Distribution of nicotinic cholinergic receptors in the rat tel- and diencephalon: a quantitative receptor autoradiographical study using [3H]-acetylcholine, [alpha-125I]bungarotoxin and [3H]nicotine

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Development of 11C-Labeled ASEM Analogues for the Detection of Neuronal Nicotinic Acetylcholine Receptors (α7-nAChR)

The homo-pentameric alpha 7 receptor is one of the major types of neuronal nicotinic acetylcholine receptors (α7-nAChRs) related to cognition, memory formation, and attention processing. The mapping of α7-nAChRs by PET pulls a lot of attention to realize the mechanism and development of CNS diseases such as AD, PD, and schizophrenia. Several PET radioligands have been explored for the detection of the α7-nAChR. ¹⁸F-ASEM is the most functional for in vivo quantification of α7-nAChRs in the human brain. The first aim of this study was to initially use results from in silico and machine learning techniques to prescreen and predict the binding energy and other properties of ASEM analogues and to interpret these properties in terms of atomic structures using ¹⁸F-ASEM as a lead structure, and second, to label some selected candidates with carbon-11/hydrogen-3 (¹¹C/³H) and to evaluate the binding properties in vitro and in vivo using the labeled candidates. In silico predictions are obtained from perturbation free-energy calculations preceded by molecular docking, molecular dynamics, and metadynamics simulations. Machine learning techniques have been applied for the BBB and P-gp-binding properties. Six analogues of ASEM were labeled with ¹¹C, and three of them were additionally labeled with ³H. Binding properties were further evaluated using autoradiography (ARG) and PET measurements in non-human primates (NHPs). Radiometabolites were measured in NHP plasma. All six compounds were successfully synthesized. Evaluation with ARG showed that ¹¹C-Kln83 was preferably binding to the α7-nAChR. Competition studies showed that 80% of the total binding was displaced. Further ARG studies using ³H-KIn-83 replicated the preliminary results. In the NHP PET study, the distribution pattern of ¹¹C-KIn-83 was similar to other α7 nAChR PET tracers. The brain uptake was relatively low and increased by the administration of tariquidar, indicating a substrate of P-gp. The ASEM blocking study showed that ¹¹C-KIn-83 specifically binds to α7 nAChRs. Preliminary in vitro evaluation of KIn-83 by ARG with both ¹¹C and ³H and in vivo evaluation in NHP showed favorable properties for selectively imaging α7-nAChRs, despite a relatively low brain uptake.

Nicotine' actions on energy balance: Friend or foe?

Obesity has reached pandemic proportions and is associated with severe comorbidities, such as type 2 diabetes mellitus, hepatic and cardiovascular diseases, and certain cancer types. However, the therapeutic options to treat obesity are limited. Extensive epidemiological studies have shown a strong relationship between smoking and body weight, with non-smokers weighing more than smokers at any age. Increased body weight after smoking cessation is a major factor that interferes with their attempts to quit smoking. Numerous controlled studies in both humans and rodents have reported that nicotine, the main bioactive component of tobacco, exerts a marked anorectic action. Furthermore, nicotine is also known to modulate energy expenditure, by regulating the thermogenic activity of brown adipose tissue (BAT) and the browning of white adipose tissue (WAT), as well as glucose homeostasis. Many of these actions occur at central level, by controlling the activity of hypothalamic neuropeptide systems such as proopiomelanocortin (POMC), or energy sensors such as AMP-activated protein kinase (AMPK). However, direct impact of nicotine on metabolic tissues, such as BAT, WAT, liver and pancreas has also been described. Here, we review the actions of nicotine on energy balance. The relevance of this interaction is interesting, because considering the restricted efficiency of obesity treatments, a possible complementary approach may focus on compounds with known pharmacokinetic profile and pharmacological actions, such as nicotine or nicotinic acetylcholine receptors signaling.

Lipid-induced thermogenesis is up-regulated by the first cold-water immersions in juvenile penguins

The passage from shore to marine life is a critical step in the development of juvenile penguins and is characterized by a fuel selection towards lipid oxidation concomitant to an enhancement of lipid-induced thermogenesis. However, mechanisms of such thermogenic improvement at fledging remain undefined. We used two different groups of pre-fledging king penguins (Aptenodytes patagonicus) to investigate the specific contribution of cold exposure during water immersion to lipid metabolism. Terrestrial penguins that had never been immersed in cold water were compared with experimentally cold-water immersed juveniles. Experimentally immersed penguins underwent ten successive immersions at approximately 9-10 °C for 5 h over 3 weeks. We evaluated adaptive thermogenesis by measuring body temperature, metabolic rate and shivering activity in fully immersed penguins exposed to water temperatures ranging from 12 to 29 °C. Both never-immersed and experimentally immersed penguins were able to maintain their homeothermy in cold water, exhibiting similar thermogenic activity. In vivo, perfusion of lipid emulsion at thermoneutrality induced a twofold larger calorigenic response in experimentally immersed than in never-immersed birds. In vitro, the respiratory rates and the oxidative phosphorylation efficiency of isolated muscle mitochondria were not improved with cold-water immersions. The present study shows that acclimation to cold water only partially reproduced the fuel selection towards lipid oxidation that characterizes penguin acclimatization to marine life.

Nicotine-induced upregulation of native neuronal nicotinic receptors is caused by multiple mechanisms

Nicotine causes changes in brain nicotinic acetylcholine receptors (nAChRs) during smoking that initiate addiction. Nicotine-induced upregulation is the long-lasting increase in nAChR radioligand binding sites in brain resulting from exposure. The mechanisms causing upregulation are not established. Many different mechanisms have been reported with the assumption that there is a single underlying cause. Using live rat cortical neurons, we examined for the first time how exposure and withdrawal of nicotine shape the kinetics of native α4β2-containing nAChR upregulation in real time. Upregulation kinetics demonstrates that at least two different mechanisms underlie this phenomenon. First, a transient upregulation occurs that rapidly reverses, faster than nAChR degradation, and corresponds to nAChR conformational changes as assayed by conformational-dependent, subunit-specific antibodies. Second, a long-lasting process occurs correlating with increases in nAChR numbers caused by decreased proteasomal subunit degradation. Previous radioligand binding measurements to brain tissue have measured the second process and largely missed the first. We conclude that nicotine-induced upregulation is composed of multiple processes occurring at different rates with different underlying causes.

Nicotine excites hypothalamic arcuate anorexigenic proopiomelanocortin neurons and orexigenic neuropeptide Y neurons: similarities and differences

Two of the biggest health problems facing us today are addiction to nicotine and the increased prevalence of obesity. Interestingly, nicotine attenuates obesity, but the underlying mechanism is not clear. Here we address the hypothesis that if weight-reducing actions of nicotine are mediated by anorexigenic proopiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus, nicotine should excite these cells. Nicotine at concentrations similar to those found in smokers, 100-1,000 nM, excited POMC cells by mechanisms based on increased spike frequency, depolarization of membrane potential, and opening of ion channels. This was mediated by activation of both α7 and α4β2 nicotinic receptors; by itself, this nicotine-mediated excitation could explain weight loss caused by nicotine. However, in control experiments nicotine also excited the orexigenic arcuate nucleus neuropeptide Y (NPY) cells. Nicotine exerted similar actions on POMC and NPY cells, with a slightly greater depolarizing action on POMC cells. Immunocytochemistry revealed cholinergic axons terminating on both cell types. Nicotine actions were direct in both cell types, with nicotine depolarizing the membrane potentials and reducing input resistance. We found no differences in the relative desensitization to nicotine between POMC and NPY neurons. Nicotine inhibited excitatory synaptic activity recorded in NPY, but not POMC, cells. Nicotine also excited hypocretin/orexin neurons that enhance cognitive arousal, but the responses were smaller than in NPY or POMC cells. Together, these results indicate that nicotine has a number of similar actions, but also a few different actions, on POMC and NPY neurons that could contribute to the weight loss associated with smoking.

α7 Nicotinic Receptor Agonists: Potential Therapeutic Drugs for Treatment of Cognitive Impairments in Schizophrenia and Alzheimer's Disease

Accumulating evidence suggests that α7 nicotinic receptors (α7 nAChRs), a subtype of nAChRs, play a role in the pathophysiology of neuropsychiatric diseases, including schizophrenia and Alzheimer's disease (AD). A number of psychopharmacological and genetic studies shown that α7 nAChRs play an important role in the deficits of P50 auditory evoked potential in patients with schizophrenia, and that (α nAChR agonists would be potential therapeutic drugs for cognitive impairments associated with P50 deficits in schizophrenia. Furthermore, some studies have demonstrated that α7 nAChRs might play a key role in the amyloid-β (Aβ)-mediated pathology of AD, and that α7 nAChR agonists would be potential therapeutic drugs for Aβ deposition in the brains of patients with AD. Interestingly, the altered expression of α7 nAChRs in the postmortem brain tissues from patients with schizophrenia and AD has been reported. Based on all these findings, selective α7 nAChR agonists can be considered potential therapeutic drugs for cognitive impairments in both schizophrenia and AD. In this article, we review the recent research into the role of α7 nAChRs in the pathophysiology of these diseases and into the potential use of novel α7 nAChR agonists as therapeutic drugs.

Nicotine-induced upregulation of nicotinic receptors: underlying mechanisms and relevance to nicotine addiction

A major hurdle in defining the molecular biology of nicotine addiction has been characterizing the different nicotinic acetylcholine receptor (nAChR) subtypes in the brain and how nicotine alters their function. Mounting evidence suggests that the addictive effects of nicotine, like other drugs of abuse, occur through interactions with its receptors in the mesolimbic dopamine system, particularly ventral tegmental area (VTA) neurons, where nicotinic receptors act to modulate the release of dopamine. The molecular identity of the nicotinic receptors responsible for drug seeking behavior, their cellular and subcellular location and the mechanisms by which these receptors initiate and maintain addiction are poorly defined. In this commentary, we review how nicotinic acetylcholine receptors (nAChRs) are upregulated by nicotine exposure, the potential posttranslational events that appear to cause it and how upregulation is linked to nicotine addiction.

Chronic exposure to typical or atypical antipsychotics in rodents: temporal effects on central alpha7 nicotinic acetylcholine receptors

A decrease in alpha7 nicotinic acetylcholine receptors in the hippocampus has been hypothesized to contribute to alterations in auditory gating and other behavioral impairments in schizophrenia. However, while both typical and atypical neuroleptics are routinely used in the therapeutics of schizophrenia, little is known about their effects on auditory gating or alpha7 nicotinic acetylcholine receptor expression particularly when they are administered for extended periods of time (which is common in the clinical setting). In the present study in normal rats, the residual effects of prior chronic treatment (90 or 180 days) with representative typical and atypical neuroleptics (oral haloperidol, 2.0 mg/kg/day; chlorpromazine, 10.0 mg/kg/day, risperidone, 2.5 mg/kg/day; or olanzapine, 10.0 mg/kg/day) on prepulse inhibition of the auditory gating response were investigated. The densities of alpha7 nicotinic acetylcholine receptors were subsequently measured using [125I]-alpha-bungarotoxin autoradiography. The results indicated that none of the compounds significantly altered the startle amplitude or prepulse inhibition response either during drug treatment (day 60) or after 90 or 180 days of treatment (i.e. during a drug free washout). However, prior exposure to chlorpromazine, risperidone and olanzapine for 90 days resulted in modest but significant (P<0.01) decreases in [125I]-alpha-bungarotoxin binding sites in some brain regions (e.g. posterior cortical amygdala). After 180 days of treatment, decreases in [(125I]-alpha-bungarotoxin binding ranging from approximately 12% (lateral dentate gyrus) up to 24% (e.g. CA1 hippocampal region) were evident in the risperidone group in 13 of the 36 regions analyzed while decreases associated with the other neuroleptics agents were still present, but not statistically significant. These data indicate that the commonly used atypical neuroleptic, risperidone is associated with time dependent and persistent negative effects on an important biological substrate of memory (i.e. the alpha7 nicotinic receptor), but that the magnitude of the deficits was not sufficient to impair auditory gating.

Localization of [3H]nicotine, [3H]cytisine, [3H]epibatidine, and [125I]alpha-bungarotoxin binding sites in the brain of Macaca mulatta

We determined the localization of [(3)H]nicotine, [(3)H]cytisine, [(3)H]epibatidine, and [(125)I]alpha-bungarotoxin binding sites in the brain of rhesus monkey by means of receptor autoradiography. The labelings by [(3)H]nicotine, [(3)H]cytisine, and [(3)H]epibatidine were highly concordant, except for epibatidine. Layer IV of some cortical areas, most thalamic nuclei, and presubiculum displayed high levels of labeling for the three ligands. Moderate levels of binding were detected in the subiculum, the septum, and the mesencephalon. Low levels were present in layers I-II and VI of the cortex, the cornu Ammonis, the dentate gyrus, and the amygdala. In addition, the level of epibatidine labeling was very high in the epithalamic nuclei and the interpeduncular nucleus, whereas labeling by nicotine and cytisine was very weak in the same regions. The distribution of [(125)I]alpha-bungarotoxin binding differed from the binding of the three agonists. The labeling was dense in layer I of most cortical areas, dentate gyrus, stratum lacunosum-moleculare of CA1 field, several thalamic nuclei, and medial habenula. A moderate labeling was found in layers V and VI of the prefrontal and frontal cortices, layer IV of primary visual cortex, amygdala, septum, hypothalamus, and some mesencenphalic nuclei. A weak signal was also detected in subiculum, claustrum, stratum oriens, and stratum lucidum of cornu Ammonis and also in some mesencephalic nuclei. The distribution of nicotine, cytisine, and epibatidine bindings corresponds broadly to the patterns observed in rodents, with the marked exception of the epithalamus. However, in monkey, those distributions match the distribution of alpha2 messenger RNA, rather than that of alpha4 transcripts as it exists in rodent brains. The distribution of the binding sites for alpha-bungarotoxin is larger in the brain of rhesus monkeys than in rodent brain, suggesting a more important role of alpha7 receptors in primates.

Nicotinic receptor-mediated effects on appetite and food intake

It is well known, although not well understood, that smoking and eating just do not go together. Smoking is associated with decreased food intake and lower body weight. Nicotine, administered either by smoking or by smokeless routes, is considered the major appetite-suppressing component of tobacco. Perhaps the most renowned example of nicotine's influence on appetite and feeding behavior is the significant weight gain associated with smoking cessation. This article presents an overview of the literature at, or near, the interface of nicotinic receptors and appetite regulation. We first consider some of the possible sites of nicotine's action along the complex network of neural and non-neural regulators of feeding. We then present the hypothesis that the lateral hypothalamus is a particularly important locus of the anorectic effects of nicotine. Finally, we discuss the potential role of endogenous cholinergic systems in motivational feeding, focusing on cholinergic pathways in the lateral hypothalamus.

Alpha7-nicotinic receptor expression and the anatomical organization of hippocampal interneurons

C3H and DBA/2 mice differ in their hippocampal inhibitory function, as measured by the inhibitory gating of pyramidal neuron response to repeated auditory stimulation. This functional difference appears to be related to differences in expression of the alpha7 nicotinic cholinergic receptor, which may be generally expressed by interneurons. This study examines the relationship between genetic variation in alpha7 receptor subunit expression and GABAergic interneuron distribution in various regions and layers of the hippocampus in the two mouse strains. Subpopulations of hippocampal interneurons in both mouse strains were found to bind [(125)I]alpha-bungarotoxin. However, the distribution of the [(125)I]alpha-bungarotoxin-positive hippocampal interneurons was significantly different between C3H and DBA/2 mice. In region CA1, and to a lesser extent in region CA3, DBA/2 mice had increased numbers of [(125)I]alpha-bungarotoxin-positive neurons in stratum lacunosum-moleculare and decreased numbers in stratum oriens. Similar differences in GABAergic neuron distribution were observed in region CA1 in the two strains. C3H/DBA/2 F1 animals were backcrossed to the C3H parental strain for six generations, with selection for either the DBA/2 or C3H allelic variant of the alpha7 receptor gene. The distribution of [(125)I]alpha-bungarotoxin labeling closely resembled the DBA/2 parental phenotype in animals retaining the DBA/2 allele of the alpha7 gene. These data suggest that the alpha7 receptor gene locus may influence the anatomical organization of at least a subset of hippocampal interneurons by an as yet unidentified mechanism. This difference in interneuron anatomy may also contribute to functional differences in inhibitory sensory gating between the two strains.

Ligands for in vivo imaging of nicotinic receptor subtypes in Alzheimer brain

The neuronal nicotinic acetylcholine receptors (nAChR) are involved in functional processes in brain including cognitive function and memory. A severe loss of the nAChRs has been detected in brain of patients with Alzheimer's disease (AD). There is a great interest to image nAChRs noninvasive for detection of receptor impairments even at a presymptomatic stage of AD as well for monitoring outcome of drug treatment. (S) [11C]Nicotine, has so far been the only nAChR ligand used in positron emission tomography (PET) studies for visualizing nAChRs in human brain. In order to develop PET/SPECT nAChRs ligands for detection of subtypes of nAChRs nicotine analogues, epibatidine and A-85380 compounds have been characterized in vitro and investigated in vivo. Epibatidine and A-85380 have been found to have higher specific signals and more favorable kinetic parameters than nicotine and its analogues. The epibatidine and A-85380 compounds can also be radiolabeled with high specific radioactivity, show affinities for the nAChRs in the pM range and readily cross the blood-brain barrier. In addition they reversibly bind to the nAChRs and show low non-specific binding and moderately fast metabolism. Due to a probably high alpha4beta2 nAChR selectivity combined with low toxicity, the A-85380 analogs presently seem to be the most promising nAChR ligand imaging of subtypes of nAChRs in human brain.

Deficits in spatial learning and nicotinic-acetylcholine receptors in older, spontaneously hypertensive rats

Spontaneously hypertensive rats are often used as models of attention deficit hyperactivity disorder and to investigate the effects of hypertension on cognitive function. Along with the wide variety of cardiovascular anomalies, these animals as young adults also exhibit deficits in memory and attention and central nicotinic-acetylcholine receptor sites. These findings may have particular significance since nicotinic receptors appear to be involved in the regulation of cerebral circulation and mnemonic function. Furthermore, a lack of high affinity nicotinic receptors (in knockout mice) has also been shown to accelerate both the structural and cognitive degeneration associated with age, findings that may be especially relevant to age-related memory disorders such as Alzheimer's Disease where large deficits in nicotinic receptors are observed. Since spontaneously hypertensive rats appear to be both memory-impaired and deficient in nicotinic receptors at a young age (compared to the non-hypertensive phenotype, Wistar-Kyoto rats), we were interested to learn if these conditions were exacerbated in older animals with particular interest in specific nicotinic receptor subtypes in memory areas of the brain. Spatial learning was assessed in 15-month-old subjects of each phenotype (i.e. hypertensive and non-hypertensive) using a two-phase water maze paradigm, and nicotinic receptors were measured via autoradiography with [125I]-alpha-bungarotoxin and [3H]-epibatidine. In the water maze, both groups learned to locate a hidden platform as indicated by progressively shorter latencies across training days, however, Wistar-Kyoto rats were more efficient in both phases. While the number of both bungarotoxin and epibatidine binding sites was lower in the hypertensive rats across several brain regions, in the case of epibatidine binding, the magnitude of the difference and the number of areas affected was generally greater and included areas important for spatial learning (e.g. frontal and entorhinal cortex). In a direct comparison between 3-month-old and 15-month-old rats of each phenotype, epibatidine sites were markedly reduced by age (i.e. by greater than 50% in some cases) across multiple brain regions in both groups, although Wistar-Kyoto rats appeared to be more substantially affected by age. These data further support the use of the spontaneously hypertensive rat as model for studying learning-impairment and reduced central nicotinic receptors and also indicate that these characteristics persist and (in the case of high affinity nicotinic receptor cites) worsen with age.

Local alpha-bungarotoxin-sensitive nicotinic receptors in the nucleus accumbens modulate nicotine-stimulated dopamine secretion in vivo

Nicotinic cholinergic receptors in the ventral tegmental area are required for the accumbal dopamine response to systemic nicotine. In contrast, the role of nicotinic receptors located within the nucleus accumbens itself has not been clarified for systemically administered nicotine. In the present study, in vivo microdialysis of accumbal dopamine secretion and receptor antagonist blockade in both the ventral striatal nucleus accumbens and the midbrain ventral tegmental area were used to evaluate this question. The nicotinic receptor antagonists methyllycaconitine or mecamylamine were delivered through the accumbal dialysis probe, followed by 0.09mg/kg nicotine (i.v.). The alpha7 subunit antagonist methyllycaconitine inhibited 71% of the dopamine response (P<0.01), whereas mecamylamine was completely ineffective. In addition, the classical alpha7 subunit antagonist alpha-bungarotoxin infused into the nucleus accumbens adjacent to the microdialysis probe, significantly reduced dopamine release by 0.065 or 0.09mg/kg nicotine (i.v.; P<0. 05). Combined, these data indicate the involvement of alpha7 subunit-containing nicotinic receptors in the nucleus accumbens. In contrast, local infusion of mecamylamine into the ventral tegmental area effectively blocked nicotine-induced accumbal dopamine release. Simultaneous infusions of methyllycaconitine into the accumbens and mecamylamine into the ventral tegmental area induced greater blockade of nicotine-stimulated dopamine secretion than methyllycaconitine or mecamylamine alone. In conclusion, the present study demonstrates that different types of nicotinic cholinergic receptors, located in the ventral striatal nucleus accumbens (alpha-bungarotoxin sensitive and mecamylamine insensitive) and the midbrain ventral tegmental area (mecamylamine sensitive), may be required for the full effects of nicotine on the mesostriatal dopaminergic pathway. While activation of nicotinic cholinergic receptors in the ventral tegmentum is required for the accumbal dopamine response to systemic nicotine, accumbal nicotinic receptors themselves act as modulators of this response. This fine tuning of the dopamine reward pathway through alpha7 nicotinic cholinergic receptors in the nucleus accumbens may amplify the secretion of dopamine, allowing a subthreshold brain concentration of nicotine to become an effective stimulus for dopamine secretion.

Increase in the adenine nucleotide translocase content of duckling subsarcolemmal mitochondria during cold acclimation

Intermyofibrillar and subsarcolemmal mitochondria were isolated from duckling gastrocnemius muscle. The adenine nucleotide translocase (ANT) content of subsarcolemmal mitochondria was found to be half of that present in intermyofibrillar mitochondria. In addition, cold acclimation resulted in a 1.7-fold increase in subsarcolemmal mitochondrial ANT content, with intermyofibrillar mitochondrial ANT remaining constant. This change in mitochondrial ANT content correlates with the previously reported cold-induced change in the sensitivity of mitochondria to palmitate-inhibited ATP synthesis [Roussel et al. (1998) FEBS Lett. 439, 258-262]. It is suggested that the mitochondrial ANT content enhances or reduces the fatty acid uncoupling activity in tissue, depending on the energetic state of mitochondria.

Magnocellular neurons of the rat supraoptic nucleus are endowed with functional nicotinic acetylcholine receptors

Acetylcholine can stimulate the release of vasopressin. In organ-cultured hypothalamo-neurohypophyseal systems, acetylcholine enhanced vasopressin release by acting in or near the supraoptic nucleus Extracellular recordings suggested that acetylcholine can increase supraoptic neuron excitability. These effects could be mimicked, in part, by nicotine or blocked by nicotinic antagonists, suggesting that they might be mediated by nicotinic acetylcholine receptors. Autoradiography indicated that alpha-bungarotoxin binding sites are present in the supraoptic nucleus; however, neither acetylcholine nor nicotine binding sites could be detected. Thus, the existence, let alone the nature, of nicotinic receptors in the supraoptic nucleus has so far remained elusive. The present work attempts to determine: (i) whether functional nicotinic receptors are present in this nucleus; (ii) whether they are located on neurosecretory magnocellular cells or at presynaptic sites; (iii) what their pharmacological and biophysical properties are; (iv) whether they influence the activity of all or only part of supraoptic neurons. Whole-cell recordings were performed in hypothalamic slices or in acutely dissociated supraoptic neurons and the effect of nicotinic agonists was tested under voltage-clamp conditions. Autoradiography was done in coronal hypothalamic sections, using [3H]epibatidine and [125I]alpha-bungarotoxin as ligands. Our results indicate that supraoptic neurons possess functional nicotinic receptors containing the alpha7 subunit.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

The nicotinic acetylcholine receptor agonist (+/-)-epibatidine increases FGF-2 mRNA and protein levels in the rat brain

In a previous work, we showed that acute intermittent nicotine treatment up-regulates the level of fibroblast growth factor-2 (FGF-2) mRNA in brain regions of tel- and mesencephalon of rats suggesting that neuroprotective effect of (-)nicotine may, at least in part, involve an activation of the neuronal FGF-2 signalling. The present experiments were designed to extend the study on the nicotinic receptor mediated up-regulation of FGF-2 mRNA levels to the use of the potent nicotinic acetylcholine receptor (nAChR) agonist (+/-)-epibatidine. The (+/-)-epibatidine treatment led to a strong and long lasting up-regulation of FGF-2 mRNA expression in the cerebral cortex, in the hippocampal formation, in the striatum and in the substantia nigra. This FGF-2 mRNA induction, already statistically significant at 4 h, peaked at 12 h from treatment and was only partially returned towards normal levels at 48 h, the last time point examined. Using Western blot analysis it was found that the epibatidine-induced upregulation of FGF-mRNA is accompaned by an increase of FGF-2 protein level at the 20-h time-interval. These (+/-)-epibatidine effects on FGF-2 expression were antagonized by the non-competitive nAChR antagonist mecamylamine, indicating an involvement of nicotinic receptors. In the same brain areas examined, no changes were observed in the fibroblast growth factor receptor-1 (FGFR-1) mRNA levels, in brain-derived neurotrophic factor (BDNF) and in glial cell line-derived neurotrophic factor (GDNF) mRNA levels. In view of the neurotrophic function of FGF-2, these results, together with previous ones, could further help to understand the molecular mechanisms mediating the previously observed neuroprotective effects of (-)nicotine.

Synthesis and characterization of binding of 5-[76Br]bromo-3-[[2(S)-azetidinyl]methoxy]pyridine, a novel nicotinic acetylcholine receptor ligand, in rat brain

5-[76Br]Bromo-3-[[2(S)-azetidinyl]methoxy]pyridine ([76Br]BAP), a novel nicotinic acetylcholine receptor ligand, was synthesized using [76Br]bromide in an oxidative bromodestannylation of the corresponding trimethylstannyl compound. The radiochemical yield was 25%, and the specific radioactivity was on the order of 1 Ci/micromol. The binding properties of [76Br]BAP were characterized in vitro and in vivo in rat brain, and positron emission tomography (PET) experiments were performed in two rhesus monkeys. In association experiments on membranes of the cortex and thalamus, >90% of maximal specific [76Br]BAP binding was obtained after 60 min. The dissociation half-life of [76Br]BAP was 51 +/- 6 min in cortical membranes and 56 +/- 3 min in thalamic membranes. Saturation experiments with [76Br]BAP revealed one population of binding sites with dissociation constant (K(D)) values of 36 +/- 9 and 30 +/- 9 pM in membranes of cortex and thalamus, respectively. The maximal binding site density (Bmax) values were 90 +/- 17 and 207 +/- 33 fmol/mg in membranes of cortex and thalamus, respectively. Scatchard plots were nonlinear, and the Hill coefficients were <1, suggesting the presence of a lower-affinity binding site. In vitro autoradiography studies showed that binding of [76Br]BAP was high in the thalamus and presubiculum, moderate in the cortex and striatum, and low in the cerebellum and hippocampus. A similar pattern of [76Br]BAP accumulation was observed by ex vivo autoradiography. In vivo, binding of [76Br]BAP in whole rat brain was blocked by preinjection of (S)(-)-nicotine (0.3 mg/kg) by 27, 52, 68, and 91% at survival times of 10, 25, 40, 120, and 300 min, respectively. In a preliminary PET study in rhesus monkeys, the highest [76Br]BAP uptake was found in the thalamus, and radioactivity was displaceable by approximately 60% with cytisine and by 50% with (S)(-)-nicotine. The data of this study indicate that [76Br]BAP is a promising radioligand for the characterization of nicotinic acetylcholine receptors in vivo.

Shivering thermogenesis in the pigeon: the effects of activity, diurnal factors, and feeding state

Shivering (electromyographic activity of the pectoral muscle), oxygen consumption, and body temperature were measured from undisturbed pigeons for periods of several weeks, and segments from the midparts of each phase of the light-dark cycle were compared at various ambient temperatures and feeding regimes. Behavior was recorded with a video camera. None of the observed types of behavior (e.g., walking, preening, feeding, drinking, pecking, defecation) induced spurious electrical activity in the pectoral muscle. On the other hand, none of these behaviors directly inhibited ongoing shivering. There was no difference in the mean level of shivering between the light (L) and dark (D) phases of the day in any of the conditions, although body temperature was 2 degreesC higher during L. Measurements of integrated electromyogram (EMG) with high temporal resolution (28 samples/s) showed that, at 1 degreesC, shivering in the pectoral muscle was present for more than 98% of the time. Plots of oxygen consumption against root mean square EMG were obtained in each condition by a filtering procedure that excludes data points in which oxygen consumption is affected by motor activity. These plots showed that the increase in heat production induced by a unit increase in pectoral EMG was lower in D than in L and that it was further lowered by fasting. The amplitude spectra of raw EMG signals were similar in all conditions. Spectra of demodulated (rectified, low-pass filtered) EMG showed a distinct rhythmicity around 8 Hz at 21 degreesC that was further enhanced by fasting but absent at 1 degreesC. This suggests that the degree of synchronization and pattern of recruitment of motor units are specific for various temperatures and feeding regimes, and may partly explain the variable relation between heat production and muscle electrical activity. The results emphasize the advantages of long-term measurements for understanding the control of thermogenesis in birds.

How to stay cool in a hot desert--a lesson from the rock pigeon

This review presents an overview of the puzzle called "cutaneous water evaporation (CWE) cooling mechanism" in birds. Heat acclimation of the rock pigeon induces cellular modifications that affect the myocardium, cutaneous vasculature, and the epidermis, and hence enable the initiation of CWE. These cells are the targets for adrenergic signals that participate in the mechanism that controls the initiation and intensity of CWE. As a result the cardiac performance of the heat acclimated pigeon is intensified in response to adrenergic agents, and peripheral blood vessels and the epidermis both increase their permeability in response to heat stress. The CWE cooling mechanism is more economical in terms of water conservation, and provides more efficient protection to its owner, compared to the 'classic' respiratory cooling mechanism. Moreover, current data present the rock pigeon--a small diurnal homeotherm--as a classic model for a desert bird.

Laminar distribution of nicotinic receptor subtypes in human cerebral cortex as determined by [3H](-)nicotine, [3H]cytisine and [3H]epibatidine in vitro autoradiography

The subregional localization of different nicotinic acetylcholine receptor subtypes in human cerebral cortex was estimated by quantitative in vitro autoradiography using the nicotinic ligands [3H](-)nicotine, [3H]cytisine and [3H]epibatidine in large whole human forebrain hemispheres. Saturation experiments in frontal cortex revealed for [3H](-)nicotine two binding sites with affinity constants (Kd) of 0.45 and 6.3 nM and binding site densities (Bmax) of 3.0 and 14.2 pmol/g, for [3H]cytisine one binding site with Kd of 0.19 nM and Bmax of 21.8 pmol/g, and for [3H]epibatidine one binding site with Kd of 0.011 nM and Bmax of 20.0 pmol/g. The laminar binding distributions of the three ligands were compared in different cortical areas by creating binding profiles perpendicular to the entire cortical depth. The regional autoradiographic binding patterns of the three ligands were essentially similar, with higher receptor binding in cortical layers I, III and V. In the primary sensory cortex and inferior frontal sulcus, marked binding of all ligands was observed in layer III. [3H]Cytisine showed the lowest difference between maximal and minimal binding within the gray tissue in all other areas. In the primary motor cortex, [3H]epibatidine and [3H](-)nicotine showed high binding in layers III and V. The [3H](-)nicotine binding was higher than that of the other ligands in layers I and VI of the primary motor cortex, the deeper layer V of the primary sensory cortex, layer III of the superior temporal sulcus and layer VI of the parietal cortex. A distinct band of binding of [3H](-)nicotine and [3H]epibatidine but not of [3H]cytisine was found in layer IIlb of the occipital cortex and layer V of the superior temporal sulcus. [3H]Epibatidine showed higher binding than the other ligands in all layers of the medial frontal, superior frontal and superior temporal sulcus. The findings with the three nicotinic ligands suggest three binding sites in the cortex with different laminar distributions. All three ligands bound to an identical receptor site, most likely the alpha4 nicotinic receptor subunit. The morphological distribution of [3H]epibatidine and [3H](-)nicotine binding indicate that they bind to an additional site, especially in the primary motor cortex, in layer IIIb of the occipital cortex and layer V of the superior temporal sulcus. High binding of [3H](-)nicotine in layers I and VI of the primary motor cortex, the deeper layer V of the primary sensory cortex, layer III of the superior temporal sulcus and layer VI of the parietal cortex may indicate a third binding site.

Acute intermittent nicotine treatment produces regional increases of basic fibroblast growth factor messenger RNA and protein in the tel- and diencephalon of the rat

Several findings show a neuroprotective effect of nicotine treatment in different experimental models, and a negative correlation has been observed between cigarette smoking and the incidence of Parkinson's disease. It seems possible that nicotine may in part exert its neuroprotective actions by favouring the synthesis of neurotrophic factors. The aim of this study was to determine whether the nicotine treatment could be associated with the induction of a neurotrophic factor in brain regions with nicotinic receptors. Thus, we analysed by in situ hybridization and RNAse protection assay the effects of (-)nicotine on basic fibroblast growth factor messenger RNA and by immunocytochemistry fibroblast growth factor-2 protein in the tel- and diencephalon of rats following single or acute intermittent (-)nicotine treatment. The present results showed that acute intermittent (-)nicotine treatment (four i.p. injections at intervals of 30 min), but not single injections, lead to a substantial and dose-related (0.1-2 mg/kg) up-regulation of fibroblast growth factor-2 messenger RNA levels in the cerebral cortex, in the hippocampus, in the striatum and ventral midbrain. This induction of fibroblast growth factor-2 expression peaked 4 h after the first injection and returned to normal levels within 24 h. The change of fibroblast growth factor-2 messenger RNA levels was associated with increased fibroblast growth factor-2 immunoreactivity mainly localized to nerve cells. The treatment was effective also when repeated in the same animals three or five days after the first injection. The pre-treatment with the non-competitive (-)nicotine receptor antagonist mecamylamine blocked the (-)nicotine effects on fibroblast growth factor-2 messenger RNA levels. In the above areas, no changes were observed in the fibroblast growth factor-1, 2 and 3 receptor messenger RNA levels nor in brain-derived neurotrophic factor messenger RNA levels. The present data indicate an ability of intermittent (-)nicotine to increase fibroblast growth factor-2 in many tel- and diencephalic areas. In view of the trophic function of fibroblast growth factor-2, the previously observed neuroprotective effects of (-)nicotine may at least in part involve an activation of the neuronal fibroblast growth factor-2 signalling, and open up new avenues for treatment of Parkinson's disease and Alzheimer's disease based on the existence of nicotinic receptor subtypes enhancing fibroblast growth factor-2 signalling in many regions of the tel- and diencephalon.

Nicotinic antagonist alpha-bungarotoxin binding to rat hippocampal neurons containing nitric oxide synthase

The hippocampus is a major target of alpha-bungarotoxin (alpha-BTX) binding. This ligand binds to the alpha 7 nicotinic, cholinergic receptor, which has been implicated in hippocampal habituation to repetitive auditory stimulation, a phenomenon thought to involve inhibitory neurons. This study examined whether alpha-BTX binds to neurons containing nitric oxide synthase (NOS), a marker of one subgroup of inhibitory hippocampal neurons. Rat hippocampal sections were processed for NOS immunohistochemistry, photographed and then processed for [125I]alpha-BTX autoradiography. Comparison between the distribution of neurons immunoreactive for NOS and those positive for alpha-BTX binding in the same regions of the hippocampal formation revealed a variable degree of colocalization of NOS and alpha-BTX. Of the cells labeled with alpha-BTX, 2% in the dentate gyrus and 40% in the hippocampus proper were also immunoreactive for NOS. These NOS/alpha-BTX neurons were most prevalent in CA1 stratum oriens. The results suggest a possible role for NOS-containing neurons in alpha 7-mediated inhibition to repetitive auditory stimulation in rat hippocampus.

Cognitive impairment in spontaneously hypertensive rats: role of central nicotinic receptors. I

Both human essential hypertension and genetically induced hypertension in rats have been associated with a range of impairments of cognitive ability. The spontaneous hypertensive rat (SHR) previously has been shown to exhibit a decrease in the expression of brain nicotinic acetylcholine receptors, a factor that could play a role in the impaired ability of this strain in the performance of learning and memory-related tasks. The purpose of this study was to help determine whether task impairment by SHR was related to the reduced expression of central nicotinic acetylcholine receptors. Twelve-week-old SHR were tested in two phases of a water maze (spatial memory) task, and their performance was compared with that of two age-matched normotensive strains, Wistar Kyoto (WKY) and Wistar rats. During Phase 1, SHR exhibited significantly increased latencies to locate a hidden platform as compared with either WKY or Wistar rats. During Phase 2 (subsequent series of trials after a 4-day inter-phase period), where rats were required to find a new platform location, SHR again exhibited significantly impaired performance compared to the normotensive strains. In a single trial passive avoidance paradigm, SHR again displayed significantly reduced avoidance behavior as compared with both WKY and Wistar rats. In consecutive coronal sections, the density of [3H]cytisine binding sites was decreased in SHR by up to 25% in about half of the brain regions examined, with the deficits particularly apparent in cephalic regions. The binding of [125I]alpha-bungarotoxin to brain sections also was decreased in SHR; however, only certain brain areas exhibited significant interstrain differences. These alterations in the expression of putative nicotinic receptor subtypes in SHR were not due to changes in the density of cholinergic neurons since there were no interstrain differences in the binding densities for [3H]vesamicol, which labels the vesicular acetylcholine transporter. Moreover, the magnitude of nicotine-stimulated rubidium efflux from cortical and striatal synaptosomes in vitro was significantly reduced in samples derived from SHR as compared with those from normotensive rats. These results are consistent with the possibility that a reduction in the expression of cortical nicotinic receptors in SHR plays a role in this strain's impaired performance of both spatial and non-spatial learning and memory-related tasks.

Condition-independent sensitization of locomotor stimulation and mesocortical dopamine release following chronic nicotine treatment in the rat

Chronic nicotine (NIC) pretreatment has been shown to enhance NIC-induced locomotor stimulation, an effect that seems critically dependent on activation of brain dopamine (DA) systems. In the present study the effects of chronic, intermittent NIC treatment were examined in the rat to establish whether such behavioral sensitization is associated with specific, regional changes in brain dopaminergic activity. Male rats received daily injections in their home cage with either saline (SAL) or NIC (0.5 mg/kg, s.c.) for 12 days. Twenty-four hours later, the locomotor activity of the animals subjected to NIC challenge as well as the functional responsiveness of the mesolimbocortical dopaminergic system were assessed. To this end, microdialysis experiments were performed in awake animals, measuring extracellular concentrations of DA and its metabolites in the prefrontal cortex (PFC) and the nucleus accumbens (NAC). Extracellular single cell recordings from DA neurons in the ventral tegmental area (VTA) were also performed in anesthetized animals. NIC (0.5 mg/kg, s.c.) increased all measured parameters of locomotor activity, with the exception of rearing, in SAL-pretreated animals; these effects were substantially enhanced after pretreatment with NIC. Nicotine (0.5 mg/kg, s.c.) increased DA release in both the PFC and the NAC in SAL-treated animals. Nicotine pretreatment significantly enhanced this effect in the PFC, whereas it did not affect the response in the NAC. Low doses of intravenously administered NIC dose-dependently increased burst activity, starting at 12 micrograms/kg in the SAL pretreated animals and at 6 micrograms/kg in the NIC-pretreated animals, and also dose-dependently increased firing rate in SAL as well as NIC-pretreated animals, although starting at a higher dose level, i.e., 25 micrograms/kg. These results demonstrate that behavioral sensitization after chronic NIC treatment is accompanied by an enhanced dopamine release specifically within the PFC. This phenomenon may be highly significant for the dependence-producing effects of NIC, particularly in association with major psychiatric disorder, such as schizophrenia.

Nicotine treatment counteracts perinatal asphyxia-induced changes in the mesostriatal/limbic dopamine systems and in motor behaviour in the four-week-old male rat

In the present study, the effects of nicotine treatment on the changes induced by perinatal asphyxia in exploratory and D-amphetamine-induced behaviour, and in the number of brain tyrosine hydroxylase-immunoreactive nerve cell bodies were investigated in four-week-old male rats. Asphyxia was induced in pups by placing the fetuses, still in their uterus horns removed by hysterectomy from full-term pregnant rats, in a 37 degrees C water bath for 15-16 min or 19-20 min. Surviving male pups were treated with nicotine via suckling from surrogate mothers implanted subcutaneously with Alzet minipumps containing nicotine (0.2 mumol/kg per h) for four weeks. The minipumps implanted in the mothers of sham-treated animals contained saline only. After treatment, exploratory behaviour and D-amphetamine-induced behaviour was analysed in a computerized "activity" box. After the behavioural experiments, the rats were taken for tyrosine hydroxylase immunohistochemistry, and the total number of tyrosine hydroxylase immunoreactive cell bodies were counted in the A9 and A10 regions of the substantia nigra and the ventral tegmental area, respectively. Nicotine serum levels were measured using gas chromatography in selected asphyctic and control pups at different periods after delivery. During the exploratory phase, in saline-nurtured rats, 15-16 min of asphyxia slightly increased (approximately 25%) locomotion, motility and rearing. In contrast, 19-20 min of asphyxia reduced the locomotion and rearing by approximately 50%, as compared to controls. An increase in amphetamine-induced behaviours was observed after 15-16 min, but not after 19-20 min of asphyxia, as compared to controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence in postmortem brain tissue for decreased numbers of hippocampal nicotinic receptors in schizophrenia

This study tests the hypothesis that nicotinic cholinergic receptors, including those sensitive to the antagonist alpha-bungarotoxin, are decreased in the hippocampus of schizophrenics. The hypothesis is derived from the finding that alpha-bungarotoxin causes a defect in the inhibitory gating of auditory-evoked potentials in laboratory animals that resembles a defect in auditory sensory gating observed in schizophrenics. Nicotine transiently normalizes this psychophysiological deficit in schizophrenic patients. Postmortem brain tissue was obtained from eight schizophrenic and eight age-matched nonschizophrenic subjects. Sections of the hippocampus were labeled with [125I alpha-bungarotoxin and imagined by autoradiography. Binding of the nicotinic agonist [3H]-cytisine was determined in tissue homogenates. alpha-Bungarotoxin labeled a population of putative interneurons in the hippocampus, primarily in the dentate gyrus and the CA3 region of Ammon's horn. This labeling was significantly decreased in the tissue from the schizophrenic patients, with seven or eight patients below the range of the nonschizophrenic subjects. There was also a significant decrease in the binding of cytisine. The results were not related to generalized hippocampal cell loss, drug exposure at time of death, or smoking history. This initial study suggests that schizophrenic patients have fewer nicotinic receptors in the hippocampus, a condition which may lead to failure of cholinergic activation of inhibitory interneurons, manifest clinically as decreased gating of response to sensory stimulation.

Nicotine phase-advances the circadian neuronal activity rhythm in rat suprachiasmatic nuclei explants

In vivo studies reported that cholinergic agents affect mammalian circadian rhythmicity. To study phase resetting properties of cholinergic compounds more directly, we carried out experiments in rat suprachiasmatic nuclei slices. Compounds were added to the perfusate for 1 h at specific phases of the circadian cycle. On the following day, the time of peak neuronal activity, a measure of the phase of the endogenous circadian pacemaker, was assessed by means of extracellular recording in the suprachiasmatic nuclei. The peak of neuronal activity occurred at circadian time 5.8 +/- 0.7 (mean +/- 95% confidence limits) in the control slice (circadian time 0: lights-on). Ten-micromolar carbachol had no effect on the phase of the circadian rhythm when given at circadian times 6 and 15, while at circadian time 21 a phase advance of one hour was observed. By contrast, 10 microM nicotine significantly phase advanced (> 1 h) the neuronal circadian rhythm at all but one experimental circadian phase. The circadian times of maximal nicotinic phase advances were 15 (+2.6 h) and 21 (+2.8 h). A concentration response curve for nicotine was generated and pharmacological blocking experiments were performed at circadian time 15. The estimated maximum response of nicotine was 3.4 h, and the estimated concentration for half maximal response was 5 microM. The Hill coefficient (= 1.08) indicated that the effects of nicotine may be explained by a single receptor occupancy model. Mecamylamine (20 microM) almost completely antagonized the nicotinic phase-advances, whereas tetrodotoxin (1 microM) or high Mg2+ (10 mM) did not significantly attenuate the nicotinic phase-advances.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic and environmental aspects of the role of nicotinic receptors in neurodegenerative disorders: emphasis on Alzheimer's disease and Parkinson's disease

As neurodegenerative disorders are better characterized, the importance of genetic and environmental interactions is becoming more evident. Among the neurodegenerative disorders, Alzheimer's disease and Parkinson's disease are both characterized by large losses of nicotinic binding sites in brain. In addition, losses in nicotinic receptors occur during normal aging. Chronic administration of nicotine in man or experimental animals increases the number of nicotinic receptors in brain. Nicotine has been shown to possess some neuroprotective properties for both cholinergic and dopaminergic neurons. These neuroprotective properties, when better understood, may provide important information on normal aging and neurodegenerative disorder related neuronal cell death. Understanding the functional aspects of neuronal nicotinic receptor subtypes may lead to successful therapeutic treatments or disease preventative strategies for neurodegenerative disorders.

Localization of nicotinic cholinergic receptors in rat brain: autoradiographic studies with [3H]cytisine

There is a great deal of interest in the role of nicotinic acetylcholine receptors in the central nervous system, although their function is not well understood at present. Currently, central nicotinic receptors can be classified broadly as either alpha-bungarotoxin binding sites with low affinity for acetylcholine agonists, or as high-affinity agonist binding sites with low affinity for alpha-bungarotoxin. Neuronal nicotinic receptors with a high affinity for agonists are distributed widely in the central nervous system. Evidence from molecular biology and electrophysiology suggests that multiple nicotinic receptor types exist in the brain. In this study we have used the agonist [3H]cytisine as a ligand for autoradiography to generate a detailed quantitative map of the high-affinity agonist binding nicotinic receptor in the rat brain. Optimized binding conditions, characterization of the kinetic and equilibrium binding properties, and demonstration of the nicotinic pharmacology of this binding site in tissue sections confirm the usefulness of [3H]Cytisine as a ligand for nicotinic receptor autoradiography. [3H]Cytisine autoradiography provides excellent anatomic resolution with very low non-specific binding. This property has allowed us to describe variations in receptor density within subnuclei and gradients of receptor density in larger brain regions. Data from several studies suggest that the predominant high-affinity agonist binding nicotine receptor in the central nervous system is composed of the alpha 4 and beta 2 subunits. The data in the current study are consistent with the suggestion that [3H]cytisine labels only the alpha 4 beta 2 nicotinic receptor with high affinity, offering the possibility of localizing a specific nicotinic receptor subtype in the central nervous system. In summary, we characterize the optimum experimental conditions for the use of [3H]cytisine in tissue section autoradiography. [3H]Cytisine proves to be an excellent marker for nicotinic cholinergic receptors with a very high affinity and very low background. We provide a detailed quantitative characterization of nicotinic receptor density in the rat central nervous system and we find there are significant variations and gradients in receptor density within specific brain regions, including subregions previously thought to be homogeneous.

Function of melatonin in thermoregulatory processes

Accumulated information shows that, besides its role in the timing of seasonal reproduction, melatonin also plays an important role in seasonal thermoregulatory adjustments of animals including torpor and hibernation. Furthermore, melatonin has a crucial role in circadian thermoregulatory adjustments of body temperature (Tb). Melatonin appears to send signals to the preoptic area of anterior hypothalamus (PoAH) where it adjusts the set point of Tb consistent with the metabolic rate of the animal. This new function for melatonin as a transducer mediating information about energy balance has been suggested in this review. Melatonin also adjusts the activity of the biological clock in vertebrates.

Neurophysiological and neuropsychological evidence for attentional dysfunction in schizophrenia

The behavior of the P50 wave of the auditory evoked potential in a paired stimulus or conditioning-testing paradigm has been used as a measure of sensory gating disturbance in schizophrenia. Schizophrenics fail to decrement the P50 response to the second stimulus of the pair, so that the ratio of the test to the conditioning amplitude is elevated over normal values. The aim of this study was to compare this neurophysiological measure to neuropsychological measures of attention and memory. As expected, schizophrenics performed worse than controls on most measures. The time to complete a digit cancellation test, a measure of sustained attention, was found to be particularly longer in schizophrenics than in control subjects. Furthermore, the increased time to complete this task correlated with the increased ratio of the amplitude of the test P50 response to the conditioning response in the schizophrenics. Thus, a neurophysiological defect in sensory gating may relate to a disorder in sustained attention in schizophrenia. Although the P50 wave may come from the hippocampus, neuropsychological measures of verbal learning and memory were not correlated with alterations in the P50 ratio.

The competition of (-)-[3H]nicotine binding by the enantiomers of nicotine, nornicotine and anatoxin-a in membranes and solubilized preparations of different brain regions of rat

In order to characterize the properties of nicotinic acetylcholine receptor (nAChR) subtypes in the CNS, the enantiomers of nicotine, nornicotine and anatoxin-a were studied for their ability to displace (-)-[3H]nicotine binding to membranes and solubilized preparations of different brain regions of rats. In hippocampal membranes, (-)-[3H]nicotine binding was stereoselectively displaced from two sites by (+)- and (-)-nicotine, as well as by (+)- and (-)-anatoxin-a. (-)-Nicotine displayed a larger proportion of high affinity binding sites than did (+)-nicotine, while the proportions of high and low affinity binding sites for (+)-anatoxin-a was the same as that for (-)-anatoxin-a. In cerebellar membranes, the (-)-[3H]nicotine binding was stereoselectively displaced from a single binding site by nicotine and anatoxin-a with Ki values that did not correspond with their KH and KL values observed in hippocampus. The (-)-[3H]-nicotine binding was displaced from a single site by both (+)- and (-)-nornicotine with similar Ki values in both hippocampal and cerebellar membranes. In Triton X-100 solubilized preparations, the (-)-[3H]nicotine binding was displaced from a single site by all of the drugs tested and the Ki values for each individual drug were similar in the cortex, hippocampus and cerebellum. These results provided further evidence for pharmacological heterogeneity of membrane bound nAChRs and clearly indicated that detergent solubilization changed the binding properties of nAChRs in rat brain.

Electroencephalographic changes in experimental autoimmune myasthenia gravis

A number of reports have suggested that central disturbances of cholinergic function may occur in patients with myasthenia gravis. The present study was designed in order to examine cortical electroencephalographic (EEG) activity in Lewis rats with experimental autoimmune myasthenia gravis (EAMG). Experiments were performed on conscious rats with clinical EAMG and demonstrable antibodies against the acetylcholine receptor. The animals showed no gross changes in cortical EEG discharge in terms of cycles and durations of wake, desynchronized sleep, and synchronized sleep, as compared with control rats. However, abnormalities characterized by single spikes or waves, and by spike and wave complexes, were observed, most commonly during synchronization of the EEG. Use of computerized frequency analysis of the EEG records revealed the presence of three basic differences in EEG discharge in myasthenic animals: (1) additional high-amplitude, low frequency (< 4 Hz) activity was recorded, especially during synchronized sleep; (2) decreases in mid-range (4-7 Hz) activity were recorded, particularly during periods of wakefulness; and, (3) increases in high frequency (> 8 Hz) spike discharge were observed at all times, although this was most evident during periods of synchronized sleep. The data provide further evidence for alterations in central cholinergic function in myasthenia gravis.

Pre- and postnatal development of high-affinity [3H]nicotine binding sites in rat brain regions: an autoradiographic study

The ontogeny of high affinity nicotinic cholinergic binding sites was studied in Long-Evans rat brain by in vitro autoradiography, using [3H]nicotine (10 nM) and cold (-)nicotine bitartrate to assess specificity. The first binding sites become detectable in spinal cord and caudal medulla oblongata at gestational day (GD) 12. Until GD 14, labelling spreads throughout lower brainstem, mesencephalon and parts of diencephalon, with higher densities in ventral areas (including the area of developing mesencephalic dopamine neurons). Matrix zones remain unlabelled. Receptor sites appear in the cerebellar anlage by GD 15, and in caudal caudate-putamen by GD 16. During development from late gestational to early postnatal stages, labelling is reduced in many lower brainstem areas and increases in forebrain, in particular in neocortex. Receptor density remains high in thalamus. In neocortex, nicotinic receptor sites are first seen in the subplate layer by GD 20. Labelling of this zone remains prominent until PN 14, when an additional band of increased receptor density is seen in cortical layers III/IV which contain high receptor levels in adulthood. At PN 27, the pattern has become similar to the adult one. The development of [3H]nicotine-binding sites in individual brain regions, with a general caudo-rostral gradient, accompanies cell differentiation and early synapse formation, e.g., in neocortex. The ontogenetic pattern differs in detail from that of muscarinic-cholinergic binding sites. The early presence of binding sites provides a basis for specific actions of nicotine on the fetal brain. As a consequence of the ontogenetic changes, different brain structures become targets for the action of this drug at different stages of development.

Cholinergic gating of response to auditory stimuli in rat hippocampus

Rapid decrement of response to repeated stimuli is a characteristic of hippocampal neurons. To assess the possible role in this process of cholinergic afferents from the medial septal nucleus, a series of cholinergic antagonists were administered intraventricularly to chloral hydrate-anesthetized rats. Auditory stimuli were delivered in pairs to the rats, and the evoked response was recorded from an electrode in the CA3 layer of the hippocampus. The most prominent component of the auditory evoked potential recorded in this region (N40) showed over 60% decrement in the amplitude of the response to the second stimulus when the two stimuli were delivered 0.5 s apart. Only neuromuscular-type nicotinic antagonists, alpha-bungarotoxin and (+)-tubocurarine, disrupted this decrement of response to repeated auditory stimuli. The muscarinic antagonist, scopolamine, and the ganglionic-type nicotinic antagonists, kappa-bungarotoxin and mecamylamine, were without effect. The results suggest that a subset of nicotinic receptors mediate the gating of response to auditory stimuli in the hippocampus.

The effect of neosurugatoxin on the release of neurohypophysial hormones by nicotine, hypotension and an osmotic stimulus in the rat

1. Experiments were carried out to test whether neosurugatoxin (NSTX) which blocks autonomic ganglia also acts centrally, like hexamethonium, on nicotinic cholinoceptors involved in the neural control of release of vasopressin and oxytocin from the neurohypophysis. 2. In the water-loaded rat under ethanol anaesthesia, nicotine 100 micrograms i.v. produced a pressor and an antidiuretic response accompanied by an increase in the urinary excretion of vasopressin and of oxytocin-like radioimmunoreactivity (OLRI). This indicates release of both vasopressin and oxytocin. 3. Under conditions in which tachyphylaxis was avoided, NSTX, 80 ng i.c.v., caused a prolonged inhibition of the release of both hormones by nicotine. 4. NSTX i.c.v. caused some reduction in the pressor response to nicotine. It is suggested that this response involves both central and peripheral stimulation of the sympathetic nervous system and that the central component is blocked by neosurugatoxin. 5. Muscarine, 40 ng i.c.v., produced a pressor and an antidiuretic response with increased urinary excretion of vasopressin and OLRI. All these effects were blocked by atropine but were not inhibited by NSTX. 6. Sodium nitroprusside (SN), 200 micrograms i.v., and hypertonic saline (HS; 1.54 M NaCl solution) 4 microliters i.c.v., both produced antidiuretic responses accompanied by increased urinary excretion of vasopressin and OLRI. The ratio of the excretion of vasopressin to that of OLRI was 5.1 +/- 1.3 (mean +/- s.e.: n = 8) for SN and 1.2 +/- 0.24 (mean +/- s.e.: n = 6) for HS.NSTX 80 ng i.c.v., caused a significant reduction in the antidiuretic response to the hypotension induced with SN: the increased urinary excretion of vasopressin was also significantly reduced but not that of OLRI. NSTX had no effect on the response to HS.7. We conclude that NSTX acts centrally on nicotinic cholinoceptors to block the release of vasopressin and oxytocin by nicotine and the release of vasopressin, but not that of oxytocin, by hypotension. It does not inhibit the release of either hormone by a central osmotic stimulus.

Nicotinic and muscarinic subtypes in the human brain: changes with aging and dementia

Different effects of normal aging on muscarinic and nicotinic receptor subtypes were observed in postmortem brain tissue from different regions of the human brain. A significant decrease in M1 and M2 receptors was found in cerebral cortex, while the M1 and especially the M2 receptors increased with age in the thalamus. A similar pattern of changes was also observed when using (-)3H-nicotine as ligand for nicotinic receptors in the cortex and thalamus. No significant changes in nicotinic receptor binding were observed with age in the cortex or thalamus when using 3H-acetylcholine as ligand. Nicotinic and muscarinic receptors in the brain are not equally affected in dementia disorders. A marked loss of high affinity nicotinic receptors was observed in cortical tissue from patients with Alzheimer's disease and with multi-infarct dementia (MID). The muscarinic receptors were (both M1 and M2) increased in Alzheimer cortical tissue while they were decreased in MID.

A comparison of the binding of nicotine and nornicotine stereoisomers to nicotinic binding sites in rat brain cortex

Both stereoisomers of nicotine and nornicotine were tested for their ability to competitively displace 3H-(-)-nicotine and 3H-acetylcholine (in the presence of atropine), in rat cortex tissue. 3H-acetylcholine was displaced from two binding sites, super-high and high, by (+)-nicotine, (-)-nornicotine and (+)-nornicotine but from a high affinity site by (-)-nicotine. 3H-nicotine was displaced from two sites, high and low affinity by nicotine and nornicotine stereoisomers. The high-affinity 3H-(-)-nicotine binding site showed similar binding characteristics to one of the sites labelled by 3H-acetylcholine. IC50 values showed (-)-nicotine to be 13 and 25-fold more potent than (+)-nicotine for displacing 3H-(-)-nicotine and 3H-acetylcholine, respectively, but no difference was observed for nornicotine stereoisomers. While (-)-nicotine preferentially bound to the high affinity site of 3H-(-)-nicotine (+)-nicotine preferred the low affinity site. The study provides further evidence for multiple nicotine receptors in brain.

Effects of combined pre- and postnatal treatment with nicotine on hypothalamic catecholamine nerve terminal systems and neuroendocrine function in the 4-week old and adult male and female diestrous rat

Abstract Male and female Sprague-Dawley rats were treated with nicotine during the prenatal period and the first three postnatal weeks (the pregnant and lactating rats were given nicotine hydrogen (+) tartrate (165 mg/l) in the tap water). Catecholamine fluorescence was evaluated using quantitative histofluorometry on brain sections treated according to Falck-Hillarp methodology. In order to evaluate catecholamine utilization in discrete hypothalamic catecholamine nerve terminal networks, the alphaMT- (alpha-methyl-(+/-)-p-tyrosine methyl ester) induced catecholamine disappearance was studied 2 h following the tyrosine hydroxylase inhibition. The body weight was reduced in both the male and female rats from 3 weeks of age until 9 weeks of age following pre- and postnatal treatment with nicotine. Following one week of withdrawal from pre- and postnatal treatment with nicotine, an increased catecholamine utilization was observed in the medial and lateral palisade zones of the median eminence mainly in the female rat. In the female rat, reduced prolactin serum levels were found both in the presence and absence of alphaMT treatment as well as reduced luteinizing hormone concentration in the presence of alphaMT treatment. At 6 months of age indications of a maintained, weak activation of the catecholamine nerve terminal systems in the medial palisade zones of the median eminence were observed in male rats pre- and postnatally treated with nicotine. Furthermore, increased noradrenaline levels were found in the paraventricular hypothalamic nucleus. An increase in serum luteinizing hormone levels was also found in these rats. In the 7-month old diestrous rat, maintained marked increases in catecholamine utilization in the medial and lateral palisade zones of the median eminence were found following treatment with nicotine during the pre- and postnatal period. A significant reduction of nigral dopamine stores was also demonstrated. The serum levels of thyroid stimulating hormone, prolactin and luteinizing hormone were unchanged in these rats both in the presence and absence of tyrosine hydroxylase inhibition. Finally, pre- and postnatal treatment with nicotine did not alter [(3)H]nicotine binding (quantitative receptor autoradiography) in cortical, striatal and thalamic areas of the adult diestrous rat. The results demonstrate that pre- and postnatal treatment with nicotine in the drinking water produces permanent activations of the catecholamine nerve terminal networks of the external layer of the median eminence mainly in the female rat. These changes appear to be associated with reduced serum prolactin levels in the 4-week old female rat. Sex-specific changes occur in discrete noradrenaline nerve terminal systems. The observed changes may have functional consequences for neuroendocrine regulation and for the regulation of food and water intake as well as sex-specific responses to stress in the male versus the female rat. Nicotine-induced disturbances in brain cell replication and differentiation may underlie the permanent alteration found in discrete catecholamine neuron systems after pre- and postnatal exposure to this drug.

The effect on vasopressin release of microinjection of cholinergic agonists into the paraventricular nucleus of conscious rats

Abstract We have studied in conscious unrestrained rats under basal conditions the effect of activation of muscarinic and nicotinic receptors in the paraventricular nucleus on vasopressin secretion and mean arterial blood pressure. The microinjection of oxotremorine (0.2, 2 or 20 ng), a specific muscarinic agonist, produced a substantial, dose-dependent, transient increase in the plasma vasopressin concentration. There was also a rise in mean arterial blood pressure and a bradycardia that followed the same time-course as the change in plasma vasopressin levels. The microinjection of nicotine (0.1, 1 or 10 HQ) into the paraventricular nucleus had only questionable effects on vasopressin release and mean arterial blood pressure; heart rate was unaffected. These findings suggest that muscarinic receptors may be of primary importance in the paraventricular nucleus in the Cholinergic stimulation of vasopressin release.

Uptake and regional distribution of (+)-(R)- and (-)-(S)-N-[methyl-11C]-nicotine in the brains of rhesus monkey. An attempt to study nicotinic receptors in vivo

N-[methyl-11C] nicotine (11C-nicotine) was given intravenously to monkeys and the uptake and regional distribution of radioactivity was followed in the brain using positron emission tomography (PET). The 11C-radioactivity in the brain peaked within 1-2 min and then rapidly declined. Pretreatment with unlabelled nicotine (10 micrograms/kg) reduced the uptake of 11C-radioactivity to the brain by 30%. The uptake of radioactivity was higher following (+)11C-nicotine than (-)11C-nicotine. Both enantiomers were distributed in a similar manner within the brain. When animals were infused with a peripheral nicotinic blocker (trimetaphan) the uptake of radioactivity to the brain was lower following (+)11C-nicotine compared to (-)11C-nicotine. The amount of radioactivity was high in the occipital cortex, thalamus, intermediate in the frontal cortex and low in white matter in (-)11C injected monkeys while no regional difference in distribution of 11C-radioactivity was observed after injection of (+)11C-nicotine.

Quantitative autoradiography of nicotinic receptors in large cryosections of human brain hemispheres

In vitro quantitative autoradiography was used to visualize nicotinic receptors in large cryosections of human brain hemispheres. Sections 80 microns thick of human brain hemispheres were incubated with (-)-[3H]nicotine and the distribution of nicotinic receptors in human brain was studied. Increasing numbers of nicotinic binding sites were observed in the: hippocampus less than cortex less than cerebellum less than substantia nigra less than putamen less than periaqueductal gray. The cartography of nicotinic receptors in the normal human brain will hopefully be of use in the study of the alteration of these receptors in diseased brain.

Protective effects of chronic nicotine treatment on lesioned nigrostriatal dopamine neurons in the male rat

The present results demonstrate that chronic nicotine treatment can in part protect against mechanically-induced and neurotoxin-induced degeneration of nigrostriatal DA neurons. These results indicate that in sufficient doses chronic treatment with nicotine may be considered in the pharmacological treatment of Parkinson's disease. It remains to be demonstrated whether these protective actions can be extended to include also other injured neurons such as the cholinergic neurons, known to be severely affected in Alzheimer's disease.