Diurnal variation in 5-HT1B autoreceptor function in the anterior hypothalamus in vivo: effect of chronic antidepressant drug treatment.
1. Intracerebral microdialysis was used to examine the function of the terminal 5-hydroxytryptamine (5-HT) autoreceptor in the anterior hypothalamus of anaesthetized rats at two points in the light phase of the light-dark cycle. 2. Infusion of the 5-HT1A/1B agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU24969) 0.1, 1.0 and 10 microM through the microdialysis probe led to a concentration-dependent decrease (49, 56 and 65% respectively) in 5-HT output. The effect of RU24969 (1 and 5 microM) was prevented by concurrent infusion of methiothepin (1 and 10 microM) into the anterior hypothalamus via the microdialysis probe. Infusion of methiothepin alone (1.0 and 10 microM) increased (15 and 142% respectively) 5-HT output. 3. Infusion of RU24969 (5 microM) through the probe at mid-light and end-light resulted in a quantitatively greater decrease in 5-HT output at end-light compared with mid-light. 4. Following treatment with either paroxetine hydrochloride (10 mg kg(-1) i.p.) or desipramine hydrochloride (10 mg kg)(-1) i.p.) for 21 days the function of the terminal 5-HT1B autoreceptor was more markedly attenuated at end-light. 5. The data show that, as defined by the response to RU24969, the function of the 5-HT1B receptors that control 5-HT output in the anterior hypothalamus is attenuated following chronic desipramine or paroxetine treatment in a time-of-day-dependent manner. (+info)
Neuronal uptake affects dynamic characteristics of heart rate response to sympathetic stimulation.
Recently, studies in our laboratory involving the use of a Gaussian white noise technique demonstrated that the transfer function from sympathetic stimulation frequency to heart rate (HR) response showed dynamic characteristics of a second-order low-pass filter. However, determinants for the characteristics remain to be established. We examined the effect of an increase in mean sympathetic stimulation frequency and that of a blockade of the neuronal uptake mechanism on the transfer function in anesthetized rabbits. We found that increasing mean sympathetic stimulation frequency from 1 to 4 Hz significantly (P < 0.01) decreased the dynamic gain of the transfer function without affecting other parameters, such as the natural frequency, lag time, or damping coefficient. In contrast, the administration of desipramine (0.3 mg/kg iv), a neuronal uptake blocking agent, significantly (P < 0.01) decreased both the dynamic gain and the natural frequency and prolonged the lag time. These results suggest that the removal rate of norepinephrine at the neuroeffector junction, rather than the amount of available norepinephrine, plays an important role in determining the low-pass filter characteristics of the HR response to sympathetic stimulation. (+info)
Alpha 1-adrenergic receptor-mediated increase in the mass of phosphatidic acid and 1,2-diacylglycerol in ischemic rat heart.
OBJECTIVE: 1,2-Diacylglycerol (1,2-DAG) and phosphatidic acid (PA) are produced by phospholipase C and D activity and play a key role as second messengers in receptor-mediated signal transduction. So far, little is known about alterations of endogenous 1,2-DAG and PA production during myocardial ischemia. METHODS: Rat isolated perfused hearts were subjected to global ischemia, total lipids were extracted, and separated by thin-layer chromatography. The mass of PA and 1,2-DAG were quantified using laserdensitometric analysis of visualized lipids. RESULTS: Compared to normoxic control values (1,2-DAG 713 +/- 45 ng/mg protein, PA 171 +/- 11 ng/mg protein), the myocardial content of 1,2-DAG and PA was unaltered after 10 min of ischemia. Prolonged myocardial ischemia (20 min), however, which was accompanied by marked overflow of endogenous norepinephrine, significantly increased the mass of both second messengers (1,2-DAG 1062 +/- 100 ng/mg protein, PA 340 +/- 29 ng/mg protein). The increase in PA and 1,2-DAG in response to ischemia was abolished by inhibition of ischemia-induced norepinephrine release as well as by alpha1-adrenergic blockade but unaffected by beta-adrenergic blockade. While inhibition of diacylglycerol kinase did not affect ischemia-induced increase in PA and 1,2-DAG, inhibition of phosphatidylinositol-specific phospholipase C activity significantly suppressed ischemia-induced increase in 1,2-DAG but did not affect endogenous production of PA indicating phospholipase C-independent formation of PA and activation of both, phospholipase C and D, in the ischemic heart. CONCLUSIONS: Ischemia elicits an alpha1-adrenergic receptor-mediated increase in the mass of myocardial PA and 1,2-DAG. The increase in endogenous PA is suggested to be due to the activation of myocardial phospholipase D, whereas 1,2-DAG is formed predominantly by activation of phospholipase C in the ischemic heart. (+info)
Enhancement of the serotonin-mediated acetylcholine release by repeated desmethylimipramine treatment in the hippocampus of freely moving rats.
A possible involvement of serotonin-mediated cholinergic activation in the antidepressant effect of desmethylimipramine (DMI) was investigated by determination of the effects of a single or repeated DMI administration on acetylcholine (ACh) release in the hippocampus using an in vivo microdialysis technique and a radioimmunoassay for ACh. Rats were administered DMI (10 mg/kg, i.p.) acutely or repeatedly for 21 days. A single or repeated DMI administration did not cause any significant effects on the basal ACh release compared with the respective controls. Atropine perfusion in the acutely DMI-treated or control rats increased the ACh release to the same degree. In repeatedly DMI-treated rats, serotonin (5-HT) (1 to 10 microM) perfusion enhanced significantly the ACh release. However, 5-HT in acutely DMI-treated rats enhanced significantly the ACh release only at 10 microM. 5-HT did not cause any changes in ACh release in control rats. Hippocampal 5-HT content of acutely DMI-treated rats was significantly higher than that of saline-treated control rats, while no difference was observed between the repeatedly DMI- and saline-treated rats. These findings suggest, for the first time, that DMI induced a facilitation of cholinergic neurotransmission in the rat hippocampus through the activation of 5-HT-receptor function. (+info)
Electrochemical and electrophysiological characterization of neurotransmitter release from sympathetic nerves supplying rat mesenteric arteries.
1. Characteristic features of noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves in rat small mesenteric arteries in vitro have been investigated on an impulse-by-impulse basis. NA release was measured using continuous amperometry and ATP release was monitored by intracellular recording of excitatory junction potentials (e.j.ps). 2. Electrical stimuli evoked transient increases in oxidation current. During trains of ten stimuli at 0.5 - 4 Hz there was a depression in the amplitude of oxidation currents evoked following the first stimulus in the train. 3. The neuronal NA uptake inhibitor, desmethylimipramine (1 microM), increased the amplitude of the summed oxidation current evoked by ten stimuli at 1 Hz and slowed the decay of oxidation currents evoked by trains of ten stimuli at 1 and 10 Hz. 4. The alpha2-adrenoceptor antagonist, idazoxan (1 microM), increased the amplitudes of the oxidation currents evoked during trains of ten stimuli at 0.5 - 10 Hz but had no effect on the oxidation currents evoked by the first stimulus in the train. 5. Idazoxan (1 microM) increased the amplitude of all e.j.ps evoked during trains of stimuli at 0.5 and 1 Hz. In addition, the facilitatory effect of idazoxan on e.j.ps was significantly greater than that on oxidation currents. 6. The findings indicate that NA release from sympathetic nerves supplying small mesenteric arteries is regulated by activation of presynaptic alpha2-adrenoceptors and that clearance of released NA in this tissue depends, in part, upon neuronal uptake. The different effects of idazoxan on the oxidation currents and e.j.ps may indicate that the release of NA and ATP is differentially modulated. (+info)
Comparison of the effects of venlafaxine, desipramine, and paroxetine on noradrenaline- and methoxamine-evoked constriction of the dorsal hand vein.
AIMS: To examine whether the antidepressant venlafaxine, a novel serotonin-noradrenaline re-uptake inhibitor (SNRI), can modify alpha-adrenoceptor-mediated venoconstriction in man. The effects of venlafaxine were compared with those of desipramine, a tricyclic antidepressant with noradrenaline uptake inhibiting properties, and paroxetine, a selective serotonin re-uptake inhibitor (SSRI), on noradrenaline-and methoxamine-evoked venoconstriction using the dorsal hand vein compliance technique. METHODS: Fifteen healthy male volunteers participated in five weekly experimental sessions. Each session was associated with a clinically effective dose of an antidepressant or placebo. The following oral dosages were used: venlafaxine 75 mg, venlafaxine 150 mg, desipramine 100 mg, paroxetine 20 mg, or placebo. A double-blind, cross-over, balanced design was used. In each session, dose-response curves to both locally infused noradrenaline acid tartrate (0.1-33.33 ng min-1 ) and methoxamine hydrochloride (0.5-121.5 microg min-1 ) were constructed. Systolic and diastolic blood pressure and pulse rate were measured in the supine and erect positions. Salivation was measured by the dental roll technique. RESULTS: Venlafaxine 150 mg and desipramine 100 mg potentiated the venoconstrictor response to noradrenaline (anova of log ED50s: P<0.01; individual comparisons: venlafaxine 150 mg vs placebo: P<0.005; mean difference, 95% CI: -0. 49 (-0.81, -0.17); desipramine 100 mg vs placebo: P<0.005; mean difference, 95% CI: -0.34 (-0.60, -0.09) without affecting the response to methoxamine. Neither paroxetine nor placebo had any effects on the venoconstrictor responses. Both doses of venlafaxine increased systolic blood pressure (supine and erect) and venlafaxine 150 mg increased diastolic blood pressure (supine) (anova, P<0.05). Desipramine increased heart rate (P<0.05). Desipramine and both doses of venlafaxine reduced salivation (P<0.025). CONCLUSIONS: These results show that, similarly to desipramine 100 mg, venlafaxine 150 mg can potentiate venoconstrictor responses to noradrenaline, consistent with venlafaxine's ability to block noradrenaline uptake in man. The importance of noradrenaline uptake blockade in these observations is confirmed by the lack of effect of the antidepressants on methoxamine-evoked venoconstriction and the failure of paroxetine to modify noradrenaline-evoked venoconstriction. (+info)
Comparison of the novel antipsychotic ziprasidone with clozapine and olanzapine: inhibition of dorsal raphe cell firing and the role of 5-HT1A receptor activation.
Ziprasidone is a novel antipsychotic agent which binds with high affinity to 5-HT1A receptors (Ki = 3.4 nM), in addition to 5-HT1D, 5-HT2, and D2 sites. While it is an antagonist at these latter receptors, ziprasidone behaves as a 5-HT1A agonist in vitro in adenylate cyclase measurements. The goal of the present study was to examine the 5-HT1A properties of ziprasidone in vivo using as a marker of central 5-HT1A activity the inhibition of firing of serotonin-containing neurons in the dorsal raphe nucleus. In anesthetized rats, ziprasidone dose-dependently slowed raphe unit activity (ED50 = 300 micrograms/kg i.v.) as did the atypical antipsychotics clozapine (ED50 = 250 micrograms/kg i.v.) and olanzapine (ED50 = 1000 micrograms/kg i.v.). Pretreatment with the 5-HT1A antagonist WAY-100,635 (10 micrograms/kg i.v.) prevented the ziprasidone-induced inhibition; the same dose of WAY-100,635 had little effect on the inhibition produced by clozapine and olanzapine. Because all three agents also bind to alpha 1 receptors, antagonists of which inhibit serotonin neuronal firing, this aspect of their pharmacology was assessed with desipramine (DMI), a NE re-uptake blocker previously shown to reverse the effects of alpha 1 antagonists on raphe unit activity. DMI (5 mg/kg i.v.) failed to reverse the inhibitory effect of ziprasidone but produced nearly complete reversal of that of clozapine and olanzapine. These profiles suggest a mechanism of action for each agent, 5-HT1A agonism for ziprasidone and alpha 1 antagonism for clozapine and olanzapine. The 5-HT1A agonist activity reported here clearly distinguishes ziprasidone from currently available antipsychotic agents and suggests that this property may play a significant role in its pharmacologic actions. (+info)
LLC-PK(1) cells stably expressing the human norepinephrine transporter: A functional model of carrier-mediated norepinephrine release in protracted myocardial ischemia.
In myocardial ischemia, adrenergic terminals undergo ATP depletion, hypoxia, and intracellular pH reduction, causing the accumulation of axoplasmic norepinephrine (NE) and intracellular Na(+) [via the Na(+)-H(+) exchanger (NHE)]. This forces the reversal of the Na(+)- and Cl(-)-dependent NE transporter (NET), triggering massive carrier-mediated NE release and, thus, arrhythmias. We have now developed a cellular model of carrier-mediated NE release using an LLC-PK(1) cell line stably transfected with human NET cDNA (LLC-NET). LLC-NET cells transported [(3)H]NE and [(3)H]N-methyl-4-phenylpyridinium ([(3)H]MPP(+)) in an inward direction. This uptake was abolished by the NET inhibitors desipramine (100 nM) and mazindol (300 nM) and by extracellular Na(+) removal. Na(+)-gradient reversal induced an efflux of (3)H-substrate from preloaded LLC-NET cells. Desipramine and mazindol blocked this efflux. Because of its greater intracellular stability and higher sensitivity to Na(+)-gradient reversal, [(3)H]MPP(+) proved preferable to [(3)H]NE as an NET substrate; therefore, only [(3)H]MPP(+) was used for subsequent studies. The K(+)/H(+) ionophore nigericin (10 microM) evoked a large efflux of [(3)H]MPP(+). This efflux was potentiated by the Na(+),K(+)-ATPase inhibitor ouabain (100 microM), was sensitive to desipramine, and was blocked by the NHE inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (EIPA; 10 microM). In contrast, EIPA failed to inhibit the [(3)H]MPP(+) efflux elicited by the Na(+) ionophore gramicidin (10 microM). Furthermore, [(3)H]MPP(+) efflux induced by the NHE-stimulant proprionate (25 mM) was negatively modulated by imidazoline receptor activation. Our findings suggest that LLC-NET cells are a sensitive model for studying transductional processes of carrier-mediated NE release associated with myocardial ischemia. (+info)