Chronic nicotine and dizocilpine effects on regionally specific nicotinic and NMDA glutamate receptor binding

Chronic nicotine administration has long been known to increase the number of high-affinity alpha4beta2 nicotinic receptors with lesser effects on low-affinity alpha7 nicotinic receptors. Nicotine has been shown to promote the release of a variety of neurotransmitters including glutamate. Nicotine may also interact directly with the glutamatergic receptors. Nicotinic-glutamate interactions may be critical to the long-term effects of nicotine. Conversely, glutamatergic drugs may interact with the nicotinic system. Such interactions have important implications in interpretation of the mechanism of drug actions, especially when the drugs are given together. The current study examined the effects of chronic administration of nicotine (5 mg of the nicotine base/kg/day for 28 days), dizocilpine (MK-801) (0.3 mg/kg/day for 28 days), an NMDA receptor antagonist, as well as the combination of the two drugs on nicotinic and NMDA receptor densities in discrete brain regions. The chronic dose of dizocilpine used was behaviorally active causing a dramatic reduction in prepulse inhibition (PPI) of acoustic startle response. The nicotine dose used did not significantly affect PPI but previously we have found it to be behaviorally active in improving working memory function. High-affinity nicotinic receptor binding, as has been seen previously, was significantly increased by chronic nicotine in most areas. Chronic dizocilpine alone did not affect high-affinity nicotinic receptor binding, but it did modify the effects of chronic nicotine, attenuating nicotine-induced increases in the frontal cortex and striatum. Low-affinity nicotinic binding was significantly increased by chronic nicotine in only one area, the cerebellum. Chronic dizocilpine significantly increased low-affinity nicotinic binding in several brain areas, the colliculi, hippocampus, and the hypothalamus. The combination of nicotine and dizocilpine attenuated the effects of each with diminished nicotine-induced increased nicotinic low-affinity binding in the cerebellum and diminished dizocilpine-induced increased nicotinic low-affinity binding in the hippocampus and hypothalamus. In contrast, chronic nicotine and dizocilpine had a mutually potentiating effect of increasing nicotinic low-affinity binding in the frontal cortex. NMDA receptor binding was affected only in the hippocampus, where both dizocilpine and nicotine significantly increased binding. Chronic nicotine effects on receptor regulation are significantly affected by concurrent blockade of NMDA glutamate receptors.