Stimulation of nicotinic receptors in the lateral septal nucleus increases anxiety

The present study investigated the role of nicotinic receptors in the lateral septum in the modulation of anxiety. The effects of direct injections of nicotine into the lateral septum were first investigated in two tests of anxiety, social interaction and elevated plus-maze tests. Intra-septal injection of nicotine (1 and 4 microgram) induced consistent anxiogenic effects in both tests. The reversal of nicotinic effects with mecamylamine was then studied in the social interaction test. Intra-septal injection of mecamylamine at a low dose (15 ng) induced an anxiolytic effect, suggesting the presence of intrinsic cholinergic tone increasing anxiety. At higher doses (30-50 ng), mecamylamine was without effect in the social interaction test, but blocked the anxiogenic effects of nicotine (4 microgram). These findings provide further evidence for the role of the lateral septum in the modulation of anxiety and suggest that cholinergic projections to this brain area facilitate anxiety through nicotinic receptors.

Nociceptive and antinociceptive responses to intrathecally administered nicotinic agonists

Activation of spinal nicotinic receptors evokes a prominent algogenic response. Recently, epibatidine, a potent nicotinic agonist, was found to display an antinociceptive response after systemic administration. To examine the spinal component of this action, effects of three nicotinic agonists epibatidine, cytisine and nicotine--were given intrathecally (IT) and their antinociceptive activity was subsequently assessed. All agents elicited dose-dependent algogenic activity, characterized at lower doses by touch-evoked hyperactivity and at higher doses by intermittent vocalization and marked behavioral activity, with the rank order of potency being epibatidine > cytisine > nicotine. In addition, intrathecal epibatidine elicited a short-lasting, dose-dependent thermal antinociception. In contrast, the other nicotinic agonists at the highest usable dose failed to produce a significant antinociception. Mecamylamine, a nicotinic channel blocker, completely abolished the antinociceptive and algogenic responses of epibatidine. The competitive antagonist, alpha-lobeline, blocked both the analgesic and algogenic responses, but methyllycaconitine inhibited only the algogenic effects of epibatidine. Dihydro-beta-erythroidine, also a competitive antagonist, had no effect on the initial intense algogenic responses. The analgesic response to epibatidine was neither inhibited by naloxone nor by atropine. 2-Amino-5-phosphopentanoic acid, a competitive N-methyl-D-aspartate receptor antagonist, did not affect the analgesic response to intrathecal epibatidine or the intense initial algogenic response. Upon repeated administration (30-min interval), epibatidine (1 microg, IT) exhibited marked and rapid desensitization to both the analgesic and algogenic responses which recovered within 8 h. Pretreatment with two consecutive doses of cytisine (5 microg, IT, 30-min apart) inhibited the agitation and analgesic actions of intrathecal epibatidine. Thus, we contend that in addition to the typical nociceptive response elicited by spinal nicotinic agonists, intrathecal epibatidine also exhibits a pronounced but short-lasting antinociception. The analgesic and algogenic responses to intrathecal epibatidine may be mediated by distinct subtypes of spinal nicotinic receptors as suggested by the antagonist studies.

Differences between the antinociceptive effects of the cholinergic channel activators A-85380 and (+/-)-epibatidine in rats

(+/-)-Epibatidine (EPIB) and A-85380 are nicotinic acetylcholine receptor (nAChR) agonists that bind to the agonist ([3H]cytisine) binding site with 40 to 50 pM affinity but have different affinities in nAChR subtype selective functional receptor assays. In vivo EPIB was more (23-fold) potent than A-85380 in reducing open field activity and more (12-fold) potent in reducing nociception in the formalin test of persistent chemical pain. In the rat hot box test of thermal acute pain, both compounds produced antinociception, as indicated by an increase in the paw withdrawal latency, however EPIB was a approximately 33-fold more potent than A-85380 (ED50 = 0.004 and 0.11 micromol/kg, i.p., respectively). The systemic effects of both nAChR agonists were blocked by central (i.c.v.) administration of the nAChR antagonist chlorisondamine suggesting a central site of action for these compounds. Injections of EPIB (0.0013 to 0.013 nmol) and A-85380 (0.013 to 0.13 nmol) directly into the nucleus raphe magnus (NRM) were also effective in the hot box and could be blocked by coadministration of the nAChR antagonists chlorisondamine (0.23 nmol) or mecamylamine (0.8 nmol). The NRM was found to be critical for the antinociceptive effects of systemic EPIB but not for A-85380 in that NRM injections of either mecamylamine (0.8 nmol) or lidocaine (74 nmol) blocked the antinociceptive effects of systemic (i.p.) EPIB but not those of A-85380. These results suggest that A-85380 may act at multiple sites both within and outside the NRM, whereas EPIB acts largely via descending inhibitory pathways arising from the NRM.

The lateral hypothalamus in the modulation of tonic immobility in guinea pigs

The lateral hypothalamus has been reported to be involved in the organization of aggression and predatory attack but not in behavioral inhibition responses such as tonic immobility (TI). TI may be defined as an inborn behavioral inhibition characterized by profound physical inactivity and relative lack of responsiveness to the environment, triggered by an intense sensation of fear generated during prey-predator confrontation. Our study indicates that cholinergic stimulation of anterior regions of the lateral hypothalamus of guinea pigs potentiates the duration of TI episodes, while stimulation of medial and posterior regions of this structure promotes a decrease in TI duration, suggesting that the lateral hypothalamus modulates the duration of TI episodes in a differentiated manner.

Accumbal dopamine overflow after ethanol: localization of the antagonizing effect of mecamylamine

It has been suggested that ethanol exerts its mesolimbic dopamine activating effects and its reinforcing effects via interaction with central nicotinic acetylcholine receptors, thus providing a basis for the often observed covariation between ethanol and nicotine consumption. We have previously demonstrated that the central nicotinic acetylcholine receptor antagonist mecamylamine totally counteracts the ethanol-induced elevation of extracellular dopamine in the nucleus accumbens, as measured by in vivo microdialysis. A contribution of peripheral nicotinic receptor blockade could, however, not be excluded. In the present study, mecamylamine (1.0 mg/kg, i.p.) again totally counteracted the ethanol-induced dopamine overflow, as measured by in vivo microdialysis, while the quarternary nicotinic receptor antagonist hexamethonium (10 mg/kg, i.p.) did not. Furthermore, the increase in accumbal dopamine overflow after systemic ethanol (2.5 g/kg, i.p.) was counteracted by local perfusion of mecamylamine (50 microM) in the ipsilateral ventral tegmental area, but not by mecamylamine perfusion in the nucleus accumbens. Ethanol-induced accumbal dopamine overflow was also counteracted by perfusion of hexamethonium (250 microM) in the ventral tegmental area. These results provide further evidence that ethanol-induced activation of the mesolimbic dopamine system is mediated via stimulation of central nicotinic acetylcholine receptors, and that the receptor population within the ventral tegmental area may be the most important in this regard. It is suggested that antagonists of central nicotinic acetylcholine receptors may be useful in the treatment of alcoholism.

Role of central nicotinic and beta-adrenergic receptors in the onset and further development of tail-tremor induced by repeated nicotine administration to rats

The effects of nicotinic and beta-adrenergic receptor antagonists on tail-tremor induced by repeated nicotine administration were investigated in rats. The daily administration of nicotine (0.5 mg/kg/day, s.c.) for 8 days resulted in an augmentation of tail-tremor. However, repeated administration of dimethyl phenyl piperazinium iodide (1 mg/kg/day, s.c.) for 8 days did not cause tail-tremor. Mecamylamine (0.5 mg/kg, i.p), administered before the nicotine injection on each day, abolished the tail-tremor. After discontinuation of the mecamylamine treatment, nicotine injections caused tail-tremor augmentation. Propranolol (20 mg/kg, i.p.), administered before the nicotine on each day, suppressed the appearance of tail-tremor. After the discontinuation of propranolol treatment, the degree of tail-tremor induced by a single injection of nicotine on day 9 was much greater in the propranolol-treated group than in the saline-treated control group. Neither carteolol (20 mg/kg, i.p.) nor metoprolol (20 mg/kg, i.p.) treatment showed such effects. Intraspinal injection of 6-hydroxydopamine markedly enhanced the tail-tremor induced on the first day of nicotine injection. This effect became more intense on subsequent administration of nicotine. The enhanced tail-tremor following 6-hydroxydopamine treatment was abolished by mecamylamine (0.5 and 1 mg/kg, i.p.), and was suppressed by propranolol (5-20 mg/kg, s.c.) in a dose-dependent manner. These results suggest that central nicotinic receptors are essential for the onset and for the further development of tail-tremor induced by the repeated administration of nicotine, and that beta 2-adrenoceptors are associated with the tremor mechanism. Moreover, spinal noradrenergic mechanisms may be involved in the manifestation of this phenomenon.

Mecamylamine-induced impairment of acquisition and retrieval of olfactory conditioning in the honeybee

Mecamylamine, a nicotinic receptor antagonist, was injected into the honeybee brain haemolymph. The effects of the drug were investigated on Pavlovian conditioning of the proboscis extension reflex. The conditioned response was acquired after a one-trial learning session, consisting of an olfactory-conditioned stimulus combined with a gustatory antennal unconditioned stimulus. The drug was injected at different times before or after the learning session in order to dissociate its effects on acquisition, consolidation and retrieval processes. The performance was evaluated in short-delayed recall tasks. To control potential effects on sensory-motor activity, the effects of the drug were also investigated on sensory processes (through olfactory and gustatory functions) and on motor processes of proboscis extension. The results of conditioning experiments showed that pretrial injection induced a decrease of retention performance 1 h after the learning trial. Mecamylamine injected 20 min after the learning session induced a time-dependent impairment of retention performance, as has been shown by the performance level registered from 10 to 80 min after injection. A 5-min post-trial injection had no effect on retention performance. Control experiments did not reveal any effect of mecamylamine on the response reflex of proboscis extension and on responsiveness to olfactory stimuli (geraniol, lavender and vanillin). The absence of effects on sensory perception combined with the amnestic effect induced by pre- or late post-trial injections lead us to conclude that mecamylamine specially impaired acquisition and retrieval processes. The involvement of nicotinic-like receptors in these processes is discussed.

Evidence for adrenergic and tachykinin activity in venom of the stonefish (Synanceja trachynis)

The aim of the present study was to investigate previously suggested adrenergic and tachykinin activity, as well as the cardiovascular effects, of venom from the stonefish (Synanceja trachynis). Stonefish venom (60-120 micrograms/kg, i.v.) produced dose-dependent bronchoconstriction in anaesthetised guinea-pigs. This response (100 micrograms/kg, i.v.) was significantly reduced by the neurokinin 1 (NK1) receptor antagonist CP-99,994 (1 mg/kg, i.v.). Contractile responses to venom (4 micrograms/ml) of guinea-pig isolated ileum (GPI) were significantly inhibited by a combination of the sodium channel blocking drug tetrodotoxin (1 microM) and the ganglion blocking drug mecamylamine (10 microM). However, subsequent administration of CP-99,994 (0.1 microM) did not produce further inhibition. Endogenous tachykinin depletion with capsaicin (1 microM) also significantly attenuated responses to venom (4 micrograms/ml) in GPI. Venom (4 micrograms/ml) produced increases in rate and force of contraction of rat spontaneously beating isolated atria which were significantly inhibited by the beta-adrenoceptor antagonist propranolol (5 microM) but not by noradrenergic transmitter depletion with reserpine (4.5 mg/kg, i.p.). In the presence of the alpha 1-adrenoceptor antagonist prazosin (0.3 microM), venom (6 micrograms/ml) significantly inhibited electrically evoked twitches of prostatic segments of rat vas deferens. The inhibitory effect of venom was significantly reduced by the alpha 2-adrenoceptor antagonist idazoxan (1 microM) but not by propranolol (5 microM) or the neurokinin 2 (NK2) receptor antagonist SR-48,968 (0.1 microM). Venom (60-120 micrograms/kg, i.v.) produced dose-dependent increases in mean arterial blood pressure in anaesthetised rats. This pressor response (60 micrograms/kg, i.v.) was significantly reduced by prazosin (10-50 micrograms/kg, i.v.) and the leukotriene receptor antagonist SB205312 (1 mg/kg, i.v.), significantly increased by propranolol (2 mg/kg, i.v.), but not significantly affected by the cyclo-oxygenase inhibitor indomethacin (10 mg/kg, i.v.) or the thromboxane A2/prostaglandin H2 (TP) receptor antagonist GR32191B (1 mg/kg, i.v.). Pressor responses to venom (100 micrograms/kg, i.v.) were also observed in anaesthetised rabbits. These results suggest that stonefish venom contains a component capable of stimulating the release of endogenous tachykinins with subsequent activity at NK1 receptors. The venom also appears to act via stimulation of sodium channels on sensory nerves. The venom also has activity at alpha 2-adrenoceptors and a direct action at beta-adrenoceptors. The effect of venom on blood pressure of anaesthetised rats appears to include a pressor component that is mediated, in part,by alpha-adrenoceptors and leukotriene receptors, and a depressor component that is mediated by beta-adrenoceptors. However, the pressor response does not involve action at TP receptors, or require the production of cyclo-oxygenase metabolites.

Central cardiovascular effects of acetylcholine in the conscious dog

1. The effects of central cholinomimetic drugs on cardiovascular and vasoactive hormonal responses (blood pressure, heart rate, catecholamines, vasopressin, atrial natriuretic factor, neuropeptide Y plasma levels and plasma renin activity) were investigated in conscious Beagle dogs. For this purpose a catheter was chronically implanted into each dog's cisterna magna to allow repeated central injections in the awake animals. 2. Intracisternal acetylcholine (20 micrograms kg-1) significantly increased systolic and diastolic blood pressure. These changes were accompanied by an initial short term tachycardia followed by a long lasting bradycardia. Intracisternal acetylcholine also increased noradrenaline, adrenaline and vasopressin plasma levels, decreased plasma renin activity but did not modify plasma levels of neuropeptide Y and atrial natriuretic factor. 3. The effects of acetylcholine were completely abolished by pretreatment with intracisternal injection of the muscarinic antagonist, atropine (5 micrograms kg-1) but not by the intracisternal injection of the nicotinic antagonist, mecamylamine (25 micrograms kg-1). 4. The present results demonstrate that there are qualitative and quantitative differences between the central cardiovascular effects of acetylcholine in conscious dogs compared to what we previously reported, using a comparable protocol, in anaesthetized dogs. Under both conditions, we observed a central cholinergically mediated increase in blood pressure secondary to an increase in sympathetic tone and vasopressin release but these responses were shorter (less than 10 min) in the conscious dogs than in anaesthetized dogs (more than 10 min). Moreover, we detected in the response to the central cholinergic stimulation in the conscious dogs a significant increase in plasma adrenaline levels and biphasic changes in heart rate which were not described previously in the anaesthetized dog.

The role of alpha-adrenergic mechanisms within the area postrema in dopamine-induced emesis

Intracerebroventricular injection of dopamine (0.5-4.0 mg) produced dose-dependent and short-lasting emesis (1-8 min) in cats, which was abolished after ablation of the area postrema. Relatively selective alpha 2-adrenoceptor antagonists (yohimbine and idazoxan) and a mixed alpha 1- and alpha 2-adrenoceptor antagonist (tolazoline), but not a non-selective alpha 1-adrenoceptor antagonist (prazosin), injected intracerebroventricularly inhibited the emesis induced by intracerebroventricular dopamine. However, dopamine receptor antagonists (chlorpromazine, droperidol, spiperone, domperidone, triflupromazine, sulpiride and metoclopramide), an antimuscarinic drug (atropine), a ganglionic blocking agent (mecamylamine), an opioid receptor antagonist (naloxone) and a 5-HT receptor antagonist (methysergide), all injected intracerebroventricularly, had no significant effect on emesis evoked by intracerebroventricular dopamine. The emetic response to intracerebroventricular dopamine was attenuated in cats pretreated with intracerebroventricular reserpine, 6-hydroxydopamine, alpha-methyl-p-tyrosine and hemicholinium-3. It is postulated that dopamine-induced emesis is mediated through the release of noradrenaline acting at alpha 2-adrenoceptors and that it depends on the integrity of monoaminergic and possibly cholinergic structures within the area postrema. It appears, therefore, that the emetic effect of intracerebroventricular dopamine is mediated by adrenergic rather than dopaminergic mechanisms in the area postrema, at least in the cat.

Infusion of nicotine in the ventral tegmental area or the nucleus accumbens of the rat differentially affects accumbal dopamine release

The present study examined the effects of acute, continuous infusion of nicotine in either the ventral tegmental area or the nucleus accumbens on extracellular concentrations of dopamine in the nucleus accumbens by applying in vivo microdialysis in freely moving rats. Nicotine (1000 microM) infusion for 80 min. in the ventral tegmental area produced a long-lasting increase in accumbal dopamine, whereas similar nicotine infusion in the nucleus accumbens increased dopamine levels only within the first 20 min. of administration. This effect was blocked by systemic pretreatment with the nicotinic receptor antagonist mecamylamine (1 mg/kg, subcutaneously). In contrast to the effects of nicotine, N-methyl-D-aspartate infusion in the ventral tegmental area as well as in the nucleus accumbens produced a long-lasting increase in accumbal dopamine levels. The more procounced effect of infusion of nicotine in the ventral tegmental area as compared to the nucleus accumbens on accumbal dopamine release may be due to a lower rate or even lack of tolerance to nicotine's stimulatory action in the ventral tegmental area. These results support the notion that nicotinic receptors in the ventral tegmental area may be of greater importance than those located in the nucleus accumbens for mediating some of the stimulatory effects of nicotine on the reward-related mesoaccumbens dopamine system.

Augmented responses to intrathecal nicotinic agonists in spontaneous hypertension

Abnormal central cholinergic activity has been reported to be responsible in part for the pathogenesis of high blood pressure in spontaneously hypertensive rats (SHR). Administration of cholinergic agonists in brain and spinal cord results in exaggerated pressor responses in SHR. Studies to date have focused largely on the muscarinic cholinergic system. Recently, we demonstrated that intrathecal administration of nicotinic agonists results in pressor, tachycardic, and irritation responses. In the present study we examine the cardiovascular and behavioral responses to nicotine and cytisine administered intrathecally in La Jolla strain (LJ) SHRLJ and age-matched Wistar-Kyoto (WKYLJ) rats. Nicotinic agonists produced augmented pressor, heart rate, and irritation responses in SHRLJ compared with normotensive rats. In both SHRLJ and WKYLJ rats, cytisine elicited a greater nociceptive response and greater spinobulbar component to the pressor response than nicotine. SHRLJ and WKYLJ rats also differ in that the SHRLJ strain shows a diminished tendency for desensitization to cytisine. As in Sprague-Dawley rats, in SHRLJ and WKYLJ rats the cardiovascular and behavioral responses to intrathecal nicotine were significantly inhibited by mecamylamine, dihydro-beta-erthyroidine, and methyllycaconitine. However, methyllycaconitine, which effectively blocked cytisine-elicited cardiovascular and behavioral responses in Sprague-Dawley and WKYLJ rats, was unable to inhibit the maximal rise in cystine-elicited blood pressure, heart rate, and irritation responses in SHRLJ. In contrast to the heightened cardiovascular and behavioral responses, the number of nicotinic binding sites in spinal cord membranes was significantly decreased in the hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal of hemorrhagic shock in rats by oxotremorine: the role of muscarinic and nicotinic receptors, and AV3V region

In an experimental model of hemorrhagic shock resulting in the death of almost all rats within 20-30 min, centrally active cholinomimetic drugs are reported to induce a prompt, sustained and dose-dependent improvement in blood pressure and survival rate claimed to be due to nicotinic, but not muscarinic actions. In the present study, cholinergic receptor agonist, oxotremorine (50 micrograms/kg, i.v.) increased mean arterial pressure (from 22 +/- 1 to 123 +/- 3 mm Hg) and 60 min-survival rate (from 0% to 92%) in rats bled to hypovolemic shock. Atropine (2 mg/kg, i.v.) pretreatment inhibited the pressor effect of oxotremorine significantly, but did not modify its effect on survival rate. On the other hand, pretreatment with mecamylamine (50 micrograms, i.c.v.) almost abolished the reduction in mortality rate, but inhibited the pressor effect of oxotremorine, partially. These results indicate that oxotremorine-induced pressor response and decrease in mortality in rats with severe hemorrhagic shock are primarily mediated via central muscarinic and nicotinic receptors, respectively. AV3V region was previously reported to be involved in pressor and natriuretic effects of i.c.v. carbachol in normotensive rats. In the present study, the electrolytic lesions of AV3V region significantly inhibited oxotremorine-induced increases in both blood pressure and survival rate in rats subjected to hemorrhagic shock. These findings indicate that AV3V region plays a major role in cholinergic cardiovascular control in hypotensive animals as well as normotensives.

Acute nicotine injections induce c-fos mostly in non-dopaminergic neurons of the midbrain of the rat

Induction of c-fos gene is an immediate and early response in the cascade of molecular events that ultimately lead to long-term alterations in gene expression in neurons. The psychomotor stimulant and positive reinforcing effects of nicotine have been speculated to be mediated by the dopaminergic neurons of the ventral tegmental area (VTA). To identify the precise subsets of VTA neurons of the rat that mediate the acute nicotinergic effects, the pattern of expression of c-fos gene was mapped using immunocytochemical methods. Acute nicotine injections resulted in prominent Fos-like immunoreactivity (-LI) in the medial terminal nucleus of the accessory optic system, the interpeduncular nucleus, and in the caudal linear subnucleus of VTA. The neurons of other VTA subnuclei, viz., the rostral linear, paranigralis, nucleus parabrachialis pigmentosus, and nucleus interfascicularis or the substantia nigra pars compacta did not contain any cells with Fos-LI. Mecamylamine abolished Fos-LI in most of the VTA neurons. These results suggest that acute nicotine injections induce c-fos expression mostly in non-dopaminergic neurons of the ventral tegmental area of the rat and that nicotine induces c-fos most intensely in the interpeduncular nucleus, the superior colliculus, and several other subnuclei of the accessory optic system.

Reticular facilitation of cat visual cortical responses is mediated by nicotinic and muscarinic cholinergic mechanisms

Stimulation of the mesencephalic reticular formation facilitates responses in the visual cortex elicited from the optic radiation. Using intravenous administration of cholinergic antagonists we investigated in adult cats and two kittens whether this effect is mediated by cholinergic mechanisms. When administered alone the muscarinic antagonists atropine and scopolamine and the nicotinic antagonist mecamylamine failed to block reticular facilitation and sometimes even enhanced the effects of reticular stimulation. However, when administered in combination muscarinic and nicotinic antagonists eliminated or significantly reduced the facilitation. This was even true when the two antagonists were administered with a time lag of several hours. These results support the notion that reticular facilitation of cortical responses is mediated by cholinergic mechanisms and suggest that this effect is mediated either by a receptor with a mixed pharmacological property or by two independent pathways acting via nicotinic and muscarinic receptors. This hypothesis is discussed in the context of recent evidence on cholinergic transmission and earlier data on the pharmacology of reticular arousal.

Effects of nicotine on the threshold for rewarding brain stimulation in rats

The rewarding effects of nicotine alone and nicotine challenged with mecamylamine, a nicotine receptor blocker, or naloxone were determined using a rate-independent discrete-trial threshold measure of brain-stimulation reward in rats. If nicotine acts as other drugs of abuse, it would be expected to lower the reward threshold, that is, increase an animal's sensitivity to rewarding brain stimulation, and naloxone would be expected to block this effect, as it does other stimulants in this paradigm. Nicotine was found to significantly lower the reward threshold and mecamylamine blocked this effect. However, although naloxone increased the variability of nicotine's effect on the reward threshold, it failed to dose dependently block nicotine's threshold-lowering effect.

Somatosensory regulation of regional hippocampal blood flow in anesthetized rats

The effect of noxious or non-noxious mechanical stimulation of various cutaneous areas on cerebral blood flow in hippocampus was examined with laser Doppler flowmetry in urethane-anesthetized artificially-ventilated rats. Noxious mechanical stimulation (pinching) of the skin on the face, forepaw, chest, or hindpaw for 20s increased regional hippocampal blood flow (Hpc-BF) and systemic blood pressure, but non-noxious mechanical stimulation (brushing) had no such effect. After the spinal cord was transected at T1 level a forepaw pinch caused no change in blood pressure but still increased Hpc-BF. This suggests that cutaneous noxious stimulation can induce pressor-independent increases in Hpc-BF. The increase in Hpc-BF induced by a forepaw pinch in T1-transected rats was partially reduced by intravenous administration of mecamylamine (2 mg/kg), a nicotinic cholinergic receptor antagonist. Atropine (0.5 mg/kg), a muscarinic cholinergic antagonist was ineffective. These data indicate that the cholinergic vasodilative system is involved in the somatically-induced increase in Hpc-BF via activation of the nicotinic cholinergic receptors.

Effects of systemic and intracerebroventricular administration of mecamylamine, a nicotinic cholinergic antagonist, on spatial memory in rats

The effects of both systemic and intracerebroventricular administration of mecamylamine, a nicotinic antagonist, were tested on the Morris water maze performance of rats. In experiment 1, mecamylamine (0, 3, and 10 mg/kg, IP) was administered before daily training sessions on the Morris water maze, a task in which rats use environmental cues to learn the location of an invisible escape platform in a large pool of water. The escape latencies of rats given the higher dose of mecamylamine were significantly longer than the latencies of rats given either saline or the peripherally-acting nicotinic antagonist hexamethonium (10 mg/kg). Analysis of search patterns during a free swim trial conducted in the absence of an escape platform confirmed the disruptive effects of the higher dose of mecamylamine. Similar drug effects were not observed when these rats were trained to a visible platform, and mecamylamine did not affect the retrieval of spatial information in well-trained rats. In experiment 2, similar effects were observed with ICV administration of mecamylamine (0, 10, 30, and 100 micrograms). The two higher doses increased escape latencies during the last day of place training and all three doses significantly impaired performance on a free swim. No significant effects were noted on subsequent training to a visible platform, and only the highest dose marginally impaired the retrieval of spatial information in well-trained animals. Thus, mecamylamine appears to impair the acquisition of spatial information in the Morris water maze but does not affect retrieval of previously acquired spatial information at comparable doses.

Acute nicotine administration increases somatostatin content and binding in the rat hypothalamus

Within 4 minutes a single, intravenous injection of nicotine (0.3 mg/Kg) induced increases in somatostatin-like immunoreactivity concentrations in the rat hypothalamus but not in the striatum. These changes were associated with a significant increase in the specific binding of somatostatin to putative receptor sites in hypothalamic membranes, while no significant changes were found in striatum. The enhancement of somatostatin binding resulted from a rapid increase in the number of available receptors rather than a change in receptor affinity. This effect appears to be mediated by nicotinic cholinergic receptors, because pretreatment with a centrally active nicotinic receptor antagonist, mecamylamine (5.0 mg/Kg i.v.), prevented the nicotine-induced changes in somatostatin content and binding in the hypothalamus. Mecamylamine alone had no observable effect on the hypothalamic somatostatinergic system. These results suggest that the rat hypothalamic somatostatinergic system can be regulated by nicotine-like acetylcholine receptors.

Spinal serotonin receptors mediate descending facilitation of a nociceptive reflex from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Electrical stimulation in the nucleus reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) produces facilitation and/or inhibition of spinal nociceptive transmission in behavioral and electrophysiological studies. The present study examined spinal neurotransmitter receptors mediating descending facilitation from the NGC/NGC alpha. As previously demonstrated, electrical stimulation in the NGC/NGC alpha at low intensities (approximately equal to 10 microA) produced facilitation and at greater intensities (approximately equal to 38 microA) inhibition of the tail-flick (TF) reflex. Intrathecal pretreatment with the non-selective serotonin (5-HT) receptor antagonist methysergide attenuated or completely abolished facilitation of the TF reflex produced by electrical stimulation in the NGC/NGC alpha; intrathecal pretreatment with atropine, phentolamine, naloxone or mecamylamine was without effect on stimulation-produced facilitation. Descending inhibition from the NGC/NGC alpha produced by electrical stimulation was attenuated or completely abolished by bilateral transection of the dorsolateral funiculi (DLF) of the cervical spinal cord. Descending facilitation produced by electrical stimulation, however, was unaffected or enhanced following DLF transections. Glutamate microinjections (1.7 nmol/0.17 microliters) into the NGC/NGC alpha produced a rapid, repeatable and short-duration facilitation of the TF reflex in rats with bilateral DLF transections and such facilitation was attenuated by intrathecal pretreatment with methysergide, but not atropine, xylamidine (5-HT2 selective receptor antagonist) or MDL-72222 (5-HT3 selective receptor antagonist). These findings suggest that facilitation of the TF reflex from the activation of the cell bodies in the NGC/NGC alpha is mediated by a descending serotonergic pathway traveling in the ventrolateral funiculi and by spinal 5-HT1 receptors.

Locomotor activity in rats after administration of nicotinic agonists intracerebrally

1. Nicotine (0.13 and 0.4 mg kg-1, s.c.) increased the ambulatory component of locomotor activity in rats previously exposed to the drug. Nicotine did not increase repeated movements reliably. 2. An infusion of either nicotine (8 micrograms) or the potent nicotinic agonist cytisine (4 micrograms) into the ventral tegmental area of the forebrain increased ambulation but not repeated movements. 3. An infusion of nicotine or cytisine into the nucleus accumbens, striatum, dorsal hippocampal formation or motor thalamus did not increase ambulatory or repeated movements. 4. Mecamylamine (0.1-1.0 mg kg-1, s.c.) blocked increases in locomotor activity produced by an infusion of nicotine or cytisine into the ventral tegmental area. 5. The locomotor activity produced by systemically administered nicotine may be mediated, in part, through nicotinic receptors located in the ventral tegmental area of the mesolimbic dopamine system.

Protection against diisopropylfluorophosphate intoxication by pyridostigmine and physostigmine in combination with atropine and mecamylamine

Atropine (A), mecamylamine (M), pyridostigmine (Py) and physostigmine (Ph) are pretreatment components of Mix I (A = 0.79, M = 0.79, Py = 0.056 mg/kg) and Mix II (A = 0.79, M = 0.79, Ph = 0.026 mg/kg). They have been successfully used in antagonizing Soman intoxication in experimental animals. Rats were pretreated with either Mix I or Mix II and subsequently exposed to diisopropylfluorphosphate (DFP). Pretreatment with Mix I or Mix II (i.m.) 30 min before DFP (i.v.) protected rats from the lethal effects of DFP. The protective ratios were 2.8 (Mix I) and 6.9 (Mix II). Changes in brain levels of acetylcholine (ACh) were measured to help understand the basis for effectiveness of these pretreatments. In the absence of DFP, pretreatments had no significant (P greater than 0.05) effect on bound or free ACh. Pretreatment did not prevent the DFP-induced rise in bound and free ACh nor the agent-induced physical incapacitation at 30 min post exposure. At 2 h after DFP exposure, rats pretreated with Mix II, but not Mix I, showed significant recovery from signs of physical incapacitation. At 30 min after the administration of 3.3 mg/kg of DFP (i.v.), the levels of free and bound ACh in rats given Mix I or Mix II pretreatment increased above control levels by 705% and 116% and 120% and 43%, respectively. By 2 h after DFP, cerebral ACh levels had changed to 437% and 91% with Mix I pretreatment and 26% and 50% with Mix II pretreatment. These data suggest a correlation between DFP-induced increases in the levels of cerebral ACh, possibly free, and physical incapacitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of behavior by intravenous nicotine injections in laboratory animals

A series of recent studies are reviewed which demonstrate that behavior can be controlled by nicotine injections in different ways depending on the behavioral history of the subject and the schedule of reinforcement under which nicotine is administered. Lever-pressing responses by squirrel monkeys and beagle dogs were maintained well above saline-substitution levels by injections of 10 to 30 micrograms/kg of nicotine under fixed-ratio schedules of nicotine injection. Lever-pressing responses by squirrel monkeys also were well maintained by injections of 30 to 300 micrograms/kg of nicotine under a fixed-interval schedule of nicotine injection. The highest rates of responding were maintained by injections of 10 to 30 micrograms/kg of nicotine under second-order schedules in which responding by squirrel monkeys produced brief-light presentations which were only occasionally paired with nicotine injection. Under other conditions, however, response-produced injections of these same injection doses of nicotine (10 to 30 micrograms/kg) suppressed food-maintained fixed-ratio responding by squirrel monkeys during the punishment component of a multiple schedule. Finally, under a schedule of nicotine postponement, injection doses of 30 to 56 micrograms/kg of nicotine maintained responding that prevented, rather than produced, nicotine injections. These findings indicate that nicotine may control smoking behavior of humans in very complex and divergent ways depending on prevailing environmental conditions.