Depletion of brain norepinephrine: differential influence on anxiolyic treatment effects.

RATIONALE: Previous studies have demonstrated that anxiolytic-like anticonflict effects can be produced by either (1) acute administration of traditional anti-anxiety compounds (benzodiazepines or barbiturates) or (2) chronic administration of tricyclic (TCA) or monoamine oxidase inhibitor (MAOI) anti-depressants. OBJECTIVE: The present study determined the effects of noradrenergic neuronal depletion on these distinct anticonflict treatments. METHODS: After 3 weeks of training in a repeated measures drink suppression conflict paradigm, water-restricted rats consumed 11-14 ml water/session (unpunished responding) and accepted 25-40 shocks/session (punished responding) during control (i.e., non-drug) 10-min test sessions. The noradrenergic neurotoxin DSP4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride; 65 mg/kg, IP] or its vehicle (saline; 1 ml/kg) was administered after 3 weeks of conflict testing. Conflict behavior was then evaluated for 8 weeks post-treatment. In separate groups of DSP4- and vehicle-pretreated subjects, the effects of acute administration (10-min pretreatment) of phenobarbital (5-40 mg/kg) or alprazolam (0.3-2.5 mg/kg) were determined. In a third experiment, the effects of chronic treatment with the TCA desipramine (DMI; 5 mg/kg, twice daily for 8 weeks) or the non-selective MAOI phenelzine (4.0 mg/kg, twice daily for 8 weeks) on conflict behavior were determined in additional groups of DSP4- or vehicle-pretreated subjects. RESULTS: DSP4 treatment produced a modest yet statistically significant decrease in punished responding (i.e., anxiogenic-like effect) relative to vehicle controls. DSP4 pretreatment did not alter the anticonflict effects of acute alprazolam or phenobarbital treatment. In contrast, DSP4 treatment completely abolished the anticonflict effects produced by chronic DMI or chronic phenelzine treatment. CONCLUSIONS: Thus, noradrenergic neuronal integrity appears to be required for the anxiolytic-like effects of chronic antidepressant treatment, but not for the anxiolytic-like effects of acute treatment with barbiturates and benzodiazepines.

Enhanced delayed matching performance in younger and older macaques administered the 5-HT4 receptor agonist, RS 17017.

Recent evidence indicates that the 5-HT4 subtype of serotonin receptor may modulate central cholinergic activity in regions of the mammalian CNS important to memory such as the frontal cortex, hippocampus and amygdala. These receptors could represent targets for drugs designed for the symptomatic therapy of Alzheimer's disease (AD) and other disorders of memory. In the present study, the binding activity of RS 17017 (previously described as a selective 5-HT4 agonist) was assessed across a number of neurotransmitter receptors and binding sites, pharmacokinetic data were obtained, and the compound was evaluated in macaques for mnemonic effects via a computer-assisted delayed matching-to-sample task (DMTS). Binding data confirmed the 5-HT4 selectivity of the compound, while pharmacokinetic results revealed low oral bioavailability, but a large volume of distribution of the compound. Significant and reproducible improvements in DMTS accuracy were observed after oral administration of the compound across a dose-effect series in both younger and older monkeys. The results suggest that RS 17017 offers a potential for memory enhancement in disorders involving cognitive decline, and are consistent with a role for central 5-HT4 receptors in memory. Improvements in DMTS performance in aged monkeys may have particular implications for neurodegenerative conditions such as AD, whereas positive results in the younger monkeys indicate that RS 17017 (or similar compounds) may have additional potential in the therapeutics of memory disorders not necessarily associated with advanced age.

The effect of novel anti-epileptic drugs in rat experimental models of acute and chronic pain.

The novel anti-epileptic drugs lamotrigine, felbamate and gabapentin were compared in rat experimental models of acute (tail flick) and chronic pain: the chronic constriction injury and spinal nerve ligation models. Lamotrigine (10-100 mg/kg, s.c.), felbamate (150-600 mg/kg, i.p.) and gabapentin (30-300 mg/kg, i.p.) each reversed cold allodynia (chronic constriction injury model) with ED50 values of 28, 241 and 103 mg/kg, respectively, 1 h post-dose. However, only gabapentin reversed tactile allodynia (spinal nerve ligation model) with an ED50 of 34 mg/kg (i.p.). The established anti-epileptic drugs, carbamazepine (1-30 mg/kg, i.p.) and phenytoin (1-100 mg/kg, s.c.), were ineffective in both models. The anti-allodynic effect of the newer anti-epileptic drugs was observed at doses that were either ineffective or produced only a negligible effect on acute nociceptive function and/or locomotor activity. In conclusion, the data suggest that the newer anti-epileptic drugs appear to have the potential to be effective alternatives to either carbamazepine or phenytoin in the treatment of neuropathic pain. However, only gabapentin ameliorated both cold and touch hyperesthesias.

RS-102221: a novel high affinity and selective, 5-HT2C receptor antagonist.

The 5-HT2C receptor is one of three closely related receptor subtypes in the 5-HT2 receptor family. 5-HT2A and 5-HT2B selective antagonists have been described. However, no 5-HT2C selective antagonists have yet been disclosed. As part of an effort to further explore the function of 5-HT2C receptors, we have developed a selective 5-HT2C receptor antagonist, RS-102221 (a benzenesulfonamide of 8-[5-(5-amino-2,4-dimethoxyphenyl) 5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione). This compound exhibited nanomolar affinity for human (pKi = 8.4) and rat (pKi = 8.5) 5-HT2C receptors. The compound also demonstrated nearly 100-fold selectivity for the 5-HT2C receptor as compared to the 5-HT2A and 5-HT2B receptors. RS-102221 acted as an antagonist in a cell-based microphysiometry functional assay (pA2 = 8.1) and had no detectable intrinsic efficacy. Consistent with its action as a 5-HT2C receptor antagonist, daily dosing with RS-102221 (2 mg/kg intraperitoneal) increased food-intake and weight-gain in rats. Surprisingly, RS-102221 failed to reverse the hypolocomotion induced by the 5-HT2 receptor agonist 1-(3-chlorophenyl)piperazine (m-CPP). It is concluded that RS-102221 is the first selective, high affinity 5-HT2C receptor antagonist to be described.

The effects of novel, selective 5-hydroxytryptamine (5-HT)4 receptor ligands in rat spatial navigation.

Activation of central 5-hydroxytryptamine (5-HT4) receptors may enhance cognitive performance. In the present study, the effects of two novel, potent and selective 5-HT4 receptor agonists, RS 67333 (1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-n-burtl-4-piperidinyl)- 1-propanone) and RS 67506 (1-(4-amino- 5-chloro-2-methoxyphenyl)-3-[1-[2-[(methylsulfonyl)amino]ethyl]-4- piperidinyl]-1-propanone), were studied in a rat model of spatial learning and memory; the Morris water maze. RS 67333 (0.1, 10 and 1000 micrograms/kg, intraperitoneally (i.p.)), a highly potent, selective and hydrophobic 5-HT4 receptor agonist, reversed the decrements in cognitive performance induced by atropine (30 mg/kg, i.p.). By contrast, no effect was seen to RS 67506 (0.1, 10 and 1000 micrograms/kg, i.p.), a hydrophilic 5-HT4 receptor agonist, of equivalent potency and selectivity to RS 67333. This differential effect may reflect the enhanced ability of RS 67333 to enter the CNS, with respect to RS 67506. The ameliorative actions of RS 67333 on cognitive dysfunction were abolished by prior treatment with a selective 5-HT4 receptor antagonist, RS 67532 [1-(4-amino-5-chloro-2-(3, 5-dimethoxy benzyloxyphenyl)-5-(1-piperidinyl)-1-pentanone; 1 mg/kg, i.p.]. When given alone, or in naive rats, RS 67532 (0.1, 10 and 1000 micrograms/kg, i.p.), was without effect. None of the compounds tested affected the swim speed at any of the doses used. In separate locomotor studies, RS 67532 reduced activity at 1 and 10 mg/kg, i.p., although no effect was seen with RS 67333 or RS 67506 (0.01-10 mg/kg, i.p.). These data suggest that RS 67333 reversed the cognitive deficit induced by atropine and support a role of 5-HT4 receptors in rat spatial learning and memory.

Assessment of the role of alpha2-adrenoceptor subtypes in the antinociceptive, sedative and hypothermic action of dexmedetomidine in transgenic mice.

1. The role of alpha2-adrenoceptor (AR) subtypes in the modulation of acute nociception, motor behaviour and body temperature, has been investigated by determining the activity of the alpha2AR selective agonist dexmedetomidine (Dex) in mice devoid of individual alpha2AR subtypes through either a point (alpha2A) or null (alpha2B/alpha2C) mutation ('knock-out'). 2. In a rodent model of acute thermal nociception, the mouse tail immersion test, Dex, in wild type (WT) control animals, produced a dose-dependent increase in the threshold for tail withdrawal from a 52 degrees C water bath with mean ED50 values of 99.9+/-14.5 (alpha2A), 94.6+/-17.8 (alpha2B) and 116.0/-17.1 (alpha2C) microg kg(-1), i.p. 3. In comparison to the WT controls, Dex (100-1000 microg kg(-1), i.p.), was completely ineffective as an antinociceptive agent in the tail immersion test in the alpha2A AR D79N mutant animals. Conversely, in the alpha2B AR and alpha2C AR knock-outs, Dex produced a dose-dependent antinociceptive effect that was not significantly different from that observed in WT controls, with ED50 values of 85.9+/-15.0 (P>0.05 vs WT control) and 226.0+/-62.7 (P>0.05 vs WT control) microg kg(-1) i.p., respectively. 4. Dex (10-300 microg kg(-1), i.p.) produced a dose-dependent reduction in spontaneous locomotor activity in the alpha2A, alpha2B and alpha2C AR WT control animals with ED50 values of 30.1+/-9.0, 23.5+/-7.1 and 32.3+/-4.6 microg kg(-1), i.p., respectively. Again, Dex (100-1000 microg kg(-1), i.p.) was ineffective at modulating motor behaviour in the alpha2A AR D79N mutants. In the alpha2B AR and alpha2C AR knock-out mice, Dex produced a dose-dependent reduction in spontaneous locomotor activity with ED50 values of 29.1+/-6.4 (P>0.05 vs WT control) and 57.5+/-11.3 (P>0.05 vs WT control) microg kg(-1), respectively. 5. Dex was also found to produce a dose-dependent reduction in body temperature in the alpha2A, alpha2B and alpha2C AR WT control mice with ED50 values of 60.6+/-11.0, 16.2+/-2.5 and 47.2+/-9.1 microg kg(-1), i.p., respectively. In the alpha2A AR D79N mutants, Dex had no effect on body temperature at a dose (100 microg kg(-1), i.p.) that produced a significant reduction (-6.2+/-0.5 degrees C; P<0.01 vs vehicle) in temperature in WT controls. However, higher doses of Dex (300 and 1000 microg kg(-1), i.p) produced a small, but statistically significant decrease in temperature corresponding to -1.7+/-0.4 degrees C and -2.4+/-0.3 degrees C (both P<0.01 vs vehicle), respectively. In the alpha2B AR and alpha2C AR knock-out mice, Dex produced a dose-dependent reduction in body temperature with ED50 values of 28.4+/-4.8 (P>0.05 vs WT control) and 54.1+/-8.0 (P>0.05 vs WT control) microg kg(-1), respectively. 6. In conclusion, the data are consistent with the alpha2A AR being the predominant subtype involved in the mediation of the antinociceptive, sedative and hypothermic actions of Dex. This profile would appear to indicate that an alpha2A AR subtype selective analgesic will have a narrow therapeutic window, particularly following systemic administration.

Developmental abnormalities and age-related neurodegeneration in a mouse model of Down syndrome.

To study the pathogenesis of central nervous system abnormalities in Down syndrome (DS), we have analyzed a new genetic model of DS, the partial trisomy 16 (Ts65Dn) mouse. Ts65Dn mice have an extra copy of the distal aspect of mouse chromosome 16, a segment homologous to human chromosome 21 that contains much of the genetic material responsible for the DS phenotype. Ts65Dn mice show developmental delay during the postnatal period as well as abnormal behaviors in both young and adult animals that may be analogous to mental retardation. Though the Ts65Dn brain is normal on gross examination, there is age-related degeneration of septohippocampal cholinergic neurons and astrocytic hypertrophy, markers of the Alzheimer disease pathology that is present in elderly DS individuals. These findings suggest that Ts65Dn mice may be used to study certain developmental and degenerative abnormalities in the DS brain.

Stereoselective effects of (R)- and (S)-zacopride on cognitive performance in a spatial navigation task in rats.

In the present studies we investigated the actions of (R)- and (S)-zacopride, potent 5-HT3 receptor antagonists with 5-HT4 receptor agonists properties, on performance in a spatial learning and memory task in rats, the Morris water maze. A significant cognitive/performance deficit, as indicated by the increased escape latency across several trials, was produced by systemic administration of the muscarinic receptor antagonist atropine (30 mg/kg, IP). (R)-zacopride (0.001-1 microgram/kg, but not 10 or 100 micrograms/kg) significantly reduced escape latency in atropine-treated animals. (S)-Zacopride was inactive over the entire dose range examined (0.001-100 micrograms/kg, i.p.). Moreover, pretreatment with (S)-zacopride (1 or 100 micrograms/kg) did not alter the procognitive effects of (R)-zacopride (1 microgram/kg). These data demonstrate that the cognition enhancing properties of zacopride in this model of cholinergic hypofunction are exclusive to its (R)-enantiomer and imply that this action is unrelated to 5-HT, receptor antagonism or 5-HT4 receptor agonism. The possibility that the procognitive effects of (R)-zacopride may be related to actions at the novel "(R)-zacopride site" is discussed.

Ondansetron improves cognitive performance in the Morris water maze spatial navigation task.

In the present studies we investigated the actions of ondansetron, a prototypic 5-hydroxytryptamine3 (5-HT3) receptor antagonist, on performance in a complex spatial navigation/memory task in rats. Specifically, we compared the activity of ondansetron to that of the cholinesterase inhibitor physostigmine in attenuating two distinct cognitive deficits in the Morris water maze. In the first model, rats treated with the muscarinic receptor antagonist atropine (30 mg/kg) had significantly longer latencies to find the submerged platform across two days of testing. Physostigmine (0.03, 0.1 and 0.3 mg/kg) and ondansetron (0.03-1 mg/kg) significantly reduced the latencies to find the submerged platform in atropine-treated animals, suggesting an increase in cognitive performance. There was little evidence of a dose-response relationship for either compound, and a loss of efficacy for ondansetron was seen at 3 mg/kg. In the second model, pre-screened, aged (23 months), cognition-impaired and nonimpaired rats were tested. Ondansetron (0.1 mg/kg), but not physostigmine (0.1 mg/kg), decreased the latencies to find the submerged platform in the aged-impaired rats, while neither compound improved performance of aged-nonimpaired rats. These data suggest that ondansetron may have cognition enhancing properties in animal models of aging and cholinergic hypofunction.

Characterization of the electrophysiological, biochemical and behavioral actions of epibatidine.

Epibatidine has been reported to be a potent, nonopioid analgesic. In this study we further characterized its receptor interactions and its analgesic properties. Radioligand binding assays demonstrated that epibatidine has high affinity for nicotinic receptors (Ki = 0.12 nM) but low affinity for opioid and other receptors (Ki > 3.0 microM). In vitro functional assays demonstrated that the compound is a potent agonist at both neuronal and neuromuscular nicotinic receptors. Epibatidine depolarized rat isolated vagus nerve with an EC50 of 33.1 nM and contracted guinea pig ileum with an EC50 of 6.1 nM. Epibatidine contracted frog rectus abdominis muscle with an EC50 of 18.2 nM. In vivo, epibatidine demonstrated short-lived analgesic actions. Epibatidine (10 and 30 micrograms/kg), at 5 but not 20 min after dosing, increased the threshold for vocalization evoked by foot shock. Epibatidine, at 5 and 20 but not 60 min after dosing, also increased the latency to a nociceptive response in a hot-plate assay. Both (+)- and (-)-enantiomers of epibatidine were active in these assays. The action of epibatidine in the hot-plate test was reversed by the nicotinic receptor antagonist mecamylamine but not by the opioid receptor antagonist naloxone. In contrast to morphine, epibatidine failed to increase locomotor activity. These findings demonstrate that epibatidine is a potent agonist at both neuronal and neuromuscular nicotinic receptors. These findings also demonstrate a short-lived, naloxone-insensitive, analgesic action for both the (+)- and (-)-enantiomers of epibatidine.(ABSTRACT TRUNCATED AT 250 WORDS)