Mark D. Tricklebank

The behavioural effects of MK-801: a comparison with antagonists acting non-competitively and competitively at the NMDA receptor

The selective non-competitive NMDA receptor antagonist, MK-801, potently blocked convulsions induced in the mouse by N-methyl-DL-aspartic acid (NMDLA) with an i.v. ED50 dose of 0.2 mg/kg. Similar doses of MK-801 were also effective in blocking seizures induced by pentylenetetrazol (PTZ), electroshock and by sound in audiogenic seizure-prone animals. Other less selective non-competitive NMDA receptor antagonists including phencyclidine (PCP), thienylcyclohexylpiperidine (TCP), (+)-N-allylnormetazocine [+)-NANM, (+)-SKF 10,047) and ketamine also blocked NMDLA-induced seizures with a rank order of potency of MK-801 greater than PCP greater than TCP = (+)-NANM greater than ketamine. The competitive NMDA receptor antagonist, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) blocked NMDLA-induced seizures with an ED50 of 4.5 mg/kg, 22- and 560-fold more potently than the competitive antagonists, 2-DL-amino-7-phosphonoheptanoic acid (2-APH) and 2-DL-amino-5-phosphonovaleric acid (2-APV), respectively. MK-801 was the most potent of the non-competitive antagonists to induce a motor syndrome including head weaving, body rolling, increased locomotion and ataxia, characteristic of the behavioural response to PCP in the mouse. The syndrome was also present following injection of the competitive NMDA receptor antagonists, although they were generally less potent (probably a reflection of poor brain penetration) and less efficacious than the non-competitive antagonists. For all compounds except CPP, the anticonvulsant ED50 dose was close to the minimum effective dose to induce motor stimulation: CPP was 5- to 10-fold more potent as an anticonvulsant.(ABSTRACT TRUNCATED AT 250 WORDS)

The behavioural response to 5-HT receptor agonists and subtypes of the central 5-HT receptor

Abstract The analysis of the role of 5-hydroxytryptamine (5-HT)-containing neurons in the control of behaviour is complicated by the diverse functional effects that may follow the activation of the multiple types of recognition site for 5-HT that have recently been identified in the CNS, and the lack of specificity inherent in many of the 5-HT receptor agonists and antagonists that are currently available for use as pharmacological tools. Mark Tricklebank explains some of the pitfalls that can complicate the investigation of 5-HT-mediated behaviours and discusses recent progress that has been made in assigning some of these behaviours to activation of putative subtypes of the central 5-HT receptor.

Activation of the 5-HT1A receptor subtype increases rat plasma ACTH concentration

Serotonergic (5-HT) neuronal pathways regulate the release of adrenocorticotropin hormone (ACTH) from the pituitary gland probably through the action of hypothalamic corticotropin-releasing hormone (CRH). 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A receptor agonist, dose dependently (0.016-3 mg/kg s.c.) increased rat plasma ACTH concentration. This response was blocked stereoselectively by (-)-pindolol, known to have 5-HT1 antagonist properties, but not by (+)-pindolol, beta 1-, beta 2- or alpha 1-adrenoceptor, dopamine, muscarinic, 5-HT2 or 5-HT3 receptor antagonists. Similar increases of plasma ACTH were induced by other 5-HT1A receptor ligands (buspirone, ipsapirone and gepirone). These results suggest that activation of the 5-HT1A receptor induces the secretion of ACTH from the rat pituitary gland.

Subtypes of the 5-HT receptor mediating the behavioural responses to 5-methoxy-N,N-dimethyltryptamine in the rat

The 5-HT receptor subtypes involved in the mediation of reciprocal forepaw treading and the flat body posture induced by the central 5-HT receptor agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), were examined in intact rats and in rats depleted of monoamines with reserpine. Forepaw treading in non-reserpinised rats was antagonised by the 5-HT2 receptor antagonist, ketanserin, only at doses in excess of those required for occupation of a large proportion of 5-HT2 receptors in brain, and at which there was significant inhibition of stereotyped sniffing induced by the dopamine receptor agonist, apomorphine. Since forepaw treading induced by 5-MeODMT was also blocked in intact rats by haloperidol, blockade of the behaviour by ketanserin may more accurately reflect antagonism at dopamine receptors than at 5-HT2 receptors. In reserpinised rats, i.e. with minimised contributions from other monoamine systems, neither forepaw treading nor the flat body posture were significantly altered by ketanserin, haloperidol or the beta 1- and beta 2-selective adrenoceptor antagonists, betaxolol and ICI 118.551, making a key role for 5-HT2 receptors, dopamine receptors and beta-adrenoceptors unlikely. In contrast, forepaw treading in both reserpinised and non-reserpinised rats was antagonised stereoselectively by pindolol and by spiperone, which interact with 5-HT1 and 5-HT1A recognition sites. The results are consistent with the hypothesis that forepaw treading induced by 5-MeODMT arises by activation of the putative 5-HT1A receptor. Antagonism of the flat body posture by pindolol could be demonstrated only in non-reserpinised rats and the mechanism of induction of this behaviour remains to be established.

Evidence that the anxiolytic-like effects of chlordiazepoxide on the elevated plus maze are confounded by increases in locomotor activity

In exploratory animal models of anxiety, such as the elevated plus maze, the anxiogenic- and anxiolytic-like effects of drugs may be confounded by changes in locomotor activity. In the present experiments, the sensitivity of several measures of anxiety and locomotor activity in the elevated plus maze were assessed. Both chlordiazepoxide hydrochloride (CDP, 7.5 mg/kg) andd-amphetamine sulphate (AMP, 0.75, 1.5 mg/kg) increased the percent time on the open arms and doses of 7.5 mg/kg and 1.5 mg/kg CDP and AMP, respectively, increased the number of entries into the open arms. The increase in these measures might suggest that both compounds induced an anxiolytic-like effect. Although FG 7142 (30.0 mg/kg) did not decrease the number of entries to the open arms, it did decrease the time on the open arms, which might suggest that it had anxiogenic-like effects. Similarly, buspirone reduced both the number of entries into the open arms and the time spent on the open arms. However, all the compounds significantly affected locomotor activity. CDP (3.0 and 7.5 mg/kg) increased the total number of arm entries, the distance travelled on the open arms and the mean speed of the animals on the open, and in the closed arms. Moreover, the distance travelled by the animals in the closed arms was increased by 1.0 mg/kg CDP, a dose that had no measurable effects on the indices of anxiety. Similarly, although AMP failed to increase the total number of arm entries, it did increase the distance travelled in the closed arms (0.75 and 1.5 mg/kg), on the open arms (1.5 mg/kg) and the speed of the animals in the closed arms (1.5 mg/kg), a measure that is independent of the time spent in the closed arms. By contrast, both FG 7142 (30.0 mg/kg) and buspirone decreased the total number of arm entries (0.3–8.0 mg/kg), the speed of the animals in the closed arms and the distance travelled in the closed arms (1.0–4.0 mg/kg). These experiments suggest that: (i) the anxiogenic- and anxiolytic-like effects of drugs in the elevated plus-maze are confounded by changes in locomotor activity and that “total arm entries” is a relatively insensitive measure of drug-induced changes in locmotor activity; (ii) psychostimulant compounds, such as AMP, at doses that increase locomotor activity have an anxiolytic-like profile in the elevated plus maze and (iii) the measurement of speed of movement is a more sensitive index of changes in locmotor activity than the conventional measure of “total arm entries”.

Centrally active 5-HT receptor agonists and antagonists.

Eleven subtypes of central 5-HT receptor have so far been postulated, four of which have been cloned (5-HT1A, 5-HT1C, 5-HT1D and 5-HT2) and a fifth (the 5-HT3 receptor) purified. The present review discusses the agonists and antagonists which act at these subtypes with respect to their degree of selectivity and in vivo potency. Selective agonists exist for the 5-HT1A, 5-HT1B and 5-HT3 receptors and selective antagonists for the 5-HT2 and 5-HT3 receptors.

Mediation of the discriminative stimulus properties of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) by the putative 5-HT1A receptor

Male Sprague-Dawley rats were trained to discriminate the putative 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) from saline in a 2-lever operant drug discrimination paradigm. The 8-OH-DPAT cue was found to be highly selective; neither the 5-HT receptor agonists, quipazine, LSD, MK 212 and RU 24969, the 5-HT releasing agent, p-chloroamphetamine, nor the alpha 2-adrenoceptor agonist, clonidine, were able to substitute for 8-OH-DPAT in tests of generalization. In contrast, both buspirone and TVX Q 7821, which like 8-OH-DPAT have high affinity and selectivity for the 5-HT1A recognition site, generalized to the 8-OH-DPAT cue in a dose-dependent manner. The discriminative stimulus properties of 8-OH-DPAT were not antagonized by the 5-HT2 receptor antagonist, ketanserin, or the selective beta 1- and beta 2-adrenoceptor antagonists, betaxolol and ICI 118.551, indicating that neither 5-HT2 receptors, nor beta-adrenoceptors play a significant role in the behaviour. However, the 8-OH-DPAT cue was antagonized stereoselectively by pindolol and alprenolol, which have relatively high affinity and stereoselectivity for 5-HT1, but not 5-HT2, recognition sites. Similarly, the capacity of TVX Q 7821 to generalize to the 8-OH-DPAT cue could be blocked by pindolol. In view of the fact that 8-OH-DPAT has negligible affinity for the 5-HT1B site, the above results are consistent with its discriminative stimulus properties being mediated by the putative 5-HT1A receptor. Moreover, agonist activity at central 5-HT1A receptors may be an important mechanism contributing to the anxiolytic properties of buspirone and TVX Q 7821.

Modulation of seizure susceptibility in the mouse by the strychnine-insensitive glycine recognition site of the NMDA receptor/ion channel complex.

1 In order to determine whether the strychnine‐insensitive glycine modulatory site on the N‐methyl‐d‐aspartate (NMDA) receptor/ion channel complex is fully activated in vivo, the ability of the selective glycine receptor agonist, d‐serine, to modulate seizure susceptibility in the mouse has been examined. 2 d‐Serine (10–200 μg per mouse, i.c.v.) dose‐dependently increased the potency of NMDLA in inducing seizures in Swiss Webster mice by approximately 3 fold. l‐Serine was without significant effect. 3 The potency of pentylenetetrazol in inducing seizures was also enhanced by d‐, but not l‐serine, although the magnitude of the shift (1.6 fold) was considerably less than for NMDLA. 4 Similar doses of d‐serine were also able to block the anticonvulsant effect of the non‐selective glycine receptor antagonist, kynurenic acid, against seizures induced by NMDLA, but were without effect on the anticonvulsant effect of the competitive NMDA receptor antagonist, 3‐((+)‐2‐carboxypiperazin‐4‐yl)‐propyl‐1‐phosphonic acid (CPP). 5 d‐Serine completely antagonized the protective effect of the selective glycine receptor antagonist, 7‐chlorokynurenic acid, against sound‐induced seizures in DBA/2 mice, but was less effective in this model against the less selective antagonist, kynurenic acid. 6 The results indicate that in vivo, NMDA receptors are not maximally potentiated by endogenous glycine and suggest an important involvement of the glycine modulatory site on the NMDA receptor/ion channel complex in the pathophysiology of epilepsy.

Characterization of MDL 73005EF as a 5‐HT1A selective ligand and its effects in animal models of anxiety: comparison with buspirone, 8‐OH‐DPAT and diazepam

1 With radioligand binding techniques, MDL 73005 EF (8‐[2‐(2,3‐dihydro‐1,4‐benzodioxin‐2‐yl‐methylamino)ethyl]‐8‐azaspiro[4,5]decane‐7,9‐dione methyl sulphonate) shows high affinity (pIC50 8.6) and selectivity (>100 fold compared to other monoamine and benzodiazepine receptor sites) for the 5‐hydroxytryptamine (5‐HT)1A recognition site; it was both more potent and more selective than buspirone in this respect. 2 In rats pretreated with reserpine, 8‐hydroxy‐2‐(di‐n‐propyl‐amino) tetralin (8‐OH‐DPAT) induced forepaw treading and flat body posture; in the same model, MDL 73005EF and buspirone showed minimal agonist activity and at high doses MDL 73005EF inhibited responses to 8‐OH‐DPAT. 3 In rats trained to discriminate 8‐OH‐DPAT from saline in a drug discrimination paradigm, both MDL 73005EF and buspirone generalized dose‐dependently and completely to the 8‐OH‐DPAT cue. 4 To define the anxiolytic potential of MDL 73005EF, it was examined in the elevated plus‐maze test and in the water‐lick conflict test in comparison with diazepam and buspirone. In both tests MDL 73005EF induced effects similar to those seen following diazepam. Buspirone had similar effects to both MDL 73005EF and diazepam in the water‐lick conflict test but opposite effects in the elevated plus‐maze. 8‐OH‐DPAT also had opposite effects in the elevated plus‐maze test to MDL 73005EF and diazepam. 5 The anti‐conflict effects of MDL 73005EF were reversed by low doses of the 5‐HT1A receptor agonist, 8‐OH‐DPAT; those of buspirone were neither antagonised nor mimicked by 8‐OH‐DPAT. 6 These results suggest that an interaction with 5‐HT1A receptors is the basis of the anxiolytic‐like activity of MDL 73005EF. However, its mechanism of action is clearly different from that of buspirone, possibly reflecting a greater selectivity for the 5‐HT1A receptors located presynaptically on central 5‐hydroxytryptaminergic neurones.

One-trial tolerance to the effects of chlordiazepoxide on the elevated plus maze may be due to locomotor habituation, not repeated drug exposure.

The phenomenon of “one-trial tolerance” to the effects of chlordiazepoxide hydrochloride (CDP) in the elevated plus maze was re-examined. Unlike previous experiments, pre-exposure to the maze resulted in habituation and a consequential reduction in time spent on the open arms. The habituation effect was measured by recording the actual distance travelled by the rats in the maze and this was found to be significantly reduced by pre-exposure. Pre-exposure to the maze in the presence of CDP resulted in a reduced response to its “anxiolytic-like” effects (increasing time on the open arms compared to vehicle control rats). However, although the time spent on the open arms was reduced by pre-exposure, CDP significantly increased the time spent on the open arms by rats pre-exposed under a non-drugged state. These results suggest that rats do not become tolerant to the effects of CDP, but rather the reduced response to CDP after pre-exposure is due to habituation of exploratory behaviour.