Pharmacological comparison of transient and persistent [3H]dopamine release from mouse striatal synaptosomes and response to chronic L-nicotine treatment.

L-Nicotine stimulates a biphasic release of [3H]dopamine from mouse striatal synaptosomes which does not persist after agonist is removed. Approximately 80% of the initial release is transient and disappears with a half-time of less than 1 min; the other 20% persists for several minutes (t(1/2), 5-10 min). Both the transient and persistent phases were investigated by 10-min exposures to agonists with an in vitro perfusion technique. A series of nicotinic agonists and antagonists were used to determine the pharmacological relationship of the two phases. Parameters measured included EC50 and Vmax values and desensitization rates for both phases for agonists, Ki values for antagonists and Ki values for low concentrations of agonists. The results are consistent with both phases being mediated by a single type of receptor. In addition, the effects of chronic nicotine treatment on transient and persistent [3H]DA release were measured. For both phases, release was decreased approximately 15% by chronic infusion of 4.0 mg/kg/hr L-nicotine. Correlation of the results with inactivation of a portion of the receptors rather than a reversible desensitization is discussed.

ST/b and DBA/2 mice differ in brain alpha-bungarotoxin binding and alpha 7 nicotinic receptor subunit mRNA levels: a quantitative autoradiographic analysis.

Inbred mouse strains vary in sensitivity to a number of behavioral and physiological effects produced by nicotine. Differences in sensitivity to nicotine are correlated with variance in the number of brain nicotinic receptors as measured in regionally dissected brain tissue. The studies reported here used quantitative autoradiography and in-situ hybridization methods to measure regional levels of alpha-bungarotoxin (alpha BTX) binding and alpha 7 mRNA levels. Two inbred mouse strains, ST/b and DBA/2, were compared because these strains differ maximally in sensitivity to nicotine-induced seizures and in alpha BTX binding measured in regional brain homogenates. The binding of alpha BTX was significantly greater in the St/b strain in 42 of 127 brain regions that were analyzed, and a trend towards increased binding was seen in many additional brain regions. The most consistent strain differences were found in hippocampal, thalamic and pontine nuclei. Strain differences in alpha 7 mRNA levels were also detected, but these were not as widespread as were the alpha BTX binding differences. The alpha 7 mRNA levels were significantly correlated with alpha BTX binding in both mouse strains which suggests that the strain differences in binding are related, in part, to the levels of alpha 7 mRNA.

Changes in sensitivity to nicotine and brain nicotinic receptors following chronic nicotine and corticosterone treatments in mice.

Chronic nicotine treatment often results in tolerance to nicotine as well as increases in brain [3H]-nicotine binding and [125l]-alpha-bungarotoxin (alpha-BTX) binding. Chronic corticosterone (CCS) treatment also produces tolerance to nicotine, but it does not change [3H]-nicotine binding; decreases in alpha-BTX binding are observed, which suggests that tolerance to nicotine may be related to decreases in the number of this nicotinic receptor subtype. In the studies reported here, C57BL/6 mice were implanted subcutaneously with cholesterol or 60% CCS/40% cholesterol-containing pellets and were infused continuously with saline (control) or nicotine for a total of 9 days. Effects of acute nicotine challenge on Y-maze crossing and rearing activities, heart rate, and body temperature were measured. Both chronic nicotine and CCS treatment resulted in tolerance to nicotine for all of the measures, and some evidence for additivity was seen in the animals that were cotreated with CCS and nicotine. Chronic nicotine infusion increased brain nicotine binding and CCS treatment reduced alpha-BTX binding. Decreases in alpha-BTX binding were not detected in the cotreated animals. The latter finding argues that changes in alpha-BTX binding are not reliable predictors of or a cause of tolerance to nicotine.

Corticosterone reversibly alters brain alpha-bungarotoxin binding and nicotine sensitivity.

Previous studies have shown that chronic corticosterone (CCS) treatment via subcutaneous pellets elicits reduced sensitivity to many actions of nicotine in mice as well as decreased brain alpha-bungarotoxin (alpha-BTX) binding. We report here the time courses of altered sensitivity to nicotine, as measured by acoustic startle, Y-maze crossing and rearing activities, heart rate, and body temperature, and alpha-BTX binding during and after CCS treatment. CCS treatment resulted in rapid decreases in sensitivity to nicotine for four of the five responses that were measured, as well as rapid changes in alpha-BTX binding. Sensitivity to nicotine returned to control levels within 3 days following pellet removal, but alpha-BTX binding returned to control levels in most brain regions 9-11 days after pellet removal. Because the restoration of control sensitivity to nicotine occurred long before alpha-BTX binding returned to control levels, it seems likely that factors other than changes in alpha-BTX binding cause chronic CCS-induced changes in sensitivity to nicotine.

Tolerance to nicotine following chronic treatment by injections: a potential role for corticosterone.

C57BL/6 male mice were injected intraperitoneally with nicotine (2.0 mg/kg) or saline three times each day (0800 h, 1300 h and 1800 h) for a period of 12 days and then tested for nicotine tolerance using a series of behavioral and physiological tests. For each of these tests, animals that received chronic nicotine treatment were significantly less sensitive to nicotine challenge than were animals that received chronic saline treatment, as indicated by shifts to the right of dose-response curves. Animals were retested for nicotine sensitivity 2 weeks following cessation of chronic nicotine injections. Tolerance to acute nicotine challenge persisted in nicotine-treated animals. Chronic nicotine treatment by injections did not alter the binding of L-[3H]-nicotine or alpha-[125I]-bungarotoxin in any of eight brain regions. Plasma corticosterone (CCS) levels were determined in animals prior to the initiation of the injection series (day 0), and on days 4, 8 and 12 of chronic treatment, immediately before the first injection of the day. CCS levels in nicotine-treated animals were elevated as compared to saline-injected controls by day 12 of treatment. Nicotine-treated animals also had elevated CCS levels 2 weeks after the last chronic injection. Nicotine-treated animals were, however, tolerant to nicotine-induced CCS release. Since previous studies from our laboratory have demonstrated that plasma CCS levels are inversely correlated with sensitivity to nicotine, it is possible that the tolerance to nicotine measured following chronic treatment by injections is due, at least in part, to the elevation in plasma CCS levels.

Chronic corticosterone administration modulates nicotine sensitivity and brain nicotinic receptor binding in C3H mice.

Glucocorticoid regulation of nicotine sensitivity was investigated in adrenalectomized (ADX) and sham-operated C3H mice administered chronic corticosterone (CCS) replacement therapy. Hormone pellets (60% CCS or pure cholesterol) were implanted at the time of surgery and animals were tested for nicotine sensitivity in a battery of behavioral and physiological tests. ADX-induced increases in nicotine sensitivity were reversed by chronic CCS replacement. Sham-operated animals that received CCS supplementation were subsensitive to the effects of nicotine. In both ADX and sham-operated animals, chronic CCS administration induced a decrease in the number of CNS nicotinic cholinergic receptors labeled by alpha-[125I]-bungarotoxin. Binding was decreased by 30-60% depending on brain region; no changes in affinity (KD) were detected. The number of brain nicotinic sites labeled by [3H]-nicotine was unaltered following 1 week of chronic CCS administration. These data support the hypothesis that glucocorticoids modulate nicotine sensitivity in the C3H mouse. In animals chronically treated with CCS, nicotine tolerance may be due to CCS-induced changes in nicotinic cholinergic receptor binding or the presence of high CCS titers at the time of testing.