A comparison of multiple injections versus continuous infusion of nicotine for producing up-regulation of neuronal [³H]-epibatidine binding sites

Chronic nicotine exposure in the rat produces a characteristic increase in neuronal nicotinic binding sites in many brain regions. The conventional method for inducing such increases utilizes twice daily subcutaneous injections of a near maximal, sub-convulsive dose of nicotine. Alternatively, nicotine may be chronically infused via an osmotic mini-pump. However, little is known about how administration of nicotine by chronic infusion compares to multiple injections in producing nicotinic receptor upregulation. This study used [³H]-epibatidine, a high potency neuronal nicotinic agonist radioligand, to compare the increases in receptor levels in rat brain, spinal cord and trigeminal ganglion tissues following chronic nicotine administration via either twice daily injections (2 mg/kg s.c.) or an osmotic mini-pump (1 mg/kg/hr) for 10 days. All central and peripheral nervous system tissues examined demonstrated significant neuronal nicotinic receptor up-regulation following chronic infusion of nicotine. Only the cerebral cortex and hippocampus displayed significant up-regulation following nicotine administration by injections. Moreover, in all tissues studied, the receptor levels measured were significantly higher in the animals that received nicotine by chronic infusion compared with multiple injections. These data indicate that chronic infusion of nicotine is a convenient and efficacious alternative to multiple injections for producing neuronal nicotinic receptor up-regulation in both central and peripheral nervous tissues.