LSD-potentiated apomorphine hypermotility: a model for differentiating antipsychotic drugs

Nicotine-induced enhancement of a sensory reinforcer in adult rats: antagonist pretreatment effects

RATIONALE AND OBJECTIVES

The reinforcement-enhancing effect (REE) of nicotine refers to the drug's ability to enhance the strength of other primary and conditioned reinforcers. The main aim was to investigate neuropharmacological mechanisms underlying nicotine's strengthening of a primary visual reinforcer (i.e., a light cue), using a subcutaneous (SC) dose previously shown to provide plasma nicotine levels associated with habitual smoking.

METHODS

Adult male rats pressed an "active" lever to illuminate a brief cue light during daily 60-min sessions. Rats that showed a clear REE were tested with systemically administered pretreatment drugs followed by nicotine (0.1 mg/kg SC) or saline challenge, in within-subject counterbalanced designs. Pretreatments were mecamylamine (nicotinic, 0.1-1 mg/kg SC), SCH 39166 (D1-like dopaminergic, 0.003-0.2 mg/kg SC), naloxone (opioid, 1 and 5 mg/kg SC), prazosin (alpha1-adrenergic antagonist, 1 and 2 mg/kg IP), rimonabant (CB1 cannabinoid inverse agonist, 3 mg/kg IP), sulpiride (D2-like dopaminergic antagonist, 40 mg/kg SC), or propranolol (beta-adrenergic antagonist, 10 mg/kg IP).

RESULTS

The nicotine REE was abolished by three antagonists at doses that did not impact motor output, i.e., mecamylamine (1 mg/kg), SCH 39166 (0.01 and 0.03 mg/kg), and naloxone (5 mg/kg). Prazosin and rimonabant both attenuated the nicotine REE, but rimonabant also suppressed responding more generally. The nicotine REE was not significantly altered by sulpiride or propranolol.

CONCLUSIONS

In adult male rats, the reinforcement-enhancing effect of low-dose nicotine depends on nicotinic receptor stimulation and on neurotransmission via D1/D5 dopaminergic, opioid, alpha1-adrenergic, and CB1 cannabinoid receptors.

The role of dopaminergic transmission through D1-like and D2-like receptors in amphetamine-induced rat ultrasonic vocalizations

RATIONALE

Systemic amphetamine (AMPH) administration increases the rate of 50-kHz ultrasonic vocalizations (USVs) in adult rats and preferentially enhances the 'trill' subtype; these effects of AMPH critically depend on noradrenergic transmission, but the possible contributions of dopamine are unclear.

OBJECTIVE

To assess the role of dopamine in 50-kHz USVs emitted drug-free and following systemic AMPH administration.

METHODS

Adult male Long-Evans rats pre-selected for high AMPH-induced calling rates were tested with AMPH (1 mg/kg, intraperitoneal (IP)) and saline following pretreatment with the following dopamine receptor antagonists: SCH 23390 (0.005-0.02 mg/kg, subcutaneous (SC)), SCH 39166 (0.03-0.3 mg/kg, SC), haloperidol (0.1, 0.2 mg/kg, IP), sulpiride (20-80 mg/kg, SC), raclopride (0.1-0.5 mg/kg, SC), clozapine (4 mg/kg, SC), risperidone (0.5 mg/kg, SC), and pimozide (1 mg/kg, IP). The dopamine and noradrenaline reuptake inhibitors (GBR 12909 and nisoxetine, respectively) were also tested, alone and in combination.

RESULTS

SCH 23390, SCH 39166, haloperidol, and raclopride dose-dependently inhibited vocalizations under AMPH and suppressed the proportion of trill calls. Sulpiride, however, had no discernable effect on call rate or profile, even at a high dose that reduced locomotor activity. Single doses of clozapine, risperidone, and pimozide all markedly decreased calling under saline and AMPH. Finally, GBR 12909 and nisoxetine failed to promote 50-kHz USVs detectably or alter the subtype profile, when tested alone or in combination.

CONCLUSIONS

The rate of 50-kHz USVs and the call subtype profile following systemic AMPH administration depends on dopaminergic neurotransmission through D1-like and D2-like receptors. However, inhibiting dopamine and/or noradrenaline reuptake appears insufficient to induce calling.

2-Bromolisuride, an ergot derivative, with dopamine antagonistic and serotonin agonistic properties

The open-field test was used to study the involvement of dopaminergic and serotonergic mechanisms in the effects of 2-bromolisuride on locomotor activity in the rat. 2-Bromolisuride produced a dose-dependent inhibition of spontaneous locomotor activity. This is most likely due to an antagonistic action at postsynaptic dopamine receptors. Low doses of 2-bromolisuride potentiated apomorphine-induced hypermotility. This potentiating effect was not mediated by a blockade of presynaptic dopamine receptors, because it was not prevented by 6-OHDA lesion of the nucleus accumbens. The potentiating effect of 2-bromolisuride was completely blocked by the serotonin antagonists cyproheptadine and ritanserin. It is suggested that 2-bromolisuride possesses dopamine antagonistic and serotonin agonistic properties.

Effects of apomorphine on novelty-induced place preference behavior in rats

Adult male rats were exposed to one of two different stimulus compartments by being placed into the compartment for 30 min on each of eight consecutive days. Following this exposure, each rat was administered 0, 0.05, 0.1, 0.5 or 5.0 mg/kg apomorphine. Thirty min after injection, each animal was given free-choice access to the familiar (exposed) compartment and to the novel (nonexposed) compartment. As expected, saline-injected control animals displayed a preference for the novel compartment. This novelty preference was disrupted in animals given either 0.05 or 0.1 mg/kg apomorphine, but not in animals given either 0.5 or 5.0 mg/kg apomorphine. The disruption in novelty preference by the low doses of apomorphine did not reflect a disruption of locomotor activity, as there was no direct relationship between the preference for novelty and the rate of horizontal or vertical activity among the different treatment groups. Instead, the low doses of apomorphine may have inhibited dopamine function by blocking presynaptic autoreceptors selectively, and thus the reinforcing effect of the novel stimulation may have been attenuated.

Atypical neuroleptics suppress dopaminergic behavioral supersensitivity

Seven days after bilateral 6-OHDA denervation of the nucleus accumbens locomotor activity was recorded in rats. 6-OHDA lesion strongly enhanced hypermotility induced by apomorphine (1.0 mg/kg IP) as a sign of behavioral dopaminergic supersensitivity. The potency of the classical neuroleptic haloperidol (0.03-0.25 mg/kg IP) to antagonize apomorphine-induced hypermotility was reduced in 6-OHDA-pretreated rats. The atypical neuroleptics sulpiride (5.0-20.0 mg/kg IP), thioridazine (1.0-5.25 mg/kg IP) and clozapine (0.5-2.0 mg/kg IP) and the 5-HT antagonists cyproheptadine (0.2 mg/kg IP) and ritanserin (0.01 mg/kg IP) suppressed the augmented apomorphine response in 6-OHDA-lesioned animals to the level of the apomorphine effect in controls. It is concluded that the model of denervation supersensitivity is capable of differentiating typical and atypical neuroleptics. The abolition of the 6-OHDA-induced increase of the apomorphine hypermotility by the atypical neuroleptics cannot be explained solely by postsynaptic dopamine receptor antagonism. Serotonergic mechanism may be involved in this action.

Sulpiride blocks postsynaptic dopamine receptors in the nucleus accumbens

Intra-accumbens injection of sulpiride, tiapride, and metoclopramide antagonized locomotor hyperactivity induced by intraperitoneal administration of apomorphine in rats and measured over the first five minutes after introducing the animal to an open-field cage. Sulpiride was slightly more potent than tiapride which was more than 10 times more potent than metoclopramide and haloperidol. The threshold dose of sulpiride was as low as 0.001 microgram, bilaterally. Intra-accumbens injection of sulpiride also blocked exploratory hypermotility induced by bilateral intra-accumbens injections of apomorphine and picrotoxin. The threshold dose of sulpiride for blocking these two effects was about 0.01 microgram, bilaterally. Sulpiride was more than 10 times more potent than haloperidol in blocking this apomorphine-induced hypermotility. Haloperidol did not influence the picrotoxin hypermotility. The results obtained indicate strong postsynaptic dopamine antagonist properties of sulpiride, tiapride and metoclopramide.

Locomotor effects of lisuride: a consequence of dopaminergic and serotonergic actions

The open-field test was used to study the involvement of serotonergic and dopaminergic mechanisms in the action of lisuride on locomotor activity in the rat. Lisuride produced a biphasic locomotor effect. The maximum locomotor stimulatory response of lisuride was stronger than that of apomorphine and comparable with that of apomorphine and LSD combined. Hypermotility induced by high doses of lisuride was partially suppressed by the serotonin antagonist cyproheptadine and not further enhanced by LSD. A moderate dose of lisuride potentiated apomorphine-induced hypermotility in the same manner as has been shown for LSD. Lesion of dopaminergic structures within the median raphe nucleus by 6-OHDA produced a potentiation of lisuride-induced hypermotility. This effect was suppressed by cyproheptadine. The locomotor inhibitory effect of low doses of lisuride may be related to a stimulation of presynaptic mesolimbic dopamine receptors. It is concluded that the locomotor stimulant effect of higher doses of lisuride may depend on stimulation of postsynaptic dopamine receptors and a serotonergic action and that the locomotor effects of lisuride reflect a complex interaction at dopaminergic and serotonergic transmission systems.

Drug-induced modulation of locomotor hyperactivity induced by picrotoxin in nucleus accumbens

Locomotor hyperactivity was induced in rats by bilateral injection of picrotoxin (PIC) into the nucleus accumbens (NAC) followed by intraperitoneal (IP) or intra-accumbens (IA) injection of agents affecting dopamine (DA), acetylcholine, serotonin, or GABA receptors. IP injection of haloperidol and diazepam attenuated PIC-induced hypermotility in a dose-dependent manner. Low (sedative) doses of the DA agonists apomorphine (APO) and lisuride, or pretreatment with reserpine abolished PIC-induced hypermotility. Independent of a preceding IA injection of PIC, higher IP doses of APO produced the well-known locomotor effect. LSD, and the atypical neuroleptic, sulpiride, potentiated PIC-induced hypermotility strongly whereas clozapine was ineffective. IA injection of carbachol or haloperidol, in doses which antagonized hypermotility induced by APO IP, did not influence PIC-induced hypermotility. The atypical neuroleptics, clozapine and sulpiride, and the benzodiazepine, diazepam, inhibited PIC-induced hypermotility. The results suggest that there is a complex involvement of GABA, DA and serotonin functions in the effectuation of PIC-induced hypermotility and that PIC-induced hypermotility may be affected by DA-sensitive structures situated outside the NAC.

Atypical neuroleptics clozapine and thioridazine enhance amphetamine-induced stereotypy

The effects of the atypical neuroleptics clozapine and thioridazine and the typical neuroleptic pimozide on amphetamine-induced behavior were examined. Pimozide, as expected, blocked both amphetamine-induced locomotion and stereotypy. Thioridazine and clozapine antagonized the increases in locomotion produced by amphetamine, but produced increases in amphetamine-induced stereotypy and lowered the threshold dose for stereotypy. It is suggested that the increased stereotypy might partly account for the decreased locomotion, and that this might be a primary effect of these atypical neuroleptics. The data would also suggest that the use of amphetamine-induced stereotypy as a model for psychosis is inappropriate, as clozapine and thioridazine, which enhance stereotypy, are antipsychotic.

Clozapine--a serotonin antagonist?

The effect of clozapine on the central serotonergic transmission system was studied by investigation of open-field motility of rats after microinjection of drugs into nucleus accumbens and median raphe nucleus. Previous work has shown that LSD in low doses potentiates apomorphine-induced hypermotility and that this LSD effect is induced by a serotonin agonist action in median raphe nucleus. Clozapine, injected into median raphe nucleus (0.05 micrograms), suppressed the LSD effect in the same manner as serotonin antagonists did. Since alpha-adrenergic drugs, injected into median raphe nucleus, caused locomotor stimulant effects, an alpha- adrenalytic action of clozapine was excluded. Clozapine, injected into nucleus accumbens (0.2 micrograms), increased apomorphine-induced hypermotility, whereas the dopamine antagonist haloperidol suppressed it. Our results suggest a serotonin antagonist action of clozapine.