not change after pretreatment with NX but the opiate antagonist was fully able to antagonize the analgesic effect of the peptide in the tail-flick test. The cataleptogenic effect and the inhibition of apomorphine-induced cage-climbing demonstrated after D-Trp-LH-RH were not antagonized by NX.     

The effect of transient dopamine antagonism on thyrotropin-releasing hormone-induced prolactin release in female rats during the estrous cycle

Prolactin (PRL) release induced by TRH was examined on each day of the estrous cycle in female rats in which pituitary dopamine (DA) receptors were blocked pharmacologically. The objective was to determine if an interaction exists between hypothalamic inhibitory and releasing hormones with regard to prolactin (PRL) secretion. Domperidone (0.01 mg/rat i.v.) followed 5 minutes later by the administration of the DA agonist 2-Br-alpha-ergocryptine maleate (CB-154, 0.5 mg/rat i.v.) were used to produce a transient (less than 1 hr) dopamine blockade. One hour later, thyrotropin-releasing hormone (TRH, 1.0 microgram/rat i.v.) was given to stimulate PRL release. On the morning of proestrus, TRH released a significantly greater quantity of PRL into the plasma after DA antagonism compared to control animals which did not receive the dopamine antagonist. Dopamine antagonism also enhanced the effectiveness of TRH on the mornings of estrus and metestrus. The response on estrus was significantly greater than the response on proestrus. However by the morning of diestrus, TRH-"releasable" PRL was greatly diminished. Our results suggest that DA antagonism is able to shift differing quantities of PRL into a TRH "releasable" pool on several days of the estrous cycle and that the control of this mechanism is acute.     

A comparison of the effects of suckling or transient dopamine antagonism on thyrotropin-releasing hormone and suckling induced prolactin release in lactating rats

Prolactin (PRL) release was studied in mid-lactational female rats by comparing the stimulatory influence of suckling to a drug protocol that mimics the effect of suckling on the anterior pituitary (AP). Animals that nursed pups for 15 minutes and were allowed to suckle again 60 minutes later for 10 minutes, released PRL effectively during both nursing episodes; however, in animals that received the dopamine (DA) agonist 2-Br-alpha-ergocryptine maleate (CB-154, 0.5 mg/rat i.v.) at the end of the first nursing period did not show an increase in plasma PRL to a second suckling stimulation by the pups. When thyrotropin releasing hormone (TRH) was substituted for the second suckling period in CB-154 treated rats, a slight increase in plasma PRL occurred 5 minutes after the injection. In a third study we transiently blocked the action of DA at the AP by injecting the DA antagonist domperidone (0.01 mg/rat i.v.), followed 5 minutes later by the administration of CB-154. One hour later animals were either allowed to suckle pups for 10 minutes or were injected with TRH. Treatment with TRH resulted in an 11 fold increase in plasma PRL but suckling was completely ineffective in inducing PRL release. These data suggest that the lack of PRL release to suckling in CB-154 treated rats was due to inhibitory effects of CB-154 on neural mechanisms which link nursing to PRL release. In addition, the data show that pharmacologic DA antagonism affects TRH releasable PRL more than does suckling. This may be due to a reduction, by suckling, of the pool of PRL that is available to be released by TRH administration.     

Bremazocine induces antinociception, but prevents opioid-induced constipation and catatonia in rats and precipitates withdrawal in morphine-dependent rats

Some in vivo agonist and antagonist properties of the putative k-compound bremazocine were characterized in rats. Bremazocine, at doses from 0.015-32 mg/kg i.p., delayed nociceptive reaction on a 55 degrees C hot-plate with a dose-response curve not readily fitting a single straight line; this effect was antagonized by high doses of naloxone. In the same rats bremazocine did not delay the intestinal transit of a charcoal meal fed 5 min earlier and prevented morphine-induced constipation. This antagonism appeared to be opioid-specific and competitive, with apparent pA2 value 8.56. Catatonia induced by etorphine (0.004 mg/kg s.c.) and constipation induced by etorphine (0.004 mg/kg s.c.) and D-Ala2-D-Leu5-enkephalin (0.1 mg/kg i.p.) were completely antagonized by bremazocine (0.03-8 mg/kg i.p.). Antinociception induced by morphine (10 mg/kg i.v.) and etorphine (0.004 mg/kg s.c.) was only partly prevented. Naloxone (1 mg/kg) and bremazocine (0.015-1 mg/kg i.p.) precipitated a withdrawal syndrome, evaluated as jumping frequency, in rats rendered dependent to morphine. These data suggest the involvement of more than one opioid receptor population in bremazocine action in vivo.     

Rat-1 Fibroblasts Engineered with GAD65 and GAD67 cDNAs in Retroviral Vectors Produce and Release GABA

Abstract: The effects of the adenosine A₁ agonist N ₆-cyclohexyladenosine (CHA) on MPTP-induced dopamine (DA) depletion in the striatum of C57BL/6 mice were studied. Twenty hours after a single injection of MPTP (30 mg/kg, s.c.), the toxin caused 62% depletion of striatal DA. CHA (0.2–3 mg/kg, s.c.), when given together with MPTP, prevented the toxin-induced DA depletion in a dose-dependent manner. This protective action was apparently mediated by the A¹ receptors, because this effect was selectively antagonized by pretreating the animals with the A₁ antagonist 8-cyclopentyl-1,3-dipropylxanthine (25 mg/kg, i.p.) but not with the A₂ antagonist 1,3-dipropyl-7-methylxanthine (25 mg/kg, i.p.). When CHA (3 mg/kg) was injected 5 h after MPTP administration, at which point striatal DA levels were already reduced significantly, a rapid and complete recovery of the striatal DA levels occurred. These neurochemical data suggest that the A₁ agonist CHA is potentially useful as a neuroprotective agent against MPTP-induced toxicity.     

Effects of methylenechloride-soluble fraction of Japanese angelica root extract,ligustilide and butylidenephthalide,on pentobarbital sleep in group-housed and socially isolated mice

We previously showed that the extract of Japanese angelica root (JAR-E) reversed the decrease in pentobarbital (PB) sleep induced by isolation stress and yohimbine and methoxamine, stimulants of central noradrenergic systems, in mice. Here, we tested the effects of several fractions from JAR-E and ligustilide and butylidenephthalide, phthalide components of JAR-E, on PB sleep in isolated mice to elucidate the mechanism of the action of JAR-E. Methanol-soluble (Met-S) and -insoluble (Met-IS) fractions were obtained from JAR-E. Methylenechloride-soluble (MC-S) and -insoluble fractions (MC-IS) were prepared from Met-S. MCS (11.4–76 mg/kg, p.o.) reversed the isolation stress-induced decrease in PB sleep, but neither Met-IS (0.8–2.4 g/kg, p.o.) nor MC-IS (0.7–2 g/kg, p.o.) had the same effect. The i.p. administration of MC-S exhibited a similar activity to that observed after the p.o. administration of the same fraction. Ligustilide (5–20 mg/kg, i.p.) and butylidenephthalide (10–30 mg/kg, i.p.) reversed PB sleep decrease in isolated mice. Both components (20 mg/kg, i.p.) attenuated the suppressive effects of yohimbine (30 nmol, i.c.v.), methoxamine (200 nmol, i.c.v.) and a benzodiazepine inverse agonist FG7142 (10 mg/kg, i.p.) on PB sleep in group-housed mice. These results suggest the contribution of ligustilide and butylidenephthalide to the effect of JAR-E on PB sleep in isolated mice, and implicate central noradrenergic and/or GABA_a systems in the effects of these components.     

Cross-tolerance of dopamine metabolism to baclofen, γ-butyrolactone and HA-966 in the striatum and olfactory tubercle of the rat

Rats received 7 daily injections with baclofen (40 mg/kg), GBL (750 mg/kg) or HA-966 (100 mg/kg). Dopamine (DA) was measured in the striatum and olfactory tubercle (OT) of rats, sacrificed 0.5 h or 1 h after the last injection. Marked tolerance and cross-tolerance for the DA-elevating effect of these drugs was seen in the striatum, but not in OT. When on day 7 a unilateral lesion of the nigrostriatal pathway was made, also some tolerance to the DA increase in the striatum on the lesioned side was seen in HA-966-pretreated rats, but it was small compared to the tolerance after an additional drug administration in non-lesioned animals. A low dose of apomorphine (0.25 mg/kg, i.p.) had no effect on DA, dihydroxyphenylacetic acid DOPAC) or homovanillic acid (HVA) levels in the lesioned striata, whether the rats had been pretreated for 6 days with HA-966 or not. However, this dose of apomorphine had a significantly more lowering effect on striatal DOPAC and HVA levels on the unlesioned side of HA-966 pretreated rats. The results show that tolerance develops to the increase of DA synthesis, which is possibly receptor-mediated. This tolerance develops more readily in the striatum than in the olfactory tubercle.     

Central dopamine agonistic activity and microsomal biotransformation of lisuride, lergotrile and bromocriptine.

Lisuride (0.05 – 0.1 mg/kg), lergotrile (1 mg/kg) and bromocriptine (5 mg/kg) reduced the turnover of dopamine (DA) in rat brain, as indicated by a pronounced decrease of cerebral homovanillic acid (HVA) without appreciable changes in DA level. Time curves revealed that lisuride injected intracerebroventricularly or i.p. caused a rapid reduction of HVA lasting for a few hours, whereas after p.o. administration the decrease of HVA was delayed. Pretreatment of the rats with the microsomal enzyme inhibitor proadifen potentiated and prolonged, rather than prevented, the effect of i.p. injected lisuride on rat cerebral DA turnover. The HVA reduction obtained with lergotrile and bromocriptine was also somewhat retarded after p.o. administration; the HVA diminution seen after i.p. injection was again potentiated and prolonged by proadifen. In addition, this microsomal enzyme inhibitor prolonged and intensified, rather than prevented, the hypothermic effect of all of the 3 ergolines. It is concluded that, in the rat, the central DA agonistic activity of the ergolines studied is caused by the drugs themselves and does not require previous biotransformation into active metabolites. The retarded onset and the prolonged duration of action after oral administration is probably due to slow intestinal absorption and slow microsomal inactivation, respectively.     

Mechanisms for the stimulatory effects of opioidergic and serotonergic input signals on prolactin in pregnant rats.

Dopamine (DA) neurons participate in tonic inhibition of prolactin (PRL), whereas beta-endorphin (beta-End) and serotonin (5-HT) neurons appear to be important stimulatory links for nocturnal PRL surges that occur throughout the first half of pregnancy in the rat. The purpose of this study was to determine how these neuronal components might be organized within the pathway controlling PRL release during gestation. Maximal stimulation of DA receptors with the agonist bromocriptine mesylate (Bromo) completely blocked the PRL response to beta-End (100 ng/microliters/min for 15 min) given intracerebroventricularly (i.c.v.) on day 8 of pregnancy. DA receptor blockade, produced by implanting a 25 mg pellet of haloperidol (Hal) on day 7 of pregnancy, resulted in PRL levels of 500-600 ng/ml by the following morning. beta-End i.c.v. or 250 mg/ml/kg BW of the DA synthesis inhibitor, alpha-methyl-p-tyrosine (alpha-MPT), given during the intersurge period, were equally effective in significantly increasing PRL (p less than 0.01) above pretreatment levels. beta-End and alpha-MPT evoked similar increases in rats pretreated with Hal, suggesting the stimulatory effect of beta-End on nocturnal PRL surges may primarily be due to DA inhibition. The next objective was to determine how beta-End and 5-HT might interact to stimulate the nocturnal surge. Day 8 pregnant rats were infused continuously with the opioid receptor blocker, naloxone hydrochloride (Nal), at a rate of 2.0 mg/10 min from 1000-1300 h. The PRL response to an injection of 20 mg/kg BW 5-hydroxytryptophan (5-HTP) at 1200 h was greatly attenuated, compared to controls infused with saline instead of Nal. This suggests that 5-HT stimulates PRL, at least in part, by an action at opioid receptors. Distilled H2O or 10 mg/kg BW of the selective S2 receptor blocker, ketanserin tartrate (Ket), was given intraperitoneally (i.p.) during the intersurge period on day 8 of pregnancy. All animals demonstrated an identical response to beta-End given 2 hours later, regardless of the type of pretreatment. It appears that beta-End does not stimulate PRL by way of an S2 receptor. Although beta-End induced a significant increase in PRL on day 16 of pregnancy, the response was attenuated by more than 60% compared to the response on day 8 of pregnancy. This attenuation may involve placental lactogens, shown to be secreted during this time and to inhibit PRL secretion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Reserpine pretreatment prevents increases in extracellular striatal dopamine following L-DOPA administration in rats with nigrostriatal denervation

The influence of L-DOPA and reserpine on extracellular dopamine (DA) levels in the striatum of intact and dopaminergic denervated rats was studied using the brain microdialysis technique. In intact rats, reserpine (5 mg/kg s.c.) reduced extracellular DA levels to 4% of basal values. L-DOPA (50 mg/kg i.p.) had no effect on extracellular DA levels in reserpine-pretreated rats. In rats with 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic system, basal levels of extracellular DA were low but markedly increased by L-DOPA (50 mg/kg i.p.). In 6-hydroxydopamine-lesioned rats, pretreatment with reserpine (5 mg/kg s.c.) diminished L-DOPA (50 mg/kg i.p.)-induced increases in extracellular DA levels to 16% of those obtained in denervated animals not pretreated with reserpine (p<0.01). These results suggest that in the intact striatum, extracellular DA stems mainly from vesicular storage sites and that in the striatum with dopaminergic denervation, a large part of the L-DOPA-derived extracellular DA is also derived from a vesicular pool that is released by an exocytosis mechanism.     

Involvement of D1 dopamine receptors in the nicotine-induced noradrenaline release from hypothalamic and preoptic noradrenaline nerve terminal systems

Nicotine (4 × 2 mg/kg, i.p.) was given every 30 min for 2 h to male rats. Some rats were pretreated with the D1 dopamine (DA) receptor antagonist SCH 23390 (1 mg/kg, i.p.) or with the D2 DA receptor antagonist raclopride (1 mg/kg, i.p.), 5 min before nicotine treatment. Hypothalamic and preoptic catecholamine levels were measured by quantitative histofluorimetry in discrete DA and noradrenaline nerve terminal systems.Nicotine treatment produced a depletion of catecholamine stores in noradrenaline and DA nerve terminals of the hypothalamus, the preoptic area and the median eminence, an action which was counteracted by SCH 23390 but not by raclopride.The results indicate that hypothalamic D1 DA receptors may regulate the sensitivity of the nicotinic cholinoceptors and increase their ability to release hypothalamic noradrenaline. A possible role of D1 DA receptor antagonists to reduce the ability of nicotine treatment to produce rapid increases in LH, prolactin and corticosterone secretion and tonic arousal is implicated.     

Midazolam Ameliorates the Behavior Deficits of a Rat Posttraumatic Stress Disorder Model through Dual 18 kDa Translocator Protein and Central Benzodiazepine Receptor and Neurosteroidogenesis

Post-traumatic stress disorder (PTSD) is a debilitating anxiety disorder that may develop after an individual has experienced or witnessed a severe traumatic event. It has been shown that the 18 kDa translocator protein (TSPO) may be correlated with PTSD and that the TSPO ligand improved the behavioral deficits in a mouse model of PTSD. Midazolam, a ligand for TSPO and central benzodiazepine receptor (CBR), induces anxiolytic- and anti-depressant-like effects in animal models. The present study aimed to determine whether midazolam ameliorates PTSD behavior in rats as assessed by the single prolonged stress (SPS) model. The SPS rats received daily Sertraline (Ser) (15 mg/kg, p.o.) and midazolam (0.125, 0.25, 0.5, and 1 mg/kg, p.o.) during the exposure to SPS and behavioral assessments, which included the open field (OF) test, the contextual fear paradigm (CFP), and the elevated plus-maze (EPM). The results showed that, like Ser (15 mg/kg, p.o.), midazolam (0.25 and 0.5 mg/kg, p.o.) significantly reversed the behavioral deficiencies of the SPS rats, including PTSD-associated freezing and anxiety-like behavior but not the effects on spontaneous locomotor activity. In addition, the anti-PTSD effects of midazolam (0.5 mg/kg, p.o.) were antagonized by the TSPO antagonist PK11195 (3 mg/kg, i.p.), the CBR antagonist flumazenil (15 mg/kg, p.o.) and the inhibitor of steroidogenic enzymes finasteride (30 mg/kg, p.o.), which by themselves had no effect on PTSD-associated freezing and anxiety-like behavior. In summary, this study demonstrated that midazolam improves the behavioral deficits in the SPS model through dual TSPO and CBR and neurosteroidogenesis.     

Ramelteon (TAK-375) accelerates reentrainment of circadian rhythm after a phase advance of the light-dark cycle in rats

In vivo pharmacological effects of ramelteon (TAK-375), a novel, highly MT1/MT2-selective receptor agonist, were studied in rats to determine ramelteon's ability to reentrain the circadian rhythm after an abrupt phase advance. Experiments were also conducted to assess the potential cognitive side effects of ramelteon and its potential to become a drug of abuse. After an abrupt 8-h phase shift, ramelteon (0.1 and 1 mg/kg, p.o.) and melatonin (10 mg/kg, p.o.) accelerated reentrainment of running wheel activity rhythm to the new lightdark cycle. Ramelteon (3-30 mg/kg, p.o.) and melatonin (10-100 mg/kg, p.o.) did not affect learning or memory in rats tested by the water maze task and the delayed match to position task, although diazepam and triazolam impaired both of the tasks. Neither ramelteon (3-30 mg/kg, p.o.) nor melatonin (10-100 mg/kg, p.o.) demonstrated a rewarding property in the conditioned place-preference test, implying that MT1/MT2 receptor agonists have no abuse potential. In contrast, benzodiazepines and morphine showed rewarding properties in this test. The authors' results suggest that ramelteon may be useful for treatment of circadian rhythm sleep disorders without adverse effects typically associated with benzodiazepine use, such as learning and memory impairment, and drug dependence.     

Involvement of dopamine receptor subtypes in dopaminergic modulation of aldosterone secretion in rats

The postulation that dopamine (DA) may tonically inhibit aldosterone (ALDO) secretion has arisen from the finding that metoclopramide, a non-selective DA receptor antagonist with prominent non-dopaminergic actions, stimulates ALDO secretion. Experiments were performed to determine: (a.) the ability of several non-specific and subtype-specific DA receptor antagonists to stimulate ALDO secretion, (b.) the subtype DA receptor involved in regulating ALDO secretion, and (c.) if ALDO responses were associated with changes in plasma Na+(pNa), K+(pK), or osmolality (pOsm). Blood samples were withdrawn from carotid arterial catheters in conscious, fasted male Sprague-Dawley rats before and following intra-arterial administration of lactated Ringer's placebo, furosemide (10 mg/kg), or one of several DA receptor antagonists. Furosemide stimulated ALDO, decreased pK, and left pNa and pOsm unchanged. The non-selective DA receptor antagonists metoclopramide (0.2, 0.6 mg/kg), rs-sulpiride (0.2 mg/kg), and haloperidol (0.1 mg/kg), and the DA-2 receptor antagonists domperidone (0.1 mg/kg) and s-sulpiride (0.1 mg/kg) each stimulated ALDO, and left pNa, pK, and pOsm unchanged. Conversely, the DA-1 receptor antagonists SCH 23390 (0.03, 0.1 mg/kg) and r-sulpiride (0.1 mg/kg) failed to stimulate ALDO, and left pNa, pK, and pOsm unaltered. These studies suggest that ALDO secretion in rats is modulated by a mechanism involving DA-2, but not DA-1 subtype receptors, and that the ALDO responses to DA receptor antagonism are independent of changes in pNa, pK, and pOsm.