Quantification and pharmacological characterization of dialysate levels of noradrenaline in the striatum of freely-moving rats: release from adrenergic terminals and modulation by α2-autoreceptors

Information concerning striatal levels of noradrenaline (NA) remains inconsistent. Here we have addressed this issue using a sensitive method of HPLC coupled to amperometric detection. The NA reuptake-inhibitor, reboxetine, selectively elevated levels of NA versus dopamine (DA), and NA levels were also selectively elevated by the α₂-adrenoceptor (AR) antagonist, atipamezole. The actions of atipamezole were mimicked by the preferential α_2A-AR antagonist, BRL44408, while JO-1 and prazosin, preferential antagonists at α_2C-ARs, caused less marked elevations in NA levels. In contrast to antagonists, the α₂-AR agonist, S18616, decreased NA levels and likewise suppressed those of DA. Unilateral lesions of the substantia nigra with 6-hydroxydopamine depleted DA levels without affecting those of NA. Further, the D₃/D₂ receptor agonist, quinelorane, decreased levels of DA without modifying those of NA. However, the D₃/D₂ receptor antagonists, haloperidol and raclopride, and the DA reuptake-inhibitor, GBR12935, elevated levels of both DA and NA. Levels of 5-HT (but not of NA or DA) were increased only by the 5-HT reuptake-inhibitor, citalopram. They were decreased by S18616 and prazosin, reflecting the inhibitory and excitatory influence of α₂- and α₁-ARs, respectively, upon serotonergic pathways. In conclusion, NA in the striatum is derived from adrenergic terminals. Its release is subject to tonic, inhibitory control by α₂-ARs, possibly involving both α_2A- and α_2C-AR subtypes, though their respective contribution requires clarification. A role of dopaminergic terminals in the reuptake of NA likely explains the elevation in its levels elicited by DA reuptake-inhibitors and D₃/D₂ receptor antagonists.     

Dorsal raphé stimulation modifies striatal-evoked antidromic invasion of nigral dopaminergic neurons in vivo

Summary Extracellular single unit recordings were obtained from antidromically identified nigrostriatal dopaminergic neurons in anesthetized rats to determine the effects of dorsal raphé stimulation on the somatodendritic excitability of substantia nigra dopaminergic neurons. Stimulation of the dorsal raphé with a brief train of pulses delivered 7–2 ms prior to the neostriatal-evoked antidromic response significantly reduced the proportion of neostriatal-evoked antidromic responses that consisted of both initial segment and somatodendritic components without significantly altering the neostriatal-evoked post-stimulus inhibitory period. Raphé stimulation alone facilitated post-stimulus neuronal firing in almost half of the cells examined. The raphé-induced decrease in somatodendritic excitability was blocked by the serotonin antagonist, metergoline (0.5–2.0 mg/kg, i.v.), without significantly affecting the rate or pattern of spontaneous activity. The tryptophan hydroxylase inhibitor, parachlorophenylalanine (400 mg/kg, i.p. for three consecutive days), abolished the decrease in somatodendritic excitability following raphé stimulation which could be re-instated by intravenous administration of 5-HTP. The dopamine antagonists haloperidol (25–100 g/kg, i.v.) and sulpiride (10–30 mg/kg, i.v.) also blocked the effects of dorsal raphé stimulation on somatodendritic invasion. These data suggest that in vivo, serotonin liberated from raphé-nigral terminals facilitates the release of dopamine from nigrostriatal dendrites resulting in a local, autoreceptor-mediated reduction in somatodendritic excitability without affecting the spontaneous firing rate and excitability of the neuron as a whole.     

Pre- and postsynaptic dopaminergic activities of U-86170F

Summary The biochemical, endocrine, receptor binding, and behavioral effects of the putative dopamine autoreceptor agonist, U-86170F, were evaluated in various in vivo and in vitro models. U-86170F and apomorphine were shown to cause a significant reversal of the effects of -butyrolactone (GBL) on dopamine accumulation in mouse striata. In contrast to apomorphine, U-86170F had a ceiling effect on the extent of the reversal of GBL effects (55%), whereas apomorphine had an 82% reversal. The effect on striatal homovanillic acid (HVA) levels was also monitored, and both compounds exerted a similar and significant reduction in striatal HVA. A comparison was made between the effects of intraperitoneal (i.p.) and oral administration of U-86170F in the -methyl-p-tyrosine (-MPT)/prolactin model in rats. When administered by the i.p. route, U-86170F suppressed the effects of -MPT on prolactin level increase, having an ED₅₀ of about 0.03 mg/kg, and when administered by the oral route, its ED₅₀ was approximately 0.1 mg/kg. U-86170F has been shown to be a potent dopamine autoreceptor agonist in the GBL, prolactin, and HVA models, with an effective i.p. dose of approximately 0.03 mg/kg. When evaluated for postsynaptic dopaminergic activity in the reserpinized mouse model, and compared to apomorphine, U-86170F was found to increase locomotor activity, but its maximum effect was only 65% of that attained with apomorphine. Higher doses were needed for postsynaptic effects.In receptor binding studies using cloned D2 receptor preparations, U-86170F was found to exhibit agonist binding properties similar to dopamine as demonstrated by their inhibition of 3H-raclopride binding. Both compounds exhibited biphasic inhibition curves, with U-86170F having K_i values of 7.5 nM and 250 nM, and for dopamine the K_i values were 34.7 nM and 1031 nM. Binding studies conducted in the presence of GTP yielded only one site with Kis of 289 nM and 670 nM, for both U-86170F and dopamine, respectively.The results presented in this report demonstrated that U-86170F is a potent dopamine autoreceptor agonist, with limited activity at the postsynaptic receptor. Send offprint requests to R.A. Lahti at the above address     

Sigma1 receptor upregulation after chronic methamphetamine self-administration in rats: a study with yoked controls

Rationale Sigma₁ receptors (Sig-1R) are implicated in behavioral sensitization, conditioned place preference, and cellular restructuring induced by psychostimulants. We previously reported that rats which actively self-administered methamphetamine for 5 weeks and were then withdrawn from methamphetamine for 24 h showed downregulation of dopamine D₂ autoreceptors (approximately 30%) in the midbrain and this was not seen in rats that passively received injections of methamphetamine or saline at the same time (yoked controls). Involvement of Sig-1R in the self-administration of psychostimulants, however, has never been reported.Objectives This study examined neuroadaptive changes in Sig-1R in the brains of rats self-administering methamphetamine.Methods Three groups of rats were tested simultaneously 5 days per week, for 5 weeks (25 daily sessions). Two groups served as yoked controls and passively received an injection of either 0.1 mg/kg methamphetamine or saline (not contingent on responding) each time a response-contingent injection of 0.1 mg/kg methamphetamine was actively self-administered by the first group of rats. Protein and mRNA levels of Sig-1R were then measured by Western and Northern blottings, respectively.Results There was a marked upregulation of Sig-1R proteins (50%) in the midbrain and altered levels of Sig-1R mRNA in the frontal cortex and hippocampus of rats that learned to actively self-administer methamphetamine, but not in yoked methamphetamine- or saline-control rats.Conclusions Neuroadaptive increases in Sig-1R seen in this study may contribute to the reinforcing effects of methamphetamine. This upregulation of Sig-1R may be mediated by increased protein kinase A activity due to downregulation of dopamine D₂ autoreceptors.R.S., Z.J., T.H. and M.T. contributed equally to this work     

Additive Reduction of Alcohol Drinking by 5-HT1A Antagonist WAY 100635 and Serotonin Uptake Blocker Fluoxetine in Alcohol-Preferring P Rats

We found previously that alcohol-preferring (P) rats have fewer serotonin (5-HT) neurons and fibers in key brain regions than alcoholnonpreferring (NP) rats. Because 5-HT uptake blockers increase synaptic 5-HT content and 5-HT_1A receptor antagonists increase 5-HT release by disinhibiting 5-HT autoinnervation, in the present study, our intent was to determine whether increased synaptic 5-HT content and/or 5-HT release in P rats would effectively reduce alcohol consumption. In experiment 1, the 5-HT antagonist WAY 100635 (WAY) was tested on adult female P rats maintained on 24-hr free-choice access to ethanol (10% v/v) and water. Twice daily doses of WAY (0.05, 0.1, 0.5, and 1.0 mg/kg, subcutaneously) were administered to each rat in a counterbalanced order. Baseline ethanol intake, derived from the mean ethanol intakes of the three previous non-drug days, was approximately 8 g/kg/day. Results indicated that 0.05,0.1, and 0.5 mg/kg doses of WAY reduced 24-hr ethanol drinking by 25-30% ( p < 0.01) without affecting 24-hr water intake or body weight In the second experiment, the effects of WAY (0.5 mg/kg), fluoxetine (1.0 mg/kg), or a combination of both were tested in another group of female P rats. WAY and fluoxetine, each alone, reduced ethanol drinking by around 20% and, when combined, decreased ethanol intake by 50%, whereas the body weight and the total fluid intake were not significantly affected. Taken together, these results indicate that both fluoxetine and WAY preferentially reduce ethanol drinking in the P line of rats and, when administered together, reduce ethanol intake in an additive manner. It is proposed that coadministration of these two compounds with distinct mechanisms of action may be a new strategy for reducing alcohol intake.     

5-HT(1B) mrna regulation in two animal models of altered stress reactivity.

BACKGROUND: Acute stress has profound effects on serotonergic activity, but it is not known whether alterations in the serotonin system can predispose individuals to exaggerated stress responses. We examined the regulation of 5-HT(1B) and 5-HT(1A) mRNA in two rodent models of differential sensitivity to stress: congenital learned helplessness (cLH) and handling and maternal separation (HMS). METHODS: 5-HT(1B) and 5-HT(1A) mRNAs in brain tissue sections were quantitated by in situ hybridization from control, stress-sensitive, and stress-resistant male rats in the HMS model and stress-sensitive and stress-resistant rats (both males and females) in the cLH model. Dorsal raphe nucleus, striatum, and hippocampus were examined. RESULTS: The main result was that dorsal raphe 5-HT(1B) mRNA was substantially elevated (63-73%) in male rats in the stress-resistant group of both models compared with stress-sensitive animals. 5-HT(1B) mRNA in female rats did not differ between groups in the cLH model. There were no differences in 5-HT(1A) mRNA between HMS groups. CONCLUSIONS: These findings suggest that 5-HT(1B) autoreceptor regulation is altered in animals with diminished stress reactivity. These results suggest that 5-HT(1B) autoreceptors in unstressed and acutely stressed animals differ, indicating the importance of state versus trait changes in serotonin function in animal models of anxiety and depression.     

A role for computer simulation in solving the riddles of autoreceptor-mediated regulation of GABA release

Summary The autoreceptor-mediated control of GABA release was simulated on a personal computer using commercially available software (STELLA^TM/ITHINK^TM). The experimental data to be matched were taken from previous publications. A basic model was able to fairly accurately reproduce frequency dependencies of GABA release in the presence and absence of uptake inhibition as well as concentration-response curves for changes in release produced by the agonist, (–)-baclofen, or by relatively low concentrations of the antagonists, phaclofen and CGP 35348. Obvious mismatch was observed at high concentrations of a potent antagonist, at a stimulation frequency of 2 Hz. Whereas the experimental data indicate a 3-fold increase in release as compared to controls, simulation predicts a 7-fold increase. By adaptation of the model, simulation data were obtained indicating that this mismatch was not due to (a) the autoreceptor occurring as two subtypes with different affinities for antagonists, (b) the occurrence of an agonist and antagonist state of the autoreceptor, with the latter prevailing at low synaptic concentrations of endogenous GABA, and (c) overruling of uptake inhibition by markedly elevated synaptic GABA concentrations. On the other hand, a simple restriction of the amount of transmitter able to be released per time unit produced much better matching data. A refined model assuming a restricted replacement capacity for exocytotically emptied synaptic vesicles at their docking sites gave similar results. As a consequence, we shall attempt to address this possibility experimentally.Simulation can never prove a case in the positive sense. It can, however, help to exclude ill-matching solutions of a problem and to prioritize among possible ones, which then must be experimentally addressed. We found simulation with this user-friendly software extraordinarily useful, also and not least because it necessitates and stimulates very intense dealing with a subject.Correspondence to: P. C. Waldmeier at the above address     

Mechanism of ionophore A23187 induction of plasma protein leakage and of its inhibition by indomethacin

Calcium ionophore A23187 produced a dose-dependent increase in plasma protein leakage on intradermal injection in rats. Studies with mepyramine and cyproheptadine indicated that histamine and 5-hydroxytryptamine (5-HT) partially contribute to the ionophore action and experiments with compound 48/80 supported these findings. Depletion of plasma kininogen levels with cellulose sulphate administered indomethacin inhibited the ionophore response in a dose-dependent manner. The inhibition was not reversed by intradermally injected prostaglandin E2 (PGE2) in doses up to 50 ng/site, suggesting that PGE2 also may not be an important mediator. It is proposed that the ionophore produces plasma protein leakage by an indirect (through histamine and 5-HT release) and a direct action on vascular endothelial cells and that indomethacin antagonises both actions by inhibiting calcium transport processes.     

Human sleep EEG following the 5-HT1A antagonist pindolol: possible disinhibition of raphe neuron activity

The sleep electroencephalogram (EEG) was used to assay central effects of pindolol (10 and 30 mg p.o.), a mixed β_1/2-adrenoceptor/5-hydroxytryptamine (5-HT)_1A/1B receptor blocker, in humans. Compared to placebo, pindolol produced a dose-related suppression of rapid-eye-movement (REM) sleep, including a prolongation of REM latency, and a decrease of REM time and REM density. At the higher dose, it also reduced EEG spectral power during non-REM sleep in portions of the δ, θ, and α frequencies (1.125–5.125 Hz, 7.125–9.625 Hz). By contrast, betaxolol (20 mg p.o.), a selective β₁-antagonist devoid of serotonergic affinity, affected neither REM sleep nor EEG power. REM sleep is, in part, under the inhibitory control of serotonergic neurons projecting from the dorsal raphe nucleus to pontine cholinergic/cholinoceptive cells. The EEG power spectrum induced by pindolol tended to be opposite to what has previously been reported for ipsapirone, a 5-HT_1A agonist. Therefore, the present data, tentatively, are consistent with the contention that pindolol inhibits, possibly selectively, somatodendritic 5-HT_1A autoreceptors in humans and may antagonize self-inhibition of midbrain raphe nuclei 5-HT neurons.