Plasma level-dependent effects of methylphenidate on task-related functional magnetic resonance imaging signal changes.

RATIONALE: Methylphenidate (MPH) is a dopamine and noradrenaline enhancing drug used to treat attentional deficits. Understanding of its cognition-enhancing effects and the neurobiological mechanisms involved, especially in elderly people, is currently incomplete. OBJECTIVES: The aim of this study was to investigate the relationship between MPH plasma levels and brain activation during visuospatial attention and movement preparation. METHODS: Twelve healthy elderly volunteers were scanned twice using functional magnetic resonance imaging (fMRI) after oral administration of MPH 20 mg or placebo in a within-subject design. The cognitive paradigm was a four-choice reaction time task presented at two levels of difficulty (with and without spatial cue). Plasma MPH levels were measured at six time points between 30 and 205 min after dosing. FMRI data were analysed using a linear model to estimate physiological response to the task and nonparametric permutation tests for inference. RESULTS: Lateral premotor and medial posterior parietal cortical activation was increased by MPH, on average, over both levels of task difficulty. There was considerable intersubject variability in the pharmacokinetics of MPH. Greater area under the plasma concentration-time curve was positively correlated with strength of activation in motor and premotor cortex, temporoparietal cortex and caudate nucleus during the difficult version of the task. CONCLUSION: This is the first pharmacokinetic/pharmacodynamic study to find an association between plasma levels of MPH and its modulatory effects on brain activation measured using fMRI. The results suggest that catecholaminergic mechanisms may be important in brain adaptivity to task difficulty and in task-specific recruitment of spatial attention systems.

Adrenaline in the CNS: in vivo evidence for a functional pathway innervating the hypothalamus.

This article reviews the evidence, obtained from studies employing the technique of intracerebral dialysis, to monitor changes in extracellular adrenaline in the hypothalamus, that adrenaline is released from nerve endings in the hypothalamus and has a functional role. Studies using [3H] adrenaline to determine the specificity of uptake mechanisms for adrenaline indicate that labelled adrenaline is also taken up by noradrenergic nerve endings. This supports the need to develop techniques to monitor changes in endogenous release of adrenaline. In this study, it has been shown that inhibition of phenylethanolamine-N-methyltransferase selectively decreases tissue levels in the hypothalamus and in vivo release of adrenaline, while monoamine oxidase inhibition and antagonism of alpha 2-adrenoceptors increases the release of both adrenaline and noradrenaline. The "selective" noradrenergic neurotoxin DSP4 caused an initial increase in the release of both catecholamines, followed by a marked decrease in their release. Stimulation of the adrenaline-containing neurones in the rostral ventrolateral medulla (C1 region) increased the release of adrenaline in the posterior hypothalamus, but not that of noradrenaline, while also increasing mean arterial blood pressure. Pharmacological evidence indicates that B2-adrenoceptors in the hypothalamus and the spinal cord are involved in the rise in mean arterial pressure, as the response is reduced by a selective B2-antagonist (ICI 118551). The rise in mean arterial blood pressure during C1 stimulation is enhanced by the alpha 2-antagonist idazoxan, supporting observations that alpha 2-adrenoceptors are involved in the pre-synaptic regulation of release of adrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of 5-HT1B and 5-HT1D receptor antagonist effects on extracellular 5-HT levels in the guinea-pig brain following concurrent 5-HT1A or 5-HT re-uptake site blockade.

The effects of selective serotonin re-uptake inhibitor (SSRI), paroxetine, and 5-HT1A, 5-HT1B and 5-HT1B/1D receptor antagonists on in vivo extracellular 5-HT levels in the guinea-pig frontal cortex and dorsal hippocampus were investigated using the technique of microdialysis. The aim of the study was to further investigate the autoreceptor roles of the 5-HT1A, 5-HT1B and 5-HT1D receptors in the median vs dorsal raphe nuclei. In the frontal cortex, 5-HT1A (WAY 100635, 1 mg/kg i.p.) or 5-HT1B (SB-224289, 4 mg/kg i.p.) receptor antagonists had no effect on extracellular levels of 5-HT, whilst the mixed 5-HT1B/1D receptor antagonist (GR 127935, 0.3 mg/kg i.p) produced a significant decrease in extracellular 5-HT levels. Paroxetine (10 microM) significantly increased extracellular 5-HT levels when perfused locally into the cortex. Administration of SB-224289, followed 120 min later by WAY 100635, had no effect on extracellular 5-HT levels. In contrast, sequential administration of either WAY 100635 and GR 127935, or SB-224289 and paroxetine significantly increased extracellular 5-HT levels. In the dorsal hippocampus, whilst 5-HT1A receptor antagonism elicited by administration of WAY 100635 had no effect, both 5-HT1B and mixed 5-HT1B/1D receptor blockade significantly increased extracellular 5-HT levels. Administration of SB-224289 followed 120 min later with WAY 100635, or WAY 100635 followed 30 min later with GR 127935, potentiated the effect of the three compounds alone, significantly increasing extracellular 5-HT levels. These data demonstrate that to simultaneously increase extracellular 5-HT in both frontal cortex and dorsal hippocampus of the guinea-pig brain concurrent 5-HTA1A, 5-HT1B and 5-HT1D receptor blockade is required. Whereas in the dorsal hippocampus, 5-HT1B receptor blockade is sufficient to elicit an increase in extracellular 5-HT levels.

Importance of h5-HT1B receptor selectivity for 5-HT terminal autoreceptor activity: an in vivo microdialysis study in the freely-moving guinea-pig.

The importance of h5-HT1B receptor selectivity for 5-HT terminal autoreceptor activity was investigated with the selective h5-HT1B receptor ligands SB 219085, SB 220272, SB 224289 and SB 216641. The studies employed measurement of compound affinity and efficacy in vitro and the measurement of extracellular 5-HT in the frontal cortex of the freely-moving guinea-pig using in vivo microdialysis. All compounds had high affinity and selectivity for the h5-HT1B receptor, with SB 224289 the most selective for h5-HT1B over h5-HT1D receptors. Compounds exhibited a range of efficacies at both receptors: SB 224289 and SB 219085 were inverse agonists, SB 220272 was an antagonist and SB 216641 was a partial agonist. SB 220272, SB 216641 and SB 224289 had no effect on extracellular 5-HT following systemic administration, however, SB 219085 produced a significant increase. The SB 219085-induced increase in extracellular 5-HT was attributed to the compounds non-specific releasing properties as it was also demonstrated to increase basal release of [3H]5-HT from pre-loaded guinea-pig cortical slices. The lack of effect of the above h5-HT1B receptor selective compounds and the decrease in extracellular 5-HT elicited by the non-selective compounds GR 127935, GR125743 and methiothepin suggest that antagonism of 5-HT1D receptors may mediate this decrease in 5-HT levels. It is plausible that blockade of 5-HT1D receptors increases 5-HT levels in the raphe, this activates 5-HTtA receptors which results in an overall decrease in terminal 5-HT release. Definitive proof now awaits elucidation of the action of a selective 5-HT1D receptor antagonist.

Evidence to suggest that agonist modulation of hyperlocomotion is via post-synaptic dopamine D2 or D3 receptors.

It has been suggested that a sub-population of dopamine D3 receptors is located pre-synaptically and these serve as autoreceptors in dopamine projection areas such as the nucleus accumbens/ventral striatum. To study further the physiological role and synaptic location of the dopamine D3 receptor, we have investigated the in vivo effect of the D3/D2 receptor agonist quinelorane on amphetamine-induced hyperactivity and extracellular dopamine release from the nucleus accumbens of the conscious rat. Amphetamine increased dopamine release to 202 +/- 34% of pre-injection control values, but quinelorane at 2.5 micrograms/kg, a dose which effectively blocked amphetamine-induced hyperlocomotion, had no significant effect on amphetamine-induced dopamine release. These data suggest that hyperlocomotion is mediated via post-synaptic rather than pre-synaptic dopamine receptors. Since quinelorane has significant affinity for the dopamine D3 receptor, these effects may be via post-synaptic D3 receptors; however, D2 receptor effects cannot be disregarded. In summary, these data indicate that the quinelorane effect on amphetamine-stimulated hyperlocomotion is not mediated via D3 or D2 autoreceptors, but rather a population of receptors located post-synaptically, which appear to mediate the inhibition of rat locomotor activity.

In vivo characterization of the putative 5-HT1A receptor antagonist SDZ 216,525 using two models of somatodendritic 5-HT1A receptor function.

SDZ 216,525 has been proposed to be a silent 5-HT1A receptor antagonist. The present study examined the potential intrinsic agonist action of SDZ 216,525 using two in vivo models of somatodendritic 5-HT1A autoreceptor function: 5-HT release using microdialysis and feeding behaviour of satiated animals. SDZ 216,525 (1 mg/kg s.c.) and the alpha 1-adrenoceptor antagonist prazosin (1 mg/kg s.c.) significantly decreased hippocampal 5-HT release. In addition, SDZ 216,525 (3 and 10 mg/kg s.c.) and prazosin (3 and 10 mg/kg s.c.) significantly increased food intake in satiated rats. The selective 5-HT1A receptor antagonist (RS)-WAY100135 (10 mg/kg s.c.) which has been demonstrated to block the effects of 8-OH-DPAT on 5-HT release and food intake had no significant effect on the response induced by SDZ 216,525. In contrast, the non-selective 5-HT1A receptor antagonist (-)-pindolol (8 mg/kg s.c.) attenuated both SDZ 216,525 responses. The decrease in hippocampal 5-HT release and increase in food intake induced by SDZ 216,525 suggest that the compound may be a 5-HT1A receptor partial agonist. However, the failure of the 5-HT1A receptor antagonist (RS)-WAY100135 to block the SDZ 216,525 responses suggests that SDZ 216,525 decreases 5-HT release and increases food intake by a mechanism other than 5-HT1A receptor agonism. The high affinity of SDZ 216,525 for the alpha 1-adrenoceptor, and the ability of prazosin to decrease 5-HT release and increase food intake, suggest that the effects of SDZ 216,525 may be mediated via an alpha 1-adrenoceptor antagonist action.

Design and synthesis of trans-N-[4-[2-(6-cyano-1,2,3, 4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolinecarboxamide (SB-277011): A potent and selective dopamine D(3) receptor antagonist with high oral bioavailability and CNS penetration in the rat.

A selective dopamine D(3) receptor antagonist offers the potential for an effective antipsychotic therapy, free of the serious side effects of currently available drugs. Using clearance and brain penetration studies as a screen, a series of 1,2,3, 4-tetrahydroisoquinolines, exemplified by 13, was identified with high D(3) affinity and selectivity against the D(2) receptor. Following examination of molecular models, the flexible butyl linker present in 13 was replaced by a more conformationally constrained cyclohexylethyl linker, leading to compounds with improved oral bioavailability and selectivity over other receptors. Subsequent optimization of this new series to improve the cytochrome P450 inhibitory profile and CNS penetration gave trans-N-[4-[2-(6-cyano-1, 2,3, 4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolinecarbo xamide (24, SB-277011). This compound is a potent and selective dopamine D(3) receptor antagonist with high oral bioavailability and brain penetration in the rat and represents an excellent new chemical tool for the investigation of the role of the dopamine D(3) receptor in the CNS.

The selective 5-HT1B receptor inverse agonist 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6,7-tetrahydro- spiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289) potently blocks terminal 5-HT autoreceptor function both in vitro and in vivo.

5-HT1 receptors are members of the G-protein-coupled receptor superfamily and are negatively linked to adenylyl cyclase activity. The human 5-HT1B and 5-HT1D receptors (previously known as 5-HT1Dbeta and 5-HT1Dalpha, respectively), although encoded by two distinct genes, are structurally very similar. Pharmacologically, these two receptors have been differentiated using nonselective chemical tools such as ketanserin and ritanserin, but the absence of truly selective agents has meant that the precise function of the 5-HT1B and 5-HT1D receptors has not been defined. In this paper we describe how, using computational chemistry models as a guide, the nonselective 5-HT1B/5-HT1D receptor antagonist 4 was structurally modified to produce the selective 5-HT1B receptor inverse agonist 5, 1'-methyl-5-[[2'-methyl-4'-(5-methyl-1,2, 4-oxadiazol-3-yl)biphenyl-4-yl]carbonyl]-2,3,6, 7-tetrahydrospiro[furo[2,3-f]indole-3,4'-piperidine] (SB-224289). This compound is a potent antagonist of terminal 5-HT autoreceptor function both in vitro and in vivo.

Electrical stimulation of the C1 region of the rostral ventrolateral medulla of the rat increases mean arterial pressure and adrenaline release in the posterior hypothalamus.

By using intracerebral dialysis in combination with high performance liquid chromatography and electrochemical detection, extracellular posterior hypothalamic adrenaline, noradrenaline, 3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid were measured in the anaesthetized rat and changes in their levels monitored following administration of tranylcypromine and electrical stimulation of the rostral ventrolateral medulla. Tranylcypromine (10 mg/kg i.p.) administration decreased basal extracellular 3,4-dihydroxyphenylacetic acid and 5-hydroxyindole acetic acid levels with a simultaneous increase in adrenaline and noradrenaline levels. Electrical stimulation of the C1 area of the rostral ventrolateral medulla increased (+56.6%) extracellular adrenaline levels in the posterior hypothalamus with a simultaneous increase in mean arterial pressure (+48 mm Hg) compared to prestimulation control values. No change was seen in posterior hypothalamic extracellular levels of noradrenaline, 3,4-dihydroxyphenylacetic acid and 5-hydroxyindole acetic acid during the stimulation period. Electrical stimulation of areas close to but outside the C1 region had no effect on either mean arterial pressure or posterior hypothalamic extracellular levels of the amines or the metabolites. The increase in adrenaline levels in the hypothalamus during stimulation of the C1 region supports the evidence for an adrenergic pathway from the rostral ventrolateral medulla to the hypothalamus and suggests that the increase in mean arterial pressure during electrical stimulation to the C1 region may relate to a specific increase in adrenaline levels.

Differential effects of potassium channel blockers on extracellular concentrations of dopamine and 5-HT in the striatum of conscious rats.

1. The selective Ca(2+)-activated K+ channel blocker apamin increased extracellular 5-hydroxytryptamine (5-HT) concentrations in the striatum when administered through the microdialysis probe at doses of 0.1 mM and 1 mM. Extracellular dopamine concentrations increased only at the highest dose administered (1 mM). 2. Mast cell degranulating peptide (MCDP), which blocks the dendrotoxin sensitive delayed rectifier (DR) current, increased extracellular concentrations of dopamine at dose of 10 microM-100 microM but had no effect on 5-HT. 3. The non selective K+ channel blocker tetraethylammonium (TEA) induced a dose-dependent (1 mM-10 mM) increase in extracellular dopamine concentrations and an increase in 5-HT which showed little or no dose-dependency. 4. 4-Aminopyridine (4-AP), a blocker with some similar characteristics to MCDP, increased extracellular dopamine concentrations at doses of 10 microM-1 mM, but had no effect on 5-HT. 5. These findings suggest that dopamine release may be modulated by DR-like current and/or A-current K+ channels. However, in view of the similar effects of MCDP and 4-AP at the concentrations used it is more likely that the dendrotoxin-sensitive DR-like current is involved. In contrast, 5-HT release appears to be modulated by Ca(2+)-activated K+ channels.

The 5-HT1 receptor agonist RU-24969 decreases 5-hydroxytryptamine (5-HT) release and metabolism in the rat frontal cortex in vitro and in vivo.

K+-stimulated release of [3H]-5-hydroxytryptamine ( [3H]-5-HT) from rat frontal cortex slices was decreased by the 5-HT receptor agonists 5-methoxy-n1N-dimethyltryptamine and 5-methoxy-3(1,2,3,6,-tetrahydro-4-pyrindinyl)-1H-indole (RU-24969) (1 X 10(-5)M). RU-24969 (10 mg kg-1, i.p.) decreased extracellular 5-HT and its metabolite 5-hydroxyindoleacetic acid measured in vivo by use of intracerebral dialysis combined with high performance liquid chromatography and electrochemical detection. The decrease in extracellular 5-hydroxyindoleacetic acid in vivo after RU-24969 (10 mg kg-1, i.p.) was also observed by in vivo voltammetry. The non-selective 5-HT antagonist metergoline prevented the RU-24969-induced decrease in 5-HT release and metabolism in vivo while the 5-HT2 receptor antagonist R-55669 (ritanserin) did not. The results support the view that RU-24969 stimulates a 5-HT1 receptor that is involved in the autoregulation of 5-HT release and metabolism.

Involvement of central alpha- and beta-adrenoceptors in the pressor response to electrical stimulation of the rostral ventrolateral medulla in rats.

1. Electrical stimulation of the C1 area of the rostral ventrolateral medulla in rats elicits an increase in mean arterial pressure (MAP). 2. This increase in MAP is attenuated by intra-hypothalamic and intracisternal administration of the non-selective beta-adrenoceptor antagonist (+/-)-propranolol and the beta 2-selective antagonist ICI 118551. 3. The selective beta 1-antagonist atenolol and (+)-propranolol, which is inactive on beta-adrenoceptors, did not alter the increase in MAP produced by stimulation of the C1 area. 4. The alpha 2-adrenoceptor antagonist idazoxan enhanced the effects of C1 stimulation on MAP when administered either into the posterior hypothalamus or intracisternally. 5. The results indicate that beta 2- and alpha 2-receptors both have a role in the mediation of the rise in MAP during stimulation of the C1 area and that the receptors involved are located both within the hypothalamus and the spinal cord.

Characterization of [(125)I]-SB-258585 binding to human recombinant and native 5-HT(6) receptors in rat, pig and human brain tissue.

SB-258585 (4-Iodo-N-[4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-benzen esulphonamide) is a high affinity ligand at 5-HT(6) receptors. It displays over 100 fold selectivity for the 5-HT(6) receptor over all other 5-HT receptors tested so far. SB-258585 has been radiolabelled, to high specific activity, for its characterization as a 5-HT(6) receptor selective radioligand. [(125)I]-SB-258585 bound, with high affinity, to a single population of receptors in a cell line expressing human recombinant 5-HT(6) receptors. Kinetic and saturation binding experiments gave pK(D) values of 9.01+/-0.09 and 9.09+/-0.02, respectively. In membranes derived from rat or pig striatum and human caudate putamen, [(125)I]-SB-258585 labelled a single site with high levels (>60%) of specific binding. Saturation analysis revealed pK(D) values of 8.56+/-0.07 for rat, 8.60+/-0.10 for pig and 8.90+/-0.02 for human. B(max) values for the tissues ranged from 173+/-23 and 181+/-25 fmol mg(-1) protein in rat and pig striatum, respectively, to 215+/-41 fmol mg(-1) protein in human caudate putamen. The pK(i) rank order of potency for a number of compounds, determined in competition binding assays with [(125)I]-SB-258585, at human caudate putamen membranes was: SB-271046>SB-258585>SB-214111>methiothepin>clozapine>5-Me-OT>5-HT>Ro 04-6790>mianserin>ritanserin=amitriptyline>5-CT>mesulergine. Similar profiles were obtained from pig and rat striatal membranes and recombinant 5-HT(6) receptors; data from the latter correlated well with [(3)H]-LSD binding. Thus, [(125)I]-SB-258585 is a high affinity, selective radioligand which can be used to label both recombinant and native 5-HT(6) receptors and will facilitate further characterization of this receptor subtype in animal and human tissues.

Characterization of SB-271046: a potent, selective and orally active 5-HT(6) receptor antagonist.

SB-271046, potently displaced [(3)H]-LSD and [(125)I]-SB-258585 from human 5-HT(6) receptors recombinantly expressed in HeLa cells in vitro (pK(i) 8.92 and 9.09 respectively). SB-271046 also displaced [(125)I]-SB-258585 from human caudate putamen and rat and pig striatum membranes (pK(i) 8.81, 9.02 and 8.55 respectively). SB-271046 was over 200 fold selective for the 5-HT(6) receptor vs. 55 other receptors, binding sites and ion channels. In functional studies on human 5-HT(6) receptors SB-271046 competitively antagonized 5-HT-induced stimulation of adenylyl cyclase activity with a pA(2) of 8.71. SB-271046 produced an increase in seizure threshold over a wide-dose range in the rat maximal electroshock seizure threshold (MEST) test, with a minimum effective dose of < or =0.1 mg kg(-1) p.o. and maximum effect at 4 h post-dose. The level of anticonvulsant activity achieved correlated well with the blood concentrations of SB-271046 (EC(50) of 0.16 microM) and brain concentrations of 0.01-0.04 microM at C(max). These data, together with the observed anticonvulsant activity of other selective 5-HT(6) receptor antagonists, SB-258510 (10 mg kg(-1), 2-6 h pre-test) and Ro 04-6790 (1-30 mg kg(-1), 1 h pre-test), in the rat MEST test, suggest that the anticonvulsant properties of SB-271046 are likely to be mediated by 5-HT(6) receptors. Overall, these studies demonstrate that SB-271046 is a potent and selective 5-HT(6) receptor antagonist and is orally active in the rat MEST test. SB-271046 represents a valuable tool for evaluating the in vivo central function of 5-HT(6) receptors.

Effects of centrally administered orexin-B and orexin-A: a role for orexin-1 receptors in orexin-B-induced hyperactivity.

RATIONALE: Orexin-A and orexin-B are hypothalamic neuropeptides derived from a 130-amino acid precursor, prepro-orexin, and are potent agonists at both the orexin-1 (OX1) and orexin-2 (OX2) receptors. Orexin-A has been ascribed a number of in vivo functions in the rat after intracerebroventricular (ICV) administration, including hyperphagia, neuroendocrine modulation and a role in the regulation of sleep-wake function. The in vivo role of orexin-B is not as clear. OBJECTIVES: To investigate the behavioural, endocrine and neurochemical effects of orexin-B in in-vivo tests. In a number of experiments, these effects were compared with those of orexin-A. METHODS: Experiments were carried out in male, Sprague-Dawley rats with a guide cannula directed towards the lateral ventricle. The effects of orexin-B (ICV) upon grooming behaviour were compared with those of orexin-A. The effects of orexin-B upon the motor activity response to both novel and familiar environments were assessed in an automated activity monitor. Orexin-B was tested upon startle reactivity and body temperature. Further, plasma hormones and [DOPAC+ HVA]/[DA] and [5-HIAA]/[5-HT] ratios in six brain areas were measured 40 min post-orexin-B or orexin-A. RESULTS: The clearest behavioural response to orexin-B was increased motor activity in both novel and familiar environments. Orexin-B-induced hyperactivity was blocked by an OX1 receptor antagonist, SB-334867-A, implicating OX1 receptors in this behavioural response. In common with orexin-A, orexin-B reduced plasma prolactin and failed to influence startle reactivity. However, in contrast with orexin-A, orexin-B increased head grooming but failed to cause a robust whole body grooming response or increase plasma corticosterone levels. Further, orexin-B, but not orexin-A, increased plasma TSH and increased hypothalamic and striatal [5-HIAA]/[5-HT] ratios. CONCLUSIONS: The present study has demonstrated a number of behavioural, neuroendocrine and neurochemical effects of orexin-B that distinguish it from orexin-A. Further, we have demonstrated a role for OX1 receptors in the actions of orexin-B upon motor activity.

Development of 5-HT1A receptor antagonists.

The discovery that non-benzodiazepine anxiolytic agents such as buspirone bind with high affinity to the 5-HT1A receptor has stimulated the development of selective 5-HT1A receptor ligands as potential drug candidates. However, the lack of selective 5-HT1A receptor antagonists has hampered the elucidation of the mechanism of action of these agents, indeed, it is still unclear whether buspirone exerts its anxiolytic effects via an agonist action at presynaptic (somatodendritic) or an antagonist action at postsynaptic 5-HT1A receptors. Ligands that have been used previously to define the 5-HT1A receptor are either non-selective or have agonist activity at the presynaptic 5-HT1A receptor. It is only in the past three years that selective and silent 5-HT1A receptor antagonists have emerged. This overview compares the profiles of the first selective 5-HT1A receptor antagonists in models of pre- and postsynaptic 5-HT1A receptor function. In addition, it highlights some of the problems associated with the development of selective 5-HT1A receptor antagonists.

Comparison of the effects of selected drugs on the release of hypothalamic adrenaline and noradrenaline measured in vivo.

Intracerebral dialysis combined with high-performance liquid chromatography and electrochemical detection was used to monitor changes in extracellular posterior hypothalamic, noradrenaline, 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole acetic acid (5-HIAA) following administration of an inhibitor of phenylethanolamine-N-methyltransferase (LY87130); the alpha 2-antagonist idazoxan; the monoamine oxidase (MAO) inhibitor tranylcypromine, and a selective noradrenergic neurotoxin (DSP4) to the anaesthetised rat. LY87130 (50 mg/kg i.p.) decreased basal hypothalamic perfusate and whole-tissue levels of adrenaline by 100% and 64%, respectively, but was without effect on basal extracellular hypothalamic levels and whole-tissue levels of noradrenaline, DOPAC and 5-HIAA. Administration of the alpha 2-adrenoceptor antagonist idazoxan and the MAO inhibitor tranylcypromine elicited increases in hypothalamic extracellular levels of both adrenaline and noradrenaline by 208% and 229%, respectively. Idazoxan also increased hypothalamic extracellular 5-HIAA by 97% but was without effect on extracellular DOPAC. In contrast, tranylcypromine decreased hypothalamic extracellular levels of DOPAC and 5-HIAA by 72% and 50%, respectively. DSP4 depleted extracellular hypothalamic adrenaline and noradrenaline 360 min and 390 min postdrug, respectively, after causing an initial 3-fold increase in both these amines 150 min after drug administration. DSP4 was without effect on posterior hypothalamic extracellular DOPAC and 5-HIAA. These results demonstrate that the pharmacology of central adrenaline and noradrenaline is very similar and, with the exception of phenylethanolamine-N-methyl transferase inhibitors, none of the drugs investigated are able to differentiate between adrenergic and noradrenergic neurones.

Differential effects of 5-HT1B/1D receptor antagonists in dorsal and median raphe innervated brain regions.

The effect of SB-224289 (2,3,6.7-tetrahydro-1'-methyl-5-¿2'-methyl-4'-[(5-methyl-1,2,4-oxadiazol e-3-yl)biphenyl-4-yl]carbonyl¿Furo[2,3-F]-indole-3-spiro-4'-piperidine oxalate) (4 mg/kg i.p., 5-HT1B receptor antagonist), GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-me thyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide) (0.3 mg/kg i.p., 5-HT1B/1D receptor antagonist), and paroxetine (10 mg/kg p.o.) were investigated on extracellular 5-hydroxytryptamine (5-HT) levels in the frontal cortex, striatum and dentate gyrus of the freely moving guinea-pig with microdialysis. In the frontal cortex and striatum (dorsal raphe innervated areas), GR 127935 evoked a significant decrease in extracellular 5-HT, reaching minima of 41+/-12% and 32+/-6% of basal, respectively. This decrease may be explained by antagonism of inhibitory 5-HT1B/1D receptors on raphe cell bodies, leading to a local increase in 5-HT, which, in turn, stimulated 5-HT1A receptors to decrease cell firing, and hence 5-HT release from terminals. In contrast, SB-224289 had no effect on 5-HT levels in either region. In the dentate gyrus (median raphe innervated area), GR 127935 and SB-224289 significantly increased extracellular 5-HT, reaching maxima of 146+/-11% and 151+/-19% of basal, respectively. The ability of both compounds to increase 5-HT levels in the dentate gyrus suggests a lack of 5-HT1B/1D receptors in the median raphe nucleus. Paroxetine produced a small but non-significant increase in extracellular 5-HT in the frontal cortex, and a small decrease in the dentate gyrus. The lack of effect of paroxetine in terminal areas may be due to the limiting effects of cell body 5-HT autoreceptors. In summary, the above data demonstrate that 5-HT1B/1D receptor antagonists increase 5-HT levels in the dentate gyrus, implying that acute administration of 5-HT1B/1D receptor antagonists will achieve a similar effect to chronic selective serotonin re-uptake inhibitor treatment in median raphe innervated areas. This, in turn, suggests that such compounds may be efficacious in the treatment of depression.

Differential effects of WAY-100135 on the decrease in 5-hydroxytryptamine release induced by buspirone and NAN-190.

1-(2-Methoxyphenyl)-4-[(phthalimido)butyl] piperazine (NAN-190) and 8-[4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5] decane-7,9-dione (buspirone) are 5-HT1A receptor partial agonists which decrease 5-hydroxytryptamine (5-HT) release in vivo. In order to assess whether these ligands decrease 5-HT release by stimulating 5-HT1A receptors we examined the ability of the selective 5-HT1A receptor antagonist N-tert-butyl 3-4-(2-methoxyphenyl) piperazin-1-yl-2-phenylpropanamide dihydrochloride (WAY-100135) to block their inhibitory effects on 5-HT. NAN-190 (0.1 mg/kg s.c.) and buspirone (1.0 mg/kg s.c.) significantly decreased extracellular levels of 5-HT in hippocampal dialysates. WAY-100135 (10.0 mg/kg s.c.) attenuated the effect of buspirone but had no significant effect on the NAN-190-induced decreased in 5-HT release. These data demonstrate that buspirone is an agonist at the somatodendritic 5-HT1A receptor but that the inhibitory effects of NAN-190 on 5-HT release may be mediated via a mechanism other than, or in addition to, 5-HT1A receptor agonism.

Neurochemical profile of the selective and silent 5-HT1A receptor antagonist WAY100135: an in vivo microdialysis study.

The neurochemical profile of the selective 5-HT1A receptor antagonist WAY100135 [N-tert-butyl 3-4-(2-methoxyphenyl) piperazin-1-yl-2-phenylpropanamide dihydrochloride] and its enantiomers at the somatodendritic 5-HT1A receptor was determined by studying the effects of these compounds on 5-HT (5-hydroxytryptamine, serotonin) release in the rat hippocampus using in vivo microdialysis. (+/-)-WAY100135, (+)-WAY100135 and (-)-WAY100135 (all at 10 mg/kg s.c.) had no significant effect on extracellular levels of 5-HT in the hippocampus demonstrating that these compounds are devoid of 5-HT1A receptor agonist properties. In contrast, the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.1 mg/kg s.c.) and the partial agonists BMY 7378 (1.0 mg/kg s.c.) and buspirone (5 mg/kg s.c.) significantly decreased hippocampal 5-HT. Pretreatment with (+/-)-WAY100135 (at 10 mg/kg s.c.) and (+)-WAY100135 (at 1.0-10 mg/kg s.c.) completely blocked the 8-OH-DPAT-induced decrease in 5-HT release demonstrating that these compounds are antagonists at the somatodendritic 5-HT1A autoreceptor. (-)-WAY100135 at a dose of 10 mg/kg s.c. had no significant effect on the 8-OH-DPAT-induced inhibition of 5-HT release. (+/-)-WAY100135 had no significant effect on extracellular levels of dopamine in the rat hippocampus but significantly increased extracellular levels of noradrenaline. The mechanism underlying the increase in noradrenaline is unknown at present.(ABSTRACT TRUNCATED AT 250 WORDS)