Effect of nicotine and other drugs on the release of 3H-norepinephrine and 3H-dopamine from rat brain slices

Effect of nicotine on dopaminergic-cholinergic interaction in the striatum

We have investigated the effect of nicotinic receptor stimulation on acetylcholine (ACh) release measured by radioassay in rat striatal slices. Since the release of ACh in the striatum is tonically inhibited by endogenous dopamine and nicotine enhances the release of dopamine, we studied the release of ACh when the dopaminergic input was impaired. We used chemical denervation (6-hydroxydopamine pretreatment) or D2-receptor-blockade by sulpiride to remove the dopaminergic control of the cholinergic neurons. In our experiments nicotine failed to increase ACh release from striatal slices taken from rats whose dopaminergic-cholinergic interaction was not impaired but it enhanced the release of ACh from slices dissected from 6-hydroxydopamine pretreated rats or in the presence of sulpiride. Our results provide neurochemical evidence for the existence of nicotinic receptors on striatal cholinergic interneurons. Since the spontaneous release of ACh enhanced by nicotine was inhibited by tetrodotoxin it seems very likely that (-)-nicotine acts on the somatodendritic part of cholinergic interneurons.

Effects of chronic and subchronic nicotine on tyrosine hydroxylase activity in noradrenergic and dopaminergic neurones in the rat brain

Chronic nicotine (0.8 mg/kg by daily subcutaneous injection) over a 7 to 28-day period was found to increase the activity of tyrosine hydroxylase in predominantly noradrenergically innervated regions but not in dopaminergic projection areas. Increases in tyrosine hydroxylase activity were observed in dopaminergic cell body regions only after nicotine treatment for 3 to 5 days. The increase in tyrosine hydroxylase activity in noradrenergic neurones was evident first in the cell bodies in the locus coeruleus from 3 to 7 days, reaching 223% of control activities, and was followed by increases of up to 205% in the terminals up to 3 weeks later. It was then established that nicotine for 7 days was sufficient to increase the activity of the enzyme to the same extent in the terminals at 21 days even without further nicotine administration. This is consistent with axonal transport preceded by induction of the enzyme in noradrenergic cell bodies, whereas "delayed activation" might account for the transient effect seen in dopaminergic cell body regions. The response in the locus coeruleus to nicotine for 7 days was completely blocked by daily preinjection with mecamylamine but not with hexamethonium, which is consistent with the effect of nicotine on tyrosine hydroxylase being mediated by central nicotinic receptors.

Characterization of somatodendritic neuronal nicotinic receptors located on the myenteric plexus

The effects of nicotine and dimethylphenylpiperazinium (DMPP) on resting and stimulation-evoked release of [3H]-acetylcholine ([3H]ACh) from cholinergic interneurons and neuro-effector neurons of the ileal longitudinal muscle and the responses of the smooth muscle to nicotinic agonists were studied. (-)-Nicotine was 15 times more effective than (+)-nicotine in releasing ACh. Since tetrodotoxin (1 microM) completely antagonized the effect of nicotinic agonists, the site of action of the nicotinic agonists studied was on the somatodendritic nicotinic receptors. The electrical field stimulation-evoked release was not affected by nicotinic agonists and antagonists, indicating that the axon terminals of cholinergic interneurons are not equipped with nicotinic receptors. This preparation proved to be useful to study the effect of nicotinic agonists on somatodendritic receptors, to determine the affinity constants of nicotinic antagonists, and to characterize these receptors. The rank order of antagonists was d-tubocurarine = mecamylamine greater than pipecuronium greater than pancuronium greater than vecuronium greater than hexamethonium; the apparent affinity constants (KD) were 1.15, 1.55, 3.06, 3.98, 13.59 and 32.88 microM, respectively. alpha-Bungarotoxin had no antagonistic activity at all. This finding indicates that nicotine and the endogenous ligand ACh act via a postsynaptic, somatodendritic nicotinic receptor that is pharmacologically similar to those located on the axon terminals of sympathetic neurons or in ganglions, but is dissimilar to those located at the postsynaptic site of the neuromuscular junction.

Effects of various experimental manipulations on neostriatal acetylcholine and dopamine release

The release of endogenous acetylcholine and dopamine and the appearance of their metabolites, choline and dihydroxyphenylacetic acid (DOPAC), from neostriatal slices prepared from Fischer 344 rats was examined under various experimental conditions. There was a dose-dependent increase in the amount of neurotransmitter or metabolite as the medium potassium concentration was increased from 5 to 50 mM. Over an eight minute period in Krebs Ringer bicarbonate buffer containing 25 mM potassium, the rate of release of acetylcholine was 6 to 13 times greater than that of dopamine. The dopamine endogenous to the slice preparation appeared to have little effect on the release of endogenous acetylcholine since manipulations that significantly altered dopamine release (depletion with 6-hydroxydopamine or uptake inhibition with nomifensine) had minimal effects on the cholinergic neurons. In contrast, increasing the endogenous acetylcholine in the preparation by inhibiting acetylcholinesterase resulted in a 1.2 to 12 fold increase in dopamine release depending upon the incubation time and the potassium concentration. These studies indicate that within the neostriatal slices there is minimal influence of the endogenous dopamine on the cholinergic neurons, whereas the extracellular acetylcholine can influence dopamine release when its concentration is increased by inhibition of acetylcholinesterase.

Pharmacologically defined M1 and M2 muscarinic cholinergic binding sites in the cat's substantia nigra: development and maturity

Muscarinic cholinergic binding in the substantia nigra of the cat was documented during development and at maturity with autoradiographic methods by labeling the pharmacologically defined M1 and M2 subtypes of muscarinic binding sites. In cats from age embryonic day 40 to postnatal day 6 and at adulthood, M1 sites were labeled with [3H]pirenzepine and M2 sites were labeled with [3H]N-methylscopolamine in competition with pirenzepine. Comparisons were made among binding site distributions, acetylcholinesterase staining and tyrosine hydroxylase-like immunoreactivity in serial or neighboring nigral tissue sections. M1 and M2 binding sites were present in the substantia nigra at all ages studied. Qualitative comparisons showed that M1 binding delineated the substantia nigra more distinctly than did M2 binding. For M1 binding sites in particular, the embryonic pars reticulata of the substantia nigra was more prominently labeled than the pars compacta. At adulthood both nigral subdivisions clearly exhibited M1 and M2 binding, with the pars compacta demonstrating some internal heterogeneity of binding density. These findings provide further evidence that the substantia nigra is a site of cholinergic transmission and suggest that the functional balance between acetylcholine and dopamine in the basal ganglia acts here as well as in the striatum.

Nicotine indirectly inhibits [3H]dopamine uptake at concentrations that do not directly promote [3H]dopamine release in rat striatum

The effects of both (-)- and (+)-nicotine isomers were examined on in vitro uptake and release of [3H]dopamine in rat striatum. Both isomers inhibited uptake of [3H]dopamine in chopped tissue at concentrations well below those necessary for promoting release of preloaded [3H]dopamine. (-)-Nicotine was more potent than (+)-nicotine both at inhibiting uptake and at promoting release. Unlike other dopamine uptake inhibitors, however, nicotine inhibited only 50% of the total uptake. In the presence of 1 nM nicotine, the residual [3H]dopamine uptake was less sensitive to inhibition by cocaine than uptake in the absence of nicotine. Nicotine did not compete against the binding of [3H]GBR 12935, a selective dopamine uptake inhibitor. The nicotinic receptor agonists carbachol and 1,1-dimethyl-4-phenylpiperazinium iodide also inhibited uptake, whereas the nicotinic antagonists chlorisondamine and mecamylamine blocked nicotine's effect. Thus, the effect of nicotine on dopamine uptake appears to be mediated by a receptor similar to the nicotinic acetylcholine receptor. These receptors do not seem to be on the terminals that are accumulating dopamine, however, since tetrodotoxin prevented the effect of nicotine on [3H]dopamine uptake and nicotine had no effect on uptake in a synaptosomal preparation.

Chronic administration of nicotine fails to alter the MPTP-induced neurotoxicity in mice

1. The effects of chronic (14 day) administration of nicotine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (15 mg/kg, s.c.)-induced neurotoxicity in C57BL/6 mice were examined. 2. Nicotine pretreatment failed to alter the deficit in locomotor activity and the reduction in striatal levels of dopamine produced by MPTP. 3. Our results do not support a therapeutic action of nicotine in a Parkinsonian animal model.

Prejunctional nicotinic receptors involved in facilitation of stimulation-evoked noradrenaline release from the vas deferens of the guinea-pig

In guinea-pig prostatic vas deferens loaded with [3H]-noradrenaline ([3H]-NA), nicotinic receptor agonists, nicotine and dimethylphenylpiperazinium (DMPP) enhanced the resting and facilitated the stimulation-evoked release of [3H]-NA in a concentration-dependent fashion. The effect of nicotine on both contraction of vas deferens and release of NA in response to field stimulation was stereospecific in favour of the naturally occurring (-)-enantiomer. Prolonged (15 min) exposure to (-)-nicotine resulted in a cessation of the facilitatory effect on NA release and on responses of the vas deferens to field stimulation. 2 The rank order of agonist potency in facilitating NA release was DMPP = (-)-nicotine greater than (+)-nicotine. Cytisine had no agonistic activity. The dissociation constants (KD) of antagonists were 9.3 +/- 0.6 and 31.4 +/- 2.4 microM for (+)-tubocurarine and hexamethonium, respectively, when (-)-nicotine was used as agonist. alpha-Bungarotoxin had no antagonistic activity. These findings suggest that nicotinic receptors located on noradrenergic axon terminals are different from those located postsynaptically in striated muscle or ganglia but seem similar to those present on cholinergic axon terminals at the neuromuscular junction. 3. Cotinine, the breakdown product of nicotine failed to have any agonistic activity indicating that nicotine itself is responsible for the effects observed on axon terminals. 4 Stimulation of presynaptic muscarinic receptors by oxotremorine prevented the nicotine-induced facilitation of [3H]-NA release, indicating the presence of both inhibitory muscarinic and facilitatory nicotinic receptors on noradrenergic axon terminals.

Cholinergic-dopaminergic interactions in cognitive performance

Both acetylcholinergic (ACh) and dopaminergic (DA) systems have been found to be crucial for the maintenance of accurate cognitive performance. In a series of studies examining those aspects of cognitive function revealed by the radial-arm maze, we have found that these two neurotransmitter systems interact in a complex fashion. Choice accuracy deficits in the radial-arm maze can be induced by blockade of either muscarinic- or nicotinic-ACh receptors. The choice accuracy deficit induced by blockade of muscarinic receptors with scopolamine can be reversed by the DA receptor blocker, haloperidol. The specific DA D1 blocker SCH 23390 also has this effect, whereas the specific D2 blocker raclopride does not, implying that it is D1 blockade that is critical for reversing the scopolamine effect. On the other hand, the choice accuracy deficit induced by nicotinic blockade with mecamylamine is potentiated by haloperidol. This effect is also seen with the D2 antagonist raclopride, but not with the D1 antagonist SCH 23390, implying that it is the D2 receptor which is important for the potentiation of the mecamylamine effect. The relevance of the D2 receptor for nicotinic actions on cognitive function is emphasized by the finding that the selective D2 agonist LY 171555 reverses the choice accuracy deficit caused by mecamylamine. Nicotinic and muscarinic blockade are synergistic in the deficit they produce. Antagonist doses subthreshold when given alone produce a pronounced impairment when given together. This latter deficit can be reversed by the D2 agonist LY 171555. These studies have outlined the complex nature of ACh-DA interactions with regard to cognitive function. Possible neural circuits for these interactions are discussed. The effectiveness of these selective DA treatments in reversing cognitive deficits due to ACh underactivation suggests a novel approach to treating cognitive dysfunction in syndromes such as Alzheimer's disease.

Chronic nicotine treatment changes differentially the effects of acute nicotine on the three main dopamine metabolites in mouse striatum

The effect of chronic treatment with nicotine on striatal dopamine metabolism was studied in mice by measuring the striatal concentrations of dopamine and its metabolites 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). (-)-Nicotine was administered for 7 days using subcutaneously implanted nicotine releasing reservoirs. The release of nicotine was confirmed by measuring nicotine and cotinine concentrations in the plasma. To study the possible tolerance induced by chronic nicotine treatment, acute challenge doses of (-)-nicotine (either 3 mg/kg given once or 1 mg/kg repeated 4 times at 30 min intervals) were given to mice on the 7th day after the implantation. At an ambient temperature of 20-22 degrees C, acute nicotine treatment induced marked hypothermia (-5.2 to -6.7 degrees C) in both chronic nicotine treated and control mice, an effect that was prevented by elevating the ambient temperature to 32-34 degrees C. Chronic nicotine treatment did not per se alter striatal dopamine metabolism. Acute nicotine administration altered the striatal dopamine metabolism in a temperature-dependent manner. In mice kept at 20-22 degrees C, the DOPAC concentration rose slightly but concentrations of 3-MT and HVA fell, indicating a decrease in the release of dopamine. In contrast, in mice kept at 32-34 degrees C the DOPAC and HVA concentrations were clearly elevated by acute nicotine, whereas the concentration of 3-MT was not altered. In these normothermic mice chronic nicotine pretreatment did not alter the effects induced by acutely administered nicotine.(ABSTRACT TRUNCATED AT 250 WORDS)

Microinjections of a nicotinic agonist into dopamine terminal fields: effects on locomotion

Nicotine induces locomotion, a behavior associated with the mesocorticolimbic dopamine system. The present study determined the effects on locomotion of direct microinjections of the nicotinic agonist cytisine into four DA terminal fields were nicotinic receptors have been localized: nucleus accumbens (NAS, n = 20), caudate putamen (CPU, n = 9), olfactory tubercle (OT, n = 8), and medial prefrontal cortex (MPC, n = 12). Male Long-Evans rats were injected with cytisine (0.1, 1, 10 and 100 nanomoles per 0.5 microliters per side) or vehicle through indwelling cannulae, and locomotor activity was recorded during a 60-minute test session; each animal was tested with each dose in counterbalanced order. NAS injections of the three highest doses of cytisine increased locomotion relative to vehicle injections; injections in the CPU, dorsal to the NAS, were ineffective, as were MPC and OT injections. The data support the notion that systemic nicotine may interact with dopaminergic projections to the NAS to produce increases in locomotor activity.

Interaction between cholinergic and adrenergic pathways of the hypothalamic ventromedial nucleus on cardiovascular regulation

In this work we studied the possible interaction between cholinergic (muscarinic and nicotinic) and adrenergic (alpha 1- and beta-adrenergic) pathways of the hypothalamic ventromedial nucleus on the regulation of arterial pressure and heart rate in conscious normotensive rats. Mean arterial pressure and heart rate were recorded in rats with cerebral chronic stainless steel cannulae implanted directly into the ventromedial nucleus. The changes in arterial pressure and heart rate produced by the injection of the cholinergic agonist (carbachol or nicotine) into the ventromedial nucleus were studied before and after the injection of prazosin (an alpha 1-adrenergic antagonist) or propranolol (a beta-adrenergic antagonist) into this same area. The injection of carbachol (2 nmol) or nicotine (40 mmol) into the ventromedial nucleus induced pressor and tachycardia responses. Previous treatment with prazosin or propranolol blocked the pressor response to carbachol and nicotine. Propranolol also abolished the tachycardic response to carbachol or nicotine, but prazosin reduced only the tachycardia produced by carbachol into the ventromedial nucleus. These results show an interaction between cholinergic and adrenergic pathways of the ventromedial nucleus affecting cardiovascular regulation and suggest that the alpha 1- and beta-adrenoceptors of this nucleus are involved in these responses.

Nicotine-induced catecholamine synthesis after lesions to the dorsal or ventral noradrenergic bundle

The role of projections from coerulear (A6) and lateral tegmental (A1-A5) noradrenergic cell groups in the induced catecholamine response to (-)-nicotine was studied following lesions to the dorsal (DNAB) or ventral (VNAB) noradrenergic bundle by 6-hydroxydopamine. The lesions produced large reductions in basal noradrenaline levels in hippocampus (after DNAB lesions) and hypothalamus (after VNAB lesions), while not affecting basal levels of dopamine or 5-hydroxytryptamine. Vehicle and sham operated controls showed a significant increase in DOPA accumulation in response to (-)-nicotine (0.8 mg/kg s.c.) following inhibition of amino acid decarboxylase. In DNAB lesioned rats, the response induced by (-)-nicotine in both the hippocampus and hypothalamus was significantly attenuated, whereas in VNAB lesioned rats the induced response was still evident. The effect of (-)-nicotine was also studied in the nucleus accumbens and was found not to be affected by either lesion. These data suggest that increases in catecholamine synthesis in the hippocampus and hypothalamus reflect increased noradrenaline synthesis, and that this effect occurs specifically in noradrenergic projections originating in the locus coeruleus.

Locomotion induced by ventral tegmental microinjections of a nicotinic agonist

Bilateral microinjections of the nicotinic agonist cytisine (0.1, 1 or 10 nanomoles per side) into the ventral tegmental area increased locomotor activity. This increase in locomotion was antagonized by mecamylamine (2 mg/kg, IP), a nicotinic antagonist that readily crosses the blood-brain barrier, and by pimozide (0.3 mg/kg, IP), a central dopaminergic antagonist. Hexamethonium (2 mg/kg, IP), a nicotinic antagonist that, unlike mecamylamine, does not cross the blood-brain barrier, had no effect; this suggests that mecamylamine's attenuation of cytisine-induced locomotor activity resulted from a blockade of central and not peripheral nicotinic receptors. The data support the notion that nicotinic and dopaminergic substrates interact at the level of the VTA to produce increases in locomotor activity.

Nicotinic modulation of [3H]dopamine release from striatal synaptosomes: pharmacological characterisation

Presynaptic nicotinic acetylcholine receptors on striatal nerve terminals modulate the release of dopamine. We have compared the effects of a number of nicotinic agonists and antagonists on a perfused synaptosome preparation preloaded with [3H]dopamine. (-)-Nicotine, acetylcholine, and the nicotinic agonists cytisine and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), at micromolar concentrations, stimulated the release of [3H]dopamine from striatal nerve terminals. Carbamylcholine was a much weaker agonist. The actions of (-)-nicotine, cytisine, and DMPP were inhibited by low concentrations of the nicotinic antagonists dihydro-beta-erythroidine, mecamylamine, pempidine, and neosurugatoxin; alpha-bungarotoxin was without effect, and extending the time of exposure to this toxin resulted in only very modest inhibition. This pharmacology points to a specific nicotinic receptor mechanism that is clearly distinct from that at the neuromuscular junction. Atropine failed to antagonise the effects of acetylcholine and carbamylcholine, suggesting that no muscarinic component is involved. The nicotinic receptor ligands (-)-[3H]nicotine and 125I-alpha-bungarotoxin bound to specific sites enriched in the synaptosome preparation. Drugs tested on the perfused synaptosomes were examined for their ability to interact with these two ligand binding sites in brain membranes. The differential sensitivity to the neurotoxins alpha-bungarotoxin and neosurugatoxin of the 125I-alpha-bungarotoxin and (-)-[3H]nicotine binding sites, respectively, leads to a tentative correlation of the (-)-[3H]nicotine site with the presynaptic nicotinic receptor on striatal nerve terminals.

Aversion instead of preference learning indicated by nicotine place conditioning in rats

Although nicotine is a drug of abuse for millions of smokers, it has been difficult to demonstrate clearly the motivational properties of nicotine with rats using the conditioned place preference (CPP) paradigm. The first experiment attempted to replicate CPPs reported by other researchers using nicotine doses of 0.4, 0.8, and 1.2 mg/kg. There was a trend for all three doses to produce aversions, but it was significant only for the 0.8 mg/kg dose. Exposures to the CS alone extinguished aversions, but a "priming" dose (0.2 mg/kg) of nicotine given after extinction produced aversions only in animals exposed to 1.2 mg/kg. Experiment 2 tested whether preexposure to morphine or nicotine would sensitize animals to nicotine's reinforcing effects. In this experiment, rats were exposed to either six nicotine (0.6 mg/kg) or morphine (1.0 mg/kg) dosings prior to preference conditioning. Neither preferences nor aversions were observed in any group following subsequent conditioning with 0.6 mg/kg nicotine. The results suggest that previous observations of preference effects may have been due to specific procedural factors or may have depended on negative reinforcement due to stress reduction.

Pharmacological activity of N-methyl-carbamylcholine, a novel acetylcholine receptor agonist with selectivity for nicotinic receptors

N-Methyl-carbamylcholine (also called N-methyl-carbachol) is an analogue of the mixed muscarinic-nicotinic acetylcholine receptor agonist, carbachol. Previous studies have provided evidence that radiolabelled N-methyl-carbachol can bind selectively to nicotinic acetylcholine receptors in rat brain. To determine whether N-methyl-carbachol acts as an agonist or an antagonist at nicotine and/or muscarinic receptor sites, the present study examined the pharmacological activity of this compound on some cholinergically innervated tissues. N-Methyl-carbachol, like carbachol, depolarized rat isolated sympathetic ganglia and these effects were inhibited by a nicotinic antagonist, d-tubocurarine, but not by a muscarinic antagonist, atropine. Exposure of rat sympathetic ganglia to N-methyl-carbachol blocked the compound action potential generated in ganglia by stimulation of the pre-ganglionic trunk; this effect of N-methyl-carbachol was likely due to desensitization of the nicotinic response. N-Methyl-carbochol, like carbachol, stimulated the release of [3H]noradrenaline from cultured adrenal medullary cells that had been pre-loaded with [3H]noradrenaline; these effects were largely inhibited by a nicotinic antagonist, mecamylamine, while atropine produced less blockade. N-Methyl-carbachol contracted the frog isolated rectus abdominis muscle and the effect was completely blocked by d-tubocurarine. By contrast, contracture of the rectus abdominis produced by carbachol was partially inhibited by either atropine or d-tubocurarine. N-Methyl-carbachol, like carbachol, contracted the rat isolated ileum and these effects were completely blocked by atropine; however, N-methyl-carbachol was about 42 times less potent than carbachol for this effect. Intravenous injection of N-methyl-carbachol, like nicotine, to the rat produced a transient decrease followed by a more sustained rise in blood pressure while carbachol produced only a sustained decrease in blood pressure. The effects of N-methyl-carbachol and nicotine on blood pressure were blocked by pretreatment of the animal with a nicotinic antagonist, hexamethonium. N-methyl-carbachol, like nicotine, stimulated the release of [3H]dopamine from rat striatal synaptosomes, pre-loaded with [3H]dopamine; release induced by either N-methyl-carbachol or nicotine was inhibited by mecamylamine but not by atropine. In rat cerebral cortical slices pre-loaded with [3H]inositol, carbachol, but not N-methyl-carbachol, stimulated the accumulation of [3H]inositol-1-phosphate, an effect blocked by atropine but not by mecamylamine.(ABSTRACT TRUNCATED AT 400 WORDS)

A direct nicotinic receptor-mediated inhibition recorded intracellularly in vitro

Acetylcholine activates both nicotinic and muscarinic receptors in the central nervous system. Although the action of acetylcholine at muscarinic receptor has been well characterized, relatively little is known at the cellular level concerning nicotinic receptor stimulation in brain. Central nicotinic receptors have been implicated in Alzheimer's disease, seizure activity, the generation of slow-wave theta rhythm in the hippocampus and the potential abuse liability of nicotine. At the neuronal level, nicotinic agonists have been most often associated with postsynaptically mediated excitation and membrane depolarization at various sites, including Renshaw spinal motoneurons, locus coeruleus and the medial habenular nucleus. Nicotine acting presynaptically can produce either excitation or inhibition indirectly through the release of endogeneous transmitters or modulators. Whereas a direct inhibitory effect of nicotine has been suggested by one in vivo extracellular recording study in rat cerebellar Purkinje neurons, the mechanism(s) underlying this action is not yet known. We now report our findings obtained using in vitro intracellular methods in a submerged brain slice preparation in which application of nicotinic agonists to rat dorsolateral septal neurons reveal a direct membrane hyperpolarization mediated by an increase in potassium conductance.

Regionally specific effects of acute and chronic nicotine on rates of catecholamine and 5-hydroxytryptamine synthesis in rat brain

Acute (-)-nicotine administration (0.4 and 0.8 mg/kg s.c.) produced a regionally specific increase in the rate of catecholamine synthesis in the rat nucleus accumbens, hypothalamus and hippocampus but not elsewhere, including the caudate-putamen. In all regions rates of 5-hydroxytryptamine synthesis were unaffected. (-)-Cotinine (0.4 and 0.8 mg/kg), the major metabolite of (-)-nicotine was without effect. (-)-Nicotine-induced increase in catecholamine synthesis occurred by a direct stimulation of central nicotinic receptors, as mecamylamine (5 mg/kg) but not hexamethonium (5 mg/kg) was an effective antagonist. Following repeated daily injections of (-)-nicotine (0.8 mg/kg) for up to 28 days, the induced catecholamine response following a subsequent challenge was unaffected in the nucleus accumbens and hypothalamus, but was increased in the hippocampus. This effect persisted for up to 14 days following withdrawal. Rates of 5-hydroxytryptamine synthesis remained unaltered after chronic pretreatment.

Interaction of noradrenergic and cholinergic systems in regulation of ocular dominance plasticity

We studied interactions among the noradrenergic (NA) and the muscarinic cholinergic (ACh) systems in the regulation of ocular dominance plasticity in kitten visual cortex. The cortex was bilaterally infused with 6-hydroxydopamine (6-OHDA) for a week. Upon termination of the 6-OHDA infusion, one hemisphere was infused with a muscarinic ACh agonist, bethanechol, through the same, chronically implanted cannula for the second week together with monocular lid suture. The other hemisphere received an infusion of the vehicle solution alone. (1) Only in the hemisphere infused with bethanechol at relatively high concentrations did we obtain a clear shift in ocular dominance. We also found that the effect of bethanechol was concentration-dependent. (2) By comparing necessary concentrations of bethanechol and NA for the respective maximal effects, we noted that the former was at least 100-fold less effective than the latter in restoring the plasticity. (3) The cortical infusion of bethanechol did not restore the plasticity to the propranolol-pretreated cortex; the ocular dominance distribution remained virtually unchanged. This result was interpreted as suggesting that functioning beta-adrenoreceptors are needed for the cortical effect of activating the muscarinic ACh receptors to become detectable. (4) The expected shift in ocular dominance following monocular deprivation was partially suppressed, when highly concentrated scopolamine, a muscarinic ACh antagonist, was used, indicating that the involvement of the ACh system in this matter was indirect. The concentration of scopolamine needed for the half-maximum effect was 172-fold higher than that of propranolol. We thus conclude that the involvement of the muscarinic ACh system in ocular dominance plasticity is secondary to that of the NA-beta-adrenoreceptor system.

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.

Neuronal bungarotoxin blocks the nicotinic stimulation of endogenous dopamine release from rat striatum

Nicotinic receptors in the brain are receiving increased attention due in part to the recent cloning of receptor subunits and to postmortem studies revealing alterations in receptor density associated with Alzheimer's disease. The peptide neurotoxin neuronal bungarotoxin (NBT) has been shown to block nicotinic cholinergic responses in autonomic ganglia and in retinal ganglion cells. These findings suggest that NBT may be a useful probe for studying nicotinic receptors in the brain. Therefore, we have investigated the effects of NBT on the nicotine-mediated enhancement of endogenous dopamine release from rat striatal slices. It was found that the transient increase in dopamine release caused by 100 microM nicotine was completely blocked by 100 nM NBT, indicating that NBT is a functional nicotinic antagonist in this system.

Nicotine infused into the nucleus accumbens increases synaptic dopamine as measured by in vivo microdialysis

It has been postulated that addiction to nicotine is mediated by dopamine release in the mesolimbic system. It is possible that nicotine might act directly on the dopamine terminals to release dopamine. This hypothesis was tested by infusing nicotine through a microdialysis probe into the nucleus accumbens of freely moving rats. Dopamine, dihydroxyphenylacetic acid, and homovanillic acid from the extracellular space were collected by microdialysis and measured by high pressure liquid chromatography. Nicotine increased extracellular dopamine in a dose-related manner. Systemic injection of the nicotine antagonist mecamylamine blocked the dopamine increase induced by local nicotine. These results suggest that nicotine releases dopamine by a local action in the nucleus accumbens terminal area of the mesolimbic system. Presynaptic induction of dopamine release might play a role in nicotine addiction.

Conditioning of nicotine effects on motility and behaviour in rats

Nicotine produces behavioural signs which are, in part, characteristic of dopaminergic activation. In the present study, it was investigated, to which degree these signs can be conditioned. The drug produced dose-dependent (0.15-0.60 mg/kg s.c.) increases in locomotor activity, hyperkinesia and stereotyped sniffing. The effects produced by 0.6 mg/kg nicotine were significantly inhibited by mecamylamine (1 mg/kg i.p.), but only in part by haloperidol (0.2 mg/kg i.p.). In a subsequent series, the administration of nicotine (0.6 mg/kg s.c.) was repeatedly associated with well-defined environmental (conditioned) stimuli: a wire cage associated with an auditory and an olfactory stimulus. Another group was pseudoconditioned, a third group remained drug-naive. When the animals were given saline in presence of the conditioned stimuli 24 h after the last conditioning session, locomotor activity, hyperkinesia and stereotyped sniffing were significantly higher in conditioned than in pseudoconditioned and drug-native rats. Similarly, when the rats were injected with nicotine (0.6 mg/kg s.c.) in presence of the conditioned stimuli 24 h after the last conditioning session, locomotor activity and stereotyped sniffing were most pronounced in the conditioned animals. These results demonstrated that behavioural effects of nicotine can be conditioned. Phenomena of this kind might contribute to the addictive behaviour to nicotine.

Effect of nicotine on human blood platelet serotonin uptake and efflux

1. Physostigmine administration has been previously shown to decrease the uptake of serotonin in human platelets. In order to test whether uptake could be inhibited as a nicotinic-cholinergic effect, the in vitro effects of nicotine on platelet 5HT uptake and efflux were examined. 2. Nicotine stimulated release of serotonin from human blood platelets, and competitively inhibited human platelet serotonin uptake in a concentration-dependent fashion at in vitro concentrations as low as 20 microM for uptake. 3. The kinetics of the nicotine effects on uptake were different from those of physostigmine. Unlike the effects of physostigmine, nicotine produced different kinetic changes, with an increase in Km and no consistent change in Vmax. 4. The efflux and inhibition of uptake paralleled that previously reported in rat brain in vitro, and was likewise similar to concentrations found previously to augment extracellular amine in other tissue preparations. However, the effects of nicotine in human platelets were not reversible by nicotinic antagonism with hexamethonium. 5. The results distinguish human platelet from rat brain with respect to nicotinic antagonism, and suggest that, at similar concentrations, nicotine may increase extracellular serotonin through differing mechanisms.

Neuroendocrine actions of nicotine and of exposure to cigarette smoke: medical implications

Over many years a large number of studies have demonstrated that nicotine and exposure to cigarette smoke produce marked neuroendocrine changes in animals and in man. The initial effects of nicotine are characterized by a marked hypersecretion of ACTH, vasopressin, beta-endorphin, prolactin and LH. Many of these very acute stimulatory effects of nicotine rapidly disappear, probably due to a desensitization of the central nicotinic cholinergic receptors involved. Instead, upon acute intermittent treatment with nicotine or exposure to cigarette smoke, an inhibition of prolactin, LH and TSH secretion occurs, which is associated with maintained hypersecretion of corticosterone. These effects are probably mediated via activation of central cholinergic receptors of the ganglionic type. Evidence indicates that the inhibitory effects of nicotine on LH and prolactin secretion are produced via an activation by these nicotinic receptors of the tubero-infundibular dopamine neurons, releasing dopamine as a prolactin inhibitory factor. Dopamine inhibits LHRH release via an axonic interaction involving D1-like dopamine receptors in the median eminence. It therefore seems possible that the reduced fertility found in heavy smokers may be counteracted by D1 receptor antagonists. The symptoms associated with glucocorticoid hypersecretion induced by nicotine is discussed considering not only the peripheral side effects but also permanent deficits in hippocampal glucocorticoid receptors and loss of hippocampal neurons. In view of the important influence of hormones on immune functions, it seems likely that smoking will cause disturbances in immune responsiveness. Finally, the nicotine-induced alterations of neuroendocrine function, especially in the pituitary-adrenal axis and in vasopressin release, may also lead to behavioural consequences in smokers, especially in the withdrawal phase.

Neuroregulatory effects of nicotine

The impact of nicotine on the central nervous system is, in an important sense, neuroregulatory, with cascading effects on physiological and biochemical function as well as on behavioral activity. Accordingly, the neurotransmitter and neuroendocrine effects of nicotine constitute a critical part of its biological action, which includes reinforcing as well as pathophysiological consequences. This review focuses on nicotine's effects on cholinergic and non-cholinergic nicotine receptors and on the responses of catecholamines, monoamines, hypophyseal hormones, and cortisol. The contribution of critical variables, such as timing and duration of neuroregulator release and the patterns that make up the total response, is still largely unknown, particularly with regard to the effects of environmental context, history of nicotine use, and mode of administration. The evidence suggests that by altering the bioavailability of the above-listed neuroregulators, nicotine serves as a pharmacological "coping response", providing immediate though temporary improvement in affect or performance in response to environmental demands. Much of what is known to date is based on studies involving the administration of agonists and antagonists under different environmental conditions. Newer technological approaches such as autoradiography and positron emission tomography show potential for determining the neuroregulatory patterns involved and specifying nicotine's locus of action relevant to its behavioral and physiological effects.

Drug effects on active immobility responses: what they tell us about neurotransmitter systems and motor functions

The literature reviewed indicates that active immobility can be promoted by systemic injections of various neurotransmitter systems, as follows: (1) Dopaminergic blockade of both D1 and D2 receptor subtypes. (2) Cholinergic agonism of both muscarinic and nicotinic receptors. (3) Noradrenergic agonism of both alpha-1 and alpha-2 receptors (but these agonists may interfere with haloperidol- and reserpine-induced catalepsy). (4) GABA agonism. (5) Histamine agonism, particularly at the H1 receptor. (6) Opiate agonism, including action of many endogenous opiate peptides, particularly those affecting mu and delta receptors. (7) Agonism by certain other peptides (neurotensin, cholecystokinin). Among the major interactions of neurotransmitter systems that regulate immobility, are the following: (1) Cholinergic-dopaminergic (cholinolytics disrupt catalepsy of dopaminergic blockade and dopaminergic agonists tend to disrupt cholinomimetic catalepsy). (2) Opiate-induced catalepsy is antagonized by the dopamine agonist, apomorphine, but is enhanced by amphetamine. It is also antagonized by certain alpha-2 adrenergic agonists, while it does not seem to be antagonized by anticholinergics. (3) Numerous other interactions have been reported, involving opiates and MSH, serotonin and dopamine mimetics, serotonin and ketamine, GABA and neuroleptics, neurotensin and anticholinergics and histamine. The significance of the multiple neurotransmitter systems is unknown. One possible explanation is that the various neurotransmitter systems participate in mediating the sensory inputs that are involved in triggering immobility and regulate the higher-order limbic and basal ganglia processing reactions that engage a final motor output pathway from the brainstem. The brain is assumed to contain two sets of systems, each with its own, or possibly overlapping, set of neurotransmitter systems, that promote either active immobility or locomotion. The systems reciprocally inhibit each other. Another view, not mutually exclusive, is that output from the locomotor-promoting system provides a negative feedback, via the active immobility pathways, to act as a "brake" on movement, while at the same time maintaining the muscular tonus that is characteristic of active immobility.

Effect of nicotine pretreatment on striatal dopaminergic system in rats

Rats were pretreated with saline or nicotine (1.5 mg/kg/day) by subcutaneously implanting each animal with an Alzet osmotic minipump for 1 or 14 days. Short-term (1-day) administration of nicotine to rats reduced the stimulatory effect of (+)-amphetamine on locomotor activity. This was correlated with an attenuation in the ability of (+)-amphetamine to stimulate [3H]dopamine formation from [3H]tyrosine in rat striatal slices of these nicotine-treated animals. In long-term (14-day) nicotine-pretreated animals, both the apomorphine- and (+)-amphetamine-induced locomotor activity were potentiated. This behavioral potentiation was associated with an increase in the total number of postsynaptic dopaminergic receptor binding sites in the striatum. The development of striatal dopamine receptor supersensitivity may be caused by a decrease in the rate of dopamine turnover in the striatum.

Postnatal behavioural effects of maternal nicotine exposure in rats

The effects of nicotine on locomotor activity have been studied in neonate rats exposed to nicotine (1.5 mg kg-1 day-1) throughout the gestational period. Both 14 day old male and female offspring demonstrated an increase in spontaneous locomotor activity when compared with saline-exposed controls. However, systemic administration of (+)-amphetamine was effective in attenuating the hyperactivity of these nicotine-exposed pups.

Muscarinic agonists evoke neurotransmitter release: possible roles for phosphatidyl inositol bisphosphate breakdown products in neuromodulation

Carbachol (CCh), a muscarinic agonist that elicits the formation of inositol trisphosphate (IP3) and diacylglycerol (DG), induces a calcium-dependent [3H]norepinephrine ([3H]NE) release [IC50 = (2.7 +/- 0.5) X 10(-4) M] in rat brain slices. Similarly, other muscarinic agonists evoke [3H]NE release which is specifically inhibited by muscarinic antagonists such as 3-quinuclidinyl benzilate, atropine, and N-methyl-4-piperidyl benzilate. The atropine-sensitive evoked release is effectively inhibited by neomycin (IC50 = 50 microM), a phospholipase C inhibitor that interferes with IP3-dependent cellular processes. In addition, polymyxin B, a rather selective inhibitor of protein kinase C (PK-C), abolishes the agonist-mediated release with a half-maximal effective concentration of 0.53 microM (750 ng/ml). These results have a significant implication for the mechanism by which agonists generating IP3 and DG act as inducers of neurotransmitter release in the CNS. However, since both neomycin and polymyxin B act also as N-calcium-channel blockers, other possible mechanisms are discussed. The CCh-induced release suggests that in the CNS an agonist-receptor interaction leads to a calcium-dependent neurotransmitter release, most likely via promoting the IP3/DG as second messengers followed by activation of PK-C.

Uptake, release, and modulation of release of noradrenaline in rabbit superior colliculus

The noradrenaline content, the uptake of [3H]noradrenaline, and the release of previously incorporated [3H]noradrenaline were studied in slices of rabbit superior colliculus. The concentration of endogenous noradrenaline was higher in superficial than in deep layers of the superior colliculus. Upon incubation with [3H]noradrenaline, tritium was accumulated by a mechanism that was strongly inhibited by oxaprotiline but little inhibited by 6-nitroquipazine. Electrical stimulation at 0.2 or 3 Hz increased the outflow of tritium from slices preincubated with [3H]noradrenaline; the increase was almost abolished by tetrodotoxin or a low calcium medium. Clonidine reduced the evoked overflow of tritium, whereas yohimbine increased it and antagonized clonidine. The evoked overflow was also reduced by the dopamine D2-receptor-selective agonists apomorphine and quinpirole, an effect antagonized by sulpiride. The preferential opioid kappa-receptor agonist ethylketocyclazocine produced an inhibition that was counteracted by naloxone. Nicotine accelerated the basal outflow of tritium; part of the acceleration was blocked by hexamethonium. The muscarinic agonist oxotremorine slightly diminished the electrically evoked overflow, and its effect was abolished by atropine. The oxaprotiline-sensitive uptake of [3H]noradrenaline as well as the tetrodotoxin-sensitive and calcium-dependent overflow of tritium upon electrical stimulation (presumably reflecting the release of [3H]noradrenaline) indicate that noradrenaline is a neurotransmitter in the superior colliculus. The release of [3H]noradrenaline is modulated through alpha 2-adrenoceptors as well as dopamine D2-receptors, opioid kappa-receptors and nicotine and muscarine receptors. No clear evidence was found for modulation through beta-adrenoceptors, D1-receptors, serotonin receptors, opioid mu- or delta-receptors or receptors for GABA or glutamate. Only the alpha 2-adrenoceptors receive an endogenous agonist input, at least under the conditions of these experiments. The pattern of presynaptic modulation resembles that found for noradrenaline release in other rabbit brain regions, suggesting that all noradrenergic axons arising in the locus coeruleus possess similar presynaptic receptor systems.

Two alpha-adrenergic contractions with different Ca2+ activation mechanisms in response to field nerve stimulation in the circular smooth muscle of guinea-pig vas deferens

1. Two noradrenergic components of contraction were induced by field nerve stimulation of the circular smooth muscle of guinea-pig vas deferens. One component occurred during the stimulation (former-response) and the other component occurred after stimulation had ceased (after-response), with a distinct tension fall between these two responses. 2. The effects of stimulation frequency and duration upon these components were examined. 3. Prazosin (0.05-0.5 microM) suppressed both responses, whereas nifedipine (0.02 microM) suppressed only the after-response. 4. It is suggested that in this tissue there are two neurogenic alpha-adrenergic contractions which possess different Ca2+ activation mechanisms. The former-response is due to an increase in cytoplasmic Ca2+ concentration through a pathway independent of action potentials, while the after-response is attributed to action potentials arising as a secondary response to the released noradrenaline.

Pharmacological treatment of tobacco dependence

Pharmacologically based approaches for the treatment of tobacco dependence are reviewed. The rational basis for pharmacologic treatment approaches is that tobacco dependence is partially, and critically, mediated by the actions of tobacco-delivered nicotine to the central nervous system. These actions include direct reinforcing properties of nicotine itself, tolerance and physiologic dependence, possible beneficial effects of nicotine in the alleviation of anxiety and control of weight, and neurohormonal regulation which can become important to the maintenance of emotional well-being and performance at work. Insofar as tobacco abstinence leads to negative consequences, via these biobehavioral mechanisms, pharmacologic intervention should be able to assist in initial tobacco detoxification and help tobacco abstinent persons to avoid subsequent relapse. The purpose of this review is to survey some of the efforts to develop such interventions, as well as to elucidate some of the issues relevant to such development. Four distinct approaches are discussed: (1) Nicotine replacement, in which physiologic dependence is transferred to a safer and more therapeutically manageable nicotine delivering formulation; this category includes nicotine polacrilex gum; (2) Blockade therapy, in which a drug is taken that blocks the reinforcing properties of nicotine should relapse occur; (3) Nonspecific pharmacotherapy, in which the biobehaviorally mediated correlates of tobacco abstinence are treated on a symptomatic basis; (4) Deterrent therapy, in which a drug is taken prior to smoking such that any tobacco use would produce reliable aversive effects.

Stereoselective nicotine-induced release of dopamine from striatal synaptosomes: concentration dependence and repetitive stimulation

Using a sensitive perfusion system we have studied the nicotine-induced release of [3H]dopamine ([( 3H]DA) from striatal synaptosomes. Nicotine-evoked release was concentration dependent with an EC50 of 3.8 microM. The response to 1 microM nicotine was comparable to that to 16 mM K+; 10 microM veratridine evoked a larger response. All three stimuli were Ca2+ dependent but only the response to veratridine was blocked by tetrodotoxin. Repetitive stimulations by 1 microM (-)-nicotine (100 microliters) at 30-min intervals resulted in similar levels of [3H]DA release; higher concentrations of (-)-nicotine resulted in an attenuation of the response particularly following the third stimulation. This may reflect desensitisation or tachyphylaxis of the presynaptic nicotinic receptor. The action of nicotine was markedly stereoselective: a 100-fold higher concentration of (+)-nicotine was necessary to evoke the same level of response as 1 microM (-)-nicotine. It is proposed that these presynaptic nicotinic receptors on striatal terminals are equivalent to high-affinity nicotine binding sites described in mammalian brain.

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.

Nicotine-induced alterations in brain regional concentrations of native and cryptic Met- and Leu-enkephalin

The distribution of cryptic forms (larger enkephalin-containing peptides) in neostriatum, hypothalamus, spinal cord T3-L1 and neurointermediate lobe of pituitary were determined by radioimmunoassay. Optimal conditions for enzymic hydrolysis of the cryptic enkephalins by trypsin and carboxypeptidase B were established. The proportion of total Met- and Leu-enkephalin represented by native pentapeptide varied markedly among these central nervous system regions. Also, the distributions of native and cryptic Met-enkephalin were distinct from that of Leu-enkephalin. Chromatographic separation by HPLC of immunoreactive Met-enkephalin peptides revealed only two peaks corresponding to Met-enkephalin and Met-enkephalin sulfoxide in rather equal amounts. Hydrolysis of cryptic Met-enkephalin also produced only two HPLC-separable peaks of immunoreactive Met-enkephalin, again corresponding to Met-enkephalin and Met-enkephalin sulfoxide. Bioactivity of cryptic striatal Met-enkephalin after hydrolysis was demonstrated by antinociception and catalepsy in rats following its intracerebroventricular injection. Repeated short-term administration of nicotine, 0.1 mg/kg IP six times at 30 min intervals, produced significant increases in native and cryptic Met-enkephalin in striatum, consistent with an increase in neuronal release of Met-enkephalin together with increases in synthesis and processing of proenkephalin A in this brain region. This regimen of nicotine also decreased levels of native Met-enkephalin and of both native and cryptic Leu-enkephalin in neurointermediate lobe, consistent with nicotine-induced release of both proenkephalin A- and prodynorphin-derived peptides from neurointermediate lobe.

Stimulation of [3H]dopamine release by nicotine in rat nucleus accumbens

The mesolimbic system of the brain has been shown to be involved in the reward properties of a number of agents. It is possible that release of monoamines by nicotine in this brain area could be related to the pleasurable aspects related to cigarette smoking. In this investigation, the effect of nicotine on the release of [3H]dopamine in the nucleus accumbens of the rat was studied. It was shown that nicotine produced a concentration-dependent increase in [3H]dopamine release at concentrations of 0.1 microM and above. The increase in release was found to be almost completely calcium dependent. The nicotine-induced release was only partially blocked by the nicotinic antagonists hexamethonium and d-tubocurarine. A number of cholinergic agonists, as well as other compounds, were tested for their capacity to mimic the effect of nicotine. At equimolar concentrations there was, at most, only 50% of the activity of nicotine. The results of this study demonstrate that nicotine stimulates the release of dopamine in the nucleus accumbens at concentrations similar to those in the blood of cigarette smokers. This suggests that the release of monoamines in specific nuclei of the mesolimbic system may be an important determinant of the desire to smoke cigarettes.

Norepinephrine in the interpeduncular nucleus of the rat: normal distribution and the effects of deafferentation

We used correlative biochemical and histochemical methods to examine (1) the norepinephrine (NE) projection from the paired locus coeruleus (LC) to the midline interpeduncular nucleus (IPN) of the adult rat and (2) the ability of the LC to respond to denervation of their target following removal of noradrenergic afferents (6-hydroxydopamine lesions of the LC) or non-noradrenergic afferents (lesion of the paired fasciculi retroflexi(FR]. Histofluorescence revealed that the NE innervation from the two LC to the IPN is symmetric and overlapping. This projection is confined to rostral, central, and intermediate subnuclei and is absent from lateral and dorsal subnuclei. We found no evidence for homotypic collateral sprouting of undamaged LC neurons into the IPN following unilateral LC lesion. Bilateral LC lesions also did not induce sprouting by NE-containing neurons from other systems (e.g. the superior cervical ganglion or the lateral tegmental group) or from those LC neurons that survived the 6-hydroxydopamine lesion. Histofluorescence following bilateral FR lesions confirmed an earlier observation that apparent hyperinnervation of the IPN by LC afferents is elicited following removal of non-noradrenergic afferents. Measurements of the turnover rate of NE in the IPN of control animals and those that received bilateral FR lesions indicate an increased NE content and increased turnover rate of NE in the IPN of lesioned animals. Taken together these results suggest an increased number of NE terminals and an increase in the activity of tyrosine hydroxylase. No change in NE content or turnover rate was seen in the frontal cortex from these same animals. This is consistent with a target-dependent regulation of heterotypic collateral sprouting.

Modulation of dopamine release from neuron-enriched tissue cultures by cholinergic agents

Modulation of [3H]dopamine release by cholinergic agents (acetylcholine, atropine, d-tubocurarine, oxotremorine, and nicotine) was studied in primary cell cultures derived from whole brains of foetal rats (17 days of gestation). Monolayer and aggregated neuron-enriched cultures were maintained for 17 days in vitro [3H]Dopamine basal outflow was enhanced by acetylcholine, nicotine, and atropine and was unaffected by oxotremorine, hexamethonium, and d-tubocurarine. The action of nicotine was antagonized by d-tubocurarine, and that of atropine was partially blocked by oxotremorine. A similar picture was seen when the influence of cholinergic agents was studied under depolarizing conditions. The action of oxotremorine was dependent on nerve activity. The presence of both muscarinic and nicotinic antagonists was necessary for abolishing the effect of acetylcholine on the dopamine outflow. These results show that dopamine release in both types of neuron-enriched cultures can be influenced by cholinergic agents and that both muscarinic and nicotinic receptors are involved in regulation of the amine's outflow.

Locomotor activity as a predictor of times and dosages for studies of nicotine's neurochemical actions

Nicotine's action on the central nervous system is complex and likely involves an interaction of neurotransmitters. To determine the time after administration of nicotine and dosage for neurochemical studies, locomotor activity of CD-1 mice was determined at 5 min intervals between 0-60 min. A low nicotine dosage (0.05 mg/kg) did not alter activity 5-15 min after drug injection, but increased activity 28% at 15-25 min post-injection. A high dosage (0.8 mg/kg) reduced total distance 62% and rearing 87% at 5-15 min; at 15-25 minutes total distance declined 56% and rearing 69%; all measures returned to control values after 30 minutes; rearing then increased at 40 min after nicotine. Pretreatment (15 min before nicotine) with mecamylamine (1.0 mg/kg), but not hexamethonium (1.0 mg/kg), prevented the depressant effect of nicotine. Dopamine (DA) and its metabolites as well as acetylcholine (ACh) synthesis were measured at the point of nicotine's maximal depressant action. Striatal levels of dihydroxyphenylacetic acid (DOPAC) were increased and ACh utilization was reduced in striatum (-25%) and cortex (-24%) 10 min after nicotine (0.8 mg/kg). Mecamylamine, while preventing the depressant effect of nicotine on locomotor activity, did not alter its effects on DA metabolism. These results demonstrate that the behavioral outcome of acute nicotine treatment is time and dose-dependent. Nicotine's depressant action appears not to be due to altered DA but may be related to changes in carbohydrate and acetylcholine metabolism.