Antidepressant-like effect of 17beta-estradiol: involvement of dopaminergic, serotonergic, and (or) sigma-1 receptor systems

17beta-estradiol has been reported to possess antidepressant-like activity in animal models of depression, although the mechanism for its effect is not well understood. The present study is an effort in this direction to explore the mechanism of the antidepressant-like effect of 17beta-estradiol in a mouse model(s) of behavioral depression (despair behavior). Despair behavior, expressed as helplessness to escape from a situation (immobility period), as in a forced swim test in which the animals are forced to swim for a total of 6 min, was recorded. The antiimmobility effects (antidepressant-like) of 17beta-estradiol were compared with those of standard drugs like venlafaxine (16 mg/kg, i.p.). 17beta-estradiol produced a U-shaped effect in decreasing the immobility period. It had no effect on locomotor activity of the animal. The antidepressant-like effect was comparable to that of venlafaxine (16 mg/kg, i.p.). 17beta-estradiol also exhibited a similar profile of antidepressant action in the tail suspension test. When coadministered with other antidepressant drugs, 17beta-estradiol (5 microg/kg, i.p.) potentiated the antiimmobility effect of subeffective doses of fluoxetine (5 mg/kg, i.p.), venlafaxine (2 mg/kg, i.p.), or bupropion (10 mg/kg, i.p.), but not of desipramine (5 mg/kg, i.p.) or tranylcypromine (2 mg/kg, i.p.), in the forced swim test. The reduction in the immobility period elicited by 17beta-estradiol (20 microg/kg, i.p.) was reversed by haloperidol (0.5 mg/kg, i.p.; a D(2) dopamine receptor antagonist), SCH 23390 (0.5 mg/kg, i.p.; a D(1) dopamine receptor antagonist), and sulpiride (5 mg/kg, i.p.; a specific dopamine D(2) receptor antagonist). In mice pretreated with (+)-pentazocine (2.5 mg/kg, i.p.; a high-affinity sigma-1 receptor agonist), 17beta-estradiol (5 microg/kg, i.p.) produced a synergistic effect. In contrast, pretreatment with progesterone (10 mg/kg, s.c.; a sigma-1 receptor antagonist neurosteroid), rimcazole (5 mg/kg, i.p.; another sigma-1 receptor antagonist), or BD 1047 (1 mg/kg, i.p.; a novel sigma-1 receptor antagonist) reversed the antiimmobility effects of 17beta-estradiol (20 microg/kg, i.p.). Similarly, in mice pretreated with a subthreshold dose of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, a 5-HT1A serotonin receptor agonist), 17beta-estradiol (5 microg/kg, i.p.) produced an antidepressant-like effect. These findings demonstrate that 17beta-estradiol exerted an antidepressant-like effect preferentially through the modulation of dopaminergic and serotonergic receptors. This action may also involve the participation of sigma-1 receptors.     

Caffeine-induced locomotor activity: Possible involvement of GABAergic-dopaminergic-adenosinergic interaction

Caffeine (10–40 mg/kg, p.o.) enhanced locomotor activity (LA). Administration of GABA antagonist, bicuculline (0.5–1.0 mg/kg, i.p.), potentiated this caffeine-induced increase of LA, as well as LA of control rats. Treatment with the GABA agonist, muscimol (0.25–1 mg/kg, i.p.) or dopaminergic antagonist, haloperidol (0.25–1 mg/kg, i.p.) or muscarinic receptor blocker, atropine (3.75–5 mg/kg, i.p.), or inhibitor of acetylcholine esterase physostigmine (0.05–0.30 mg/kg, i.p.) or nicotine (0.5–1.5 mg/kg, i.p.) an nicotinic receptor agonist all decreased the LA of both caffeinetreated and control rats. Haloperidol-induced reduction in caffeine-induced increase in LA was found to be withdrawn with higher dose of caffeine. The dopamine agonist L-Dopa (75–150 mg/kg, p.o.) along with carbidopa (10 mg/kg, p.o.) increased the LA in control rats and potentiated the LA of caffeine treated rats. The haloperidol attenuated the bicuculline-induced increase in LA and atropine or physostigmine attenuated the bicuculline or L-Dopa+carbidopa-induced increase in LA in both caffeine treated and control rats when those drugs were administered concomitantly with bicuculline or L-Dopa+carbidopa. These results suggest that (a) the GABAergic system has direct role in the regulation of LA, and (b) caffeine potentiates LA by antagonism of the adenosine receptor and activation of the dopaminergic system which, in turn, reduces GABAergic activity through the reduction of cholinergic system.     

Implication of alpha-2 adrenergic post-synaptic receptors in the central catecholaminergic stimulation of the corticotropic axis in rats

We have recently assigned a major stimulatory role to the brain catecholamines (CA) via alpha 1 and beta receptors on CRH-ACTH secretion, e.g. in the physiological response to stress. In the present study, we explored the possible participation in this regulation of post-synaptic alpha 2 receptors in free moving rats, one week after CA denervation of the hypothalamus by bilateral neurotoxic lesions of the noradrenergic ascending brain stem bundles (NAB). Intracerebroventricular (i.c.v.) injection of clonidine (alpha 2 agonist; 1 nmol) induced a 3 fold rise of ACTH release (measured by RIA) above vehicle (PBS) injected controls (p less than 0.001). This stimulatory effect was completely reversed by an i.c.v. pretreatment with the alpha 2 antagonist idazoxan (10 nmol; without action by itself), whereas it was only slightly affected by an i.c.v. pretreatment with a combination of an alpha 1 and beta blocker (prazosin + propranolol; 5/5 nmol; p greater than 0.1). The results strongly suggest the participation of alpha 2 post-synaptic receptors in the central catecholaminergic activation of ACTH secretion.     

α2-Adrenergic stimulation enhances growth hormone secretion in the dog: A presynaptic mechanism?

Intravenous administration of clonidine (CLO), (2,4 and 8/micrograms/Kg), a predominantly alpha 2-adrenergic receptor agonist, induced in unanesthetized dogs clear-cut and dose-related rises in plasma GH (cGH) levels. Pretreatment with the selective antagonist of alpha 1-adrenergic receptors prazosin (0.1 mg/Kg iv) left unaltered the cGH rise induced by 4/micrograms/Kg of CLO whilst blockade of alpha 2-adrenergic receptors by yohimbine (2.5 mg/Kg iv) completely prevented it. In dogs treated 24 h previously, with reserpine (0.5 mg/Kg iv), a depletor of brain catecholamine stores, CLO was ineffective to stimulate cGH release. These data indicate that in the dog the GH-releasing effect of CLO occurs via stimulation of alpha 2-adrenergic receptors and suggest that the latter are located presynaptically in relation to norepinephrine neurons.     

Involvement of monoaminergic system in the antidepressant-like effect of the hydroalcoholic extract of Siphocampylus verticillatus

The antidepressant-like effect of the hydroalcoholic extract obtained from aerial parts of Siphocampylus verticillatus, a Brazilian medicinal plant, was investigated in two models of depression in mice and against synaptosomal uptake of serotonin, noradrenaline and dopamine. The immobility times in the forced swimming test (FST) and in the tail suspension test (TST) were significantly reduced by the extract (dose range 100-1000 mg/kg, i.p.), without accompanying changes in ambulation when assessed in an open-field. In addition when given orally the extract was also effective in reducing the immobility time in the TST. The efficacy of extract in the TST was comparable to that of the tricyclic antidepressant imipramine (15 mg/kg, i.p.) and with fluoxetine (32 mg/kg, i.p.). The anti-immobility effect of the extract (600 mg/kg, i.p.) assessed in the TST was not affected by pre-treatment with naloxone (1 mg/kg, i.p., a non-selective opioid receptor antagonist) or L-arginine (750 mg/kg, i.p., a nitric oxide precursor). In contrast, the extract (600 mg/kg, i.p.) antidepressant-like effect was significantly reduced by pre-treatment of animals with p-chlorophenylalanine (PCPA, 100 mg/kg, i.p., an inhibitor of serotonin synthesis), sulpiride (50 mg/kg, i.p., a selective D2 receptor antagonist), prazosin (62.5 microg/kg, i.p., an alpha1 adrenoreceptor antagonist) or by guanosine 5'-monophosphate (GMP, 250 mg/kg, i.p., a nucleotide known to block some actions elicited by NMDA). The biochemical data show that the extract of S. verticillatus inhibited in a graded manner the uptake of monoamines. However, at the IC50 level, the extract was approximately 3.2 to 3.4-fold more potent and also more efficacious in inhibiting the synaptosomal uptake of noradrenaline and serotonin than dopamine. Taken together these data demonstrate that the extract of S. verticillatus elicited a significant antidepressant-like effect, when assessed in the TST and FST in mice. Its action seems to involve an interaction with adrenergic, dopaminergic, glutamatergic and serotonergic systems.     

Oxytocin acts as an antidepressant in two animal models of depression

In the behavioral despair test in mice, oxytocin, i.p. injected 60 min before testing, significantly reduced the duration of immobility at doses of 0.250-1.0 mg/Kg; the effect being similar to that of imipramine (7.5-30 mg/Kg i.p.). A more powerful effect was obtained with a 10-day treatment schedule. In the learned helplessness test, oxytocin (0.500 mg/Kg/day i.p. for 8 days) significantly reduced the escape failures and the latency to escape, the effect being even more intense than that of imipramine (20 mg/Kg/day i.p. for 8 days). These results show a new behavioral effect of oxytocin, and further support its role of CNS regulatory peptide.     

Effects of novel 5-HT1A receptor antagonists on measures of postsynaptic 5-HT1A receptor activation in vivo

The effects of two putative 5-HT_1A antagonists, 4-(2′-methoxyphenyl)-1-[2′-[N-(2″-pyridinyl)-p-iodobenzamido]ethyl]piperazine (p-MPPI) and 4-(2′-methoxyphenyl)-1-[2′-[N-(2″-pyridinyl)-p-flourobenzamido]ethyl]piperazine (p-MPPF), were examined in vivo in two tests of postsynaptic 5-HT_1A receptor activation, hypothermia and reciprocal forepaw treading, in the rat. Both p-MPPI (10 mg/Kg, I.p.) and p-MPPF (10 mg/Kg, I.p.) antagonized the hypothermia induced by the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.5 mg/Kg, S.c.). Neither p-MPPI nor p-MPPF administered alone at a dose of 10 mg/kg (i.p.) induced hypothermia. Similarly, both p-MPPI (10 mg/Kg, I.p.) and p-MPPF (2.5 mg/Kg, I.p.) completely antagonized 8-OH-DPAT (2 mg/Kg, S.c.)-induced forepaw treading in rats pretreated with reserpine (1 mg/Kg, S.c., 18–24 hours prior to the experiment). p-MPPI and p-MPPF, at doses of 10 mg/kg (i.p.) did not induce forepaw treading in reserpine pretreated animals. The results of the present study demonstrate that p-MPPI and p-MPPF act as 5-HT_1A receptor antagonists in these measures of postsynaptic 5-HT_1A a receptor activation.     

Long-Term Effects of Portacaval Anastomosis on the 5-Hydroxytryptamine, Histamine, and Catecholamine Neurotransmitter Systems in Rat Brain

Abstract: Portacaval anastomosis (PCA) in the rat is used as a model for portal systemic encephalopathy. Changes in the serotonergic, histaminergic, and catecholaminergic neurotransmitter systems are often found shortly after PCA. We have examined the long-term effects of PCA on the aminergic systems in brains of male Wistar rats, which 8 months previously had been subjected to PCA. Precursors, amines, and metabolites were assayed by HPLC. Eight months after PCA, the catecholamine levels were unchanged in all brain regions. In contrast, tryptophan was evenly increased throughout the brain. The accumulation of 5-hydroxytryptophan after decarboxylase inhibition (NSD-1015; 100 mg/kg i.p.) and the endogenous levels of 5-hydroxyindoleacetic acid were significantly higher in PCA rats, particularly in the hypothalamus and midbrain, whereas 5-hydroxytryptamine concentrations were unchanged. Histamine levels were elevated throughout the brain with the greatest increase found in the hypothalamus and in the striatum. tele -Methylhistamine levels were significantly elevated in cortex and hypothalamus. We conclude that 8 months after PCA, catecholaminergic systems had reestablished their homeostasis, whereas serotonergic and histaminergic systems still show profound disturbances in their function. With histamine, this is reflected as an increase in the amounts of both transmitter and metabolite; serotonergic neurons respond by increasing only the level of the metabolite.     

Cardiovascular effects of cocaine in anesthetized and conscious rats

This study examined the cardiovascular and respiratory effects of cocaine and procaine in anesthetized and conscious rats. Intravenous cocaine (0.16-5 mg/Kg) elicited a rapid, dose dependent increase in mean arterial pressure of relatively short duration. In pentobarbital anesthetized (65 mg/Kg, i.p.) animals, the pressor phase was generally followed by a more prolonged depressor phase. These effects on arterial pressure were generally accompanied by a significant tachypnea and at larger doses (2.5 and 5 mg/Kg, i.v.), bradycardia. Procaine (0.31 and 1.25 mg/Kg, i.v.) produced similar cardiovascular and respiratory effects (depressor phase, tachypnea) in pentobarbital anesthetized animals. In conscious-restrained animals, both cocaine and procaine (1.25 mg/kg, i.v.) produced pressor responses. The subsequent depressor response was, however, absent in both cases. The cardiovascular effects of cocaine (0.25-1 mg/Kg, i.v.) in urethane anesthetized (1.25 g/Kg, i.p.) animals were essentially similar to those observed in conscious animals. Procaine (1mg/Kg) did not produce any significant cardiovascular effects in urethane anesthetized animals, but did elicit tachypnea. Reserpine pretreatment (10 mg/Kg, i.p.) did not significantly attenuate the pressor response in urethane anesthetized animals. Phentolamine pretreatment (3 mg/Kg, i.v.) did significantly antagonize the pressor effect in urethane anesthetized animals. These results suggest that: the depressor phase is likely due to a interaction between local anesthetic activity (cocaine and procaine) and barbiturate anesthesia, the cardiovascular effects of cocaine in conscious animals are more similar to those observed in urethane anesthetized rats than in pentobarbital anesthetized rats and the pressor effect in urethane anesthetized rats is apparently due to a reserpine resistant catecholaminergic mechanism.     

Increased antidepressant-like effect of desipramine combined with central stimulants (caffeine and amphetamine) in mice

Desipramine is a widely used antidepressive agent that inhibits the reuptake of noradrenaline and serotonin, and central stimulants such as caffeine and amphetamine help to release noradrenaline and serotonin. This work aimed to evaluate whether the combination of these agents could produce a stronger antidepressant-like effect than either of the drugs alone. To this end, male mice were treated with different doses of desipramine, caffeine, amphetamine, desipramine-caffeine and desipramine-amphetamine. The results showed that all drugs produced decreased immobility time in the forced swimming model. The combined treatment of desipramine (0.31, 1.0 or 3.1 mg/kg i.p.) with caffeine or amphetamine (0.31 or 1 mg/kg i.p.) reduced immobility time greater than either of those drugs alone. The combined treatment of desipramine (0.31, 1 and 3.1 mg/kg i.p.) with amphetamine or caffeine (0.1 and 1 mg/kg i.p.) did not increase the motor activity significantly compared to the control. These results also suggested that drugs which promote the release of noradrenaline and serotonin could increase antidepressant-like effect of desipramine.     

The role of sensory neurons in the antiulcer effect of centrally injected amylin in rat

Central administration of amylin (2.2 microg/rat, i.c.v.) reduces (from a minimum of 67% to 83%) indomethacin (Indo, 20 mg Kg(-1), orally) induced ulcers in rats. The anti-ulcer effect of the peptide is not removed by the administration of prokinetic drugs like domperidone or neostigmine but it is reduced by 35% in rats treated with capsaicin or with the CGRP antagonist, CGRP(8-37). These data indicate that amylin gastroprotection involves capsaicin-sensitive nerve fiber leading to CGRP-dependent gastric vasodilatory effect. Additional mechanisms could involve noradrenergic alpha(2) receptors as the peptide gastroprotective activity is reduced from 67% to 20% by the alpha(2) antagonist yohimbine.     

Cardiorespiratory effects of diazepam-ketamine, xylazine-ketamine and thiopentone anesthesia in male Wistar rats--a comparative analysis

Several anesthetics are known to cause respiratory and cardiovascular depression in humans and animals; but, these diverse effects have not been extensively investigated in laboratory rodents. The objective of this study is to choose a suitable anesthetic combination for use in surgical models eg. coronary artery ligation in rats. Male Wistar rats were anesthetized with three different drugs viz. diazepam-ketamine (DK) (2.5 mg/Kg, intraperitoneally (i.p); 50 mg/Kg, i.p), xylazine-ketamine (XK) (5 mg/Kg i.p; 50 mg/Kg i.p) and thiopentone (T) (40 mg/Kg i.p) and the respiratory and cardiovascular functions were assessed after coronary artery ligation. Heart rate (HR), mean arterial pressure (MAP), partial pressure of carbon dioxide (PaCO2), partial pressure of oxygen (PaO2), oxygen saturation percentage (O2 sat (%)), arterial blood pH and rectal body temperature were studied in detail. During the anesthetic regime, HR was lower till 60 min in XK and T ligated group (333 +/- 6; 304 +/- 8 beats/min) and it was near normalcy in the case of DK ligated group (394 +/- 6 beats/min). Significant respiratory depression was particularly reflected in the T ligated group with an increase in PaCO2 at 30 min (40.32 +/- 2.64 mmHg), which decreased to 38.2 +/- 2.23 mmHg at 60 min. Throughout the investigation, DK showed the least overall effects compared to XK and T on respiratory functions. Thus, DK could be considered to be a suitable anesthetic for use in a surgical model such as coronary artery ligation in albino rats.     

Study of the antinociceptive effect of tetrahydropapaveroline derivatives. Interaction with opioids

The antinociceptive effect of racemic tetrahydropapaveroline (THP), of its two R(+)- and S(−) enantiomers, of 1-2-dehydro-THP and of 1-carboxy-THP was assessed using different pain tests in mice. None of these drugs possessed a significant activity in the hot-plate and tail-flick tests. However, after i.p. injection, they reduced the number of abdominal writhes induced by phenylbenzoquinone, with ED₅₀ values of 51 ± 7, 73 ± 9 and 79 ± 7 mg/kg for the most potent compounds: 1,2-dehydro-THP, ±THP and -THP, respectively. This activity was not antagonized by naloxone (1 mg/Kg, S.c.). However combination of inactive doses of these three compounds (32 mg/Kg, I.p.) and of morphine (0.5 mg/Kg, S.c.) led to a significant antinociceptive effect (83 to 85 % of reduction of the number of writhes). This synergistic potentiation confirmed with the combination of ±THP (16 mg/Kg, I.p.) and morphine (0.5 mg/Kg, S.c.) was totally inhibited by naloxone (1 mg/Kg, S.c.). These results, although excluding a direct agonistic effect of THP derivatives on opiate receptors, suggest an indirect interaction of these drugs with the endogenous opioid system.     

Regulation of the mouse aggressive behavior (pharmacologic analysis of the GABAergic mechanism)

Ethological procedures were used to study the effects of GABA-positive drugs on aggression in male albino mice kept in isolation (opponent test). The results revealed several variants of antiaggressive effects of the tested GAB Aergic drugs: 1) antiaggressive, re-socializing of GABAA agonists muscimol (0.125 and 0.5 mg/kg) and THIP (2.0 mg/kg), and GABAB agonist baclofen (2.5-10 mg/kg); 2) antiaggressive, sedative of GABAB agonists baclofen (12.5 mg/kg), phenibut (50-100 mg/kg), and inhibitor of GABA transamininase sodium valproate (100 mg/kg); 3) antiaggressive, anxiogenic for muscimol (1 mg/kg), THIP (5 mg/kg), and sodium valproate (25-50 mg/kg).     

Effect of chlordimeform and clonidine on the turnover of P-octopamine in rat hypothalamus and striatum

The effect of the invertebrate octopamine agonists chlordimeform and clonidine on the concentration and turnover of p-octopamine and m- and p-tyramine was determined in rat hypothalamus and striatum. Clonidine (0.25 mg/Kg, s.c.) did not alter the concentration of p-octopamine in the hypothalamus or p-tyramine in the striatum. Administration of chlordimeform (50 mg/Kg, i.p.) resulted in an increase in p- and m-tyramine concentrations in the striatum but not that of p-octopamine in the hypothalamus. This increase in the tyramine isomers is consistent with the ability of chlordimeform and its metabolite, demethylchlordimeform, to inhibit monoamine oxidase (MAO). The concurrent administration of chlordimeform (50 mg/Kg, i.p.) and pargyline (75 mg/Kg, i.p.) produced a significant decrease in the accumulation of octopamine in the hypothalamus but not in the striatum. In contrast, the concurrent administration of clonidine (0.25 mg/Kg, s.c.) and pargyline (75 mg/Kg, i.p.) caused a significant decrease in the accumulation of octopamine in the striatum but not hypothalamus. These results show that the turnover of octopamine in the hypothalamus and striatum is decreased by chlordimeform and clonidine, respectively. Further, clonidine is known to modulate the turnover of amines in mammalian noradrenergic nerve terminals by an action at presynaptic adrenergic receptors. These data suggest that two mechanisms, one involving presynaptic adrenergic receptors in the striatum, and the other involving as yet unidentified receptors in the hypothalamus, modulate the turnover of octopamine in the mammalian brain.     

The role of serotonergic and catecholaminergic systems for possible antidepressant activity of apigenin

Background and objectiveAlthough, the anti-depressant like effects of apigenin (APG) are documented in the literature, the underlying mechanism for exerting such an effect is still not clear. In this research, an attempt was made to determine the possible role of APG for antidepressant activity through serotonergic and catecholaminergic systems using standardized animal models.Materials and methodsThe antidepressant property of APG was determine by involving tail suspension (TST) and modified forced swimming tests (MFST). The effect of APG was evaluated at 25 and 50 mg/kg. In mechanistic models, animals were pretreated with catecholaminergic and serotonergic antagonists prior to administration of APG. The results obtained were statistically analyzed to determine the level of significance.ResultsThe period of immobility in both models (TST and MFST) was significantly reduced by APG (25 and 50 mg/kg). The best therapetuic dose of APG (50 mg/kg) was selected for the mechanistic study. The anti-immobility effect of APG declined to a significant extent upon pretreatment with catecholaminergic antagonists (α-methyl-para-tyrosine methyl ester; SCH 23390; sulpiride; phentolamine) and serotonergic inhibitors (p-clorophenylalanine-methyl-ester; ondansetron) in both TST and MFST models. The antidepressant benefits of apigenin were only modestly reversed when rats were given propranolol.ConclusionsThe findings suggest that APG's antidepressant effect is mediated by the α-adrenergic, dopaminergic and 5-HT3 serotonergic receptors.     

Effect of ACTH on the imipramine- and desipramine-induced decrease in duration of immobility time as measured in a rat forced swimming test.

In a rat forced swimming test (FS), we examined the effect of repeated injections of ACTH (adrenocorticotropic hormone) for 14 days on the decreased duration of immobility time produced by imipramine and desipramine. Both imipramine (15 and 30 mg/kg, p.o.) and desipramine (15 and 30 mg/kg, p.o.) significantly decreased the duration of immobility time in the FS. On the other hand, ACTH (100 microg/kg, i.p.) alone did not affect the duration of immobility time in FS. When ACTH (100 microg/kg, i.p.) was injected for 14 days before the 15-min swim session, it counteracted the decreased duration of immobility time induced by both imipramine and desipramine. Thus, ACTH seems to play a key role in decreasing the duration of immobility time of antidepressants in this test.     

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.     

PGF2 alpha and breathing pattern in newborn pigs

The effect of PGF2 alpha has been evaluated in 11 unanaesthetized unrestrained piglets and in 3 anaesthetized piglets (2-3 days old) using a barometric-plethysmographic technique. PGF2 alpha (mg 0.25/pig) was administered as aerosol for 5 min. In 3 of the unanaesthetized newborn pigs the effect of PGF2 alpha aerosol has been evaluated after indomethacin (mg 1/Kg i.v.). The vagal dependent activity of the prostaglandin was also evaluated after atropine (mg 0.08/Kg i.m.). Our results show that PGF2 alpha in newborn pigs causes hypoventilation due to a decrease in respiratory rate and to a lengthening in TE. The changes in TE are due to an increase in the incidence and duration of apneic events characterizing the respiratory activity at birth. After indomethacin PGF2 alpha does not change the breathing pattern. Atropine only partially reduces the effects of PGF2 alpha while, after anaesthesia, prostaglandin does not change the breathing pattern. Consequently our results show that PGF2 alpha in newborn animals similar to other prostaglandins acts as a depressant of respiratory activity.     

Bombesin and substance P analogues differentially regulate G-protein coupling to the bombesin receptor. Direct evidence for biased agonism

Substance P analogues including [d-Arg1,d-Phe5,d-Trp7,9,Leu11]substance P (SpD) act as "broad spectrum neuropeptide antagonists" and are potential anticancer agents that inhibit the growth of small cell lung cancer cells in vitro and in vivo. However, their mechanism of action is controversial and not fully understood. Although these compounds block bombesin-induced mitogenesis and signal transduction, they also have agonist activity. The mechanism underlying this agonist activity was examined. SpD binds to the ligand-binding site of the bombesin/gastrin-releasing peptide receptor and blocks the bombesin-stimulated increase in [Ca2+]i within the same concentration range that causes sustained activation of c-Jun N-terminal kinase and extracellular signal-regulated protein kinase (ERK). The activation of c-Jun N-terminal kinase by SpD and bombesin is blocked by dominant negative inhibition of G(alpha12). The ERK activation by SpD is pertussis toxin-sensitive in contrast to ERK activation by bombesin, which is pertussis toxin-insensitive but dependent on epidermal growth factor receptor phosphorylation. SpD does not simply act as a partial agonist but differentially modulates the activation of the G-proteins G(alpha12), G(i), and G(q) compared with bombesin. This unique ability allows the bombesin receptor to couple to G(i) and at the same time block receptor activation of G(q). Our results provide direct evidence that SpD is acting as a "biased agonist" and that this has physiological relevance in small cell lung cancer cells. This validation of the concept of biased agonism has important implications in the development of novel pharmacological agents to dissect receptor-mediated signal transduction and of highly selective drugs to treat human disease.