Colin Davidson

Evidence that 5-hydroxytryptamine release in rat dorsal raphe nucleus is controlled by 5-HT1A, 5-HT1B and 5-HT1D autoreceptors.

Electrically stimulated 5‐hydroxytryptamine (5‐HT) release was monitored in slices of rat dorsal raphé nucleus (DRN) by fast cylic voltammetry. Pseudo‐single pulse stimulations (5 pulses at 100 Hz) were used to enable the effect of various receptor agonists to be seen without competition from endogenously released transmitter. The selective 5‐HT1A receptor agonist, (+)‐8‐OH‐DPAT (1.0 μm) decreased stimulated 5‐HT release to 31 ± 3% of controls. This decrease was inhibited by the 5‐HT1A receptor antagonists, (+)‐WAY‐100135 (1.0 μm) and WAY‐100635 (0.1 μm) but not by the 5‐HT1D/B antagonist, GR127935 (0.05 μm). The selective 5‐HT1B receptor agonist, CP‐93129 (0.3 μm) decreased stimulated 5‐HT release to 61 ± 4% of control. This effect was antagonized by the 5‐HT1B receptor antagonist, isamoltane (0.5 μm) but not by (+)‐WAY‐100135. The 5‐HT1D agonist, sumatriptan (0.5 μm) decreased stimulated 5‐HT release to 52 ± 2 % of controls. This decrease was blocked by GR‐127935 but not by WAY‐100635. These results suggest that 5‐HT release in the rat DRN is under the control of 5‐HT1A, 5‐HT1B and 5‐HT1D autoreceptors.

The effect of paroxetine on 5-HT efflux in the rat dorsal raphe nucleus is potentiated by both 5-HT1A and 5-HT1B/D receptor antagonists.

Serotonin (5-HT) efflux in slices of rat dorsal raphe nucleus (DRN) was evoked by pseudo one pulse electrical stimulation (20 pulses at 100 Hz, 190 ms train duration) and measured, along with 5-HT uptake, by fast cyclic voltammetry (FCV). The selective serotonin re-uptake inhibitor (SSRI) paroxetine (10(-7) M) increased 5-HT efflux to 147 +/- 6% of pre-drug values at maximum (mean +/- SEM, n = 5) and the half-life of uptake to 443 +/- 38%. The non-selective 5-HT1 antagonist methiothepin (2 x 10(-7) M) increased 5-HT efflux to 147 +/- 9% at maximum but had no effect on uptake half-life. In contrast, (+)-WAY 100135 (10(-6) M) and GR 127935 (5 x 10(-8) M), selective antagonists at 5-HT1A and 5-HT1B/D receptors, respectively, affected neither 5-HT efflux nor uptake. When given in combination with paroxetine, the antagonists significantly increased the effect of paroxetine on efflux: methiothepin to 228 +/- 24% (P < 0.001), (+)-WAY 100135 to 212 +/- 31% (P < 0.05) and GR 127935 to 203 +/- 23% (P < 0.01). These data suggest that, under these experimental conditions, DRN 5-HT autoreceptors are tonically activated in the presence of the uptake blocker and that the antagonists act by blocking this counteracting autoinhibitory tone. The data also strongly indicate that 5-HT efflux in the rat DRN is under the control not only of 5-HT1A but also of 5-HT1B/D receptors.

Developments in harmine pharmacology--implications for ayahuasca use and drug-dependence treatment.

Ayahuasca is a hallucinogenic botanical mixture originating in the Amazon area where it is used ritually, but is now being taken globally. The 2 main constituents of ayahuasca are N,N-dimethyltryptamine (DMT), a hallucinogen, and harmine, a monoamine oxidase inhibitor (MAOI) which attenuates the breakdown of DMT, which would otherwise be broken down very quickly after oral consumption. Recent developments in ayahuasca use include the sale of these compounds on the internet and the substitution of related botanical (anahuasca) or synthetic (pharmahuasca) compounds to achieve the same desired hallucinogenic effects. One intriguing result of ayahuasca use appears to be improved mental health and a reduction in recidivism to alternate (alcohol, cocaine) drug use. In this review we discuss the pharmacology of ayahuasca, with a focus on harmine, and suggest pharmacological mechanisms for the putative reduction in recidivism to alcohol and cocaine misuse. These pharmacological mechanisms include MAOI, effects at 5-HT(2A) and imidazoline receptors and inhibition of dual-specificity tyrosine-phosphorylation regulated kinase 1A (DYRK1A) and the dopamine transporter. We also speculate on the therapeutic potential of harmine in other CNS conditions.

Simultaneous “real-time” electrochemical and electrophysiological recording in brain slices with a single carbon-fibre microelectrode

Many previous studies have demonstrated the value of carbon-fibre microelectrodes (CFMs) for single-unit activity recording and for fast cyclic voltammetry. In this report we show that these two independent methodologies can be combined at a single CFM and used to study simultaneous electrochemical and electrophysiological events in brain slices. In superfused slices of rat locus coeruleus, dorsal raphe and substantia nigra, we were able to record stable electrophysiological signals and stimulated monoamine efflux for periods of at least 2 h, thereby allowing quantitative pharmacological interventions. The simultaneous recording of amine efflux and unit activity at the same locus facilitates comparison of drug effects at pre- and post-synaptic sites. Furthermore, the system described here uses commercially available instrumentation. The circuitry is described and examples of its application are shown.

New psychoactive substances

A toolkit for substance misuse commissioners New psychoactive substances: toolkit for commissioners 2 About Public Health England Public Health England exists to protect and improve the nations health and wellbeing, and reduce health inequalities. It does this through world-class science, knowledge and intelligence, advocacy, partnerships and the delivery of specialist public health services. PHE is an operationally autonomous executive agency of the Department of Health. You may re-use this information (excluding logos) free of charge in any format or medium, under the terms of the Open Government Licence v2.0. To view this licence, visit OGL or email psi@nationalarchives.gsi. gov.uk. Where we have identified any third party copyright information you will need to obtain permission from the copyright holders concerned. Any enquiries regarding this publication should be sent to

Serotonin efflux in the rat ventral lateral geniculate nucleus assessed by fast cyclic voltammetry is modulated by 5-HT1B and 5-HT1D autoreceptors

Fast cyclic voltammetry (FCV) was used to measure electrically stimulated monoamine efflux in the rat ventral lateral geniculate nucleus (vLGN). The electrochemical characteristics of the released species resembled 5-HT but not dopamine or noradrenaline. Amine efflux was abolished by the sodium channel blocker tetrodotoxin (0.1 microM), Ro 4-1284 (1.0 microM), the fast-acting reserpine analogue, and removal of Ca2+ from the superfusate. Amine efflux was unaffected by the monoamine oxidase inhibitor clorgyline (0.1 microM). Of paroxetine (0.1 microM), desipramine (50 nM) and vanoxerine (0.5 microM), selective blockers of 5-HT, noradrenaline and dopamine uptake respectively, only paroxetine increased monoamine efflux (to 194 +/- 25%, mean +/- SEM) and prolonged the removal half-life (to 638 +/- 105%). The non-specific 5-HT1 antagonist methiothepin (0.2 microM) increased 5-HT efflux on long (20 pulses at 20 Hz) but not short trains (20 pulses at 100 Hz). When tested on pseudo-one-pulse stimulations (5 pulses, 100 Hz), the selective 5-HT1A agonist 8-OHDPAT (1.0 microM) had no effect. CP 93129 (0.3 microM), the selective 5-HT1B agonist, decreased 5-HT efflux to 37 +/- 4% of control and was antagonised by the 5-HT1B blocker isamoltane (0.5 microM) and by the 5-HT1D/B antagonist GR 127935 (50 nM). The preferential 5-HT1D agonist sumatriptan (0.5 microM) also decreased 5-HT efflux, to 55 +/- 6% and was antagonised by GR 127935 (50 nM) but not isamoltane (0.5 microM). These results suggest that 5-HT released in the vLGN can be measured by FCV. Furthermore, released 5-HT is taken up by the 5-HT transporter and may be under the influence of 5-HT1B and 5-HT1D autoreceptors.

Contrasting effects of chronic paroxetine on 5-HT1A control of dorsal raphe cell firing and 5-HT release

TO test the role of 5-HT1A receptors in the action of antidepressants, we investigated the effect of chronic paroxetine (10 mg/kg, p.o. for 21 days) on functional assays of 5-HT1A sensitivity. We constructed cumulative concentration response curves to the selective 5-HT1A agonist (+)-8-OH-DPAT on both extracellular recordings of 5-HT neurones and electrically stimulated 5-HT release in dorsal raphe brain slices. Chronic paroxetine desensitized the 5-HT1A receptors controlling firing, with an increase in EC50 from 10.7 nM to 46.2 nM 8-OH-DPAT. Chronic paroxetine did not, however, desensitize the 5-HT1A receptors controlling 5-HT release but increased the 8-OH-DPAT Emax from 54.9% to 79.2% inhibition of 5-HT release. These data suggest that there are either two distinct populations of 5-HT1A receptors or separate second messenger systems, one controlling 5-HT release and another influencing firing. Furthermore chronic paroxetine treatment can differentially modulate these different populations.

Effect of chronic paroxetine treatment on 5-HT1B and 5-HT1D autoreceptors in rat dorsal raphe nucleus.

This study reports the effect of chronic paroxetine (10 mg/kg p.o., 21 days) on 5-HT1B and 5-HT1D autoreceptors controlling stimulated 5-HT efflux in slices of rat dorsal raphe nucleus. Electrically evoked 5-HT (10 pulses, 200 Hz, 0.1 ms, 10 mA) was measured using fast cyclic voltammetry. 5-HT efflux was inhibited by CP 93129 (10 nM-10 microM) and by sumatriptan (1 nM-1 microM) agonists at 5-HT1B and 5-HT1D receptors, respectively. Chronic paroxetine did not, initially, appear to alter the sensitivity of the 5-HT1B autoreceptors to CP 93129. However, when constructed in the presence of WAY 100635 (10 nM) the selective and silent 5-HT1A antagonist, there was a significant (P < 0.001) rightward shift of the CP 93129 concentration-response curve in the paroxetine-treated rats but not in the controls, implying a desensitisation of the 5-HT1B autoreceptor by paroxetine. Chronic paroxetine did not affect the sumatriptan concentration-response curve, even with WAY 100635 present, implying that there was no (de)sensitisation of the 5-HT1D autoreceptor. These data suggest that chronic paroxetine treatment may desensitise 5-HT1B autoreceptors in the dorsal raphe nucleus but that this effect is unmasked only when the dominant 5-HT1A autoreceptor control is antagonised.

Desoxypipradrol is more potent than cocaine on evoked dopamine efflux in the nucleus accumbens

Desoxypipradrol is a methylphenidate-like drug that has been recently found in a number of ‘legal highs’. Evidence from emergency room toxicology reports suggests that this drug might have led to a number of psychotic events in drug abusers in the UK and elsewhere. However, very little research has been done on the effects of this drug on the brain. Here we used rat brain slices from the nucleus accumbens core, which were exposed to either cocaine (1, 3 or 10 µM) or desoxypipradrol (1, 3 or 10 µM) for 60 min. Dopamine efflux was electrically evoked and recorded using fast cyclic voltammetry. Both drugs increased the peak dopamine efflux and also slowed dopamine re-uptake. Desoxypipradrol was more potent than cocaine causing a sevenfold increase in peak dopamine levels (versus a threefold increase for cocaine) and increasing dopamine re-uptake half-life 15-fold (versus fivefold for cocaine). These data suggest that desoxypipradrol is more potent than cocaine at dopamine terminals, and this could account for its psychotogenic effects.

(+)‐WAY 100135, a partial agonist, at native and recombinant 5‐HT1B/1D receptors

We have studied the effects of the purportedly selective 5‐HT1A receptor antagonist (+)‐WAY100135 on electrically stimulated 5‐hydroxytryptamine (5‐HT) efflux in the ventrolateral geniculate nucleus (vLGN), and its affinity at human 5‐HT1B and 5‐HT1D receptors stably expressed in Chinese hamster ovary (CHO) cells. On short ‘pseudo single pulse’ stimulations (20 pulses at 100 Hz, 190 ms train duration), (+)‐WAY100135 (1.0 μM) decreased 5‐HT efflux in the vLGN to 68±8% of pre‐drug values (P<0.01). This decrease could be blocked by the 5‐HT1D/1B receptor antagonist GR 127935 (50 nM). Conversely, when long stimulations (20 pulses at 20 Hz, 950 ms train) were used, (+)‐WAY100135 had no effect on 5‐HT efflux (84±8% of pre‐drug values) although both methiothepin (200nM) and GR 127935 (50 nM) caused significant increases (to 175±18 and 130±10% of pre‐drug values, respectively). Paroxetine (100 nM), the selective 5‐HT reuptake inhibitor, increased stimulated 5‐HT efflux and re‐uptake half‐life (to 145±18% and 649±121%, respectively) on pseudo single pulse stimulations. When (+)‐WAY 100135 was added in combination with the uptake blocker, the effect of paroxetine on stimulated 5‐HT efflux was potentiated to 282±48% (P<0.01) without further effect on the 5‐HT re‐uptake half‐life. The affinity and intrinsic activity of (+)‐WAY 100135 were determined at recombinant human 5‐HT1B and 5‐HT1D receptors expressed in CHO cells, by use of radioligand binding and [35S]‐GTPγS binding. (+)‐WAY 100135 was a partial agonist at human 5‐HT1B and 5‐HT1D receptors with moderately high affinity for 5‐HT1D receptors (pEC50=7.61). In conclusion, (+)‐WAY 100135 was found to be not a selective 5‐HT1A autoreceptor antagonist but may act as a partial agonist at the 5‐HT1B/1D receptor, displaying agonist or antagonist properties depending on the stimulation protocol used and the resultant 5‐HT ‘tone’ at the receptor.