Autoradiographic comparison of cholinergic and other transmitter receptors in the normal human hippocampus

Relationships between neurotransmitter receptor densities and expression levels of their corresponding genes in the human hippocampus

Neurotransmitter receptors are key molecules in signal transmission, their alterations are associated with brain dysfunction. Relationships between receptors and their corresponding genes are poorly understood, especially in humans. We combined in vitro receptor autoradiography and RNA sequencing to quantify, in the same tissue samples (7 subjects), the densities of 14 receptors and expression levels of their corresponding 43 genes in the Cornu Ammonis (CA) and dentate gyrus (DG) of human hippocampus. Significant differences in receptor densities between both structures were found only for metabotropic receptors, whereas significant differences in RNA expression levels mostly pertained ionotropic receptors. Receptor fingerprints of CA and DG differ in shapes but have similar sizes; the opposite holds true for their "RNA fingerprints", which represent the expression levels of multiple genes in a single area. In addition, the correlation coefficients between receptor densities and corresponding gene expression levels vary widely and the mean correlation strength was weak-to-moderate. Our results suggest that receptor densities are not only controlled by corresponding RNA expression levels, but also by multiple regionally specific post-translational factors.

Age-Dependent and Pathway-Specific Bimodal Action of Nicotine on Synaptic Plasticity in the Hippocampus of Mice Lacking the miR-132/212 Genes

Nicotine addiction develops predominantly during human adolescence through smoking. Self-administration experiments in rodents verify this biological preponderance to adolescence, suggesting evolutionary-conserved and age-defined mechanisms which influence the susceptibility to nicotine addiction. The hippocampus, a brain region linked to drug-related memory storage, undergoes major morpho-functional restructuring during adolescence and is strongly affected by nicotine stimulation. However, the signaling mechanisms shaping the effects of nicotine in young vs. adult brains remain unclear. MicroRNAs (miRNAs) emerged recently as modulators of brain neuroplasticity, learning and memory, and addiction. Nevertheless, the age-dependent interplay between miRNAs regulation and hippocampal nicotinergic signaling remains poorly explored. We here combined biophysical and pharmacological methods to examine the impact of miRNA-132/212 gene-deletion (miRNA-132/212-/-) and nicotine stimulation on synaptic functions in adolescent and mature adult mice at two hippocampal synaptic circuits: the medial perforant pathway (MPP) to dentate yrus (DG) synapses (MPP-DG) and CA3 Schaffer collaterals to CA1 synapses (CA3-CA1). Basal synaptic transmission and short-term (paired-pulse-induced) synaptic plasticity was unaltered in adolescent and adult miRNA-132/212-/- mice hippocampi, compared with wild-type controls. However, nicotine stimulation promoted CA3-CA1 synaptic potentiation in mature adult (not adolescent) wild-type and suppressed MPP-DG synaptic potentiation in miRNA-132/212-/- mice. Altered levels of CREB, Phospho-CREB, and acetylcholinesterase (AChE) expression were further detected in adult miRNA-132/212-/- mice hippocampi. These observations propose miRNAs as age-sensitive bimodal regulators of hippocampal nicotinergic signaling and, given the relevance of the hippocampus for drug-related memory storage, encourage further research on the influence of miRNAs 132 and 212 in nicotine addiction in the young and the adult brain.

Cyto- and receptor architectonic mapping of the human brain

Mapping of the human brain is more than the generation of an atlas-based parcellation of brain regions using histologic or histochemical criteria. It is the attempt to provide a topographically informed model of the structural and functional organization of the brain. To achieve this goal a multimodal atlas of the detailed microscopic and neurochemical structure of the brain must be registered to a stereotaxic reference space or brain, which also serves as reference for topographic assignment of functional data, e.g., functional magnet resonance imaging, electroencephalography, or magnetoencephalography, as well as metabolic imaging, e.g., positron emission tomography. Although classic maps remain pioneering steps, they do not match recent concepts of the functional organization in many regions, and suffer from methodic drawbacks. This chapter provides a summary of the recent status of human brain mapping, which is based on multimodal approaches integrating results of quantitative cyto- and receptor architectonic studies with focus on the cerebral cortex in a widely used reference brain. Descriptions of the methods for observer-independent and statistically testable cytoarchitectonic parcellations, quantitative multireceptor mapping, and registration to the reference brain, including the concept of probability maps and a toolbox for using the maps in functional neuroimaging studies, are provided.

Quantitative expression profile of distinct functional regions in the adult mouse brain

The adult mammalian brain is composed of distinct regions with specialized roles including regulation of circadian clocks, feeding, sleep/awake, and seasonal rhythms. To find quantitative differences of expression among such various brain regions, we conducted the BrainStars (B*) project, in which we profiled the genome-wide expression of ∼50 small brain regions, including sensory centers, and centers for motion, time, memory, fear, and feeding. To avoid confounds from temporal differences in gene expression, we sampled each region every 4 hours for 24 hours, and pooled the samples for DNA-microarray assays. Therefore, we focused on spatial differences in gene expression. We used informatics to identify candidate genes with expression changes showing high or low expression in specific regions. We also identified candidate genes with stable expression across brain regions that can be used as new internal control genes, and ligand-receptor interactions of neurohormones and neurotransmitters. Through these analyses, we found 8,159 multi-state genes, 2,212 regional marker gene candidates for 44 small brain regions, 915 internal control gene candidates, and 23,864 inferred ligand-receptor interactions. We also found that these sets include well-known genes as well as novel candidate genes that might be related to specific functions in brain regions. We used our findings to develop an integrated database (http://brainstars.org/) for exploring genome-wide expression in the adult mouse brain, and have made this database openly accessible. These new resources will help accelerate the functional analysis of the mammalian brain and the elucidation of its regulatory network systems.

Effects of transdermal nicotine on episodic memory in non-smokers with and without schizophrenia

RATIONALE

Nicotinic agonists may improve attention and memory in humans and may ameliorate some cognitive deficits associated with neuropsychiatric disorders such as schizophrenia.

MATERIALS AND METHODS

We investigated the effects of a single dose of nicotine on episodic memory performance in 10 adults with schizophrenia and 12 healthy controls. Participants were nonsmokers in order to avoid confounding effects of nicotine withdrawal and reinstatement on memory. At each of two study visits, participants performed a test of episodic memory before and 4 h after application of a 14-mg transdermal nicotine (or identical placebo) patch in counterbalanced order.

RESULTS

Compared with placebo, nicotine treatment was associated with more rapid and accurate recognition of novel items. There was a trend for a treatment by diagnosis interaction, such that the effect of nicotine to reduce false alarms was stronger in the schizophrenia than the control group. There was no effect of nicotine on accuracy or reaction time for identification of previously viewed items.

CONCLUSIONS

These data suggest that nicotine improves novelty detection in non-smokers, an effect that may be more pronounced in non-smokers with schizophrenia. Because memory deficits are associated with functional impairment in schizophrenia and because impaired novelty detection has been linked to the positive symptoms of schizophrenia, study of the effects of chronic nicotinic agonist treatment on novelty detection may be warranted.

Nonsynaptic Chemical Transmission Through Nicotinic Acetylcholine Receptors

This review attempts to organize the different aspects of nicotinic transmission in the context of nonsynaptic interactions. Nicotinic acetylcholine receptors (nAChRs) dominantly operate in the nonsynaptic mode in the central nervous system despite their ligand-gated ion-channel nature, which would otherwise be better suited for fast synaptic transmission. This fast form of nonsynaptic transmission, most likely unique to nAChRs, represents a new avenue in the communication platforms of the brain. Cholinergic messages received by nAChRs, arriving at a later phase following synaptic activation, can interfere with dendritic signal integration. Nicotinic transmission plays a role in both neural plasticity and cellular learning processes, as well as in long-term changes in basic activity through fast activation, desensitization of receptors, and fluctuations of the steady-state levels of ACh. ACh release can contribute to plastic changes via activation of nAChRs in neurons and therefore plays a role in learning and memory in different brain regions. Assuming that nAChRs in human subjects are ready to receive long-lasting messages from the extracellular space because of their predominantly nonsynaptic distribution, they offer an ideal target for drug therapy at low, nontoxic drug levels.

Postnatal alterations in dopaminergic markers in the human prefrontal cortex

Dopamine in the prefrontal cortex plays a critical role in normal cognition throughout the lifespan and has been implicated in the pathophysiology of neuropsychiatric disorders such as schizophrenia and attention deficit disorder. Little is known, however, about the postnatal development of the dopaminergic system in the human prefrontal cortex. In this study, we examined pre- and post-synaptic markers of the dopaminergic system in postmortem tissue specimens from 37 individuals ranging in age from 2 months to 86 years. We measured the levels of tyrosine hydroxylase, the rate limiting enzyme in dopamine biosynthesis, using Western immunoblotting. We also examined the gene expression of the three most abundant dopamine receptors (DARs) in the human prefrontal cortex: DAR1, DAR2 and DAR4, by in situ hybridization. We found that tyrosine hydroxylase concentrations and DAR2 mRNA levels were highest in the cortex of neonates. In contrast, the gene expression of DAR1 was highest in adolescents and young adults. No significant changes across age groups were detected in mRNA levels of DAR4. Both DAR1 and DAR2 mRNA were significantly lower in the aged cortex. Taken together, our data suggest dynamic changes in markers of the dopamine system in the human frontal cortex during postnatal development at both pre-and post-synaptic sites. The peak in DAR1 mRNA levels around adolescence/early adulthood may be of particular relevance to neuropsychiatric disorders such as schizophrenia in which symptoms manifest during the same developmental period.

Further evidence for the functional role of nonsynaptic nicotinic acetylcholine receptors

The function of nicotinic acetylcholine receptors in the main central systems has been documented in the past decade. These studies focused mostly on the synaptic functions, although acetylcholine is released dominantly into the extrasynaptic space and the majority of nicotinic acetylcholine receptors on remote neurons are found on extrasynaptic membranes. Here, we show further evidence for the role of nonsynaptic nicotinic functions in the cognitive and the reward system. Dendrites of gamma-amino-n-butyric acid (GABA)-containing interneurons of the hippocampus are densely equipped with nicotinic acetylcholine receptors. These cells play an important role in memory processing. We analysed the effects of nicotinic acetylcholine receptor stimulation on the Ca(2+) dynamics of interneurons in different dendritic compartments. We also investigated the role of nicotinic receptors in the nucleus accumbens where nicotine stimulated vesicular dopamine release via activation of receptors located on varicosities. Nicotine produced comparable effects with 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) on dopamine release. These examples demonstrate that nonsynaptic nicotinic acetylcholine receptors can effectively influence activity pattern of neural networks in key structures of central systems.

Selective vulnerability of cerebellar granule neuroblasts and their progeny to drugs with abuse liability

Cerebellar development is shaped by the interplay of genetic and numerous environmental factors. Recent evidence suggests that cerebellar maturation is acutely sensitive to substances with abuse liability including alcohol, opioids, and nicotine. Assuming substance abuse disrupts cerebellar maturation, a central question is: what are the basic mechanisms underlying potential drug-induced developmental defects? Evidence reviewed herein suggests that the maturation of granule neurons and their progeny are intrinsically affected by several classes of substances with abuse liability. Although drug abuse is also likely to target directly other cerebellar neuron and glial types, such as Purkinje cells and Bergmann glia, findings in isolated granule neurons suggest that they are often the principle target for drug actions. Developmental events that are selectively disrupted by drug abuse in granule neurons and/or their neuroblast precursors include proliferation, migration, differentiation (including neurite elaboration and synapse formation), and programmed cell death. Moreover, different classes of drugs act through distinct molecular mechanisms thereby disrupting unique aspects of development. For example, drug-induced perturbations in: (i) neurotransmitter biogenesis; (ii) ligand and ion-gated receptor function and their coupling to intracellular effectors; (iii) neurotrophic factor biogenesis and signaling; and (iv) intercellular adhesion are all likely to have significant effects in shaping developmental outcome. In addition to identifying therapeutic strategies for drug abuse intervention, understanding the mechanisms by which drugs affect cellular maturation is likely to provide a better understanding of the neurochemical events that normally shape central nervous system development.

Decreased muscarinic receptor binding in subjects with schizophrenia: a study of the human hippocampal formation

BACKGROUND

Acetylcholine is important to hippocampal function, including the processes of learning and memory. Patients with schizophrenia show impaired learning and memory and hippocampal dysfunction. Thus, acetylcholinergic systems may be primarily or secondarily disrupted in the hippocampal formation of schizophrenic patients. The present study tested the hypothesis that [(3)H]pirenzepine-labeled muscarinic cholinergic receptor levels are altered in the hippocampal formation of patients with schizophrenia.

METHODS

We have used quantitative autoradiography to measure [(3)H]pirenzepine binding to M(1) and M(4) receptors in the hippocampal formation from 15 schizophrenic and 18 nonschizophrenic subjects.

RESULTS

The mean density of [(3)H]pirenzepine binding was reduced in all regions studied, including the dentate gyrus, subdivisions of Ammon's Horn (CA1-CA4), subiculum, and the parahippocampal gyrus, of the schizophrenic cohort. Moreover, unlike controls, there was no significant variation between the mean levels of [(3)H]pirenzepine binding across the subregions of the hippocampal formation from schizophrenic subjects.

CONCLUSIONS

These findings provide support for a possible involvement of the muscarinic cholinergic system in the pathology and/or treatment of schizophrenia.

Nicotinic acetylcholine receptors during prenatal development and brain pathology in human aging

Nicotinic acetylcholine receptor (nAChRs) proteins and gene transcripts are already present in human prenatal brain and spinal cord at 4-6 weeks gestation, and a clear age-related increase in number of nAChRs was apparent during first trimester. In pons, there was also a parallel increase in the alpha7 mRNA level with age. The highest specific binding of [3H]epibatidine and [3H]cytisine was detected in spinal cord, pons and medulla oblongata, and binding of [125I]alpha-bungarotoxin was highest in spinal cord, medulla oblongata and mesencephalon. From the late fetal stage brain nAChRs have been shown to fall with increasing age. During aging (between 40 and 100 years) high affinity nicotine binding in the frontal cortex decreases in parallel with glutamate NMDA receptor binding ([3H]MK801). In the hippocampal formation and entorhinal cortex nicotine binding also declines with age, in common with [125I]alpha-bungarotoxin in the entorhinal cortex, but NMDA receptor binding remains unchanged. These reductions in nicotine binding with age may predispose the neo- and archicortex to the loss of nAChRs observed in age-associated neurodegenerative conditions. By contrast no loss in nAChR binding with aging is observed in the thalamus and only after the 70th decade in the striatum, although in Alzheimer's disease, Parkinson's disease and Lewy body dementia deficits in nAChRs are observed in these areas and may be associated with specific disease-related processes.

Localization of the A kinase anchoring protein AKAP79 in the human hippocampus

The phosphorylation state of the proteins, regulated by phosphatases and kinases, plays an important role in signal transduction and long-term changes in neuronal excitability. In neurons, cAMP-dependent protein kinase (PKA), protein kinase C (PKC) and calcineurin (CN) are attached to a scaffold protein, A kinase anchoring protein (AKAP), thought to anchor these three enzymes to specific sites of action. However, the localization of AKAP, and the predicted sites of linked phosphatase and kinase activities, are still unknown at the fine structural level. In the present study, we investigated the distribution of AKAP79 in the hippocampus from postmortem human brains and lobectomy samples from patients with intractable epilepsy, using preembedding immunoperoxidase and immunogold histochemical methods. AKAP79 was found in the CA1, presubicular and subicular regions, mostly in pyramidal cell dendrites, whereas pyramidal cells in the CA3, CA2 regions and dentate granule cells were negative both in postmortem and in surgical samples. In some epileptic cases, the dentate molecular layer and hilar interneurons also became immunoreactive. At the subcellular level, AKAP79 immunoreactivity was present in postsynaptic profiles near, but not attached to, the postsynaptic density of asymmetrical (presumed excitatory) synapses. We conclude that the spatial selectivity for the action of certain kinases and phosphatases regulating various ligand- and voltage-gated channels may be ensured by the selective presence of their anchoring protein, AKAP79, at the majority of glutamatergic synapses in the CA1, but not in the CA2/CA3 regions, suggesting profound differences in signal transduction and long-term synaptic plasticity between these regions of the human hippocampus.

Evidence disputing the importance of excitotoxicity in hippocampal neuron death after experimental traumatic brain injury

The hippocampus is selectively vulnerable to experimental traumatic brain injury (TBI). Beneficial effects of glutamate receptor antagonists and increased extracellular levels of glutamate have suggested that glutamate-mediated excitotoxicity may be responsible for this selective damage. In order to clarify this important issue, we applied a severe parasagittal fluid percussion injury (FPI) to strains of mice shown to be susceptible and resistant to kainic acid (KA)-induced excitotoxic hippocampal damage. Dystrophic neurons were present by 10 min after FPI in the hippocampi of both strains. Damaged hippocampal neurons were absent at 4 days and 7 days. Additionally, there was no significant difference (p = 1.00) in CA3 neuron survival between KA-susceptible and -resistant mice at 4 days. In conclusion, excitotoxicity does not significantly contribute to hippocampal neuron loss after FPI and, in contrast to classic studies of excitotoxicity in vivo, the pattern of hippocampal cell death after TBI is extremely acute.

Distributions of nicotinic acetylcholine receptor alpha7 and beta2 subunits on cultured hippocampal neurons

The hippocampus receives cholinergic afferents and expresses neuronal nicotinic acetylcholine receptors. In particular, the alpha7 and beta2 nicotinic subunits are highly expressed in the hippocampus. There has been controversy about the location, distribution and roles of neuronal nicotinic acetylcholine receptors [Role L. W. and Berg D. K. (1996) Neuron 16, 1077-1085; Wonnacott S. (1997) Trends Neurosci. 20, 92-98]. Using immunocytochemistry and patch-clamp techniques, we examined the density and distribution of nicotinic receptors on rat hippocampal neurons in primary tissue culture. The density and distribution of alpha7 subunits change with days in culture. Before 10 days in culture, alpha7 expression, monitored immunocytochemically, is low and nicotinic currents are small or absent. In older cultures, about two-thirds of the neurons express nicotinic currents, and alpha7 appears in small patches on the soma and out along the neuronal processes. These patches of alpha7 subunits on the surface of the neuronal processes often co-localize with the presynaptic marker, synaptotagmin. The other most common nicotinic subunit, beta2, stays confined mainly to the soma and proximal processes, and beta2 is distributed more uniformly and is not specifically localized at presynaptic areas. The two subunits, alpha7 and beta2, have different expression patterns on the surface of the cultured hippocampal neurons. Taken together with previous physiological studies, the results indicate that alpha7 subunits can be found at presynaptic terminals, and at these locations, these calcium-permeable channels may influence transmitter release.

α7 Nicotinic Receptor Subunits Are Not Necessary for Hippocampal-Dependent Learning or Sensorimotor Gating: A Behavioral Characterization of Acra7-Deficient Mice

The α7 nicotinic acetylcholine receptor (nAChR) subunit is abundantly expressed in the hippocampus and contributes to hippocampal cholinergic synaptic transmission suggesting that it may contribute to learning and memory. There is also evidence for an association between levels of α7 nAChR and in sensorimotor gating impairments. To examine the role of α7 nAChRs in learning and memory and sensorimotor gating, Acra7 homozygous mutant mice and their wild-type littermates were tested in a Pavlovian conditioned fear test, for spatial learning in the Morris water task, and in the prepulse inhibition paradigm. Exploratory activity, motor coordination, and startle habituation were also evaluated. Acra7 mutant mice displayed the same levels of contextual and auditory-cue condition fear as wild-type mice. Similarly, there were no differences in spatial learning performance between mutant and wild-type mice. Finally,Acra7 mutant and wild-type mice displayed similar levels of prepulse inhibition. Other behavioral responses in Acra7 mutant mice were also normal, except for an anxiety-related behavior in the open-field test. The results of this study show that the absence of α7 nAChRs has little impact on normal, base-line behavioral responses. Future studies will examine the contribution of α7 nAChR to the enhancement of learning and sensorimotor gating following nicotine treatments.

Systemic NMDA receptor antagonist CGP-40116 does not impair memory acquisition but protects against NMDA neurotoxicity in rhesus monkeys

A widely accepted hypothesis is that long-term potentiation (LTP) is a synaptic mechanism of memory. NMDA receptors are critically involved in induction but not maintenance of LTP; therefore, their blockade should impair memory acquisition but not retrieval. In Experiment 1, we investigated the effect of a systemic NMDA receptor antagonist, CGP-40116 [D-isomer of CGP-37849: (E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (6 mg/kg, i.m.) 60 min before the testing session] on memory acquisition and retrieval by monkeys in the "object-in-place" visual memory task, an analog of human episodic memory. Only a small increase in error rate was produced (< 3%), and this increase was observed in both retention and acquisition tests. This deficit is substantially smaller than the previously reported deficit after fornix transection in the same task, and is not specific to memory acquisition. In Experiment 2, we investigated the neuroprotective effect of CGP-40116. NMDA (68 nmol) was injected into the right hippocampus, then CGP-40116 (6 mg/kg) was given intramuscularly, and then NMDA was injected into the left hippocampus. The area of cell loss in CA1 and CA3 fields was smaller in both hemispheres compared with unprotected monkeys (without CGP-40116). Thus, CGP-40116 provides both retrograde and anterograde protection against NMDA neurotoxicity. These data (1) demonstrate that acquisition of episodic memories remains almost intact when an NMDA receptor antagonist is given in a dose sufficient to block NMDA receptors in the hippocampus, and (2) indirectly oppose the hypothesis that NMDA receptor-dependent LTP plays the key role in memory.

Glutamate receptor binding in the human hippocampus and adjacent cortex during development and aging

Distinct patterns of age-related alterations in NMDA (MK801 binding) and non-NMDA, AMPA (CNQX), and kainate binding have been identified in human hippocampus and parahippocampal gyrus in normal individuals with no evidence of degenerative brain disease ranging in age from 24 gestational weeks to 94 years. Whereas MK801 binding did not alter substantially over this age range, CNQX binding rose from low levels in the fetus to maximum levels between neonate and middle age, and kainate binding declined extensively from the perinatal to adult stage. Following maturity, there were no significant changes in kainate binding, although MK801 binding increased in CA1 and CA3 and CNQX binding declined in several regions, particularly CA2 and subiculum. For each receptor binding the timing of these fluctuations ocurring during development and aging varied within different regions of the dentate gyrus, hippocampus proper, subicular complex, and entorhinal cortex examined. The transient peaks of receptor binding are likely to reflect processes of synaptogenesis and pruning and may provide clues regarding the role of the different glutamate receptor subtypes in various pathologies of the hippocampus and adjacent cortex associated with developmental disorders (of genetic origin or due to perinatal trauma or insult). The absence of substantial changes in any subtype examined from middle to old age suggests alterations in transmitter binding to these glutamate receptors are not involved in senescent neurodegeneration.

Beta-amyloidosis in normal aging and transmitter signaling in human temporal lobe

Interactions between abnormal amyloid precursor protein metabolism and cholinergic dysfunction are increasingly apparent. Both of these major features of Alzheimer's disease occur in restricted loci in normal aging--a potential model for early Alzheimer type pathology. Entorhinal cortex is particularly vulnerable to beta-amyloidosis and compared with other cortical areas is remarkable for the relatively high density of nicotinic (3H-nicotine) but not other cholinergic or glutamate receptor binding. With increasing age, post-maturity, there is a persistent decline in nicotinic receptor binding in entorhinal cortex whereas muscarinic M1 and non-M1, glutamate NMDA and non-NMDA receptors are spared. Normal elderly individuals, distinguished by the absence of beta A4 immunoreactive plaques in this area, are differentiated from those with plaques by higher nicotine binding. Amongst individuals with an established history of smoking tobacco, nicotinic receptor binding and hippocampal choline acetyltransferase were elevated compared with non-smokers and preliminary evidence indicates a reduced density of cortical plaques. These findings are consistent with the hypothesis that down regulation of the nicotinic cholinergic receptor--a ligand gated calcium channel known to control the expression of neurotrophins--plays a role in the evolution of Alzheimer-type pathology.

Distribution of nicotinic receptors in the human hippocampus and thalamus

Neuronal nicotinic acetylcholine receptors consist of different subunits, alpha and beta, with different subtype arrangement corresponding to distinct pharmacological and functional properties. The expression of alpha 3, alpha 7 and beta 2 mRNA in the human brain was studied by in situ hybridization and compared to [3H]nicotine, [3H]cytisine and [125I]alpha-bungarotoxin binding in contiguous sections. The beta 2 probe showed a strong hybridization signal in the granular layer of the dentate gyrus and in the CA2/CA3 region of the hippocampus and in the insular cortex, and a signal of lower intensity in the subicular complex and entorhinal cortex. The alpha 3 probe showed strong hybridization in the dorsomedial, lateral posterior, ventroposteromedial and reticular nuclei of the thalamus, and a weak signal in the hippocampal region and in the entorhinal, insular and cingular cortex. The amount of alpha 7 mRNA was high at the level of the dentate granular layer and the CA2/CA3 region of the hippocampus, in the caudate nucleus and in the pulvinar and ventroposterolateral nuclei of the thalamus. [3H]Nicotine and [3H]cytisine binding appeared to be identical in anatomical distribution and relative intensity. It was high in the thalamic nuclei, the putamen and in the hippocampal formation in the subicular complex and the stratum lacunosum moleculare. The level of [125I]alpha-bungarotoxin binding was particularly high in the hippocampus and in the pyramidal cells of the CA1 region, but was relatively low in the subicular complex. Our data indicate that in the human brain nicotinic receptor subtypes have discrete distributions, which are in part different from those of other species.