Mapping the effects of the selective dopamine D2/D3 receptor agonist quinelorane using pharmacological magnetic resonance imaging.

Dopamine agonists with a high affinity for D2 and D3 receptors have a biphasic effect on rodent locomotion, inducing hypolocomotion at low doses and hyperlocomotion at higher doses. Controversy surrounds the role of the D3 receptor in mediating the hypolocomotor response to low agonist doses. This study examines patterns of neuronal activation induced by varying doses of the D2/D3 receptor agonist quinelorane using blood oxygen level dependent (BOLD) pharmacological magnetic resonance imaging (phMRI), and compares them with corresponding behavioural responses. Quinelorane (3 microg/kg) induced hypolocomotion in rats naive to the testing environment, and in phMRI experiments increased neuronal activity within the anterior olfactory nuclei, nucleus accumbens and islets of Calleja, regions containing a high density of D3 receptors. A 30 microg/kg dose of quinelorane resulted in biphasic locomotor effects, with initial hypolocomotion followed by sustained hyperlocomotion. phMRI indicated that this higher dose increased cerebral activity within limbic and olfactory regions, as did the lower drug dose, but induced additional activation in the caudate-putamen and globus pallidus, areas dense in D2 receptors but containing few D3 receptors. The more restricted pattern of activation at low agonist doses and close temporal relationship between behavioural and BOLD signal responses to quinelorane suggest that those nuclei most dense in D3 receptors play a key role in mediating the hypolocomotor effects of quinelorane. However, the presence of D3 receptors in activated brain regions may be coincidental, and further studies are required to show definitively which class of receptors mediates agonist-induced hypolocomotion. In contrast, the activation of D2 receptors within the striatum appears necessary for quinelorane-induced hyperlocomotion.

Long-term evaluation of isolation-rearing induced prepulse inhibition deficits in rats: an update.

RATIONALE: Rats reared in social isolation from weaning show prepulse inhibition (PPI) deficits which are thought to model the sensorimotor gating deficits seen in schizophrenia and other psychiatric disorders. We have previously reported that ten cohorts of Lister Hooded rats reared in isolation showed robust and reliable PPI deficits. OBJECTIVE: Our methodology differed from those used by others (Weiss and Feldon in Psychopharmacology 156(2-3):305-326, 2001), most notably in the weaning of pups at postnatal day (PND) 28 compared with PND20-22. Since our initial report, we have studied 18 more cohorts weaned at PND28 and one cohort weaned at PND21. METHOD: At weaning, male Lister Hooded pups were singly (isolates) or group (n=5) housed (grouped). Eight weeks later, startle and PPI responses of isolates and grouped rats were investigated using conditions of fixed inter-stimulus interval (ISI) (pulse=110 dB/50 ms; prepulse (PP)=75-80 dB/30 ms; ISI=100 ms). RESULTS: Isolates from 14 of the subsequent 18 cohorts demonstrated PPI deficits, giving an overall success rate of 86% for all 28 cohorts. %PPI ranged from 12 to 26% in the isolates and from 26 to 47% in the grouped for the successful cohorts, compared to 16-30% (isolates) and 19-35% (grouped) for those that failed. Only five out of the 19 subsequent cohorts demonstrated startle hyperreactivity, which was unrelated to PPI response. The isolates from the cohort weaned at PND21 did not show a significant deficit in PPI, suggesting, in our hands at least, a requirement for weaning at PND28. CONCLUSION: The data presented here reinforce our original findings that isolation-rearing of Lister Hooded rats provides a viable, non-pharmacological model of impaired PPI.

Effect of haloperidol and (-)-sulpiride on dopamine agonist-induced hypoactivity.

High doses of dopamine D2 receptor agonists produce hyperactivity in rodents whereas low doses suppress activity. In this study, low doses of a range of dopamine agonists were examined for their effects on locomotor activity in rats. All agonists caused a dose-related hypolocomotor effect with a rank order of potency of quinelorane > (-)-quinpirole > 7-OHDPAT > PBTO. (-)-Sulpiride (1.6-160 mumol/kg), a neuroleptic with atypical properties caused a dose-related reversal of the hypolocomotor effect produced by all four agonists whereas the typical neuroleptic haloperidol (0.005-0.16 mumol/kg) did not reverse hypolocomotion. Neither sulpiride (5-16 mumol/kg) nor haloperidol (0.005-0.16 mumol/kg) affected locomotor activity per se, although haloperidol (1.6-5 mumol/kg) did reduce locomotor activity. The different behavioural profiles shown by (-)-sulpiride and haloperidol in these tests may reflect some of the clinical characteristics of these neuroleptics. The question of whether these effects can be ascribed to differential actions at dopamine receptor subtypes will only be answered when more selective dopamine receptor antagonists are developed.

Isoarecolone can inhibit nicotine binding and produce nicotine-like discriminative stimulus effects in rats.

Isoarecolone methiodide has been reported previously to be a potent agonist at peripheral nicotinic-cholinergic receptors. Both isoarecolone methiodide and isoarecolone HCl can produce contractures of the frog rectus abdominis muscle and can inhibit binding of [3H]-(-)-nicotine to rat brain membranes, although the methiodide is much more potent than the hydrochloride. In rats trained to discriminate the effects of nicotine from saline, there is generalization to isoarecolone HCl at doses that reduce overall rates of responding. This effect and the similar relative potencies of isoarecolone and nicotine in the biochemical and behavioural procedures support the view that the high-affinity binding site for [3H]-(-)-nicotine is the receptor mediating the discriminative effect.

Chlorisondamine blocks acquisition of the conditioned taste aversion produced by (-)-nicotine.

Intraventricular microinjection (5 micrograms) of the bisquaternary ganglion-blocking drug chlorisondamine prevented acquisition of nicotine-induced conditioned taste aversion (CTA) in rats. This appeared to be a specific effect because chlorisondamine did not attenuate the conditioned taste aversion caused by apomorphine. These results support findings from other behavioural procedures indicating that centrally administered chlorisondamine causes a long-term blockade of central nicotinic mechanisms.

High affinity binding of [3H] (-)-nicotine to rat brain membranes and its inhibition by analogues of nicotine.

Analysis of the characteristics of cerebral binding of [3H] (-)-nicotine revealed a single population of sites with high affinity (KD = 6.0 +/- 0.6 nM). The regional distribution of the binding of [3H] (-)-nicotine was heterogeneous with the largest concentration of binding sites being in the thalamus, cortex and striatum, and a low level of binding activity in the cerebellum and hypothalamus. Competition studies showed that several metabolites and congeners of nicotine potently competed with [3H] (-)-nicotine and their in vitro activity was correlated with behavioural activity, as estimated from previously published data for rats trained to discriminate central effects of nicotine. Conversely, nicotine antagonists, with the exception of dihydro-beta-erythroidine, were weak or inactive in this binding assay. It is concluded that the binding site for [3H] (-)-nicotine investigated probably mediates at least one of the behavioural effects of nicotine, the nicotine discrimative stimulus.

Behavioural and pharmacokinetic studies on nicotine, cytisine and lobeline.

Previous work has suggested that cytisine and lobeline are of low potency in producing nicotine-like behavioural effects, despite having some nicotine-like peripheral effects and potently inhibiting the binding of tritiated nicotine to the brain of the rat. Rats were trained to discriminate nicotine from saline in a two-bar operant conditioning procedure with food reinforcement. It was confirmed that cytisine had a nicotine-like discriminative effect, but it was much less potent than nicotine itself. Lobeline failed to produce a nicotine-like discriminative effect, even at doses that greatly reduced overall rates of responding. Neither drug attenuated discriminative responses to nicotine. The concentrations of drugs in plasma and brain were determined by HPLC in rats of the same sex, strain and age as those used in the behavioural experiments. The rank order of the ratios of concentrations in brain to plasma was lobeline greater than nicotine greater than cytisine, which was directly proportional to their lipophilicity determined by reversed-phase HPLC. Based on the concentrations in brain and known affinities for high-affinity nicotine binding sites, in vivo tests should show cytisine to be slightly more potent than nicotine and lobeline to have nicotine effects in the doses used. These predictions were not fulfilled and thus, the behavioural effects of cytisine and lobeline cannot be correlated with their effects at the binding site for tritiated nicotine. Since pharmacokinetic factors do not account for this discrepancy, a pharmacodynamic explanation will be necessary.

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.

SB-243213; a selective 5-HT2C receptor inverse agonist with improved anxiolytic profile: lack of tolerance and withdrawal anxiety.

SB-243213 (5-methyl-1-[[-2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-6-trifluoromethylindoline hydrochloride) is a new, selective 5-hydroxytryptamine (5-HT)2C receptor inverse agonist. SB-243213 has high affinity for the human 5-HT2C receptor (pK(i) 9.37) and greater than a 100-fold selectivity over a wide range of neurotransmitter receptors, enzymes and ion channels. In in vitro functional studies, SB-243213 acted as an inverse agonist at the human 5-HT2C receptor with a pK(b) of 9.8. In in vivo studies, SB-243213 was a potent inhibitor of central 5-HT2C receptor-mediated function in rats, blocking meta-chlorophenylpiperazine-induced hypolocomotion with an ID50 of 1.1 mg/kg p.o. and a long duration of action (>8 h). In rats, SB-243213 exhibited anxiolytic-like activity in both the social interaction and Geller-Seifter conflict tests. Importantly, unlike diazepam, chronic administration of SB-243213 did not result in the development of either tolerance to the anxiolytic-like effects or withdrawal anxiogenesis. Furthermore, in rodents, SB-243213 did not affect seizure threshold, did not increase body weight or induce catalepsy, but attenuated the haloperidol-induced catalepsy. SB-243213 did not affect amphetamine-, MK-801- or phencyclidine-induced hyperactivity. In conclusion, SB-243213 may possess an improved anxiolytic profile compared to benzodiazepines. SB-243213 also modulates dopaminergic transmission, lacks pro-psychotic properties and may have utility in the treatment of schizophrenia and motor disorders.

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.

Localization of serotonin-4 receptors in the striatonigral pathway in rat brain.

Rats were injected unilaterally with 6-hydroxydopamine either in the medial forebrain bundle or in the dorsolateral substantia nigra. Another group was injected unilaterally with kainate in the striatum. The loss of neurons was assessed by a reduction in tyrosine hydroxylase-like immunoreactivity for dopaminergic neurons, and choline acetyltransferase-like and glutamate decarboxylase-like immunoreactivities for cholinergic and GABAergic neurons, respectively. Brain sections also were analysed by autoradiography on 20 micron sections with the radio-iodinated serotonin-4 receptor antagonist [125I]SB 207710 [Brown A. M. et al. (1993) Br. J. Pharmac. 110, 10P]. Kainate injections in the striatum resulted in loss of choline acetyltransferase- and glutamate decarboxylase-like immunoreactive cell bodies in this area. There was also a decrease in glutamate decarboxylase-like immunoreactivity on the ipsilateral side in the substantia nigra and entopeduncular nucleus. These changes were accompanied by substantial (> 50%) decreases in [125I]SB 207710 binding in both the ipsilateral striatum (confined to the lesioned area) and substantia nigra, with no change in either the nucleus accumbens or the globus pallidus. There was also significant loss of [125I]SB 207710 binding in the ipsilateral entopeduncular nucleus. 6-Hydroxydopamine lesions placed either in the medial forebrain bundle or in the substantia nigra failed to decrease [125I]SB 207710 binding in any of these areas, although there was total loss of tyrosine hydroxylase-like immunoreactive terminals in the striatum and cell bodies in the nigra. We conclude that serotonin-4 receptors are present on projection neurons, both on their perikarya in the striatum and terminals in the nigra and entopeduncular nucleus. It is likely that these receptors are located on the GABAergic projection neurons and possibly on cholinergic and GABAergic interneurons. However, serotonin-4 receptors are not located on dopaminergic neurons, either on their cell bodies in the substantia nigra or terminals in the striatum.

Up-regulation of secretoneurin immunoreactivity and secretogranin II mRNA in rat striatum following 6-hydroxydopamine lesioning and chronic L-DOPA treatment.

Destruction of the nigro-striatal pathway in Parkinson's disease and treatment with L-DOPA lead to persistent alterations in basal ganglia output pathways that are poorly characterised. Differential display mRNA analysis was used to study the effects of 6-hydroxydopamine-induced lesions of the medial forebrain bundle on gene expression in the rat striatum. One up-regulated cDNA identified in two independent groups of 6-hydroxydopamine-lesioned animals was cloned and sequence analysis showed 97% homology to secretogranin II. Differential up-regulation of secretogranin II following 6-hydroxydopamine lesioning was confirmed in a further group of 6-hydroxydopamine-lesioned rats using TaqMan real time quantitative reverse transcription-polymerase chain reaction. Following chronic L-DOPA treatment of 6-hydroxydopamine-lesioned rats, secretogranin II mRNA was further up-regulated to a similar degree to that observed for preproenkephalin A mRNA expression. Immunohistochemical analysis confirmed the increase in secretogranin II peptide levels in striatal neurones in 6-hydroxydopamine-lesioned rats following chronic L-DOPA treatment. The increase in secretogranin II mRNA occurring following destruction of the nigro-striatal pathway and chronic L-DOPA treatment may result in an increase in secretoneurin levels, which could be important for the regulation of striatal output pathways.

Locomotor activity in rats after administration of nicotinic agonists intracerebrally.

1. Nicotine (0.13 and 0.4 mg kg-1, s.c.) increased the ambulatory component of locomotor activity in rats previously exposed to the drug. Nicotine did not increase repeated movements reliably. 2. An infusion of either nicotine (8 micrograms) or the potent nicotinic agonist cytisine (4 micrograms) into the ventral tegmental area of the forebrain increased ambulation but not repeated movements. 3. An infusion of nicotine or cytisine into the nucleus accumbens, striatum, dorsal hippocampal formation or motor thalamus did not increase ambulatory or repeated movements. 4. Mecamylamine (0.1-1.0 mg kg-1, s.c.) blocked increases in locomotor activity produced by an infusion of nicotine or cytisine into the ventral tegmental area. 5. The locomotor activity produced by systemically administered nicotine may be mediated, in part, through nicotinic receptors located in the ventral tegmental area of the mesolimbic dopamine system.

Nicotine cue in rats: effects of central administration of ganglion-blocking drugs.

In rats trained to discriminate nicotine from saline, a single intraventricular injection of a small dose of the quaternary ganglion-blocking drug chlorisondamine blocked the response to nicotine for four weeks. pentolinium was only weakly active and hexamethonium was inactive as a nicotine antagonist under the conditions used, even in doses that were just below those producing myoclonic jerks. Chlorisondamine had no blocking effect in rats trained to discriminate the non-nicotinic drugs midazolam or morphine from saline. Intraventricular injections of chlorisondamine have a specific and unusually persistent nicotine-blocking action, the mechanism of which requires further investigation.

Attenuation of haloperidol-induced catalepsy by a 5-HT2C receptor antagonist.

Atypical neuroleptics produce fewer extrapyramidal side-effects (EPS) than typical neuroleptics. The pharmacological profile of atypical neuroleptics is that they have equivalent or higher antagonist affinity for 5-HT2 than for dopamine D2 receptors. Our aim was to identify which 5-HT2 receptor contributed to the atypical profile. Catalepsy was defined as rats remaining immobile over a horizontal metal bar for at least 30 s, 90 min after dosing. Radioligand binding assays were carried out with homogenates of human recombinant 5-HT2A, 5-HT2B and 5-HT2C receptors expressed in Human Embryo Kidney (HEK293) cells. Haloperidol (1.13 mg kg(-1) i.p.) induced catalepsy in all experiments. The selective 5-HT2C/2B receptor antagonist, SB-228357 (0.32-10 mg kg(-1) p.o.) significantly reversed haloperidol-induced catalepsy whereas the 5-HT2A and 5-HT2B receptor antagonists, MDL-100907 (0.003-0.1 mg kg(-1) p.o.) and SB-215505 (0.1-3.2 mg kg(-1) p.o.) respectively did not reverse haloperidol-induced catalepsy. The data suggest a role for 5-HT2C receptors in the anticataleptic action of SB-228357.

Differentiation of dopamine agonists using drug-induced rotation in rats with unilateral or bilateral 6-hydroxydopamine destruction of ascending dopamine pathways.

Eighteen compounds with dopamine agonist properties were examined in two rat rotational models. In the classical Ungerstedt model, a unilateral 6-hydroxydopamine (60HDA) lesion destroyed nigrostriatal and mesolimbic dopamine pathways on one side. Indirectly acting compounds, amphetamine, amantadine, methylphenidate and S1694, produced ipsiversive rotation, which was inhibited by pretreatment with alpha-methyl-p-tyrosine (AMPT). All other compounds produced contraversive rotation, but the rotation caused by CM 29-712, bromocriptine and ET 495 was reduced by AMPT. In animals with a bilateral 60HDA lesion removing both dopaminergic inputs to nucleus accumbens and the dopaminergic input into one striatum, indirectly acting drugs caused ipsiversive posturing prevented by AMPT, but little rotation. All other compounds produced contraversive rotation, but the effects of CM 29-712, bromocriptine and ET 495 were reduced by AMPT pretreatment. Inhibition of monooxygenase drug metabolising activity utilising SKF-525A inhibited contraversive turning induced by bromocriptine and ET 495 in the unilateral lesion model, but had no effect on rotation caused by apomorphine or CM 29-712. We conclude that, in addition to indirect pre-synaptically acting agonists and direct post-synaptic receptor agonists, there are a small group of compounds which produce rotation associated with direct receptor activation, which is inhibited by disruption of pre-synaptic dopamine function. The mechanism of action of this latter group is not understood, but cannot be attributed solely to active metabolite formation.

Repeated administration of N-methyl-4-phenyl 1,2,5,6-tetrahydropyridine to rats is not toxic to striatal dopamine neurones.

N-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine ( MPTP ) (10 mg/kg/day i.p.) was administered to rats for 16 days, which were then observed for a further 9-11 days. MPTP administration did not alter spontaneous locomotor activity or amphetamine (2.5 mg/kg ip)-induced locomotion. Apomorphine (0.25 mg/kg sc) did not alter locomotion in control rats but increased activity in MPTP treated animals. The stereotyped response to apomorphine (0.25 mg/kg sc) and amphetamine (2.5 and 5.0 mg/kg ip) was unaltered by MPTP administration. The striatal content of dopamine, HVA and DOPAC was unaltered by MPTP intake. The uptake of [3H]dopamine and [3H] 5HT in striatal synpatosomes was not changed by MPTP . The results suggest that MPTP , in the dose used, is not toxic to nigro-striatal dopamine neurones in the rat. This contrasts with its neurotoxic actions in monkeys and man.

Long-term evaluation of isolation-rearing induced prepulse inhibition deficits in rats.

RATIONALE: Rats reared in social isolation from weaning show prepulse inhibition (PPI) deficits which are thought to model the sensorimotor gating deficits seen in schizophrenia and other psychiatric disorders. However, recent studies have questioned the robustness of this paradigm. OBJECTIVE: The existence of a substantial dataset generated over 4 years in our laboratory has allowed the investigation of the robustness and reliability of the procedure under a variety of environmental conditions. The effects of atypical antipsychotics (clozapine, olanzapine and risperidone) under different experimental conditions are also reported. METHOD: At weaning, Hooded Lister pups were singly (isolates) or group (n=5) housed (grouped). Eight weeks later, the startle and PPI response of isolates and grouped rats was investigated using conditions of fixed inter-stimulus interval (ISI) (pulse=110 dB/50 ms; prepulse=80 dB/30 ms; ISI=100 ms) or variable

Behavioural effects of the nicotinic agonists N-(3-pyridylmethyl)pyrrolidine and isoarecolone in rats.

The nicotinic agonists N-(3-pyridylmethyl)pyrrolidine (PMP) and isoarecolone have been compared with (-)-nicotine in three behavioural procedures and as inhibitors of [3H]-(-)-nicotine binding. Locomotor activity was recorded as movements between beams of infra-red light (ambulation). In experimentally naive rats, nicotine, PMP and isoarecolone reduced ambulation. In rats receiving nicotine chronically and previously exposed to the activity cages, nicotine and PMP increased ambulation in a dose-related manner. However, isoarecolone did not increase ambulation at doses that were active in other procedures. In rats trained to discriminate nicotine from saline in a two-bar operant conditioning procedure with food reinforcement, there was full generalization to PMP. It was also found that PMP potently inhibited the binding of [3H]-(-)-nicotine to rat brain membranes in vitro. These results were discussed with previous data for the discriminative stimulus and ligand-binding effects of isoarecolone obtained under similar conditions. The relative potency of PMP in different behavioural procedures was similar to that of (-)-nicotine. However, isoarecolone was relatively 10 times more potent in decreasing ambulation than would have been expected from its potency in the ligand-binding and discriminative stimulus procedures. The results suggest that the pharmacodynamic action of isoarecolone differs from that of nicotine and PMP, and that it will be a useful probe in further analyses of central nicotinic mechanisms.

Nicotine and some related compounds: effects on schedule-controlled behaviour and discriminative properties in rats.

Behavioural effects of d- and l-nicotine, d- and l-nornicotine and l-cotinine were studied in two paradigms. In experiment 1, rats responded under a multiple fixed-interval (FI) 5 min, fixed-ratio (FR) 20 schedule of food presentation. Aside from differences in potency and time course, l-nicotine and the stereoisomers of nornicotine produced qualitatively similar effects on rates of responding. With increasing doses of drugs, FI response rates first increased and then decreased, while FR response rates only decreased. In contrast, d-nicotine did not significantly increase FI response rates at lower doses, and only decreased FI and FR response rates at higher doses. At doses up to 100 mg/kg, cotinine produced only dose-dependent increases in FI response rates and had no effect on FR response rates. Rate-increasing effects of cotinine were not blocked by mecamylamine. In experiment 2, rats were trained to discriminate between l-nicotine (0.1 mg/kg SC) and saline (0.1 ml/kg SC) in a two-bar, operant conditioning procedure under a tandem variable-interval (VI) 1 min, FR 10 schedule of food presentation. Full generalization was obtained to d-nicotine and to l- and d-nornicotine. Generalization to cotinine occurred only with large doses that contained significant amounts of nicotine present as an impurity. There was no generalization to non-nicotinic drugs (morphine and clenbuterol), even at doses that reduced response rates.(ABSTRACT TRUNCATED AT 250 WORDS)