Role of α4- and α6-containing nicotinic receptors in the acquisition and maintenance of nicotine self-administration.

Tobacco smoking is a major cause of death and disease and as such there is a critical need for the development of new therapeutic approaches to treat nicotine addiction. Here, we utilize genetic and pharmacological tools to further investigate the nicotinic acetylcholine receptor (nAChR) subtypes that support intravenous self-administration of nicotine. α4-S248F mice contain a point mutation within the α4 nAChR subunit which confers increased sensitivity to nicotine and resistance to mecamylamine. Here, we show that acute administration of mecamylamine (2 mg/kg, i.p.) reduces established nicotine self-administration (0.05 mg/kg/infusion) in wild-type (WT), but not in α4-S248F heterozygous mice, demonstrating a role for α4* nAChRs in the modulation of ongoing nicotine self-administration. Administration of N,N-decane-1,10-diyl-bis-3-picolinium diiodide (bPiDI), a selective α6ß2* nAChR antagonist, dose dependently (5 and 10 mg/kg, i.p.) impairs the acquisition of nicotine self-administration and reduces established nicotine self-administration in WT mice when administered acutely (10 mg/kg, i.p.). This was not due to a general reduction in locomotor activity and the same dose of bPiDI did not affect operant responding for sucrose. bPiDI treatment (10 mg/kg, i.p.) also impaired both the acquisition and maintenance of nicotine self-administration in α4-S248F heterozygous mice. This provides further evidence for the involvement of α6ß2* nAChRs in the reinforcing effects of nicotine that underlies its ability to support ongoing self-administration. Taken together, selective targeting of α6ß2* or α4α6ß2* nAChRs may prove to be an effective strategy for the development of smoking cessation therapies.

Proconvulsant-induced seizures in alpha(4) nicotinic acetylcholine receptor subunit knockout mice.

The genetic basis of a number of epilepsy syndromes has been identified but the precise mechanism whereby these mutations produce seizures is unknown. Three mutations of the alpha(4) subunit of the neuronal nicotinic acetylcholine receptor (nAChR) have been identified in autosomal dominant nocturnal frontal lobe epilepsy. In vitro studies of two mutations suggest an alteration of receptor function resulting in decreased ion channel current flow. We investigated the response of alpha(4) nAChR subunit knockout mice to the gamma-aminobutyric acid (GABA) receptor antagonists; pentylenetetrazole (PTZ) and bicuculline (BIC), the glutamate receptor agonist kainic acid (KA), the glycine receptor antagonist strychnine and the K(+) channel blocker 4-aminopyridine (4-AP). Mutant (Mt) mice had a greater sensitivity to PTZ and BIC, with an increase in major motor seizures and seizure-related deaths. Furthermore, Mt mice were more sensitive to KA and strychnine, but the effects were much smaller compared to those seen with the GABA receptor antagonists. Paradoxically, Mt mice appeared to be relatively protected from 4-AP-induced major motor seizures and death. The results show that a functional deletion of the alpha(4) nAChR subunit in vivo is associated with a major increase in sensitivity to GABA receptor blockers.

Essential conservation of D1 mutant phenotype at the level of individual topographies of behaviour in mice lacking both D1 and D3 dopamine receptors.

RATIONALE: In the absence of agonists and antagonists evidencing appropriate selectivities, individual and interactive properties of D(1) and D(3) dopamine receptors would be illuminated most powerfully by their co-deletion. OBJECTIVES: To define and contrast the behavioural phenotype of D(1)/D(3) double knockout mice in comparison with wild types, and with individual D(1) and D(3 )mutants. METHODS: Behavioural phenotype was characterised using an ethologically based topographical technique. RESULTS: On comparison with wild types, D(1)/D(3) double mutants were characterised topographically as follows: increases in sniffing and locomotion, which evidenced delayed habituation; reductions in rearing free, rearing seated, grooming, chewing and stillness. Though the D(1)/D(3) double mutant ethogram comprised elements of both single mutant D(1) and D(3) lines, this phenotype was largely reflective of the D(1) mutant component. CONCLUSIONS: Distinct patterns of initial exploratory behaviour and of temporal change over subsequent habituation were evident across the three genotypes, with particular conservation of the D(1) phenotype in D(1)/D(3 )double mutants. Under the present conditions, there was little systematic evidence for D(1):D(3) interactions in the regulation of these aspects of behaviour.

Neurochemical changes in dopamine D1, D3 and D1/D3 receptor knockout mice.

Neurochemical changes were examined in dopamine D1 receptor knockout (D1(-/-)), dopamine D3 receptor knockout (D3(-/-)) and dopamine D1/D3 receptor double knockout (D1(-/-)D3(-/-)) mice. The level of dopamine D1- and D2-like receptors and gamma-aminobutyric acid (GABA(A)) receptor was assessed by ligand autoradiography and dopamine D1- and D2 receptor, enkephalin, dynorphin and substance P transcripts measured by in situ hybridization. D1(-/-) mice had normal GABA(A) receptor levels, reduced dynorphin and substance P, and increased enkephalin mRNA and dopamine D2-like binding. D1(-/-)D3(-/-) mice evidenced decreased dynorphin and substance P but normal enkephalin expression, whereas dopamine D2-like and GABA(A) receptor binding were increased. Major changes occur in substance P and dynorphin expression in D1(-/-) mice and these changes are unaffected by loss of dopamine D3 receptors. Upregulated dopamine D2-like binding and enkephalin in D1(-/-) mice may be due to decreased dopamine turnover. Upregulated enkephalin in D1(-/-) mice is dependent on functional dopamine D3 receptors.