Localization of cholinergic muscarinic receptors in rat brain by light microscopic radioautography

Selective location of acceptors for botulinum neurotoxin A in the central and peripheral nervous systems

The main site of action for botulinum neurotoxin is cholinergic motor nerve terminals where specific acceptors concentrate the toxin on the cell surface, thereby facilitating its internalization and inactivation of a component essential for transmitter release. In this study, the interaction in vitro of [125I]botulinum neurotoxin type A with central and peripheral nerve terminals of different types was investigated using Ultrofilm and electron-microscope autoradiography. It was found that: (i) The neurotoxin binds to synapse-rich areas of rat brain, particularly in the hippocampus and cerebellum; identity of the neuron types labelled is unclear although cholinergic nerves seem to be labelled, perhaps not exclusively, in many areas. (ii) Toxin uptake at central nerve terminals appears to be minimal and its penetration into intact brain slices is restricted; this may account for the toxin's lower central toxicity. (iii) Selective labelling of cholinergic nerves but not purinergic, peptidergic or adrenergic nerve terminals in mouse ileum suggests that the toxin may be a specific marker for cholinergic nerves in the periphery. Based on these localization studies and published pharmacological observations, it is concluded that efficient toxin-induced blockade of neurotransmission depends on the presence of specific acceptors of high affinity for the toxin and of an effective neuronal uptake mechanism. Inhibition of the release of numerous transmitters from different kinds of nerve terminals lacking one of these features can be produced by high toxin concentrations when uptake occurs via low affinity acceptors or by non-specific means. Notably, this widespread action of the toxin indicates the occurrence of a common intracellular target in several, possibly all, nerve types.

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.

Local cholinergic mechanisms participate in the increase in cortical cerebral blood flow elicited by electrical stimulation of the fastigial nucleus in rat

We sought to determine whether the increase in regional cerebral blood flow (rCBF) elicited within the cerebral cortex (CX) by electrical stimulation of the fastigial nucleus (FN) of the cerebellum is: prevented by local application of the muscarinic cholinergic receptor antagonist, atropine and temporally correlated with a stimulus-locked release of acetylcholine (ACh) from the cortical surface. Rats were anesthetized, paralyzed, ventilated, with arterial blood gases controlled and arterial pressure maintained within the autoregulated range. Bilateral craniotomies were performed over a standardized region of the sensory motor CX and superfusion devices stereotaxically positioned on the cortical surface. Cortical surface temperature, as well as pH, pCO2 and pO2 of the solutions applied to the cortex were also carefully controlled. rCBF was measured in dissected regions of frontal (FCX), parietal (PCX), and occipital cortices (OCX), caudate nucleus (CN), and hippocampus (HIPP) by the Kety principle using [14C]iodoantipyrine as indicator. Resting rCBF (ml/100 g/min) in unoperated control animals ranged from 70 +/- 5 in HIPP to 95 +/- 7 in PCX and was unaffected by bilateral craniotomies and placement of superfusion devices containing Kreb's bicarbonate buffer (vehicle) on the cortical surface. Local application of atropine (ATR, 100 microM) to the right PCX via the superfusion device did not affect resting rCBF. With FN stimulation rCBF increased bilaterally and symmetrically in all areas up to 227% in PCX. ATR application attenuated by 59% the FN-elicited increase in rCBF on the ipsilateral frontoparietal CX, without affecting blood flow in adjacent structures. ATR did not affect cortical cerebrovasodilation produced by hypercarbia (arterial pCO2 = 59.0 +/- 1.4 mm Hg). FN-stimulation resulted in a small (22%) but significant (P less than 0.05, n = 9) reduction in the release of [3H]ACh from the cortical surface, while supramaximal depolarization with 55 mM K+ increased [3H]ACh release by 251%. These studies indicate that: increases in cortical rCBF elicited by FN stimulation, but not hypercarbia, are in large part mediated by local muscarinic cholinergic receptors; resting rCBF is not tonically affected by muscarinic receptor activation; and the release of ACh from the cortical surface is, in general, reduced during FN-stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Selectivity of pirenzepine in the central nervous system. I. Direct autoradiographic comparison of the regional distribution of pirenzepine and carbamylcholine binding sites

The binding capacities of the novel antagonist pirenzepine and the agonist carbamylcholine were examined autoradiographically to compare their abilities to reduce the binding of 1-[3H]quinuclidinyl benzilate ([3H]-1-QNB). This technique, which is applicable to any muscarinic ligand, permits a direct comparison between the binding of carbamylcholine and pirenzepine in the same assay. Analysis of the binding curves generated by standard scintillation counting of whole-brain slices indicated that the ligands bound heterogeneously to muscarinic receptors in the brain. Following apposition of slides to tritium-sensitive film, the binding profile for each ligand was examined visually and by microdensitometry. Regional analyses indicated that the agonist carbamylcholine displayed highest potency for thalamic nuclei, lower potency for cortical regions, and the lowest affinity for layers of the hippocampus. The M1-selective ligand pirenzepine displayed the highest potency for the dentate gyrus of the hippocampus, with lower inhibition levels in the cortex, and the lowest levels of inhibition found in the thalamus. The distribution of high affinity agonist sites was found to be distinct from the distribution of high-affinity antagonist binding sites. In a separate assay, the regional inhibition of pirenzepine and scopolamine was compared for the hippocampus and the forebrain. While scopolamine did not distinguish between muscarinic receptor sites in the hippocampus and cortex, pirenzepine inhibited [3H]-1-QNB labeling in the hippocampus significantly greater than in the cerebral cortex, providing additional evidence for the hypothesis that pirenzepine is a selective antagonist.

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

The distribution of muscarinic cholinergic receptors in the human forebrain and cerebellum was studied in detail by quantitative autoradiography using N-[3H]methylscopolamine as a ligand. Only postmortem tissue from patients free of neurological diseases was used in this study. The highest densities of muscarinic cholinergic receptors were found in the striatum, olfactory tubercle and tuberal nuclei of the hypothalamus. Intermediate to high densities were observed in the amygdala, hippocampal formation and cerebral cortex. In the thalamus muscarinic cholinergic receptors were heterogeneously distributed, with densities ranging from very low to intermediate or high. N-[3H]Methylscopolamine binding was low in the hypothalamus, globus pallidus and basal forebrain nuclei, and very low in the cerebellum and white matter tracts. The localization of the putative muscarinic cholinergic receptors subtypes M1 and M2 was analysed in parallel using carbachol and pirenzepine at a single concentration to partially inhibit N-[3H]methylscopolamine binding. Mixed populations of both subtypes were found in all regions. M1 sites were largely predominant in the basal ganglia, amygdala and hippocampus, and constituted the majority of muscarinic cholinergic receptors in the cerebral cortex. M2 sites were preferentially localized in the diencephalon, basal forebrain and cerebellum. In some areas such as the striatum and substantia innominata there was a tendency to lower densities of muscarinic cholinergic receptors with increasing age. In general, we observed a slight decrease in M2 sites in elderly cases. Muscarinic cholinergic receptor concentrations seemed to be reduced following longer postmortem periods. The distribution of acetylcholinesterase was also studied using histochemical methods, and compared with the localization of muscarinic cholinergic receptors and other cholinergic markers. The correlation between the presence of muscarinic cholinergic receptors and the involvement of cholinergic mechanisms in the function of specific brain areas is discussed. Their implication in neurological diseases is also reviewed.

Autoradiographic localization of M1 and M2 muscarine receptors in the rat brain

The distribution of M1 and M2 muscarine receptors in the rat brain was investigated by in vitro autoradiography. Muscarine receptors were visualized after complete receptor uncoupling in ethylenediaminetetraacetic acid buffer containing 1 mM N-ethyl maleimide and saturation with the ligand [3H]quinuclidinyl benzilate. Pirenzepine, an M1-selective antagonist, was used in our assays as a counter ligand to occlude M1 sites, allowing the primary ligand, [3H]quinuclidinyl benzilate, to label the remaining M2 muscarine receptors. In adjacent section, M1 muscarine receptors were labelled with [3H]quinuclidinyl benzilate in the presence of sufficient carbachol, and M2-selective agonist, to inhibit the binding to M2 sites. Our results reveal a heterogeneous distribution of M1 and M2 receptors. Increased densities of carbachol-resistant and pirenzepine-sensitive sites (M1 receptor subtype) were apparent over many forebrain structures including the olfactory tubercle, caudate-putamen, nucleus accumbens, hippocampus, amygdala and cerebral cortex. In contrast, pirenzepine-resistant and carbachol-sensitive sites (M2 receptor subtype) were distributed throughout the brain with increased densities apparent over regions known to contain large numbers of cholinergic cell bodies. M2 receptor localization patterns were largely coincident with the regional distribution and intensity of acetylcholinesterase positive sites. Since the M2 receptor pattern appears to parallel regional innervation densities, we conclude that the M2 receptor may serve as a marker for cholinergic pathways. The findings also suggest that M1 muscarine receptors are involved in the presumptive postsynaptic actions of acetylcholine in many forebrain structures.

Autoradiographic distribution of high affinity muscarinic and nicotinic cholinergic receptors labeled with [3H]acetylcholine in rat brain

The relative distribution of muscarinic and nicotinic cholinergic receptors labeled with [3H]acetylcholine was determined using autoradiography. [3H]Acetylcholine binding to high affinity muscarinic receptors was similar to what has been described for an M-2 distribution: highest levels of binding occurred in the pontine and brainstem nuclei, anterior pretectal area and anteroventral thalamic nucleus, while lower levels occurred in the caudate-putamen, accumbens nucleus and primary olfactory cortex. Nicotinic receptors were labeled with [3H]acetylcholine to the greatest extent in the interpeduncular nucleus, several thalamic nuclei, medial habenula, presubiculum and superior colliculus, and to the least extent in the hippocampus and inferior colliculus. By using autoradiography to localize cholinergic binding sites throughout the brain it was observed that the distributions of high affinity muscarinic and nicotinic sites labeled with the endogenous ligand, [3H]acetylcholine are different from each other and are different from distributions of muscarinic and nicotinic sites labeled with muscarinic and nicotinic antagonists.

Muscarinic cholinergic receptors mediate the cerebrovasodilation elicited by stimulation of the cerebellar fastigial nucleus in rat

We have investigated the effects of systemic administration of atropine sulfate on the global cerebrovascular vasodilation elicited by electrical stimulation of the cerebellar fastigial nucleus (FN) in chloralose-anesthetized rat. Atropine (0.3 mg/kg, i.v.) abolished the cerebrovasodilation elicited from FN but did not affect the concomitant elevation in arterial pressure and the EEG changes. We conclude that the cerebrovascular effect of FN stimulation, but not the peripheral cardiovascular or EEG changes, are mediated by cholinergic muscarinic receptors associated with cerebral vessels and/or intrinsic neural pathways.

Muscarinic receptors in slice cultures of rat brain

Muscarinic acetylcholine receptors in organotypic slice cultures of hippocampus of the rat, have been examined using the tritiated muscarinic antagonist quinuclidinylbenzilate [( 3H]QNB) as a as a marker. Maximum specific binding of [3H]QNB in mature explants of hippocampus amounted to 316 fmol/mg protein and a dissociation constant (KD) of 185 pM was determined. Scatchard analysis suggested binding to one single binding site. In younger cultures smaller KDs were registered. This decrease in ligand affinity in maturer cultures possibly reflects a decrease in the turnover of acetylcholine. Muscarinic antagonists inhibited the total binding of [3H]QNB significantly, whereas muscarinic agonist, nicotinic antagonists and cholinesterase inhibitors had no influence whatsoever on the total binding of [3H]QNB. The content of muscarinic acetylcholine receptors varied between cultures with explants from different brain areas: hippocampus greater than striatum greater than septum greater than spinal cord greater than cerebellum. These in vitro results are generally in good agreement with results obtained in situ by other investigators and suggest that the binding of [3H]QNB observed in these cultures is indeed correlated to specific muscarinic receptor sites.

Septo-hippocampal neurons in the rat: further study of their physiological and pharmacological properties

Medial septal-nucleus of the diagonal band of Broca area (MS-nDBB) neurons, identified by their antidromic response to the electrical stimulation of the fimbria and/or hippocampus, were studied in the rat under various conditions of anesthesia. These septo-hippocampal neurons (SHNs) were classified into 4 groups on the basis of: (i) their antidromic latency; and (ii) the presence or absence of a rhythmically bursting pattern of spontaneous discharge. The rhythmically bursting activity (43.5% of the SHNs) was highly dependent on the anesthetic conditions. The groups of SHNs differed in their mean conduction velocity and rate of spontaneous activity. In contrast, irrespective of their classification in a particular group, the large majority of the SHNs could be excited by the iontophoretic application of cholinergic agonists. Beside the SHNs, two other populations of MS-nDBB neurons could be identified by electrical antidromic stimulation: neurons projecting to the amygdala (Am) and neurons projecting through the medial forebrain bundle (MFB). Half of the MS-nDBB neurons projecting to Am were also antidromically driven from the fimbria. The axonal branch projecting to Am had a slower conduction velocity than that projecting to hippocampus. In contrast MS-nDBB neurons projecting through the MFB were never antidromically driven from the fimbria, although they received orthodromic inputs. They had a slower conduction velocity than the other groups of MS-nDBB neurons.

An analysis of cholinoceptive neurons in the hippocampal formation by direct microinfusion

Microinfusions of cholinergic agents were made in various sites in the dorsal hippocampal formation of urethane anaesthetized rats. Infusions of eserine or carbachol elicited hippocampal theta activity when made in areas containing high levels of cholinergic markers: the stratum oriens and radiatum of the CA1 and CA3, the stratum moleculare and stratum granulosum of the dentate gyrus and the infragranular region of the hilus. Subsequent infusions of atropine sulfate antagonized the theta activity. Control infusions of equal volumes of saline in active sites were without effect. Infusions of eserine or carbachol in the vicinity of the hippocampal fissure, the stratum lacunosum/moleculare of the CA1 or CA3, in the deep regions of the hilus, and in the lateral ventricle and overlying neocortex, were also without effect. Furthermore, in active sites, the latency to onset of theta and subsequent theta frequency, were both directly related to the total amount of carbachol infused. Thus, areas in which theta could be elicited with a cholinergic agonist (carbachol), or an anticholinesterase (eserine) and antagonized with atropine, were found to correspond well to areas previously found to contain a high density of cholinoceptive neurons, using autoradiographic and immunohistochemical techniques. These results provide further support for the involvement of acetylcholine as a neurotransmitter in the generation of type 2 theta in the hippocampal formation.

Basal forebrain neurons projecting to the rat frontoparietal cortex: electrophysiological and pharmacological properties

Neurons located in the ventromedial globus pallidus (nucleus basalis) and substantia innominata, that were antidromically driven by electrical stimulation of the frontoparietal cortex, were recorded in the urethane anesthetized rat. The basalocortical neurons (BCNs) were antidromically driven with latencies of 1.1-13.5 ms, giving conduction velocities of 0.6-6.8 m/s. Many BCNs had regular patterns of spontaneous discharge (mean spontaneous activity: 20 impulses/s). Most BCNs were not responsive to non-noxious peripheral somatic stimulation. BCNs were readily excited by the iontophoretic application of glutamate and strongly inhibited by GABA. Eighty-five percent of the BCNs could be excited by acetylcholine. They could also be excited by cholinergic agonists. Muscarinic agonists excited a higher proportion of BCNs than nicotinic agonists. Excitatory responses to acetylcholine, carbachol and muscarinic agonists were abolished by atropine.

Development of cholinergic markers in mouse forebrain. II. Muscarinic receptor binding in cortex

The distribution of muscarinic receptor sites throughout the ontogeny of cerebral cortex in the BALB/c mouse have been labeled, placing special emphasis on binding site development in parietal neocortex and hippocampus. We describe a new procedure for the use of [3H]propylbenzilylcholine mustard as a muscarinic cholinergic ligand in an in vitro binding assay on brain sections. Muscarinic binding sites, as visualized by autoradiography, can be seen in cortex as early as embryonic day 18. They achieve maximal labeling density and adult distribution in neocortex by the end of the first postnatal month. The adult distribution pattern in hippocampus is reached by the second postnatal week, but the maximal density of label is not achieved until 4 weeks of age. Changes in the receptor binding pattern are illustrated at 5 different ages between birth and adulthood. We conclude that muscarinic cholinergic receptors develop late in cortical ontogeny as do other cholinergic markers. The distribution pattern of muscarinic binding sites in mouse cortex is puzzling because it does not correspond to the reported distribution of cells physiologically responsive to applied acetylcholine. These results are compared to the onset of choline acetyltransferase activity and acetylcholine esterase staining. The ontogenesis of the cortical cholinergic system is compared with other features of general cortical morphogenesis.

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.

Muscarinic cholinergic receptors in human infant forebrain: [3H]quinuclidinyl benzilate binding in homogenates and quantitative autoradiography in sections

The ontogeny of muscarinic receptors in human brain was studied by comparing [3H]quinuclidinyl benzilate [( 3H]QNB) binding in postmortem tissue from infants 1 week to 3 months of age with binding in adult specimens. Saturation analysis with [3H]QNB and displacement studies with muscarinic antagonists and agonists in tissue homogenates demonstrated that binding sites in the infants' forebrain regions were present in adult or higher than adult concentrations (Bmax). Binding affinity (Kd) and pharmacological characteristics were nearly identical at the two ages. Quantitative receptor autoradiography demonstrated more [3H]QNB binding in the gray matter of infants than adults and revealed a marked difference between the two ages in the laminar distribution of binding sites in neocortex. In contrast to the adult pattern with higher binding in superficial layers 1-3 than in layers 4-6, the distribution in the immature cortex was inverted. These results suggest that muscarinic receptors in infants resemble closely those in mature brain. However, the topography of receptors in the immature neocortex is distinct and they are redistributed in a gradient from inside outward during postnatal development.

Organization and morphological characteristics of choline acetyltransferase-containing fibers in the visual thalamus and striate cortex of the cat

The morphological characteristics and the distribution of cholinergic fibers in the cat visual system were studied with immunohistochemical methods employing monoclonal antibodies directed against choline acetyltransferase (ChAT). ChAT-positive fibers form dense meshworks within the main laminae of the dorsal lateral geniculate nucleus (LGNd), the perigeniculate nucleus (PGN) and the visual cortex. The fibers are of fine caliber, branch extensively and possess numerous varicosities in close contact with cellular elements. The density of ChAT + fibers is not uniform and is particularly high in cortical laminae I-III, the PGN complex and laminae A and A1 of the LGNd.

Autoradiographic distribution of nicotine receptors in rat brain

The autoradiographic visualisation of 90%-specific tritiated nicotine binding to slide-mounted sections of rat brain is reported. Tritiated nicotine bound with high affinity (nanomolar Kd) and was selectively displaced by nicotinic agonists (e.g. L-nicotine approximately ACh greater than D-nicotine). The strikingly discrete distribution pattern obtained deviates from that of alpha-bungarotoxin, and suggests several possible roles for nicotinic transmission in the brain.

Septo-hippocampal and other medial septum-diagonal band neurons: electrophysiological and pharmacological properties

Neurons located in the medial septum-nucleus of the diagonal band (vertical limb) area and antidromically activated by electrical stimulation of the fimbria were recorded in urethane anesthetized rats. Forty-five percent of these septo-hippocampal neurons (SHNs) discharged rhythmically in short bursts (mean burst frequency, 4 Hz). They were antidromically driven at short latencies from the fimbria. SHNs driven at long latencies (above 5 ms) were never bursting neurons. Fimbria stimulation also had a powerful inhibitory effect on the spontaneous activity of SHNs. The vast majority of the septo-hippocampal neurons were excited by the iontophoretic application of acetylcholine or cholinergic agonists, carbachol being the most effective. The acetylcholine-induced excitations were readily abolished by atropine. In contrast hexamethonium and mecamylamine were less effective. The rhythmic bursting activity could not be consistently altered by the iontophoretic application of cholinergic agonists or antagonists or of divalent cations. SHNs were also sensitive to various other putative neurotransmitters (substance P, GABA) known to play a role in the medial septal area. Bursting neurons and ACh-sensitive neurons were less frequent among unidentified medial septal neurons. The regulation of the septo-hippocampal cholinergic pathway is therefore not likely to be due to a direct feedback inhibition by locally released acetylcholine. However a strong inhibitory feedback could be exerted by the hippocampo-septal pathway impinging directly or indirectly on septo-hippocampal neurons.

Cholinergic synapses in the rat brain: a correlated light and electron microscopic immunohistochemical study employing a monoclonal antibody against choline acetyltransferase

Using a monoclonal antibody to choline acetyltransferase, immunoreactive synaptic boutons were identified in the neostriatum, cingulate cortex, basolateral nucleus of the amygdala, hippocampus and interpeduncular nucleus of the rat. The synapses were generally symmetrical although some asymmetrical membrane specializations were observed. Postsynaptic targets included perikarya, dendritic shafts and dendritic spines.

Kinetics of binding to opiate receptors in vivo predicted from in vitro parameters

In order to determine the relationship between in vitro and in vivo binding of opiates in the CNS, the in vitro and in vivo binding properties of 9 opiates, including agonists and antagonists, were studied in mouse brain. Equilibrium dissociation constants (KD) were measured in the presence and absence of 100 mM NaCl and 100 microM 5-guanylylimidodiphosphate (GMP-PNP) at 37 degrees C in mouse brain homogenates. The rate constants describing the exponential clearance of the specifically-bound (i.e. receptor-bound) fraction of ligand in vivo were measured in the striatum, thalamus and cortex. A mathematical model was formulated which relates the in vivo clearance rate for the specifically-bound fraction to the association and dissociation rate constants, the local density of receptors and a parameter which determines the rate at which ligand can diffuse out of the local receptor compartment. The diffusion parameter takes on two forms depending on whether ligand is transported out of the local receptor compartment by simple diffusion or by permeation of a non-polar membrane barrier. In the case of membrane permeation, the model is dependent on the diffusion constant and the lipophilicity of the ligand. A correlation between the in vitro and in vivo measurements is obtained only when KD is measured in the presence of 100 mM NaCl and 100 microM GMP-PNP and when the lipophilicity of the ligand is incorporated into the model. The model corresponds to the physical situation where continuous rebinding of ligand to receptors takes place in the vicinity of a high local density of receptors (possibly at a synapse), thereby retarding the clearance of the specifically-bound ligand from the brain. The rebinding effect is accentuated by: (1) a high local density of receptors; (2) a small KD (measured in the presence of 100 mM NaCl and 100 microM GMP-PNP); (3) a low lipophilicity; and (4) a high molecular weight.

Atlas of cholinergic neurons in the forebrain and upper brainstem of the macaque based on monoclonal choline acetyltransferase immunohistochemistry and acetylcholinesterase histochemistry

Choline acetyltransferase immunohistochemistry was used to map the cholinergic cell bodies in the forebrain and upper brainstem of the macaque brain. Neurons with choline acetyltransferase-like immunoreactivity were seen in the striatal complex, in the septal area, in the diagonal band region, in the substantia innominata, in the medial habenula, in the pontomecencephalic tegmentum and in the oculomotor and trochlear nuclei. The ventral striatum contained a higher density of cholinergic cell bodies than the dorsal striatum. All of the structures that contained the choline acetyltransferase positive neurons also had acetylcholinesterase-rich neurons. Choline acetyltransferase positive neurons were not encountered in the cortex. Some perikarya in the midline, intralaminar, reticular and limbic thalamic nuclei as well as in the hypothalamus were rich in acetylcholinesterase but did not give a positive choline acetyltransferase reaction. A similar dissociation was observed in the substantia nigra, the raphe nuclei and the nucleus locus coeruleus where acetylcholinesterase-rich neurons appeared to lack perikaryal choline acetyltransferase activity.

In vivo effects of beta-bungarotoxin on the acetylcholine system in different brain areas of the rat

The in vivo effects of beta-bungarotoxin (beta-BT) on the acetylcholine (ACh) system were studied in the whole cerebrum and in different brain regions. The effect of beta-BT on cerebral ACh and choline (Ch) contents was time-dependent. The results show that a single intracerebroventricular injection of 1 microgram toxin increased both the ACh and Ch contents in the cortex, hippocampus, and cerebellum, while in the striatum the ACh level was decreased. Ten nanograms of toxin injected into the lateral ventricle twice, on the first and third days, led to a reduced ACh level 2 days after the last treatment. In animals treated with the same dose three times, on the first, third, and fifth days, and sacrificed 2 days after the last injection, the choline acetyltransferase and acetylcholinesterase activities were reduced and the number of muscarinic acetylcholine receptors was decreased. A biphasic effect of the toxin was therefore demonstrated. It is suggested that in the first phase of the toxin effect the increased levels of ACh and Ch may be due to the inhibition of neuronal transmission, while in the second phase, when the elements of the ACh system are reduced, the neuronal degenerating effect of beta-BT plays a significant role.

Ontogenesis of muscarinic acetylcholine binding sites in cat visual cortex: reversal of specific laminar distribution during the critical period

Acetylcholine appears to act as a modulator of neuronal activity in cat visual cortex and, like noradrenaline, may be involved with cortical plasticity mechanisms during the critical period. To explore possible ACh involvement in these mechanisms we have examined acetylcholine binding sites in cat visual cortex during development using [3H]QNB, a muscarinic antagonist. At 3 days postnatal [3H]QNB preferentially labelled binding sites in layer IV. During development the pattern of binding reversed, so that by 95 days postnatal layer IV was the least densely labelled. The number of binding sites increased during development peaking at 1 month postnatal. The Kd of [3H]QNB binding sites increased to 95 days postnatal, with a peak value of 0.76 nM. The results show that during development, and especially within the critical period, changes in [3H]QNB binding site distribution, number and affinity occur.

Muscarinic cholinergic mediation of opiate and non-opiate environmentally induced analgesias

Previous studies have demonstrated that brief front paw shock and brief hind paw shock produce prolonged opiate and non-opiate analgesia, respectively. Additionally, opiate analgesia can be classically conditioned by using either front paw shock or hind paw shock as the unconditioned stimulus. However, beyond this point little is known regarding the neurochemistry of these phenomena. The present series of studies examined the potential involvement of nicotinic and muscarinic cholinergic systems in these 3 forms of environmentally induced analgesia. These experiments demonstrate that muscarinic cholinergic sites within the central nervous system are critically involved in the mediation of both hind paw (non-opiate) foot shock-induced analgesia ( FSIA ) and classically conditioned (opiate) analgesia since scopolamine, but not equimolar methylscopolamine, significantly attenuated analgesia. Furthermore, the primary muscarinic site(s) appears to exist at a supraspinal, rather than spinal, level since delivery of scopolamine directly to the lumbosacral cord produced, at most, only a slight decrease in analgesia. Nicotinic systems do not appear to be importantly involved in any of these forms of environmentally induced analgesias since mecamylamine had no effect on either front paw FSIA or hind paw FSIA and, at most, produced only a slight reduction in classically conditioned analgesia. These data and a review of the literature suggest that the critical cholinergic sites involved in hind paw FSIA exist within the caudal brainstem whereas cholinergic sites within more rostral brain levels probably mediate classically conditioned analgesia.

Quantitative autoradiography of muscarinic cholinergic receptors in the rat brain

Using the in vitro autoradiographic technique with tritium-sensitive LKB sheet film and the liquid scintillation counting method, the distribution and the binding parameters of the muscarinic cholinergic receptors (MChR) were determined in various discrete regions of the rat brain. The results obtained in the present study were as follows: (1) Specific binding of [3H]QNB to the slide-mounted tissue sections increased slowly when incubated at room temperature; saturation occurred 2 h after incubation. Only 23% of [3H]QNB bound to the tissue section was dissociated 5 h after the addition of 20 microM atropine to the medium. These findings were very different from those obtained in the study using the tissue homogenates. (2) The regional distribution of MChR was determined using both autoradiographic and liquid scintillation counting methods. The distribution of MChR was heterogeneous, with highest densities in the striatum and nucleus accumbens and lowest in the globus pallidus, nucleus interpeduncularis and nucleus septi. Moreover, MChR were unevenly distributed within the subfields of each region. (3) In saturation binding studies using the slide-mounted tissue sections of 20 micron thickness the (Kd)app-values were similar but not exactly identical in 5 discrete regions, i.e. the striatum, somatosensory cortex, hippocampus (the subiculum + CA1 field), nucleus accumbens and gyrus dentatus, determined in the present study. The (Kd)app-value of each region was about 700 pM which was about 20 times higher than that obtained in the study using the tissue homogenates. However (Kd)app-values obtained with 5 and 10 micron tissue sections were approximately 3-fold lower.

Experimental febrile convulsions in the developing rat: effects on the cholinergic system

The effects of hyperthermia-induced convulsions (HCs) on nicotinic and muscarinic receptor sites, choline acetyltransferase (ChAT), and acetylcholinesterase (AChE) in the rat were investigated. A series of 10 convulsions, evoked between 5 and 16 days of age, had marked effects on the development of the cholinergic system in the cerebellum and frontal cortex, but not in the hippocampus or hypothalamus. The concentration of muscarinic receptor sites in the cerebellum of the HC group was similar to that in controls at 2 days, greater than in controls at 8 days, but less than in controls at 55 days after the last convulsion. ChAT and AChE activities were increased at 2 days, similar at 8 days, and less than those in controls at 55 days. A decrease in muscarinic receptors and a decline in ChAT activity in the frontal cortex of the HC group were observed at 55 days after the last convulsion. The concentration of nicotinic receptor sites did not distinguish HC from control groups. A simple relationship between the experimental febrile convulsion and the cholinergic system was not found. The greatest effects were noted at 55 days after the last HC, which suggests that these may reflect secondary and possibly transsynaptic influences of the convulsion on cholinergic activity.

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.

Limbic muscarinic cholinergic and benzodiazepine receptor changes with chronic intravenous morphine and self-administration

Muscarinic cholinergic and benzodiazepine receptor affinities and densities were evaluated in membranes from seven brain regions of rats intravenously self-administering morphine and in littermates receiving yoked-morphine or yoked vehicle infusions to identify neuronal systems potentially involved in mediating opiate reinforcement processes. Passive morphine infusion resulted in increases in muscarinic cholinergic receptor densities in the pyriform cortex and in decreases in the cingulate cortex while benzodiazepine receptor densities were decreased in both the hippocampal formation and entorhinal-subicular cortex compared to littermates receiving passive infusions of vehicle. Morphine self-administration resulted in decreased muscarinic cholinergic receptor densities in the frontal and entorhinal-subicular cortices and increases in the amygdaloid complex compared to littermates receiving yoked passive drug. These data are in agreement with acetylcholine turnover rate measurements in these animals and support the proposed role of cholinergic innervations of the frontal and entorhinal-subicular cortices and amygdaloid complex in opiate reinforcement processes.

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.

A comparative study of two populations of acetylcholine-sensitive neurons in rat somatosensory cortex

The properties of two populations of acetylcholine-sensitive neurons, defined by their different laminar distribution (group A, layers V and upper VIa; group B, layer VIb and underlying white matter) were studied in the SmI cortex of urethane-anesthetized rats. The spontaneous activity of group B neurons was significantly lower than in group A. The proportion of neurons excited by iontophoretic acetylcholine, the duration and the pharmacological properties of the excitations were the same in both groups. The dose-response curves obtained for sets of neurons of both groups, recorded with the same electrode, had comparable slopes. Carbachol had a greater potency than acetylcholine in both groups. In contrast, the latency of the response to acetylcholine was significantly shorter in group B. The amount of current required to increase the spontaneous discharge rate of a given neuron by 200% was lower in group B than in group A. These data suggest that the relative change in firing frequency induced by a given cholinergic input is likely to be larger in group B than in group A. Finally, neuronal excitations could still be obtained in both groups when acetylcholine was applied 300 micron, but not 600 micron, from the cell body, suggesting that the spatial distribution of acetylcholine receptors is similar in groups A and B.

Distribution of muscarinic receptors in the caudal cerebellum and electrosensory lateral line lobe of gymnotiform fish

Muscarinic binding sites were found in the electrosensory lateral line lobe (ELLL) and vestibulo cerebellum (LC) of certain gymnotid fish; these binding sites were not present in significant numbers in the corpus cerebelli. Autoradiography of [3H]quinuclidinylbenzilate and [3H]propylbenzylcholine mustard binding confirmed these results and also demonstrated that, within the ELLL the region with muscarinic binding sites was coextensive with the region of cholinergic input. We did not find any evidence for nicotinic receptors (alpha-bungarotoxin binding) in ELLL, LC, or corpus cerebelli.

Autoradiographic localization of a binding protein(s) specific for prostaglandin D2 in rat brain

The specific [3H]prostaglandin (PG) D2 binding was detected by using the slide-mounted sections of rat brain fixed by perfusion with 2% paraformaldehyde. The binding was reversible, saturable, high affinity, Na+ dependent, and highly specific for PGD2. These binding characteristics are essentially similar to those observed with the synaptic membrane of rat brain as previously reported. Using autoradiographic image analyses by computerized densitometry and color coding, we visualized the localization of [3H]PGD2 binding in rat brain. A high density of the binding sites was observed in the cerebral cortex, preoptic area, amygdala, hypothalamic nuclei (arcuate nucleus, ventromedial nucleus, and posterior hypothalamic nucleus), thalamic nuclei (reuniens nucleus and rhomboid nucleus), hippocampus, pineal body, and cerebellar cortex. The binding was not significantly observed in the striatum and also was negative in the white matter, arachnoid membranes, and vasculatures.

Involvement of central muscarinic cholinergic mechanisms in opioid stress analgesia

Stress has been shown capable of differentially activating opioid- and non-opioid-mediated endogenous analgesia systems. In this study, the muscarinic cholinergic antagonist, scopolamine, but not the centrally inactive methylscopolamine, blocks opioid, but not non-opioid stress analgesia. Additionally, naltrexone, an opiate antagonist, attenuates analgesia induced by oxotremorine, a cholinergic agonist. These findings support the existence of a muscarinic cholinergic synapse in a central nervous system opioid pain-inhibitory pathway.

High density of cholinergic muscarinic receptors accompanies high intensity acetylcholinesterase-staining in layer IV of infant rat somatosensory cortex

In this study the laminar localization of binding sites for the muscarinic radioligand [3H]3-quinuclidinylbenzilate was studied in somatosensory cortex of infant rat. Using light autoradiographic techniques, [3H]QNB was found to be concentrated in layer IV. Atropine-pretreated pups showed evenly dispersed, nonspecific binding in all layers. We conclude that cholinergic muscarinic receptors are concentrated in layer IV of infant rat somatosensory cortex. It is also in this lamina that an acetylcholinesterase-rich pathway terminates.

Cholinergic receptor binding and autoradiography in brains of non-neurological and senile dementia of Alzheimer-type patients

Muscarinic and nicotinic cholinergic receptor distribution was studied by dry-mount autoradiography in brains obtained postmortem from patients with senile dementia of Alzheimer-type (SDAT) and non-neurological controls. Sections were incubated with either [N-methyl-3H]scopolamine, ([3H]NMS) or [125I]alpha-bungarotoxin, ([125I]alpha-BTX). No significant difference in the affinity and number of muscarinic and nicotinic receptors was found in hippocampus, frontal, temporal and cingulate cortex between SDAT patients and non-neurological controls. However, some SDAT cases showed diffuse instead of laminar [3H]NMS labeling in cortical regions. The labeling pattern was not affected by the presence of neuritic plaques and neurofibrillary tangles.

Distribution of cholinergic neurons in rat brain: demonstrated by the immunocytochemical localization of choline acetyltransferase

The neuroanatomical location and cytological features of cholinergic neurons in the rat brain were determined by the immunocytochemical localization of the biosynthetic enzyme, choline acetyltransferase (ChAT). Perikarya labeled with ChAT were detected in four major cell groups: (1) the striatum, (2) the magnocellular basal nucleus, (3) the pontine tegmentum, and (4) the cranial nerve motor nuclei. Labeled neurons in the striatum were observed scattered throughout the neostriatum (caudate, putamen) and associated areas (nucleus accumbens, olfactory tubercle). Larger ChAT-labeled neurons were seen in an extensive cell system which comprises the magnocellular basal nucleus. This more or less continuous set of neuronal clusters consists of labeled neurons in the nucleus of the diagonal band (horizontal and vertical limbs), the magnocellular preoptic nucleus, the substantia innominata, and the globus pallidus. Labeled neurons in the pontine tegmentum were seen as a group of large neurons in the caudal midbrain, dorsolateral to the most caudal part of the substantia nigra, and extended in a caudodorsal direction through the midbrain reticular formation into the area surrounding the superior cerebellar peduncle. The neurons in this latter group constitute the pedunculopontine tegmental nucleus (PPT). An additional cluster of cells was observed medially adjacent to the PPT, in the lateral part of the central gray matter at the rostral end of the fourth ventricle. This group corresponds to the laterodorsal tegmental nucleus. Large ChAT-labeled neurons were also observed in all somatic and visceral motor nerve nuclei. The correspondence of the distribution of ChAT-labeled neurons identified by our methods to earlier immunocytochemical and acetylcholinesterase histochemical studies and to connectional studies of these groups argues for the specificity of the ChAT antibody used.

Muscarinic receptors in lung and trachea: autoradiographic localization using [3H]quinuclidinyl benzilate

[3H]Quinuclidinyl benzilate binding to slide-mounted frozen sections of ferret lung was of high affinity (KD 63 +/- 14 pM, mean +/- S.E., n = 4), and characteristic of interaction with muscarinic cholinergic receptors. Light microscopic autoradiography showed muscarinic receptors to be localized predominantly to smooth muscle of trachea and intrapulmonary cartilaginous airways, and to submucosal glands. There was much less labelling of bronchiolar smooth muscle, airway epithelium and vascular smooth muscle and no labelling of alveoli. This distribution of receptors parallels that of cholinergic innervation.

Localization of hippocampal muscarinic receptors after kainic acid lesion of CA3 and fimbria-fornix transection

Bilateral intraventricular injections of 0.5 microgram of kainic acid were used to selectively destroy CA3 hippocampal pyramidal neurons, in an effort to clarify the possible localization of muscarinic cholinergic receptors in the rat hippocampal formation. Thirty days after treatment, there was 43% decrease in the total number of [3H]L-QNB binding sites per hippocampus, with no change in affinity. Histological examination confirmed the selective loss of pyramidal neurons in subareas CA3a-b while other regions of the hippocampal formation were spared. The unilateral transection of the fimbria-fornix, done 14 days after kainic acid, produced a further reduction in binding in relation to control hippocampi (-57%). The results demonstrate that in the pyramidal cells of CA3 there is a high concentration of postsynaptic muscarinic receptors. However, the slight further decrease, found after fimbria-fornix transection, suggests the possible existence of a small population of presynaptic receptors that, hitherto, had only been demonstrated indirectly by physiological methods.