Light autoradiographic localisation of cholinergic muscarinic receptors in rat brain by specific binding of a potent antagonist

Cholinergic and Glutaminergic Excitation of Neuronal Cells

Excessive cholinergic or glutaminergic brain stimulation may result in seizures, excitotoxicity and neuronal damage. Cholinergic neuronal excitation is mediated via muscarinic receptors which couple with GTP-binding proteins (G–proteins), activate phospholipase C, and produce the inositol lipid second messengers, inositol-1,4,5,-trisphosphate (InsP3) and diacyl-glycerol (DG). InsP3 facilitates intracellular Ca2+ metabolism and DG activates protein kinase C (PKC). Glutaminergic neuronal stimulation is mediated through ionotropic N-methyl-D-aspartate (NMDA) receptors, which increase Ca2+ influx, and kainate α-amino-3-hydroxy-5-methyl-4-isoxalolproprionic acid receptors, which mainly regulate Na+ fluxes. Glutaminergic metabotropic receptors are also coupled to a G-protein, and their stimulation activates neurons through increased production of InsP3 and DG. A salient feature in glutamate-induced excitotoxicity is the induction of an oxidative burst, subsequent oxidative stress, and damage to the neurons. The glutamate-induced oxidative burst can be amplified by lead, a direct activator of PKC, and the oxidative burst can be blocked by a PKC inhibitor, suggesting an important role for PKC. Carbachol also induces an oxidative burst in neuronal cells and this is associated with elevations of free intracellular calcium. The ability of an NMDA receptor antagonist, AP-5, to block carbachol-induced elevations of free intracellular calcium, suggests that activation of muscarinic receptors is associated with a simultaneous glutamate receptor activation. Thus, cross-talk between cholinergic muscarinic and glutaminergic receptors may be an important contributing factor in cholinergic and glutaminergic excitotoxicity.

Nitric Oxide Signaling in Neurodegeneration and Cell Death

In this tribute to Solomon H. Snyder (Sol) we discuss the mechanisms by which nitric oxide (NO) kills neurons. We provide a historical perspective regarding the discovery that glutamate excitotoxicity is mediated by NO. It also contains a discussion of the discovery that neuronal nitric oxide synthase (nNOS) catalytic activity accounts for NADPH diaphorase activity and its localization in the central nervous system. NADPH diaphorase/nNOS neurons are unique in that they are resistant to toxic effects of excess glutamate and that they are resistant to neurodegeneration in a variety of neurodegenerative diseases. NADPH diaphorase/nNOS neurons are resistant to neurotoxicity and neurodegeneration through the overexpression of manganese superoxide dismutase. The review also delves into the mechanisms by which NO kills neurons including NO's activation of the glyceraldehyde-3-phosphate dehydrogenase-dependent cell pathway. In addition, there is a review of parthanatos in which NO combines with the superoxide anion ( [Formula: see text] ) to form peroxynitrite (ONOO^-) that damages DNA and activates poly (ADP-ribose) (PAR) polymerase (PARP). This ultimately leads to activation of the PARP-dependent apoptosis-inducing factor-associated nuclease, the final executioner in NO-dependent cell death. Finally, there is a discussion of potential targets that are under development that target the mechanisms by which NO kills neurons.

Tactics for preclinical validation of receptor-binding radiotracers

INTRODUCTION

Aspects of radiopharmaceutical development are illustrated through preclinical studies of [125I]-(E)-1-(2-(2,3-dihydrobenzofuran-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-BF-PE-PIPZE), a radioligand for sigma-1 (σ1) receptors, coupled with examples from the recent literature. Findings are compared to those previously observed for [125I]-(E)-1-(2-(2,3-dimethoxy-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-DM-PE-PIPZE).

METHODS

Syntheses of E-IA-BF-PE-PIPZE and [125I]-E-IA-BF-PE-PIPZE were accomplished by standard methods. In vitro receptor binding studies and autoradiography were performed, and binding potential was predicted. Measurements of lipophilicity and protein binding were obtained. In vivo studies were conducted in mice to evaluate radioligand stability, as well as specific binding to σ1 sites in brain, brain regions and peripheral organs in the presence and absence of potential blockers.

RESULTS

E-IA-BF-PE-PIPZE exhibited high affinity and selectivity for σ1 receptors (Ki = 0.43 ± 0.03 nM, σ2/σ1 = 173). [125I]-E-IA-BF-PE-PIPZE was prepared in good yield and purity, with high specific activity. Radioligand binding provided dissociation (koff) and association (kon) rate constants, along with a measured Kd of 0.24 ± 0.01 nM and Bmax of 472 ± 13 fmol/mg protein. The radioligand proved suitable for quantitative autoradiography in vitro using brain sections. Moderate lipophilicity, Log D7.4 2.69 ± 0.28, was determined, and protein binding was 71 ± 0.3%. In vivo, high initial whole brain uptake, >6% injected dose/g, cleared slowly over 24 h. Specific binding represented 75% to 93% of total binding from 15 min to 24 h. Findings were confirmed and extended by regional brain biodistribution. Radiometabolites were not observed in brain (1%).

CONCLUSIONS

Substitution of dihydrobenzofuranylethyl for dimethoxyphenethyl increased radioligand affinity for σ1 receptors by 16-fold. While high specific binding to σ1 receptors was observed for both radioligands in vivo, [125I]-E-IA-BF-PE-PIPZE displayed much slower clearance kinetics than [125I]-E-IA-DM-PE-PIPZE. Thus, minor structural modifications of σ1 receptor radioligands lead to major differences in binding properties in vitro and in vivo.

Differential modulation of muscarinic receptors in the rat brain by repeated exposure to methyl parathion

The neurochemical and behavioral effects of repeated subdermal administration of methyl parathion (MP) at low doses were investigated. Adult male rats were treated repeatedly with either vehicle or MP subcutaneously (3 mg/kg/day) and observed for the signs of toxicity during the treatment period. The toxic sign, tremor, reached maximum right after 9th injection in MP-treated rats, and declined thereafter. Animals were sacrificed and brains were taken 1 week or 3 weeks after the daily treatment for measurement of acetylcholinesterase (AChE) activity and binding of radioligands, [3H]QNB (nonselective), [3H]pirenzepine (M1-selective), and [3H]AF-DX384 (M2-selective) to muscarinic receptors. With this treatment regimen, the AChE activity in the blood dropped quickly and maintained at 30% of the control level after 6 injections. After 3 weeks of treatment, MP caused 80-90% AChE inhibition and substantial reductions in [3H]QNB binding (9-33%), [3H]pirenzepine binding (9-22%) and [3H]AF-DX384 binding (6-38%) in different brain regions, including striatum, hippocampus, frontal cortex, thalamus and midbrain. After 1 week of treatment, the inhibition of AChE in brain regions was from 54 to 74%, whereas receptor densities were only marginally affected in a few regions. The timing of the changes in receptor population correlates well with the changes in behaviors during the repeated MP exposure. Our findings suggest that down-regulation of muscarinic receptors plays a role in the development of tolerance to MP. And, the regulations of muscarinic receptors were different among receptor subtypes and brain regions.

Synthesis and autoradiographic localization of muscarinic cholinergic antagonist (+)N-[11C]methyl-3-piperidyl benzilate as a potent radioligand for positron emission tomography

(+)N-[11C]methyl-3-piperidyl benzilate, a relatively low affinity muscarinic cholinergic receptor antagonist was synthesized by N-[11C]methylation of (+)3-piperidyl benzilate using [11C]methyl iodide. The product was isolated by HPLC, and obtained with radiochemical yield of 60-70% from [11C]methyl iodide, and a specific activity of 500-1000 Ci mmol-1 (18.5-37 GBq mumol-1) at EOS and radiochemical purity of > 98%. In vitro autoradiographic studies showed selective binding for this radiotracer in the different regions of the rat brain: high in corpus striatum, hippocampus and cerebral cortex, and low in cerebellum, consistent with muscarinic cholinergic receptor distributions. This radiotracer thus had potential as radioligand for positron emission tomography.

Activation of muscarinic receptors during blockade of GABA(A)-mediated inhibition induces synchronous epileptiform activity in immature rat hippocampus

We investigated the effects of the cholinergic agonist carbachol (25 microM) on the synaptic potentials recorded extracellularly and intracellularly from the CA3 area of immature hippocampal slices of the rat (postnatal days 10-20). In control conditions, carbachol reduced the amplitude of evoked synaptic responses (n=8) and did not induce any spontaneous synchronous activity (n=12); the depressant effect of carbachol was mimicked by acetylcholine (100 microM, in eserine 10 microM, n=5) and was reversed by the muscarinic antagonist atropine (1 microM, n=2). The GABA(A)-receptor antagonist bicuculline (10 microM) enhanced the amplitude and duration of the evoked synaptic responses and induced infrequent (0.016-0.045 Hz) spontaneous synchronous discharges in 23/37 of the slices. Application of carbachol in the presence of bicuculline reduced the amplitude of the evoked synaptic responses (n=21) and in addition induced synchronous discharges with rates of occurrence 0.075-0.225 Hz, in 64/68 slices. Both effects were mimicked by acetylcholine and eserine, and antagonized by atropine. The specific muscarinic antagonists pirenzepine (M1-type), tripitramine (M2-type), 4-diphenylacetoxy-N-methylpiperidine methiodide (M3-type) and tropicamide (M4-type) (all tested at 0.1-1 microM) reversibly reduced the frequency of synchronous carbachol-induced discharges. In addition, these discharges were reversibly blocked by high Ca2+ perfusion medium (7 mM CaCl2, n=4) and by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM, n=7). Synchronous epileptiform discharges were recorded from both CA1 and CA3 areas in intact slices (n=3), but only from CA3 following disruption of the CA1-CA3 synaptic connections (n=3). These experiments suggest that activation of muscarinic receptors during blockade of GABA(A)-mediated potentials, may enhance synchronous epileptiform activity in immature (postnatal days 10-20) hippocampus, through activation of local excitatory circuits and that endogenous acetylcholine may be sufficient to play this role.

Ketoprofen, a non-steroidal anti-inflammatory drug prevents the late-onset reduction of muscarinic receptors in gerbil hippocampus after transient forebrain ischemia

Ischemia-induced hippocampal late-onset reduction of muscarinic acetylcholine receptors (LORMAR) begins as late as 7 days after transient forebrain ischemia in the gerbil, but it precedes to completion of neuronal death in the CA1 region. We previously reported that post-ischemic administration of cyclosporin A prevented LORMAR with suppression of astroglial and microglial activation. In the present study, we showed that the chronic post-ischemic administration of a non-steroidal anti-inflammatory drug, ketoprofen (5 mg/kg, subcutaneously, twice a day for 14 days) significantly reduced LORMAR both 14 days and 21 days after 5-min transient ischemia. This protective effect of ketoprofen against LORMAR suggests that the non-steroidal anti-inflammatory drugs is clinically efficacious in the treatment of LORMAR, a sequela of cerebral ischemia.

Autoradiographical and immunohistochemical analysis of receptor localization in the central nervous system

Quantitative receptor autoradiographic methods have been widely used over the past two decades. Some of the advantages and limitations of these techniques are reviewed here. Comparison with immunohistochemical and in situ hybridization methods is also highlighted, as well as the use of these approaches to study receptor gene over-expression in cell lines. Together, data obtained using these various methodologies can provide unique information on the potential physiological roles of a given receptor protein and/or binding sites in various tissues.

Comparative ontogenic profile of cholinergic markers, including nicotinic and muscarinic receptors, in the rat brain

The ontogenic profiles of several cholinergic markers were assessed in the rat brain by using quantitative in vitro receptor autoradiography. Brain sections from animals at different stages of development were processed with [3H]AH5183 (vesamicol; vesicular acetylcholine transport sites), [3H]N-methylcarbamylcholine (alpha(4)beta(2) nicotinic receptor sites), [3H]hemicholinium-3 (high-affinity choline uptake sites), [3H]3-quinuclidinyl benzilate (total population of muscarinic receptor sites), [3H]4-DAMP (muscarinic M1/M3 receptor sites), [3H]pirenzepine (muscarinic M1 receptor sites), and [3H]AF-DX 116 and [3H]AF-DX 384 (muscarinic M2 receptor sites) as radiolabeled probes. The results revealed that, by the end of the prenatal period (embryonic day 20), the densities of nicotinic receptor and vesicular acetylcholine transport sites already represented a considerable proportion of those observed in adulthood (postnatal day 60) in different laminae of the frontal, parietal, and occipital cortices, in the layers of Ammon's horn fields and the dentate gyrus of the hippocampal formation, as well as in the amygdaloid body, the olfactory tubercle, and the striatum. In contrast, at that stage, the densities of total muscarinic, M1/M3, M1, and possibly M2 receptor and high-affinity choline uptake sites represent only a small proportion of levels seen in the adult. Differences were also observed in the postnatal ontogenic profiles of nicotinic, muscarinic, vesamicol, and high-affinity choline uptake sites. For example, between postnatal weeks 3 and 5, the levels of M1/M3 and M1 sites were at least as high as in the adult, whereas M2 and high-affinity choline uptake site densities appeared to be delayed and to reach adult values only after postnatal week 5. With regard to cholinergic innervation in the developing rat brain, the present findings suggest a temporal establishment of several components of the cholinergic systems. The first components are the vesicular acetylcholine transporter and nicotinic sites; these are followed by M1/M3 and M1 sites and, finally, by M2 and high-affinity choline uptake sites.

Distribution of seven major neurotransmitter receptors in the striate cortex of the New World monkey Callithrix jacchus

The distribution of seven different binding sites for the transmitters L-glutamate (L-glutamate binding sites and N-methyl-D-aspartate receptor), GABA (GABAA receptor), noradrenaline (alpha 1 receptor), acetylcholine (muscarinic M1 and M2 receptors) and serotonin (5-hydroxytryptamine1 receptor) are analysed in the primary visual cortex (area 17) of the common marmoset, Callithrix jacchus, using quantitative autoradiography. All binding sites show a well-defined laminar pattern, which changes sharply at the cytoarchitectonic border to area 18. The quantitative data show that the distribution of different receptors is relatively invariant across the cortical layers. Almost all receptors show a maximum in supragranular layers, low densities in layers IVA/IVB and a second maximum in layer IVC. Statistical analysis of these similarities in laminar distribution patterns of different receptors (co-distribution) reveals, as in other brain regions and species, that L-glutamate binding sites are co-distributed with N-methyl-D-aspartate, GABAA, and muscarinic M1 and M2 receptors. This may reflect the structural basis of a possible interaction between these receptors and their respective transmitters on the level of single cortical layers. Further co-distributions are found between N-methyl-D-aspartate, GABAA and M1, as well as between alpha 1 and M1 and finally between M1 and M2 receptors. Since not all receptors are co-distributed, the similarities in laminar patterns reveal specific aspects of the neurochemical organization of the cortex when receptors of different transmitter systems are analysed in the same brain.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of hyperphenylalaninaemia on the muscarinic acetylcholine receptor in the HPH-5 mouse brain

Previous studies on the effect of hyperphenylalaninaemia on the development of the muscarinic acetylcholine receptor in the cerebrum of the rat, using alpha-methylphenylalanine-induced hyperphenylalaninaemia, have shown a gradual and steady decrease in the number of binding sites for this neurotransmitter. The HPH-5 mouse, a phenylalanine hydroxylase mutant, can be hyperphenylalaninaemic without the use of a hydroxylase inhibitor. By employing quantitative autoradiography using [3H]quinuclinidylbenzilate to label muscarinic acetylcholine receptors, a refined analysis of this decrease in neurotransmitter binding sites can be made. The decrease was confirmed and is therefore due to the hyperphenylalaninaemia per se and not to the use of the inhibitor. Various areas of the brain reacted differently to hyperphenylalaninaemia, from no change (putamen) to a gradual decrease (external layer of the olfactory bulb, parietal, occipital and cingulate areas of the cerebral cortex, CA1 and CA3 layer of the hippocampus) to a decrease preceded by a transient increase (frontal area of the cerebral cortex, caudate nucleus). The extent of these changes depends on the duration of exposure to hyperphenylalaninaemia as well as on the degree of brain maturation, but can even be observed in the brain of the adult mouse on a hyperphenylalaninaemic regimen for 11 days. Since the hippocampus has been shown to be involved in the long-term storage of information, damage to this structure by hyperphenylalaninaemia may provide a clue to the global mental retardation observed in untreated PKU.

In vivo visualization of central muscarinic receptors using [11C]quinuclidinyl benzilate and positron emission tomography in baboons

The muscarinic antagonist, quinuclidinyl benzilate (QNB), labeled with carbon 11 was used as a radioligand to visualize in vivo by positron emission tomography (PET) the central muscarinic acetylcholine receptors (mAChR) in baboons (Papio papio). The binding characteristics of [11C]QNB showed its specific binding to central mAChR. [11C]QNB brain uptake was high in cerebral cortex and striatum, areas that are rich in mAChR, whereas it decreased rapidly in cerebellum, evidencing non-specific binding in this structure that is almost devoid of mAChR. These results are consistent with the known cerebral distribution of mAChR in primates. [11C]QNB specific cerebral binding was enhanced by pretreatment with methyl-QNB, a peripherally acting muscarinic antagonist. Specifically labeled binding sites alone were blocked by prior administration of dexetimide, a muscarinic antagonist. Specific radioactivity was driven out from mAChR-rich regions by atropine and dexetimide, drugs with high affinity for mAChR. This competition was stereospecific since only dexetimide, the pharmacologically active isomer of benzetimide, was able to compete with the radioligand on its binding sites. A relationship between the occupancy of [11C]QNB-labeled receptors by atropine or dexetimide and the concomitant induction of a pharmacological effect was also detected by simultaneous PET scanning and electroencephalographic recording. Since mAChR form an important part of choline receptors in the central nervous system, [11C]QNB appears to be a suitable radiotracer to monitor cerebral physiological or pathological phenomena linked to the cholinergic system in living subjects.

Detection and quantification of opiate receptors in man by positron emission tomography. Potential applications to the study of pain

Opiate receptors in the brain are the target of endogenous opioids and of exogenous synthetic opiates. It is well established that these receptors play a major role in the modulation of pain perception. With positron emission tomography (PET) and the appropriate radioligands, it is now possible to image and quantify neuroreceptors in vivo. We used 11C-diprenorphine and the extremely potent mu opiate receptor agonist 11C-carfentanil to image the distribution of opiate receptors in the human brain and to quantify their density, affinity, and occupancy. Several important methodological aspects of the in vivo opiate receptor labeling with PET in relation to the study of pain are considered in this paper. Monitoring receptor occupancy by opiate drugs as a function of pain relief has the potential to reveal better ways to treat pain.

Fetal ethanol exposure diminishes hippocampal beta-adrenergic receptor density while sparing muscarinic receptors during development

Because of ostensible effects of fetal exposure to ethanol on cardiac and memory functions, beta-adrenergic and muscarinic receptor binding were surveyed in hippocampus and heart in 8- and 17-day-old rat pups. Pregnant, multiparous rats were intubated with either 6 g/kg ethanol or isocaloric dextrose twice daily from gestational days 10-16. At birth, offspring were fostered to untreated mothers. Pups exposed to ethanol had diminished birth weights, although there was no difference in the amount of weight gain by ethanol and control dams during gestation, nor in litter size. Ethanol pups remained smaller than control pups, but this difference was significant only until 8 days of age. At 17 days of age, ethanol pups had fewer hippocampal beta-adrenergic receptors than age-matched controls; muscarinic receptors and CA1 cell densities were not disparate. Parallel studies suggested that approximately 50% of the hippocampal beta-adrenergic receptors in 8-day-olds were of the beta 1 and beta 2 subtypes, while by 17 days of age approximately 70% of the receptors were beta 1. There was an ontogenetic increment in both beta-adrenergic and muscarinic binding from 8 to 17 days of age in hippocampus. No differences between age or drug groups were found in the binding measures in heart tissue. The present findings indicate that fetal ethanol treatment affects developmental measures and beta-adrenergic receptors in the hippocampus in a quasi-selective manner, but not hippocampal CA1-cell density.

Muscarinic cholinergic receptors in the songbird and quail brain: a quantitative autoradiographic study

In order to clarify the neuroanatomical basis for postulated muscarinic cholinergic control of a wide array of physiological processes in birds, the distribution of muscarinic cholinergic receptors in the brain of three avian species was investigated by quantitative autoradiography. The species consisted of two passerines (songbirds), the European starling (Sturnus vulgaris) and the song sparrow (Melospiza melodia), and one galliform, the Japanese quail (Coturnix coturnix japonica). [3H]N-methyl scopolamine (NMS), a muscarinic cholinergic antagonist was used as the ligand to label the receptors. Initial experiments demonstrated that the binding of this ligand in the three species is saturable in the nanomolar range and has a high affinity (Kd = +/- 0.6 nM). Displacement experiments revealed that three muscarinic ligands competed in an order of potency characteristic of the mammalian muscarinic receptor (i.e., atropine greater than oxotremorine greater than carbachol) for NMS binding in the avian brain. In all three species, portions of the basal ganglia, such as the parolfactory lobe and the paleostriatum augmentatum, exhibited the highest density of binding. On the other hand, the paleostriatum primitivum, the avian homologue of the mammalian globus pallidus, contained very few binding sites. Other telencephalic sites, such as the ventral and dorsal hyperstriatum, also revealed relatively high receptor density. However, the neostriatum and especially the ectostriatum showed much lower levels. In the hypothalamus, in all three species, specific binding could be observed in the ventromedial nucleus and adjacent areas. The paraventricular nucleus also showed moderate levels of binding density, especially in the two songbird taxa. At a more rostral level, the preoptic area showed low levels of binding. In the quail, the sexually dimorphic nucleus of the preoptic area was clearly outlined in the autoradiograms by the low level of binding sites compared to the surrounding areas. In the two passerine species, nuclei of the song system were identified by either high or low levels of NMS binding. High binding defined area X and the mesencephalic nucleus, intercollicularis (ICo). In contrast, the robust nucleus of the archistriatum and the magnocellular nucleus of the anterior neostriatum showed low levels of binding in comparison with the surrounding tissue. None of these nuclei were visible in the quail autoradiograms except for ICo, which appeared as in the passerines as a heavily labelled area surrounding the lightly labelled nucleus mesencephalicus lateralis pars dorsalis. In all three species, the hippocampal complex was devoid of NMS binding except for two lateral dark bands that were present along the entire rostral to caudal extent of the hippocampus.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoradiographic imaging of phosphoinositide turnover in the brain

With [3H]cytidine as a precursor, phosphoinositide turnover can be localized in brain slices by selective autoradiography of the product [3H]cytidine diphosphate diacylglycerol, which is membrane-bound. In the cerebellum, glutamatergic stimulation elicits an increase of phosphoinositide turnover only in Purkinje cells and the molecular layer. In the hippocampus, both glutamatergic and muscarinic cholinergic stimulation increase phosphoinositide turnover, but with distinct localizations. Cholinergic stimulation affects CA1, CA3, CA4, and subiculum, whereas glutamatergic effects are restricted to the subiculum and CA3. Imaging phosphoinositide turnover in brain slices, which are amenable to electrophysiologic studies, will permit a dynamic localized analysis of regulation of this second messenger in response to synaptic stimulation of specific neuronal pathways.

Effects of diisopropyl phosphorofluoridate (DFP) on CA3 and CA1 responses in rat hippocampus

Diisopropyl phosphorofluoridate (DFP), an insecticide, is a potent anticholinesterase that binds essentially irreversibly to acetylcholinesterase, resulting in severe, acute neurologic pathology, and less severe, but longer-lasting, delayed neuropathy. We report here on the short-term effects of bath-applied DFP on extracellularly recorded responses from CA3 and CA1 of rat hippocampus. Exposure to 10 microM DFP evokes low amplitude, spontaneous bursts in CA3 generally within 10 minutes, and the bursting does not reverse with washing. The CA1 neuronal population usually bursts synchronously with CA3, but the population events are of low amplitude and sometimes not detectable, implying a differential sensitivity to DFP. These effects were partially blocked by the muscarinic antagonist atropine, while the cholinergic antagonist gallamine had little effect. Also, the reversible anticholinesterase physostigmine could, within temporal limits, protect slices from DFP's effects, implicating the cholinergic system as the probable mediator in the first stages of DFP-induced epileptogenesis.

Cholinergic innervation of the rat cerebellum: qualitative and quantitative analyses of elements immunoreactive to a monoclonal antibody against choline acetyltransferase

Cholinergic innervation of the rat cerebellum was investigated immunohistochemically by using a monoclonal antibody against choline acetyltransferase. Immunoreactive structures included: 1) a subpopulation of mossy fibers and glomerular rosettes; 2) thin varicose fibers, which were closely associated with the Purkinje cell layer and also found in the molecular layer; and 3) relatively dense networks of varicose fibers distributing in the cerebellar nuclei. Quantitative analysis indicated that a great many immunoreactive rosettes were localized in lobules IXc and X, although their density in lobule X was approximately four times that in the lobule IXc. A considerable number of immunoreactive structures were also present in all other lobules. In the hemispheres they were confined to a zone immediately beneath the Purkinje cell layer, whereas in the vermis they were scattered throughout the granular layer. Most of the immunoreactive fibers found in the molecular layer coursed toward the pial surface and were distributed within the inner half of the molecular layer. In the cerebellar nuclei, portions of the medial nucleus and magnocellular portion of the lateral nucleus had moderately dense networks of immunoreactive fibers, whereas loose networks of fibers were observed in the posterior interposed nucleus. Other parts of the cerebellar nuclei contained a smaller number of varicose fibers.

Time-course of malaoxon-induced alterations in brain regional inositol-1-phosphate levels in convulsing and nonconvulsing rats

The potential of a single dose of malaoxon (26.2 or 39.2 mg/kg i.p.) to produce convulsions and to increase cerebral levels of inositol-1-phosphate (Ins1P), an intermediate in phosphoinositide (PI) cycle, was followed for 1, 4, or 72 hr. The lower dose of malaoxon did not produce convulsions whereas the higher dose induced convulsions in 60% of the exposed rats. Malaoxon caused a dose-dependent, at most 2-fold, increase in brain regional Ins1P levels in nonconvulsing rats as compared to controls. At the higher dose of malaoxon, in convulsing rats, the Ins1P-levels increased 4-fold above the control Ins1P-levels. In nonconvulsing rats, the Ins1P-levels reached their maximum 1-4 hr after the administration of malaoxon, whereas in convulsing rats the levels increased for 72 hr. The results suggest that PI-signalling is associated with convulsions produced by malaoxon.

Sensory modulation of hippocampal transmission. II. Evidence for a cholinergic locus of inhibition in the Schaffer-CA1 synapse

The present work studied the neurotransmitter mediating the depressive effect of sensory stimulation on the Schaffer-CA1 transmission. Field responses of the CA1 region evoked by ipsilateral CA3 stimuli were recorded in paralyzed, locally anesthetized rats following the same experimental paradigm as in the previous work. The tissue zone under recording was perfused in vivo by an implanted hollow fiber (brain dialysis device) with either Krebs-Ringer bicarbonate (KRB), or KRB with penicillin, atropine, acetylcholine or eserine. Results were the following: (1) atropine increased the field excitatory postsynaptic potential (EPSP) amplitude in a dose-dependent manner and totally abolished the modulatory action of sensory stimulation; (2) both the field EPSP and the modulatory action of sensory stimulation remained unaltered during the blockade of GABAergic activity by penicillin; (3) acetylcholine as well as eserine induced a great diminution of both field EPSP and population spike amplitudes, without altering the effect of sensory stimulation; (4) penicillin and atropine induced multiple population spikes, reversing the effect of sensory stimulation and increasing the cell excitability. These results demonstrate that the sensory modulation of information transfer through the Schaffer-CA1 synapse is mediated by a muscarinic cholinergic mechanism. The dose-dependent increase in the field EPSP by muscarinic blockade is evidence for the existence of a cholinergic presynaptic inhibition on the Schaffer collateral terminals.

Reversal of scopolamine-induced amnesia and alterations in energy metabolism by the nootropic piracetam: implications regarding identification of brain structures involved in consolidation of memory traces

Pretreatment with scopolamine, 3 mg/kg, prevented the acquisition of a passive avoidance task in rats. These amnesic effects of scopolamine could largely be overcome by treatment with 100 mg/kg of the nootropic drug piracetam. In order to identify the brain structures involved, the effects of these drugs on regional energy metabolism were measured throughout the brain, utilizing Sokoloff's 2-deoxyglucose autoradiographic procedures. Scopolamine, 3 mg/kg, reduced glucose utilization in several areas of the cerebral cortex. These effects were largest in the parietal and temporal cortices. Other areas affected included the sensorimotor and cingulate cortices, the ventral and lateral thalamus, and the dendritic neuropil of the CA1, CA2, and CA3 regions of the hippocampus. The regional depressions in glucose metabolism observed following scopolamine treatment in the rat had some resemblance to depressions in glucose metabolism reported for Alzheimer's disease patients in positron emission tomography studies. Piracetam, 100 mg/kg, did not alter the energy metabolism of any of the 41 brain regions examined. However, this dose of piracetam completely reversed the scopolamine-induced depressions in the hippocampus. Piracetam partially but significantly reversed the scopolamine effects in the cingulate cortex. It is concluded that the data provide support for the hippocampal-cholinergic theory of memory as originally formulated by Meyers and Domino in 1964 and give insight into the mechanisms by which nootropics work.

Pharmacological characteristics and anatomical distribution of [3H]oxytocin-binding sites in the Wistar rat brain studied by autoradiography

Oxytocin-binding sites were detected by autoradiography on rat brain sections incubated in the presence of the [3H]oxytocin. These sites were characterized pharmacologically using quantitative autoradiography. High pressure liquid chromatography controls of the incubation media indicated that labelling was due to the intact [3H]oxytocin molecule. Pharmacological analysis of different locations (central amygdaloid nucleus, ventral subiculum and ventromedial hypothalamic nucleus) showed that the sites detected had a high affinity for oxytocin and also for arginine-vasopressin. In contrast, some areas known to bind vasopressin intensely, such as suprachiasmatic and lateral septum nuclei, had little or no affinity for oxytocin. Autoradiographs revealed [3H]oxytocin-binding sites in already known brain areas (olfactory centres, ventral subiculum, central amygdaloid nucleus, bed nucleus of the stria terminalis) albeit with more extensive labelling of some of these formations, in particular, the amygdaloid complex. In addition, specific [3H]oxytocin-binding sites were found in areas not yet reported to bind oxytocin, such as the paraventricular thalamic and caudate nuclei. In the hypothalamus, specific binding sites were not detected in the supraoptic and paraventricular nuclei: the only structure labelled was the ventrolateral part of the ventromedial nucleus. Discrepancies between the concentrations of [3H]oxytocin-binding sites, the known distribution of oxytocin-containing endings and electrophysiological data indicate that autoradiography, under our conditions, apparently only reveals some of the oxytocin receptors in the brain. Thus, in the hypothalamus, no relationship can be established between the known effect of oxytocin on oxytocinergic magnocellular neurons and detection of specific [3H]oxytocin-binding sites. Autoradiography may reveal mainly oxytocin-binding sites in areas receiving diverse "parasynaptic" information, where oxytocin might play a modulatory role rather than exerting rapid, short-term effects of the neurotransmitter type.

In vitro autoradiographic localization of angiotensin-converting enzyme in sarcoid lymph nodes

Angiotensin-converting enzyme (ACE) was localized in sarcoid lymph nodes by an in vitro autoradiographic technique using a synthetic ACE inhibitor of high affinity, 125I-labelled 351A. The lymph nodes were from seven patients with active sarcoidosis who underwent mediastinoscopy and from six control subjects who had nodes resected at either mediastinoscopy or laparotomy. Angiotensin-converting enzyme was localized in the epithelioid cells of sarcoid granulomata in markedly increased amounts compared with control nodes, where it was restricted to vessels and some histiocytes. In sarcoid lymph nodes, there was little ACE present in lymphocytes or fibrous tissue. Sarcoid nodes with considerable fibrosis had much less intense ACE activity than the nonfibrotic nodes. The specific activity of ACE measured by an enzymatic assay in both the control and sarcoid lymph nodes closely reflected the ACE activity demonstrated by autoradiography. Sarcoid lymph nodes with fibrosis had an ACE specific activity of half that of nonfibrotic nodes (p less than 0.05). There was a 15-fold increase in specific ACE activity in sarcoid nodes (p less than 0.05) compared to normal. Serum ACE was significantly higher in those sarcoid patients whose lymph nodes were not fibrosed compared with those with fibrosis (p less than 0.01). This technique offers many advantages over the use of polyclonal antibodies. The 351A is a highly specific ACE inhibitor, chemically defined and in limitless supply. This method enables the quantitation of results, and autoradiographs may be stored indefinitely for later comparison.

Patterns of muscarinic cholinergic binding in the striatum and their relation to dopamine islands and striosomes

The distribution of muscarinic cholinergic binding sites in the striatum was studied in relation to the locations of other neurochemical markers in the developing rat, cat, ferret, and human. In addition, patterns of striatal muscarinic binding were studied in the adult cat. Receptor binding autoradiography was carried out with tritiated propylbenzilylcholine mustard [( 3H]-PrBCM), an irreversible muscarinic antagonist, and subsequent serial section analyses involved comparisons among patterns of muscarinic binding, catecholamine histofluorescence, acetylcholinesterase (AChE) staining, Nissl staining, and cell labeling with [3H]-thymidine. Muscarinic binding in the immature striatum was characterized by local patchiness as well as regional density gradients in all species, with the most complex patterns appearing in the human. Patches of dense muscarinic binding were shown to lie in register with fluorescent dopamine islands (rat, cat, ferret), with AChE-positive patches (all species), and with clusters of neurons pulse-labeled by exposure to [3H]-thymidine on embryonic day 27 (ferret). At the developmental stages examined, the [3H]-PrBCM-positive patches were roughly aligned with regions of weak Nissl staining (cat, human). Striatal [3H]-PrBCM binding in the adult cat was dense, and though it usually appeared nearly homogeneous, in some sections patches of elevated binding were present. These had as counterparts, in neighboring sections, AChE-poor striosomes. We conclude that during development muscarinic cholinergic function is compartmentalized in the striatum in association with dopamine-containing afferents, and that this compartmentalization may persist to some degree in the adult.

Autoradiographic localisation of 3H-apomorphine binding sites in rat brain

The best experimental conditions for a selective binding of 3H-apomorphine to dopamine receptors on cryostat sections were first selected by liquid scintillation quantification of the bound radioactivity. In the corpus striatum, a specific binding occurred with a half-maximal saturation concentration of about 1 nM and a maximal capacity of 180 fmol/mg of slice protein, both values in agreement with previous binding data on either membranes or slices incubated in a physiological medium. Inhibition with domperidone was clearly biphasic, indicating two classes of sites corresponding to the D-2 and D-3 sites as previously defined on membranes. When 3H-apomorphine was used at low concentrations (0.8-1.5 nM), a condition ensuring a preferential labelling of D-2 sites, rather well contrasted autoradiographic pictures were generated. The major dopaminergic projection fields in telencephalon (caudate-putamen, nucleus accumbens, olfactory tubercles) were visualised as well as other catecholaminergic regions such as the superficial gray layer of superior colliculi. Within the striatum, differences in density of these sites were observed in three perpendicular planes and confirmed by a computer densitometric image analysis. Labelling of areas of origin of the cerebral dopaminergic neurons in substantia nigra or ventral tegmental area were also observed. When a higher concentration of 3H-apomorphine (3.5 nM) was used in the presence of domperidone, another, but autoradiographically less distinct subclass of sites (D-3 sites) was demonstrated.

Effects of intracellular antibodies to cGMP on responses of cortical neurons of awake cats to extracellular application of muscarinic agonists

Intracellular injection of specific antibody to cyclic 3',5'-guanosine monophosphate (cGMP-Ab) produced substantial decreases in input resistance (Rm) selectively in neurons of the motor cortex that had responded with increased resistance to prior application of muscarinic agents. Intracellular injection of nonspecific immunoglobulins (IgG) did not produce this effect. (Some nonspecific effects on spike production occurred in cells given IgG or cGMP-Ab.) The decrease in Rm may be interpreted as being consequential to a reduction in baseline amounts of active cGMP due to binding of cGMP with the injected antibody. In cells which demonstrated a prior increase in Rm following extracellular application of the muscarinic agonist, aceclidine, or acetylcholine, injection of antibody to cGMP also resulted in suppression of the increase in Rm to subsequent applications of these muscarinic agents. Some increases in firing rate to these agents continued to be observed after injection of cGMP-Ab. The results support the hypothesis that cGMP mediates effects of muscarinic neurotransmission on the conductances of neurons of the motor cortex of awake cats. Intracellular injection of antibodies to specific cellular elements is shown to be feasible in cortical neurons of awake cats and may prove a useful adjunct to future studies of neurotransmitter mechanisms.

Quantitative light microscopic autoradiographic localization of cholinergic muscarinic receptors in the human brain: brainstem

We have investigated the localization of muscarinic cholinergic receptors in the brainstem of eight patients free of neurological disease following quantitative autoradiography of microtome sections of postmortem tissue labeled in vitro with N-[3H]methyl scopolamine as a ligand. Receptor densities were quantified by microdensitometry with the aid of a computer assisted image analysis system. Our results reveal a heterogeneous distribution of receptor sites. High concentrations of muscarinic cholinergic receptor sites were associated with many nuclei and areas of the brainstem including the nucleus facialis (VII), hypoglossus (XII), ambiguus, the motor trigeminal nucleus (V), the nucleus solitarius, the nucleus of the lateral lemniscus, the superior and inferior colliculi, the sensory trigeminal nucleus (substantia gelatinosa), the pontine nuclei, the parabrachial nuclei, some tegmental nuclei and the periaqueductal gray matter. Very high concentrations of N-[3H]methyl scopolamine binding sites were also localized in the ventral tegmental area, the nucleus paranigralis and the nucleus ovalis. Receptor densities varied between individual brains although the relative distribution of the densities in the different nuclei was the same for all of the brains examined. Most of the brainstem nuclei containing muscarinic cholinergic receptors were enriched in high affinity agonist binding sites as shown by characteristic displacement of the ligand with carbachol. Exceptions were the substantia nigra, the nucleus olivaris inferior and the substantia gelatinosa of the fifth nerve. Receptor density values and pharmacological characteristics obtained in the cortex and basal ganglia in our cases are in good agreement with previously reported values in humans, using conventional biochemical methods. This indicates that procedures used in the autoradiographic technique are not detrimental to the pharmacological characteristics and densities of muscarinic cholinergic receptors. Our results thus clearly show the feasibility of using these techniques for the localization and quantification of muscarinic cholinergic receptors in human brain postmortem material. Furthermore, our findings indicate the potential involvement of the muscarinic cholinergic effect of acetylcholine in the normal function of many brainstem centers, including motor and sensory nuclei, visual and auditory relay nuclei and cardiovascular and respiratory-related nuclei.

Muscarinic receptor during postnatal development of rat cerebellum: an index of cholinergic synapse formation?

Quantitative and qualitative modifications of the specific binding sites for [3H]quinuclidinyl benzylate (QNB), a muscarinic antagonist, were studied during rat cerebellar postnatal development. Specific binding sites for QNB (QNB-sbs), regardless of whether they correspond to muscarinic acetylcholine receptors, are present with the highest density in the archicerebellar cortex, but the total amount per region is about the same in the archi -, paleo-, and neocerebellar cortex regions. Large amounts of QNB-sbs are also present in a cerebellar fraction including central white matter and deep cerebellar nuclei. QNB-sbs are low but present at birth and then accumulate during ontogenic development according to a curve which duplicates, with a delay of a few days, the curve of DNA accumulation. Dissection studies indicated that this curve does not depend on the preferential localization of QNB-sbs in a specific cerebellar region nor on the particular development of this region. The similarity of the QNB-sbs and the DNA developmental curves might indicate that the QNB-sbs are present on granule cells; however, a comparative analysis of the data in the literature suggests that a great many QNB-sbs are located on the Purkinje cell dendrites in the molecular layer, where all or some of them might correspond to the extrajunctional muscarinic acetylcholine receptor detected there by electrophysiology. It would appear that only a small percentage of cerebellar QNB-sbs corresponds to the cholinergic synapses present in cerebellar cortex; hence, the question of muscarinic receptors in the cerebellum should be re-examined.

Distribution of muscarinic cholinergic high and low affinity agonist binding sites: a light microscopic autoradiographic study

The distribution of high vs. low affinity muscarinic agonist binding sites has been determined using quantitative techniques of receptor autoradiography. The low affinity agonist sites predominate in many regions of the forebrain including the cerebral cortex, striatum, hippocampus, amygdala and thalamus. The high affinity agonist sites predominate in the brainstem and represent exclusively the type of muscarinic cholinergic receptor normally present in the principal nucleus of the trigeminal nerve, facial nerve nucleus, hypoglossal nerve nucleus, and in the ventral horn of the spinal cord. The regional localization of these subpopulations provides valuable information for future studies which seek to determine the functional importance of subtypes of muscarinic agonist binding sites.

Topographical distribution of down-regulated muscarinic receptors in rat brains after repeated exposure to diisopropyl phosphorofluoridate

Quantitative receptor autoradiography demonstrated that muscarinic receptors were down-regulated in Wistar rats after repeated exposure to diisopropyl phosphorofluoridate. The density of receptors was decreased to 60-85% of the controls. Reductions in muscarinic receptor binding were observed in cortex, caudate-putamen, lateral septum, hippocampal formation, superior colliculus, and pons. The density of muscarinic receptors was unchanged in thalamic and hypothalamic nuclei, periaqueductal grey, cerebellum, inferior colliculus and reticular formation of the brain stem. The down-regulation of muscarinic receptors in forebrain structures, such as cortex, caudate-putamen and hippocampus, may be important in the adaptation to the behavioral effects of organophosphate poisons.

Receptor autoradiography: the last ten years

The development of techniques for the microscopic visualization of receptor binding and other binding sites using autoradiography is reviewed. The application of these techniques to a number of pharmacological problems, particularly in the field of neuropharmacology is illustrated. The limitations and future trends in the field of receptor visualization are commented upon.

Computer-assisted autoradiographic localization of subtypes of serotonin1 receptors in rat brain

In vitro light microscopic receptor autoradiography was carried out to localize two subtypes of 5-HT1 receptors in certain regions of the rat central nervous system (CNS). The data were analyzed by computerized digital image processing to reveal subfields of high receptor densities within the septal area, the prefrontal and frontal cortex, and the hippocampus. The septum and hippocampus showed interesting localization of serotonin binding which can be subtyped into 5-HT1-A and 5-HT1-B sites. The differential localization of these sites may be helpful in shedding some light as to the functional significance of multiple serotonin receptors.

Kindled seizure-induced reduction of muscarinic cholinergic receptors in rat hippocampal formation: evidence for localization to dentate granule cells

The binding of [3H] quinuclidinyl benzilate ( [3H] QNB) to muscarinic cholinergic receptors in dentate gyrus of rat hippocampal formation was analyzed by membrane binding assay and in vitro autoradiography. The destruction of dentate granule cells, either by neonatal irradiation or colchicine injection, resulted in nearly complete elimination of [3H] QNB binding sites in the molecular and granule cell layers. By contrast, neither perforant path transection nor destruction of the septal-hippocampal cholinergic afferents caused a decline of [3H] QNB binding sites. Amygdala kindled seizures resulted in a 30% reduction of [3H] QNB binding sites which was distributed uniformly across the entire molecular and granule cell layers. Thus, most, if not all, of the muscarinic cholinergic receptors present in dentate gyrus appear to reside on the somata and dendritic trees of the dentate granule cells. We propose that this kindled seizure-induced decline of muscarinic receptors represents an endogenous compensatory mechanism designed to stabilize granule cell excitability.

Cellular bases of hippocampal EEG in the behaving rat

Rats implanted with recording and stimulating electrodes were trained to run in an activity wheel for a water reward. Unitary discharges and slow activity were recorded by a movable tungsten microelectrode and by fixed electrodes. Single cells were classified according to their spontaneous and evoked response properties as pyramidal cells, granule cells and interneurons. Unit activity, EEG and their interrelations were studied by spectral and spike-triggered averaging methods. Gradual phase-shifts of RSA were observed both in CA1 and the dentate gyrus. Movement-related RSA was correlated with a decrease in firing rate of pyramidal cells and an increase in the firing of both interneurons and granule cells. In the CA1 region pyramidal cells and interneurons fired preferentially on the negative and positive phases of the locally derived RSA, respectively. In the dentate gyrus both granule cells and interneurons discharged mainly on the positive portion of the local RSA waves, about 90 degrees before the CA1 pyramidal cells. Fourier analysis of the spike trains of interneurons and granule cells showed high power at RSA frequency, coherent with the concurrent EEG. Phase relations between discharges of interneurons and RSA remained unchanged following urethane anesthesia. In waking rats, atropine administration resulted in a decreased discharge of interneurons at RSA frequency, and reduced coherence with RSA. Lesions of the septum or the fimbria-fornix abolished RSA and the rhythmic discharges of the interneurons. Isolation of the entorhinal cortex (EC) from its cortical inputs did not change either EEG or neuronal firing. However, in such a preparation atropine completely abolished RSA and related rhythmicity of interneurons. During drinking and immobility but not during walking, sharp waves (SPW) of about 40-100 ms duration appeared in the EEG. SPWs were invariably accompanied by synchronous discharges of several pyramidal cells and interneurons. CA3 pyramidal cells also discharged in synchronous bursts but without local SPWs. Laminar profiles of SPWs and the field potentials evoked by stimulation of Schaffer collaterals were essentially identical. The behavior-dependent occurrence of SPWs was retained following atropine administration, septal lesion or EC isolation but was lost after fimbria-fornix-neocortex lesion or following atropine administration in EC isolated rats. In addition to relations to RSA and SPWs, interneurons were phase-locked to the fast EEG pattern (25-70 Hz). This relationship was preserved following lesions of the septum or the fimbria-fornix complex.(ABSTRACT TRUNCATED AT 400 WORDS)

Modifications in recognition sites for neurotransmitters in rat hippocampus by kainic acid lesion

The specific binding of the tritiated radioligands of dexetimide, serotonin, clonidine, prazosin, WB-4101 and dihydroalprenolol to hippocampal membranes was determined two weeks after producing a virtual complete degeneration of perikarya by the local application of 0.5 micrograms of kainic acid in the dorsal and ventral parts of the hippocampus. Afferent terminals were unaffected by the neurotoxin since the contents of noradrenaline, serotonin and acetylcholine, as well as the activity of choline acetyltransferase, were not modified. Scatchard analysis revealed that the kainic acid lesion produced a 60% decrease in the density of both cholinergic muscarinic binding sites and serotonin binding sites. A significant portion of alpha 1- and alpha 2-adrenoceptor binding sites are also associated with intrinsic neurons of the hippocampus, as shown by the approximately 30% reduction in the densities of tritiated WB-4101, prazosin and clonidine produced by the action of kainic acid. By contrast, the affinity and density of beta-adrenoceptor binding sites were unaffected by the lesion. It is suggested that the recognition sites of the different receptor populations surviving the lesion most likely reside on homologous and/or heterologous nerve terminals.

Origin of cingulate cortex cholinergic innervation in the rat

The relative contribution of the n. diagonal band and thalamic nuclei to the cholinergic innervation of the cingulate cortex was examined. Lesions were placed in the n. diagonal band, anterior thalamus, and medial thalamus of rats, and changes in choline acetyltransferase in discrete regions of the cingulate cortex were determined. The n. diagonal band lesion produced a large decrease in choline acetyltransferase activity while the thalamic lesions produced no significant change in activity.