Resiniferatoxin-type phorboid vanilloids display capsaicin-like selectivity at native vanilloid receptors on rat DRG neurons and at the cloned vanilloid receptor VR1.

1 Although the cloned rat vanilloid receptor VR1 appears to account for both receptor binding and calcium uptake, the identification of vanilloids selective for one or the other response is of importance because these ligands may induce distinct patterns of biological activities. 2 Phorbol 12,13-didecanoate 20-homovanillate (PDDHV) evoked 45Ca(2+)-uptake by rat dorsal root ganglion neurons (expressing native vanilloid receptors) in culture with an EC50 of 70 nM but inhibited [3H]-resiniferatoxin (RTX) binding to rat dorsal root ganglion membranes with a much lower potency (Ki>10,000 nM). This difference in potencies represents a more than 100 fold selectivity for capsaicin-type pharmacology. 3 45Ca2+ influx by PDDHV was fully inhibited by the competitive vanilloid receptor antagonist capsazepine, consistent with the calcium uptake occurring via vanilloid receptors. 4 PDDHV induced calcium mobilization in CHO cells transfected with the cloned rat vanilloid receptor VR1 with an EC50 of 125 nM and inhibited [3H]-RTX binding to these cells with an estimated Ki of 10,000 nM. By contrast, PDDHV failed to evoke a measurable calcium response in non-transfected CHO cells, confirming its action through VR1. 5 We conclude that PDDHV is two orders of magnitude more potent for inducing calcium uptake than for inhibiting RTX binding at vanilloid receptors, making this novel vanilloid a ligand selective for capsaicin-type pharmacology. These results emphasize the importance of monitoring multiple endpoints for evaluation of vanilloid receptor structure-activity relations. Furthermore, PDDHV now provides a tool to explore the biological correlates of capsaicin-type vanilloid pharmacology.     

Varenicline is a partial agonist at alpha4beta2 and a full agonist at alpha7 neuronal nicotinic receptors

Varenicline, a new nicotinic ligand based on the structure of cytisine, has recently been approved by the U.S. Food and Drug Administration for use as a smoking cessation aid. Varenicline has been shown to be a partial agonist of alpha4beta2 receptors, and in equilibrium binding assays, it is highly selective for the alpha4beta2 receptor. In this study, we have examined the functional activity of varenicline at a variety of rat neuronal nicotinic receptors expressed in Xenopus laevis oocytes and assayed under two-electrode voltage clamp. We also find that varenicline is a potent, partial agonist at alpha4beta2 receptors, with an EC50 of 2.3 +/- 0.3 microM and an efficacy (relative to acetylcholine) of 13.4 +/- 0.4%. Varenicline has lower potency and higher efficacy at alpha3beta4 receptors, with an EC50 of 55 +/- 8 microM and an efficacy of 75 +/- 6%. Varenicline also seems to be a weak partial agonist at alpha3beta2 and alpha6-containing receptors, with an efficacy <10%. It is remarkable that varenicline is a potent, full agonist at alpha7 receptors with an EC50 of 18 +/- 6 microM and an efficacy of 93 +/- 7% (relative to acetylcholine). Thus, whereas varenicline is a partial agonist at some heteromeric neuronal nicotinic receptors, it is a full agonist at the homomeric alpha7 receptor. Some combination of these actions may be involved in the mechanism of varenicline as a smoking cessation aid.     

ZK 91296, a partial agonist at benzodiazepine receptors

ZK 91296 (ethyl 5-benzyloxy-4-methoxymethyl--carboline-3-carboxylate) is a potent and selective ligand for benzodiazepine (BZ) receptors. Biochemical investigations indicate that ZK 91296 may be a partial agonist at BZ receptors. Such partial agonism may explain to some extent why ZK 91296 needs higher BZ receptor occupancy than diazepam for the same effect against chemical convulsants and for behavioural effects. The lack of sedatiye effects, and the very potent inhibition of reflex epilepsy, spontaneous epilepsy and DMCM-induced seizures suggest, furthermore, that ZK 91296 may possess pharmacological selectivity for a particular type of BZ receptor interaction, perhaps including topographic as well as receptor subtype differentiation.     

Methanandamide activation of a novel current in mouse trigeminal ganglion sensory neurons in vitro

Anandamide is an endogenous agonist for cannabinoid receptors and produces analgesia by acting at these receptors in several sites in the brain and peripheral nervous system. Anandamide is also an agonist at the TRPV1 receptor, a protein that serves as an important integrator of noxious stimuli in sensory neurons. Although anandamide actions at CB1 and TRPV1 receptors can explain many of its effects on sensory neurons, some apparently CB1- and TRPV1-independent effects of anandamide have been reported. To explore possible mechanisms underlying these effects we examined the actions of the stable anandamide analog methanandamide on the membrane properties of trigeminal ganglion neurons from mice with TRPV1 deleted. We found that methanandamide and anandamide activate a novel current in a subpopulation of small trigeminal ganglion neurons. Methanandamide activated the current (EC(50) 2 microM) more potently than it activates TRPV1 under the same conditions. The methanandamide-activated current reverses at 0 mV and does not inactivate at positive potentials but declines rapidly at negative membrane potentials. Activation of the current is not mediated via cannabinoid receptors and does not appear to involve G proteins. The phytocannabinoid Delta(9)-tetrahydrocannabinol, the endocannabinoid-related molecules N-arachidonoyl dopamine and N-arachidonoyl glycine and the non-specific TRPV channel activator 2-aminoethoxydiphenyl borate do not mimic the effects of methanandamide. The molecular identity of the current remains to be established, but we have identified a potential new effector for endocannabinoids in sensory neurons, and activation of this current may underlie some of the previously reported CB1 and TRPV1-independent effects of these compounds.     

2-Phenylmelatonin: a partial agonist at enteric melatonin receptors

The effect of the melatonin receptor ligand, 2-phenylmelatonin, has been assessed in isolated strips of the guinea-pig proximal colon. 2-Phenylmelatonin (0.01 nM-1 microM) caused a concentration-dependent contractile response. The potency value (-log EC50) was 9.3 +/- 1.0. The maximum effect was 25 +/- 4%, of that elicited by the maximally effective concentration (0.3 microM) of 5-HT and 43 +/- 3%, of that by the maximally effective concentration (10 microM) of melatonin. When used as an antagonist, 2-phenylmelatonin (0.01 nM and 0.1 nM) concentration-dependently inhibited melatonin-induced contractions with depression of the maximum response by 25% and 54%, respectively. Higher (1 nM) 2-phenylmelatonin concentrations failed to antagonize melatonin-induced response. Prazosin (0.3 microM), a selective antagonist of melatonin MT3 sites, antagonized melatonin-induced contractions to an extent similar to that induced by 0.01 nM 2-phenylmelatonin (with 30% reduction of the maximum effect to melatonin). The combination of 0.3 microM prazosin and 0.01 nM 2-phenylmelatonin caused antagonism similar in extent to that caused by each individual antagonist. 2-Phenylmelatonin at subnanomolar concentrations behaves as an antagonist of melatonin-induced contractile responses while at nanomolar/micromolar concentrations it behaves as a weak contractile agonist.     

Ergometrine--a partial agonist at 5-HT receptors in the uterus isolated from the oestrogen-primed rat

The mechanism of action of ergometrine has been studied in isolated uterus from the oestrogen-primed non-pregnant rat. Ergometrine (30 nM-1 microM) induced spasm and was antagonised selectively by methysergide and ICI 169,369, a proposed competitive antagonist at 5-hydroxytryptamine (5-HT2) receptors. The pA2 values of ICI 169,369 against 5-HT and ergometrine were not significantly different. Ergometrine (0.1-10 microM) was also a selective antagonist of 5-HT with no effect against acetylcholine or potassium chloride. It is suggested that ergometrine is a partial agonist involving 5-HT receptors in rat uterus.     

Effect of oxytocin as a partial agonist at vasoconstrictor vasopressin receptors on the human isolated uterine artery

• The effect of oxytocin on endothelium-intact and endothelium-denuded segments of the human uterine artery rings was investigated.
• In both types of preparation oxytocin induced contraction of human uterine artery with similar potency and efficacy (pEC₅₀ values: 6.95±0.05 vs 7.06±0.01; maximal response values: 61±4.1% vs 63±5.1% for arteries with and without endothelium, respectively).
• In contrast, human uterine arteries, both intact and denuded of endothelium, did not respond to the addition of the selective oxytocin receptor agonist, [Thr⁴, Gly⁷]oxytocin (10 nM–1 μM).
• The vasopressin receptor antagonists, [d(CH₂)₅Tyr(Me)]AVP (10–100 nM) and [d(CH₂)₅,D-Ile²,Ile⁴]AVP (300 nM–3 μM) produced parallel rightward shifts of the curves for oxytocin. The Schild plots constrained to a slope of unity gave the following −log K_B values: [d(CH₂)₅Tyr(Me)] AVP vs [d(CH₂)₅,D-Ile²,Ile⁴] AVP 9.24 vs 6.91 and 9.26 vs 6.84 for human uterine artery with intact and those denuded of endothelium, respectively. In contrast, in both types of preparations the oxytocin receptor antagonist, [d(CH₂)₅Tyr(OMe), ²Orn⁸]vasotocin (1 μM), did not significantly affect oxytocin-induced contractions.
• The calculated pK_A values for oxytocin itself also did not differ between preparations: 6.56 and 6.43 for human uterine artery with and without endothelium, respectively. In both types of preparations, the receptor reserve (K_A/EC₅₀) was close to unity (intact vs denuded: 3.9 vs 3.0).
• It is concluded that, in human uterine artery, oxytocin induces contractions that are not modulated by the endothelium. It is likely that oxytocin acts as a partial agonist on human uterine artery, regardless of the endothelial condition. On the basis of differential antagonists affinity and affinity of oxytocin itself, it is probable that receptors involved in oxytocin-induced contraction in human uterine arteries belong to the V_1A vasopressin receptors.

     

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

1. We have studied the effects of the purportedly selective 5-HT_1A receptor antagonist (+)-WAY100135 on electrically stimulated 5-hydroxytryptamine (5-HT) efflux in the ventrolateral geniculate nucleus (vLGN), and its affinity at human 5-HT_1B and 5-HT_1D receptors stably expressed in Chinese hamster ovary (CHO) cells.
2. 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-HT_1D/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).
3. 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.
4. The affinity and intrinsic activity of (+)-WAY 100135 were determined at recombinant human 5-HT_1B and 5-HT_1D receptors expressed in CHO cells, by use of radioligand binding and [³⁵S]-GTPγS binding. (+)-WAY 100135 was a partial agonist at human 5-HT_1B and 5-HT_1D receptors with moderately high affinity for 5-HT_1D receptors (pEC₅₀=7.61).
5. In conclusion, (+)-WAY 100135 was found to be not a selective 5-HT_1A autoreceptor antagonist but may act as a partial agonist at the 5-HT_1B/1D receptor, displaying agonist or antagonist properties depending on the stimulation protocol used and the resultant 5-HT ‘tone'' at the receptor.

     

Sazetidine-A is a potent and selective agonist at native and recombinant alpha 4 beta 2 nicotinic acetylcholine receptors

Sazetidine-A has been recently proposed to be a "silent desensitizer" of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), implying that it desensitizes alpha4beta2 nAChRs without first activating them. This unusual pharmacological property of sazetidine-A makes it, potentially, an excellent research tool to distinguish between the role of activation and desensitization of alpha4beta2 nAChRs in mediating the central nervous system effects of nicotine itself, as well as those of new nicotinic drugs. We were surprised to find that sazetidine-A potently and efficaciously stimulated nAChR-mediated dopamine release from rat striatal slices, which is mediated by alpha4beta2(*) and alpha6beta2(*) subtypes of nAChR. The agonist effects on native striatal nAChRs prompted us to re-examine the effects of sazetidine-A on recombinant alpha4beta2 nAChRs in more detail. We expressed the two alternative stoichiometries of alpha4beta2 nAChR in Xenopus laevis oocytes and investigated the agonist properties of sazetidine-A on both alpha4(2)beta2(3) and alpha4(3)beta2(2) nAChRs. We found that sazetidine-A potently activated both stoichiometries of alpha4beta2 nAChR: it was a full agonist on alpha4(2)beta2(3) nAChRs, whereas it had an efficacy of only 6% on alpha4(3)beta2(2) nAChRs. In contrast to what has been published before, we therefore conclude that sazetidine-A is an agonist of native and recombinant alpha4beta2 nAChRs but shows differential efficacy on alpha4beta2 nAChRs subtypes.     

Is Ro15-1788 a partial agonist at benzodiazepine receptors?

The ability of the imidazodiazepine derivative Ro15-1788, a potent and specific displacer of benzodiazepine binding, to induce and reduce benzodiazepine-like activity has been evaluated against the convulsant activity evoked by leptazol, bicuculline and the convulsant benzodiazepine Ro5-3663, in mice. Ro15-1788 (10-50 mg/kg) produced a dose dependent reduction in the activity of all 3 convulsants (significant at 50 mg/kg), when they were given at just maximal convulsant doses, but this effect was lost with supramaximal doses of the convulsants, as used in other reported experiments. In addition to this weak benzodiazepine-like activity, Ro15-1788 also reduced the anticonvulsant activity of diazepam and a triazolopyridazine derivative but not that of phenobarbitone. This effect was more marked at 10 than 25 or 50 mg/kg Ro15-1788. It is concluded that although Ro15-1788 has a specific effect on drugs acting at the benzodiazepine receptor it should be classified as a weak partial agonist rather than an antagonist at this site.     

Putative partial agonist 1-aminocyclopropanecarboxylic acid acts concurrently as a glycine-site agonist and a glutamate-site antagonist at N-methyl-D-aspartate receptors

1-Aminocyclopropanecarboxylic acid (ACPC) has been shown to protect against neuronal cell death after ischemic insult in vivo. Such results can be correlated with in vitro assays in which ACPC protected neurons against glutamate-induced neurotoxicity by reducing the activity of N-methyl-D-aspartate (NMDA) channel activation. Electrophysiological studies have determined that ACPC inhibits NMDA receptor activity by acting as a glycine-binding site partial agonist. In this study, rapid drug perfusion combined with whole-cell voltage-clamp was used to elicit and measure the effects of ACPC on NMDA receptor-mediated responses from cultured hippocampal neurons and cerebellar granule cells. The ACPC steady-state dose-response curve had both stimulatory and inhibitory phases. Half-maximal activation by ACPC as a glycine-site agonist was 0.7 to 0.9 microM. Half-maximal inhibition by ACPC was dependent on NMDA concentration. Peak responses to a >100 microM ACPC pulse in the presence of 1 microM glutamate were similar to those of glycine but decayed to a steady-state amplitude below that of glycine. The removal of ACPC initially caused an increase in inward current followed by a subsequent decrease to baseline levels. This suggests that relief of low-affinity antagonism occurs before high-affinity agonist dissociation. Simulations of ACPC action by a two glutamate-binding site/two glycine-binding site model for NMDA channel activation in conjunction with the concurrent role of ACPC as a glycine-site full agonist and glutamate-site competitive antagonist were able to successfully approximate experimental results.     

Non-competitive inhibition of kainate-induced currents by diethylstilbestrol in acutely isolated mouse CA1 hippocampal neurons

The effect of a synthetic estrogen, diethylstilbestrol (DES), on kainate-induced currents was investigated in the hippocampal CA1 pyramidal neurons acutely dissociated from the mice using the nystatin-perforated patch-clamp recording configuration under voltage-clamp conditions. DES inhibited the current evoked by 100 microM kainate in a concentration-dependent manner with a half-maximum inhibitory concentration of 8.8 microM. The action of DES was voltage-independent. Since DES produced a suppression of the maximum response of the kainate concentration-response curve, the inhibition by DES of the kainate-induced current appears to be non-competitive.     

AMP is a partial agonist at the sheep cardiac ryanodine receptor.

We have investigated the ability of AMP to modulate the native sheep cardiac ryanodine receptor (RyR) channel at various cytosolic [Ca2+]. Channels were incorporated into planar phospholipid bilayers and current fluctuations through the bilayer were monitored under voltage clamp conditions. We demonstrate that AMP only exhibits agonist activity if the cytosolic [Ca2+] is sufficiently high. Even in the presence of a high cytosolic [Ca2+] (65 microM), AMP cannot fully open the channel and the maximum open probability (Po) observed is approximately 0.3 at 2 mM AMP. Concentrations of AMP above the maximally activating level cause inactivation of the channel. Our experiments indicate that AMP is an agonist with such low efficacy at the ATP sites on the cardiac RyR that it is effectively an antagonist of ATP-induced increases in Po. Our study demonstrates that the number of phosphates attached to the 5'-carbon of the ribose ring of adenine-based compounds determines the efficacy of the ligand to increase the Po of the cardiac RyR. Substitution of groups at this position may lead to the identification of potent antagonists at ATP sites on RyR.     

The endogenous lipid anandamide is a full agonist at the human vanilloid receptor (hVR1)

The endogenous cannabinoid anandamide was identified as an agonist for the recombinant human VR1 (hVR1) by screening a large array of bioactive substances using a FLIPR-based calcium assay. Further electrophysiological studies showed that anandamide (10 or 100 microM) and capsaicin (1 microM) produced similar inward currents in hVR1 transfected, but not in parental, HEK293 cells. These currents were abolished by capsazepine (1 microM). In the FLIPR anandamide and capsaicin were full agonists at hVR1, with pEC(50) values of 5. 94+/-0.06 (n=5) and 7.13+/-0.11 (n=8) respectively. The response to anandamide was inhibited by capsazepine (pK(B) of 7.40+/-0.02, n=6), but not by the cannabinoid receptor antagonists AM630 or AM281. Furthermore, pretreatment with capsaicin desensitized the anandamide-induced calcium response and vice versa. In conclusion, this study has demonstrated for the first time that anandamide acts as a full agonist at the human VR1.     

Gamma-hydroxybutyrate is a weak agonist at recombinant GABA(B) receptors

Gamma-hydroxybutyrate (GHB) is a neuromodulator with high affinity binding sites in the mammalian brain. However, the receptor for GHB has not yet been identified. There are indications that GHB and gamma-aminobutyric acid (GABA) mediate their effects via the same receptor. We tested this hypothesis using GABA(B)R1/R2 receptors co-expressed with Kir3 channels in Xenopus oocytes. GHB activated these receptors with an EC50 of approximately 5 mM and a maximal stimulation of 69% when compared to the GABA(B) receptor agonist L-baclofen. GHB and L-baclofen did not amplify each others effect nor did they stimulate the GABA(B) receptor in a linearly additive manner. CGP54626A, 2-OH saclofen and CGP35348, three competitive GABA(B) receptor antagonists, inhibited the GHB induced response completely. A concentration of 30 mM GHB displaced [125I]CGP64213 binding at GABA(B)R1 expressed in COS cells by 21%. These results indicate that GHB is a weak partial agonist at the GABA binding site of GABA(B)R1/R2.     

Characterization of aripiprazole partial agonist activity at human dopamine D3 receptors

Aripiprazole is the first dopamine D2/D3 receptor partial agonist approved for use in the treatment of psychiatric disorders, including schizophrenia, bipolar disorder, and unipolar depression in the US. To explore the functional activity of aripiprazole at dopamine D3 receptors, we established Chinese hamster ovary (CHO) cell lines stably expressing high and low densities of Ser-9 and Gly-9 variants of human dopamine D3 receptors and compared aripiprazole's dopamine D3 pharmacological properties with other marketed and non-approved dopamine D3 receptor modulating agents on inhibition of forskolin-stimulated cAMP accumulation. Maximal cell responses for dopamine were dependent on receptor expression levels, and all cells had similar potency for dopamine responses. Aripiprazole, terguride, bifeprunox, OPC-4392 (7-(3-[4-(2,3-dimethylphenyl)piperazinyl]propoxy)-2(1H)-quinolinone), (-)-3-PPP ((-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine), SDZ 208-912 (N-[(8 alpha)-2-chloro-6-methylergolin-8-yl]-2,2-dimethylpropanamide), BP897 (N-[4-[4-(2-Methoxyphenyl)-1-piperazinyl]butyl]naphthalene-2-carboxamide) and GR103691 (4'-Acetyl-N-[4-[4-(2-methoxyphenyl)piperazin-1-yl]butyl]biphenyl-4-carboxamide) behaved as partial agonists. Aripiprazole's intrinsic activity was similar to that of BP897 and GR103691, lower than that of terguride, bifeprunox, OPC-4392, and (-)-3-PPP, and higher than that of SDZ 208-912. The Gly-9 variant did not differ from the Ser-9 variant with respect to those agonist potencies and intrinsic activities. These compounds blocked the action of dopamine with a maximum effect equal to that of each compound alone. ACR16 (4-(3-Methanesulfonyl-phenyl)-1-propyl-piperidine), quetiapine, clozapine, olanzapine, ziprasidone, risperidone, and haloperidol acted as antagonists. Aripiprazole's unique activity at dopamine D3 receptors may translate into clinically relevant outcomes in patients with a variety of neuropsychiatric disorders.     

Antiarrhythmic actions of meptazinol, a partial agonist at opiate receptors, in acute myocardial ischaemia.

1 The intravenous administration, to anaesthetized rats, of meptazinol (1 and 2 mg kg-1), a partial agonist at opiate receptors, greatly reduced the incidence of ventricular extrasystoles that resulted from acute coronary artery occlusion. The incidence of ventricular fibrillation (VF) was reduced from 50% (in the controls) to 10% and the mortality from 30% to zero. 2 In similar doses, pretreatment with meptazinol also reduced ventricular arrhythmias, including fibrillation, in conscious rats subjected to coronary artery occlusion. In this model, survival at 16 h was increased from 27% in the controls to 50% and 83% respectively in rats pretreated with 1 and 2 mg kg-1 of the drug. 3 In antiarrhythmic doses, meptazinol had little effect on either heart rate or systemic arterial blood pressure. 4 Intracellular action potential recordings from papillary muscle removed from rats given meptazinol (2 mg kg-1) 15 min previously showed an increase in APD50 and APD90 of more than 40%. There was no effect on dV/dtmax. When superfused with meptazinol in vitro normal rat papillary muscle stimulated at 1 or 3 Hz showed an increase in APD90 and a decrease in dV/dtmax. 5 The antiarrhythmic effect of meptazinol in these models can probably be explained by direct actions on the cardiac muscle action potential (increase in APD) although effects on opiate receptors cannot be ruled out. It is suggested that meptazinol might be useful in relieving pain, and in reducing the severity of arrhythmias in the early stages of acute myocardial infarction.     

Eosine-induced blockade of N-methyl-D-aspartate channels in acutely isolated rat hippocampal neurons

Acutely isolated rat hippocampal neurons were voltage-clamped in the whole-cell configuration. The currents through N-methyl-D-aspartate (NMDA) channels were elicited by fast application of aspartate in a Mg(2+)-free 3 microM glycine-containing solution. Eosine, known as a potent reversible inhibitor of the plasma membrane Ca(2+) pump, proved to be able to induce a blockade of NMDA channels. The eosine-induced inhibition of NMDA-mediated currents enhanced with eosine concentration (IC(50) = 248 microM) but did not depend on the membrane potential, agonist (aspartate) or coagonist (glycine) concentrations, pH, or the presence of spermine, ethanol, and the disulfide-reducing agents dithiothreitol and glutathione. Zn(2+) inhibited NMDA channels with equal efficiency both in the presence and absence of eosine. These results suggest that eosine interacts with a new, previously unknown NMDA receptor regulatory site.     

Isapirone is a partial agonist at 5-hydroxytryptamine 1A (5-HT1A) receptors in the rat hippocampus: electrophysiological evidence

Using iontophoretic techniques we observed that in vivo isapirone (TVX Q 7821), a selective ligand for the 5-HT1A binding site, at low ejection currents (5-30 nA) antagonised 5-HT and 8-hydroxy-2-(di-n-propylamino) tetralin (DPAT)-induced suppression of hippocampal unit activity, with little effect on baseline firing rate itself. At higher ejection currents (20-100 nA) or following prolonged application, isapirone inhibited unit firing. Responses to GABA were unaffected by isapirone. These data demonstrate that isapirone is a 5-HT1A receptor antagonist with partial agonist properties on 5-HT sensitive neurones in the rat hippocampus.