Metitepine distinguishes two receptors mediating inhibition of [3H]-5-hydroxytryptamine release in guinea pig hippocampus

Summary Inhibition of [³H]-5-hydroxytryptamine ([³H]-5-HT) release from guinea pig brain slices via activation of the terminal 5-HT autoreceptor has previously been characterised as a model of 5-HT_1D receptor activation, based on the rank potencies of a range of agonists, and the potent antagonism of the inhibitory effects of 5-HT by metitepine. The present study uses this model, in slices of the guinea pig hippocampus, to examine the antagonist potency of metitepine against the 5-HT receptor agonists sumatriptan, 5-carboxamidotryptamine (5-CT) and 5-HT Addition of metitepine to the perfusion buffer (30, 300 and 1000 nmol/l) significantly shifted the concentration-response curve to 5-HT, producing a Schild slope of 1.1, and a pA₂ value of 7.6. However, the ability of metitepine to antagonise the effects of sumatriptan or 5-CT in this model was less marked. A clear-cut shift in the concentration-response curve to sumatriptan was only achieved at1000 nmol/l metitepine (apparent pA₂ = 6.7),and this was similar to the ability of metitepine to attenuate the effects of 5-CT (apparent pA₂ 7.0 at 300 nmol/l and 6.7 at 1000 nmol/l). These findings suggest heterogeneity in the receptor mediating inhibition of [³H]-5-HT release in guinea pig hippocampus.Send offprint requests to L. O. Wilkinson at the above address     

Electrophysiological evidence for locus coeruleus norepinephrine autoreceptor subsensitivity following subchronic administration of -amphetamine

The effect produced by subchronic administration of -amphetamine ( -AMP) on the sensitivity of norepinephrine (NE) autoreceptors in the rat locus coeruleus (LC) was studied by means of single unit recording and microiontophoretic techniques. Twice daily i.p. administration of 5 mg/kg -AMP for one week markedly reduced the ability of i.v. -AMP and microiontophoretic application of clonidine to suppress the firing of LC NE neurons, suggesting strongly that NE autoreceptors became subsensitive. In addition, the firing pattern of NE neurons became ‘disorganized’ following subchronic AMP treatment.     

Acetylcholine release from frontal cortex in the waking rat measured by microdialysis without acetylcholinesterase inhibitors: effects of diisopropylfluorophosphate

Acetylcholine (ACh) release was measured in frontal cortex of awake quietly resting rats by microdialysis without using cholinesterase blockers in the perfusate. Resting release was 16.61 ± 2.05 fmol/h (±.S.E.M., n = 18). Injection of sublethal doses of the acetylcholinesterase blocker, diisopropylfluorophosphate produced dose-dependent increases in ACh release, reaching 79.9 fmol/h with a dose of 0.7-times the LD₅₀ Although this irreversible inactivation of acetylcholinesterase increased ACh recovery to more than 700% of control values, levels of ACh in the perfusate never reached those seen in physostigmine-treated animals. The relationship between the amount of acetylcholinesterase inactivation and the quantity of ACh in the perfusate suggests that the extracellular ACh concentrations are controlled by simple enzyme kinetics. Within 2 h after enzyme inactivation, extracellular choline levels fell significantly, suggesting that ACh degradation by acetylcholinesterase plays an important role in regulating the amount of choline in the extracellular space.     

Dopamine “Autoreceptors”: Pharmacological characterization by microiontophoretic single cell recording studies

Summary The effects on the firing of single dopamine (DA) neurons in the substantia nigra (and adjacent ventral tegmental area) of a representative group of catecholamine agonists and antagonists were studied in rats using single cell recording and microiontophoretic techniques. Microiontophoretic application of DA or the DA agonist apomorphine depressed the firing of these cells; the DA antagonist trifluoperazine blocked this effect. However, the -agonist clonidine had no depressant effect and the -agonist isoproteronol had only a weak depressant action on DA neurons. Furthermore, the -antagonist piperoxane and the -antagonist sotolol were completely ineffective in blocking the depressant effects of DA. These results show that DA-sensitive receptors on the soma of DA neurons are pharmacologically distinct from or adrenoreceptors. Because of their location and selective responsiveness to DA agonists, the catecholamine receptors on the soma of DA neurons appear best classified as DA autoreceptors.     

A Single, Moderate Ethanol Exposure Alters Extracellular Dopamine Levels and Dopamine D2 Receptor Function in the Nucleus Accumbens of Wistar Rats

Background: The nucleus accumbens (NAc) has been implicated in the neurochemical effects of ethanol (EtOH). Evidence suggests that repeated EtOH exposures and chronic EtOH drinking increase dopamine (DA) neurotransmission in the NAc due, in part, to a reduction in D₂ autoreceptor function. The objectives of the current study were to evaluate the effects of a single EtOH pretreatment and repeated EtOH pretreatments on DA neurotransmission and D₂ autoreceptor function in the NAc of Wistar rats. Methods: Experiment 1 examined D₂ receptor function after a single intraperitoneal (i.p.) injection or repeated i.p. injections of 0.0, 0.5, 1.0, or 2.0 g/kg EtOH to female Wistar rats. Single EtOH pretreatment groups received 1 daily i.p. injection of 0.9% NaCl (saline) for 4 days, followed by 1 day of saline or EtOH administration; repeated EtOH pretreatment groups received 5 days of saline or EtOH injections. Reverse microdialysis experiments were conducted to determine the effects of local perfusion with the D₂‐like receptor antagonist (‐)sulpiride (SUL; 100 uM), on extracellular DA levels in the NAc. Experiment 2 evaluated if pretreatment with a single, moderate (1.0 g/kg) dose of EtOH would alter levels and clearance of extracellular DA in the NAc, as measured by no‐net‐flux (NNF) microdialysis. Subjects were divided into the EtOH‐naïve and the single EtOH pretreated groups from Experiment 1. Results: Experiment 1: Changes in extracellular DA levels induced with SUL perfusion were altered by the EtOH dose (p < 0.001), but not the number of EtOH pretreatments (p > 0.05). Post‐hoc analyses indicated that groups pretreated with single or repeated 1.0 g/kg EtOH showed significantly attenuated DA response to SUL, compared with all other groups (p < 0.001). Experiment 2: Multiple linear regression analyses yielded significantly (p < 0.05) higher extracellular DA concentrations in the NAc of rats receiving EtOH pretreatment, compared with their EtOH‐naïve counterparts (3.96 ± 0.42 nM and 3.25 ± 0.23 nM, respectively). Extraction fractions were not significantly different between the 2 groups. Conclusions: The present results indicate that a single EtOH pretreatment at a moderate dose can increase DA neurotransmission in the NAc due, in part, to reduced D₂ autoreceptor function.     

Extracellular serotonin in the prefrontal cortex is limited through terminal 5-HT<Subscript>1B</Subscript> autoreceptors: a microdialysis study in knockout mice

RATIONALE: Serotonin (5-HT) autoreceptors regulate extracellular 5-HT levels and have been suggested to limit the effects of acute treatment with selective serotonin reuptake inhibitors (SSRIs). OBJECTIVES: The role of terminal 5-HT(1B) autoreceptors was assessed by comparing the effects of a SSRI on extracellular 5-HT in wild-type and 5-HT(1B) receptor knockout (KO) mice and by using a 5-HT(1B) receptor antagonist. Since systemic SSRI administration also activates somatodendritic 5-HT(1A) autoreceptors, a SSRI was administered locally to study the role of terminal 5-HT(1B) autoreceptors. METHODS: In vivo microdialysis in wild-type and 5-HT(1B) receptor KO mice was used to study the effects of the 5-HT(1B) receptor agonist CP93129 (1 micro M), the SSRI fluvoxamine (0.3 micro M and 1.0 micro M) and the 5-HT(1B) receptor antagonist NAS-181 (1 micro M) on extracellular 5-HT in the medial prefrontal cortex. RESULTS: The 5-HT increase induced by local SSRI administration was augmented in 5-HT(1B) KO mice relative to wild-type mice and was augmented by simultaneous administration of a 5-HT(1B) receptor antagonist in the latter genotype. Basal 5-HT levels did not differ between the two genotypes. Activation of 5-HT(1B) receptors by CP93129 decreased extracellular 5-HT, whereas 5-HT levels in wild-type mice were not affected by the 5-HT(1B) receptor antagonist NAS-181. In 5-HT(1B) KO mice, NAS-181 did not affect extracellular 5-HT and did not further increase the effect of fluvoxamine, showing that NAS-181 is a selective 5-HT(1B) receptor antagonist. The greater increase in 5-HT levels following combined administration of a SSRI with NAS-181 in wild-type mice, relative to 5-HT(1B) KO mice, suggests possible adaptive changes in the KO mice. CONCLUSIONS: The present study shows that terminal 5-HT(1B) autoreceptors play a significant role in the regulation of 5-HT release in the prefrontal cortex.     

Electrophysiological and pharmacological properties of putative A13 incertohypothalamic dopamine neurons in the rat

A13 incertohypothalamic dopamine (DA) neurons were labelled with antibodies raised to tyrosine hydroxylase in the male rat. Electrophysiologically, these neurons could be distinguished from their neighboring non-DA cells by their wide action potentials ( > 2 ms), slow firing rates (0–3.8 impulses/s) and by the ability of iontophoresed DA and systemically administered apomorphine to inhibit impulse flow. Low doses of the antipsychotic drug haloperidol attenuated DA's response and reversed the apomorphine-inhibition of impulse flow.     

Basal and activity-induced release of substance P from primary afferent fibres in NK1 receptor knockout mice: evidence for negative feedback

The concept that NK₁ receptors are located pre-junctionally on substance P (SP)-containing nerves, acting as autoreceptors to inhibit SP release, has been suggested, but remains a controversial issue. To further investigate the existence of this receptor on central and peripheral terminals of primary afferent fibres, NK₁ receptor knockout mice and an NK₁ receptor antagonist were used in nerve-attached tissue preparations. These were the isolated dorsal horn of the spinal cord with dorsal roots attached, and the hairy skin of the hind paw with attached saphenous nerve. The results reveal that in the dorsal horn preparation, basal release of SP is significantly higher in NK₁^−/− mice than NK₁^+/+ mice (P<0.05, n=7 mice/strain). However, a difference in SP release evoked in the dorsal horn by electrical stimulation of the dorsal roots or capsaicin application was not observed. In contrast, antidromic electrical stimulation of the saphenous nerve caused a substantially greater release of SP in the skin of NK₁^−/− mice than in NK₁^+/+ mice (P<0.05, n=5 to 6 mice/strain). These results provide evidence for the existence of NK₁ autoreceptors on sensory nerves in skin, which may be relevant to the modulation of their peripheral pathophysiological effector functions.     

Differential effects of 7-OH-DPAT on amphetamine-induced stereotypy and conditioned place preference

 Low doses of the dopamine D₃-preferring agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) produce a behavioral profile that is opposite to that produced by the psychomotor stimulants cocaine and amphetamine. For example, low doses of 7-OH-DPAT produce conditioned place aversion and hypolocomotion, whereas psychomotor stimulants produce conditioned place preference (CPP) and hyperlocomotion. In experiment 1, the effects of low doses of 7-OH-DPAT (0.01–0.1 mg/kg) on d-amphetamine-induced (1 mg/kg) motor behaviors and CPP were assessed. In experiment 2, the effects of 0.1 mg/kg 7-OH-DPAT on d-amphetamine (0–10 mg/kg) dose-response curves for the same behaviors were examined. During conditioning, drug injections were paired with a distinct compartment, whereas saline injections were paired with another compartment. Locomotion and headbobbing were measured following acute and repeated drug administration during conditioning and place conditioning was assessed 24 h following the last conditioning day. In experiment 1, d-amphetamine-induced locomotion was dose-dependently decreased by 7-OH-DPAT following repeated administration, which was probably due to the emergence of headbobbing, a behavior not observed with d-amphetamine alone. d-Amphetamine-CPP was not altered by co-administration of 0–0.03 mg/kg 7-OH-DPAT, but was attenuated by co-administration of 0.1 mg/kg 7-OH-DPAT. In experiment 2, 7-OH-DPAT co-administered with low doses of d-amphetamine (0–0.5 mg/kg) produced a decrease in locomotion following acute administration. However, 7-OH-DPAT produced sensitization of locomotion at the 0.5 mg/kg dose of d-amphetamine and an increase in headbobbing at the 0.5–10 mg/kg doses of d-amphetamine following repeated administration. In contrast, d-amphetamine-CPP was attenuated by co-administration of 7-OH-DPAT. These findings suggest that 0.1 mg/kg 7-OH-DPAT attenuates the reinforcing effects of d-amphetamine despite enhancing stereotypic behaviors. Received: 14 May 1997 / Final version: 22 January 1998     

Does 5-carboxamidotryptamine-induced turning in guinea-pigs involve 5-HT1D autoreceptors?

Contralateral turning induced by unilateral injection of 5-carboxamidotryptamine (5-CT) into guinea-pig substantia nigra could result from unilaterally reduced inhibition of the dopaminergic pathway as a consequence of a decreased release of inhibitory 5-hydroxytryptamine (5-HT) through stimulation of 5-HT_1D autoreceptors. This hypothesis was tested by administering 5-CT unilaterally to freely moving guinea pigs through a microdialysis probe, and simultaneously measuring extracellular 5-HT in the dialysate and rotation. A concentration of 250 mN 5-CT was required to produce significant turning. This also induced a massive increase in extracellular 5-HT, introduced as an impurity of the 5-CT. The presence of high concentrations of extracellular 5-HT makes it unlikely that 5-CT-induced rotation is mediated through decreased 5-HT release resulting from 5-HT_1D autoreceptor stimulation.