Influence of chronic administration of nicotine on the turnover and metabolism of noradrenaline in the rat brain

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

The enhancement of retention induced by vasopressin in mice may be mediated by an activation of central nicotinic cholinergic mechanisms

Immediate post-training subcutaneous administration of lysine vasopressin (LVP, 0.003-1.00 microgram/kg) enhanced retention, whereas the vasopressin antagonist AAVP (0.01-0.30 microgram/kg) impaired it, in male Swiss mice tested 48 h after training in an inhibitory avoidance task. Both effects were dose-dependent. Neither LVP nor AAVP affected response latencies in mice not given the footshock on the training trial. The simultaneous administration of AAVP at a dose (0.01 microgram/kg) which had no effect on retention shifted the dose-response curve of LVP to the right. Nicotine (1.0-30.0 micrograms/kg, sc), a central nicotinic cholinergic agonist, also facilitated retention in a dose-related manner without affecting the retention performance of unshocked mice. The effect of nicotine was prevented by the central acting nicotinic cholinergic receptor antagonist mecamylamine (5 mg/kg, sc.). In contrast, neither hexamethonium (5 mg/kg, sc), a peripheral acting nicotinic receptor blocker, nor atropine (0.5 mg/kg, sc) or methylatropine (0.5 mg/kg, sc), two anticholinergic drugs which are known to act on muscarinic cholinergic receptors, prevented the effect of post-training nicotine. The effects of LVP and nicotine were time-dependent, suggesting that both treatments enhanced retention by influencing post-training processes involved in memory storage. Low doses of nicotine (1.50 microgram/kg, sc) or the central anticholinesterase physostigmine (35 micrograms/kg, sc) and LVP (0.003 microgram/kg, sc), which had no effect on retention when administered alone, produced a synergistic interaction when given together following training. The influence of LVP (0.03 microgram/kg, sc) on retention was prevented not only by AAVP (0.01 microgram/kg, sc) but also by mecamylamine (5 mg/kg, sc), whereas the effects of nicotine (10.0 micrograms/kg, sc) were prevented only by mecamylamine. These results suggest that the enhancement of retention induced by vasopressin is probably due to an activation of central nicotinic cholinergic mechanisms which are critical for memory formation.

Pharmacology of nicotine

In recent years progress in basic neuropsychopharmacology and clinical addiction research have allowed the conclusion that tobacco smoking essentially represents an addiction to nicotine. Parallel to this work, experimental research in biochemistry, physiology and pharmacology has provided detailed descriptions of the structure and function of the nicotinic receptor, the biologic mediator of the many actions of nicotine. This article reviews current knowledge of nicotinic mechanisms in the peripheral and central nervous systems as well as some implications for the notion of smoking as an addiction to nicotine. In particular this review will focus on the effects of nicotine on brain dopamine and noradrenaline systems since these neuronal systems appear to be crucially involved in the rewarding and stimulant effects of addictive drugs.

Role of primary sensory neurons in the central effects of nicotine

Utilizing single unit recording techniques the nicotine-induced excitation of noradrenaline (NA)-containing neurons in the locus coeruleus (LC) was analyzed. Low doses of nicotine (40-160 micrograms, IV) were found to dose-dependently increase the LC firing rate. The effect was antagonized by pretreatment with the quaternary ganglionic blockers hexamethonium (12 mg/kg, IP) and chlorisondamine (0.3 mg/kg, IV). Also, neonatal treatment with capsaicin, a procedure that is associated with a selective degeneration of primary sensory C-fibre afferents, clearly antagonized the effect of nicotine on LC neurons. The typical effect of nicotine on LC discharge was, in all essentials, mimicked by the quaternary nicotinic agonist tetramethylammonium (TMA). We here propose that the action of nicotine on central NA neurons is primarily executed peripherally via activation of primary sensory C-fibre afferents.

Alteration of alpha and muscarinic receptors in rat brain and heart following chronic nicotine treatment

Adrenergic and muscarinic binding sites in 4 brain regions (cerebral cortex, corpus striatum, hypothalamus/thalamus and brainstem) and in heart ventricles were measured in rats chronically treated with nicotine added to the drinking water in doses ranging from 6 to 8 mg/kg/day, for 4 weeks. Control rats received only tap water. The nicotine treatment led to increases in the specific binding of both [3H]prazosin and [3H]clonidine in the cerebral cortex. An increase in [3H]prazosin binding was also observed in the hypothalamus/thalamus of nicotine-treated rats. These changes were all due to an increase of about 23% in Bmax. In the brainstem and heart left ventricle, respectively, an increase and a decrease in the affinity of [3H]quinuclidinyl benzilate binding were observed. There were no changes of the binding parameters for the 3 radioligands in other regions tested, and no alteration of [3H]dihydroalprenolol binding was detected in any region examined. These results indicate that chronic administration of nicotine causes an increase in the density of alpha 1- and alpha 2-binding sites in some brain regions and reciprocal changes of the affinity of muscarinic binding sites in the brain and in the heart.

Effects of nicotinic and muscarinic compounds on biting attack in the cat

Predatory-like biting attack on a rat, as well as hissing, growling, and other threat behaviors, could be induced in normally non-aggressive cats by systemic administration of the muscarinic agonist, arecoline (7-12 mg/kg). In contrast to arecoline, nicotine was found to suppress aggressive behaviors. Systemic administration of nicotine (0.5 mg/kg) prior to arecoline injection resulted in a significant reduction in elicited attack and threat behaviors. Furthermore, nicotine (0.075-0.500 mg/kg) was found to produce a dose-dependent suppression of natural predatory behavior as well. This nicotine-produced suppression of attack did not appear to be due to the induction of general malaise, since attack suppression could be seen in the absence of general behavioral inhibition, and doses of nicotine resulting in complete suppression of attack had little effect on food intake. Results indicate that muscarinic and nicotinic compounds can exert antagonistic control over some types of aggressive behaviors.

Nicotine-induced hypophagia and hypodipsia in deprived and in hypothalamically stimulated rats

Nicotine, in doses of 0.15 and 0.45 mg/kg, induced hypophagia and hypodipsia in 20 hrs-deprived rats and elevated the threshold currents for hypothalamically induced feeding and drinking in satiated rats. These effects were blocked by pretreatment with 0.5 mg/kg mecamylamine, but not by pretreatment with 3.0 or 9.0 mg/kg hexamethonium. These results indicate a centrally-located mechanism for the hypophagic and hypodipsic effect of single injections of nicotine.