Ian A. Pullar

Modulation of neurotransmitter release induced by brain-derived neurotrophic factor in rat brain striatal slices in vitro.

This study examined the influence of brain-derived neurotrophic factor (BDNF) on the basal and depolarisation-induced release of the neurotransmitters GABA, dopamine and serotonin from rat striatal brain slices in vitro. BDNF potentiated the potassium or veratrine-stimulated release of GABA, dopamine and serotonin. This potentiation was shown to be dependent on activation of the high-affinity tyrosine kinase-linked receptor TrkB, as K252a (a potent TrkB antagonist) largely prevented the effects. BDNF potentiated the release of each neurotransmitter to similar extents irrespective of the type of depolarising stimulus used. In all cases the potentiation of neurotransmitter release caused by BDNF was dependent on membrane depolarisation as BDNF alone was incapable of causing potentiation. These results, obtained using striatal slices in vitro, suggest that BDNF may be acting via the specific receptor TrkB to modulate synaptic performance in the corpus striatum in vivo.

Signalling pathways involved in the short-term potentiation of dopamine release by BDNF

Brain-derived neurotrophic factor (BDNF) has been shown to modulate synaptic plasticity in the corpus striatum in vitro by activation of the tyrosine kinase linked receptor, TrkB. However, the signalling pathways that mediate this modulation of plasticity are poorly understood. Three proteins mediating signalling pathways are activated by the binding of BDNF to TrkB: phosphoinositol-3 kinase (PI3K); Ras-MEK and phospholipase C-gamma (PLCgamma). The present study investigates which of these pathways are necessary for BDNF-mediated potentiation of synaptic output of dopamine from slices and synaptosomes of rat corpus striatum. The results indicate that activation of the PI3K and Ras-MEK pathways, but not PLCgamma, are involved. Inhibitors of transcription and translation had no effect on the potentiation of depolarisation-stimulated (15 mM KCl) dopamine release mediated by BDNF.

Actions of arginine polyamine on voltage and ligand-activated whole cell currents recorded from cultured neurones.

1 Toxins from invertebrates have proved useful tools for investigation of the properties of ion channels. In this study we describe the actions of arginine polyamine which is believed to be a close analogue of FTX, a polyamine isolated from the American funnel web spider, Agelenopsis aperta. 2 Voltage‐activated Ca2+ currents and Ca2+‐dependent Cl− currents recorded from rat cultured dorsal root ganglion neurones were reversibly inhibited by arginine polyamine (AP; 0.001 to 100 μm). Low voltage‐activated T‐type Ca2+ currents were significantly more sensitive to AP than high voltage‐activated Ca2+ currents. The IC50 values for the actions of AP on low and high voltage‐activated Ca2+ currents were 10 nm and 3 μm respectively. AP was equally effective in inhibiting high voltage‐activated currents carried by Ba2+, Sr2+ or Ca2+. However, AP‐induced inhibition of Ca2+ currents was attenuated by increasing the extracellular Ca2+ concentration from 2 mm to 10 mm. 3 The actions of AP on a Ca2+‐independent K+ current were more complex, 1 μm AP enhanced this current but 10 μm AP had a dual action, initially enhancing but then inhibiting the K+ current. 4 γ‐Aminobutyric acid‐activated Cl− currents were also reversibly inhibited by 1 to 10 μm AP. In contrast N‐methyl‐d‐aspartate currents recorded from rat cultured cerebellar neurones were greatly enhanced by 10 μm AP. 5 We conclude that at a concentration of 10 nm, AP is a selective inhibitor of low threshold T‐type voltage‐activated Ca2+ currents. However, at higher concentrations 1–10 μm AP interacts with ion channels or other membrane constituents to produce a variety of actions on both voltage and ligand gated ion channels.

LY367265, an inhibitor of the 5-hydroxytryptamine transporter and 5-hydroxytryptamine2A receptor antagonist: a comparison with the antidepressant, nefazodone

The potential antidepressant, LY367265 (1-[2-[4-(6-fluoro-1H-indol-3-yl)-3, 6-dihydro-1(2H)-pyridinyl]ethyl]-5,6-dihydro-1H,4H-[1,2, 5]thiadiazolo[4.3.2-ij]quinoline-2,2,-dioxide) has been shown to have a higher affinity for the 5-hydroxytryptamine (5-HT) transporter (K(i)=2.3 nM) and 5-HT(2A) (K(i)=0.81 nM) receptor than the clinically effective antidepressant, nefazodone. It is a potent inhibitor of [3H]5-HT uptake into rat cortical synaptosomes (IC(50)=3.1 nM) and shows selectivity over that for [3H]noradrenaline (IC(50)>1000 nM). It potentiates potassium-induced [3H]5-HT outflow from prelabelled guinea pig cortical slices both in the presence (EC(50)=950 nM) and absence (EC(50)=250 nM) of a saturating concentration of the 5-HT transport inhibitor, paroxetine, indicating a low level of activity at the 5-HT(1B/1D) autoreceptor. These studies indicate that LY367265 is a putative antidepressant which, because of its 5-HT(2A) receptor antagonist activity, has the potential to produce less sleep disturbance and sexual dysfunction than selective serotonin uptake inhibitors.

ω-Agatoxin IVA identifies a single calcium channel subtype which contributes to the potassium-induced release of acetylcholine, 5-hydroxytryptamine, dopamine, γ-aminobutyric acid and glutamate from rat brain slices

The voltage-dependent calcium channels (VDCCs) involved in K+-induced transmitter release have been studied. A maximally effective concentration of the N-type VDCC inhibitor, ω-conotoxin GVIA (GVIA) blocked the release of 5-HT (30%), DA (30%) and ACh (60%) but not that of GABA or glutamate. The O, P and Q-type VDCC inhibitor, ω-agatoxin IVA (Aga IVA, 1 μM), blocked 100% of GABA and glutamate, 70% of DA and about 50% of 5-HT and ACh release. The slopes of the inhibition curves indicate that it acts on the same, single type of VDCC in all cases. ω-Conotoxin MVIIC (MVIIC) completely inhibited the release of all the transmitters. It is concluded that a single GVIA-insensitive type of VDCC is involved in the K+-induced release of all the transmitters and, in addition, N-type VDCCs, with a higher affinity for GVIA than MVIIC, are required for the release of 5-HT, DA and ACh. The non-N-type VDCC is not the O-type as it is not blocked by low (< 10 nM) concentrations of MVIIC. Further resolution of this VDCC into P or Q-type requires more selective antagonists.

Functional responses and subunit composition of presynaptic nicotinic receptor subtypes explored using the novel agonist 5-iodo-A-85380

The novel compound 5-iodo-A-85380 binds with higher affinity to alpha4beta2* nicotinic acetylcholine receptors (nAChR), compared with other nAChR subtypes (Mukhin et al., 2000). In the present study, we have confirmed that in competition binding assays for three major nAChR subtypes, 5-iodo-A-85380 is 850 and 27,000-fold more potent at rat brain alpha4beta2* binding sites than at alpha3beta4 and alpha7 subtypes, respectively. In functional assays, 5-iodo-A-85380 potently activated (EC50 12-35 nM) both alpha-CTx-MII-sensitive and -insensitive components of [3H]dopamine release from rat striatal synaptosomes, corresponding to alpha6beta2* and alpha4beta2* nAChR, respectively. 5-Iodo-A-85380 was markedly less potent at eliciting [3H]ACh release from rat interpeduncular nucleus synaptosomes, [3H]noradrenaline release from rat hippocampal slices, and Ca2+ increases in a cell line expressing rat alpha3beta4 nAChR (EC50 = 5, 3.2, 1.6 microM, respectively). As predicted by ligand binding studies, 5-iodo-A-85380 is a more discriminating agonist than the parent compound epibatidine. However, it is not specific for alpha4beta2* nAChR as it also potently activates alpha6beta2* nAChR.

The effects of standard neuroleptic compounds on the binding of 3H-spiroperidol in the striatum and mesolimbic system of the rat in vitro

Abstract Neuroleptics are reported to produce their antipsychotic activity and extrapyramidal side effects by blocking dopamine receptors in the mesolimbic system and striatum respectively. We have thus looked at the characteristics of the binding of 3 H-spiroperidol to specific binding sites in these two areas of rat brain and the ability of a number of neuroleptics to displace it from these sites. The 3 H-spiroperidol binding sites in the striatum and mesolimbic area are different and evidence has been obtained for an involvement of 5-HT receptors, particularly in the latter area. In the striatum the order of activity of neuroleptics in displacing 3 H-spiroperidol binding parallels their clinical potency. This is not the case in the mesolimbic system. Also the ratio of activity of a neuroleptic in the two brain areas does not correlate with its ability to produce extrapyramidal disturbance in man. This may be due to the interaction of neuroleptics, particularly in the mesolimbic system, with receptors not involved in the expression of antipsychotic activity.

Two dopamine receptors: Supportive evidence with the rat rotational model

Di-isobutyryl apomorphine (5 mg/kg i.p.) and lergotrile (5 mg/kg i.p.) produce long lasting turning behaviour in rats with 6-hydroxydopamine lesions at the level of the substantia nigra. Haloperidol (1.5 and 3.0 mg/kg p.o.) blocks the rotational behaviour due to the apomorphine ester but has no effect on lergotrile turning. Clozapine (10 and 20 mg/kg p.o.) blocks the lergotrile turning but stimulates the rotational behaviour produced by the apomorphine ester. The results support the concept of two anatomically separate dopamine receptors and their relevance to the study of antipsychotic activity is discussed.

Bicyclo[2.2-1]heptanes as novel triple re-uptake inhibitors for the treatment of depression

A series of substituted naphthyl containing chiral [2.2.1] bicycloheptanes were prepared utilizing asymmetric Diels-Alder chemistry. This paper describes structure-activity relationships in this series. The N-methyl 2-naphthyl analogue (16d) and its des-methyl analogue (17d) are active triple re-uptake inhibitors both in vivo and in vitro.

Effect of voltage-sensitive calcium channel antagonists on the release of 5-hydroxytryptamine from rat hippocampus in vivo.

Abstract: The effect of calcium channel antagonists on the release of 5‐hydroxytryptamine from the hippocampus of the chloral hydrate‐anaesthetised rat was studied using the technique of intracerebral microdialysis. As the basal concentration of 5‐hydroxytryptamine was close to the limit of detection of the HPLC method (8 fmol), the 5‐hydroxytryptamine reuptake inhibitor, fluoxetine (10 μM], was included in the perfusion fluid. The l‐type voltage‐sensitive calcium channel antagonists, PN200‐110, diltiazem, and verapamil, all passed through the dialysis membrane, giving a recovery of 20‐30%. The N‐type voltage‐sensitive calcium channel antagonist, ω‐conotoxin, penetrated less readily (12% recovery). The dihydropyridine, PN200‐110, adhered to the probe, resulting in an effective concentration at the membrane 30% of that in the perfusion fluid. The concentration of 5‐hydroxytryptamine in the dialysate samples was reduced by 60% in the absence of calcium. The L channel antagonists had little effect on the release of 5‐hydroxytryptamine, which was inhibited, in a dose‐dependent manner, to a maximum of 40% by w‐conotoxin. It is concluded that, under physiological conditions, the release of 5‐hydroxytryptamine from the rat hippocampus is dependent on the entry of calcium through N‐type voltage‐sensitive calcium channels, although another calcium channel may also be involved.