Evidence for an agonist independent down regulation of hippocampal muscarinic receptors in kindling

The subiculum and its role in focal epileptic disorders

The subicular complex (hereafter referred as subiculum), which is reciprocally connected with the hippocampus and rhinal cortices, exerts a major control on hippocampal outputs. Over the last three decades, several studies have revealed that the subiculum plays a pivotal role in learning and memory but also in pathological conditions such as mesial temporal lobe epilepsy (MTLE). Indeed, subicular networks actively contribute to seizure generation and this structure is relatively spared from the cell loss encountered in this focal epileptic disorder. In this review, we will address: (i) the functional properties of subicular principal cells under normal and pathological conditions; (ii) the subiculum role in sustaining seizures in in vivo models of MTLE and in in vitro models of epileptiform synchronization; (iii) its presumptive role in human MTLE; and (iv) evidence underscoring the relationship between subiculum and antiepileptic drug effects. The studies reviewed here reinforce the view that the subiculum represents a limbic area with relevant, as yet unexplored, roles in focal epilepsy.

Asymmetric accumulation of hippocampal 7S SNARE complexes occurs regardless of kindling paradigm

Modifications of neurotransmission may contribute to the synchronization of neuronal networks that are a hallmark of epileptic seizures. In this study we examine the synaptosomal proteins involved in neurotransmitter release to determine if alterations in their interactions correlate with the chronic epileptic state. Using quantitative western blotting, we measured the levels of 7S SNARE complexes and SNARE effectors in the effected hippocampi from animals that were electrically kindled through stimulation from one of three different foci. All three kindling paradigms, amygdalar, entorhinal, and septal, were associated with an accumulation of 7S SNARE complexes in the ipsilateral hippocampus, measured 1 month after completion of kindling. Of the eight SNARE effectors examined (alpha-SNAP, NSF, SV2A/B, Munc18a/nSec1, Munc13-1, Complexins 1 and 2, and synaptotagmin I), there was a statistically significant bihemispheric increase of hippocampal SV2 and decrease of NSF upon kindling; neither by itself would be expected to account for the asymmetry of SNARE complex distribution. These data suggest that an ipsilateral hippocampal accumulation of SNARE complexes is a permanent alteration of kindling-induced epilepsy, regardless of stimulation pathway. The significance of these findings toward a molecular understanding of epilepsy will be discussed.

Muscarinic receptor loss and preservation of presynaptic cholinergic terminals in hippocampal sclerosis

PURPOSE

Prior single-photon emission tomography studies showed losses of muscarinic acetylcholine receptor (MAChR) binding in patients with refractory mesial temporal lobe epilepsy. Experimental animal studies demonstrated transient losses of MAChR due to electrically induced seizures originating in the amygdala. However, the relations between cholinergic synaptic markers, seizures, and underlying neuropathology in human temporal lobe epilepsy are unknown. We tested the hypotheses that human brain MAChR changes are attributable to hippocampal sclerosis (HS), and that HS resembles axon-sparing lesions in experimental animal models.

METHODS

We measured MAChR binding-site density, an intrinsic neuronal marker, within the hippocampal formation (HF) in anterior temporal lobectomy specimens from 10 patients with HS and in 10 autopsy controls. Binding-site density of the presynaptic vesicular acetylcholine transporter (VAChT) was measured as a marker of extrinsic cholinergic afferent integrity. MAChR and VAChT results were compared with neuronal cell counts to assess their relations to local neuronal losses.

RESULTS

Reduced MAChR binding-site density was demonstrated throughout the HF in the epilepsy specimens compared with autopsy controls and correlated in severity with reductions in cell counts in several HF regions. In contrast to MAChR, VAChT binding-site density was unchanged in the epilepsy specimens compared with autopsy controls.

CONCLUSIONS

Reduction in MAChR binding in HS is attributable to intrinsic neuronal losses. Sparing of afferent septal cholinergic terminals is consistent with the hypothesis that an excitotoxic mechanism may contribute to the development of HS and refractory partial epilepsy in humans.

Widespread increase of nerve growth factor protein in the rat forebrain after kindling-induced seizures

Nerve growth factor (NGF) protein levels were determined in various forebrain regions using a two-site immunoassay following kindling-induced seizures. In the dentate gyrus the NGF content was significantly elevated 7 days after the last seizure (to 152% of control). In the piriform and parietal cortices, maximal increases were seen at 12 h (to 261% of control) and at 24 h (to 169% of control), respectively, and the NGF content was then normalized at 7 days. The increased production of NGF might be a protective response or could be involved in plastic changes underlying kindling epileptogenesis.

Autoradiographic analysis of benzodiazepine binding in entorhinal-kindled rat brains

A quantitative autoradiographic analysis of binding to 'central' benzodiazepine ([3H]flunitrazepam) receptors was carried out in 95 brain areas from entorhinal-kindled and handled control rats. Animals were sacrificed either 24 h or 28 days after the last kindled seizure. A significant post-kindling change was observed in only one area, the fascia dentata, which showed an elevation of 31% at 24 h, but not at 28 days. This change, which appeared in both the dorsal and ventral divisions of the hippocampus, was largest in the suprapyramidal layer. These data confirm previous reports of transient post-kindling binding changes in the fascia dentata, and extend them by suggesting that the fascia dentata is the only site in the brain which shows altered benzodiazepine binding after kindling. These transient and circumscribed changes are unlikely to contribute significantly to the 'kindled state', although they may relate to the enhancement of inhibitory effects observed in fascia dentata after kindling.

The electroencephalographic pattern during electroconvulsive therapy: V. Observations on the origins of phase III delta energy and the mechanism of action of ECT

The generation of the spike-wave activity of Phase III of ECT seizures is attributed to the recurrence of synchronized, prolonged periods of intense inhibitory current flow (hyperpolarization), and associated rebound spike bursts, produced by the inhibitory circuit relationships and intrinsic electrophysiological properties of thalamic neurons. An anatomical and neurophysiological model of the development of generalized, synchronous 3-Hz spike-wave seizure activity is proposed which outlines the origin, maintenance, slowing, and termination of this fundamental seizure rhythm. Phase III inhibitory current flow (delta energy) and/or spike bursts may bring about therapeutic benefit by initiating a chain of agonist-independent and agonist-dependent events which results in long-term augmentation of serotonergic and noradrenergic neurotransmission and diminution of cholinergic neurotransmission in the forebrain. A specific anatomical and functional model of the mechanism of action of ECT is proposed, in which: (1) adrenergic and cholinergic pathways in the forebrain are assumed to be massively stimulated during ECT seizures, whereas serotonergic pathways are assumed to be inhibited during these seizures; (2) the beneficial effects of ECT are considered to be more dependent upon ECT-induced changes in 5-HT neurotransmission than upon alteration of noradrenergic function; (3) these beneficial effects involve up-regulation of 5-HT2 and down-regulation of M1- and M2-muscarinic receptor densities by both agonist-independent and agonist-dependent mechanisms, coupled with functional augmentation of noradrenergic neurotransmission; and (4) these effects may be brought about by Phase III inhibitory current flow- and/or spike burst-induced alteration of the function of second-messenger generator systems.

Atropine slows olfactory bulb kindling while diminished cholinergic innervation does not

The development of kindled seizures elicited through electrical stimulation of the rat olfactory bulb (OB) was examined under two conditions which decrease cholinergic neurotransmission. Atropine sulfate (25 mg/kg, IP) administered 1 hr prior to stimulation of the OB was found to significantly delay the acquisition of the fully kindled state. In a second experiment, diminished cholinergic innervation of the OB was established using chemical lesions of the basal forebrain cholinergic system. Despite the depletion of acetylcholine (Ach), as determined by acetylcholinesterase (AchE) and choline acetyltransferase (ChAt) assays, no significant alterations in kindling parameters were observed. Based upon these findings we suggest that Ach is not critical to the establishment of an OB kindled focus but is important for the propagation and generalization of epileptiform activity initiated through OB stimulation.

An in vitro autoradiographic analysis of mu and delta opioid binding in the hippocampal formation of kindled rats

Recent studies have shown that opioid peptide levels are altered in hippocampal formation of kindled animals. We therefore studied the distributions of mu and delta opioid binding sites in hippocampal formation of kindled and control rats using quantitative in vitro autoradiography. Animals received daily stimulations of the amygdala until they experienced 3 class 5 seizures. Paired control animals underwent implantation of electrodes but were not stimulated. Mu binding sites were labeled with 125I-FK-33824. Twenty-four hours after the last kindled seizure, mu binding was decreased by 32% in stratum pyramidale of CA1 and stratum radiatum of CA2 and by 17-27% throughout most of the rest of CA1, CA2, and CA3. Few, if any, differences were seen between kindled and control animals at 7 or 28 days after the last kindled seizure. Delta binding sites were labeled with 125I-[D-Ala2,D-Leu5]enkephalin in the presence of the morphiceptin analog PL-032. Twenty-four hours after the last kindled seizure, delta binding was decreased only in stratum moleculare of the dentate gyrus. Seven days after the last kindled seizure, delta binding was decreased by 11-17% throughout CA1, CA3, and the dentate gyrus. At 28 days after the last seizure, however, no differences were found between kindled and control animals. Since the decreases in mu and delta opioid binding are transient, they are unlikely to be the molecular basis of the permanent kindling phenomenon. Rather, these changes in opioid binding may represent responses to repeated seizures.

Cholinergic mechanisms in affective disorders. Future directions for investigation

Advances in clinical and basic research methodology combined with clearly articulated concepts create new opportunities for researching the roles of cholinergic mechanisms in the pathophysiology of affective disorders. Areas for study include: roles of cholinergic mechanisms in mediating effects of stress and cholinergic mechanisms linking the pathophysiologies of affective and panic disorders, use of pharmacologic agents to produce cholinergic system supersensitivity in modeling biologic aspects of affective illness, use of multigenerational intrapedigree studies of cholinergic markers associated with affective disease, research into the neurobiology of lithium and ECT as they pertain to muscarinic cholinergic mechanisms, study of the interrelationship of sodium, calcium and lithium ion metabolism and their relationship to cholinergic-monoaminergic interaction, the development of brain imaging strategies and techniques, e.g., positron emission tomography (PET), to measure changes in cholinergic receptor density and affinity as a function of clinical state, identification and validation of a peripheral model of the central muscarinic receptor, study of the pharmacology of abusable substances and its relationship to mechanisms regulating mood, affect, psychomotor function and other variables related to the affective disorders, and development of in vitro and in vivo models useful in studying the physiology and biochemistry of the interaction of cholinergic and monoaminergic neurons. These models may allow us to bridge the traditional cholinergic and monoamine hypotheses of affective disorders.

Covariation of depressive symptoms, parkinsonism, and post-dexamethasone plasma cortisol levels in a bipolar patient: simultaneous response to ECT and lithium carbonate

A patient presented with concurrent mood congruent delusions, parkinsonism, and elevated post-dexamethasone plasma cortisol levels. This triad could result from simultaneous development of cholinergic-monoaminergic dysfunction within critical limbic and extrapyramidal loci. The magnitude of each abnormality decreased in concert during a course of electroconvulsive therapy (ECT). Remaining abnormalities disappeared during treatment with lithium. Actions of ECT and lithium on muscarinic systems are reviewed, and a strategy for testing the hypothesis that dysfunction of cholinergic-monoaminergic mechanisms develops in parallel in different neural networks is considered.

Pathophysiology of "cholinoceptor supersensitivity" in affective disorders

Phenomenological and physiological variables demonstrate supersensitive changes to cholinergic challenge in affective disorder subjects. Theorists generally assume the primary defect is the postsynaptic muscarinic receptor. However, in addition to defectiveness or up-regulation of this receptor, the appearance of postsynaptic "cholinoceptor supersensitivity" can result from abnormal presynaptic mechanisms, membrane "pathology," derangement of intrasystolic mechanisms that amplify effects of receptor-agonist coupling, or aberrant cholinergic-monoaminergic interaction. This article discusses abnormalities of the postsynaptic receptor, regulation of postsynaptic receptor density, the presynaptic muscarinic receptor, and other mechanisms regulating the release of acetylcholine, membrane dynamics, and "cascade" mechanisms-specifically the phosphatidylinositol (PI) cycle, Ca2+ mobilization, and cyclic guanosine monophosphate (GMP) generation-as causes of cholinergic system "supersensitivity." It is suggested that an approach to the topic emphasizing site of abnormality will encourage greater clarity of thought in the study of the cholinergic component of the pathophysiology of affective illness.

Decreased muscarinic acetylcholine receptor number in the central nervous system of the tottering (tg/tg) mouse

The tottering mouse (tg/tg) is a single-locus mutant, phenotypically characterized by the development of epilepsy associated with distinct electroencephalographic abnormalities. Because of reported alterations in muscarinic receptor (mAChR) number in various seizure states, mAChR density was examined in discrete brain regions of tottering (tg/tg) and coisogenic wild-type (+/+) mice. Saturation binding experiments revealed a widespread decrease in membrane mAChR density in the CNS of adult tottering (tg/tg) mice as compared with age-matched control wild-type (+/+) mice. The decrease was most pronounced in the hippocampus, where tg/tg mice exhibited a 40-60% reduction in mAChR density with no change in the affinity of the receptor for antagonists or agonists. At postnatal day 10, before the reported onset of electroencephalographic abnormalities, 114 and 65% increases in mAChR density were observed in the tg/tg hippocampus and cortex, respectively. Following the development of seizure activity at postnatal day 22, mAChR density in the tg/tg hippocampus was reduced by 29%. No change in brain mAChR density was seen in adult heterozygotes (+/tg), which do not develop electroencephalographic or seizure abnormalities. These results indicate that the development of reduced mAChR number in the CNS of the tg/tg mouse is secondary to abnormal neuronal activity, providing further support for the hypothesis that membrane depolarization can cause a decrease in neuronal mAChR density.

Combined treatment with muscarinic and nicotinic cholinergic antagonists slows development of kindled seizures

Rats were given daily injections of atropine, mecamylamine or both drugs in combination, 1 h prior to daily electrical amygdaloid kindling stimulation. Neither drug was effective alone, but the two drugs in combination significantly increased the latency to develop stage 5 kindled seizures.

Relationship between nicotine-induced seizures and hippocampal nicotinic receptors

A controversy has existed for several years concerning the physiological relevance of the nicotinic receptor measured by alpha-bungarotoxin binding. Using mice derived from a classical F2 and backcross genetic design, a relationship between nicotine-induced seizures and alpha-bungarotoxin nicotinic receptor concentration was found. Mice sensitive to the convulsant effects of nicotine had greater alpha-bungarotoxin binding in the hippocampus than seizure insensitive mice. The binding sites from seizure sensitive and resistant mice were equally affected by treatment with dithiothreitol, trypsin or heat. Thus it appears that the difference between seizure sensitive and insensitive animals may be due to a difference in hippocampal nicotinic receptor concentration as measured with alpha-bungarotoxin binding.

Interruption of cholinergic afferent pathways to the amygdala failed to alter electrical kindling

We investigated the hypothesis that the cholinergic system is involved in the process of amygdala kindling. Electrical kindling of the amygdala was associated with an increase in the concentration of acetylcholine in the kindled amygdala and the ipsilateral hippocampus but was not associated with any alteration of choline acetyltransferase activity. Destruction of cholinergic neurons in the lateral preoptic area significantly decreased the activity of choline acetylase in the ipsilateral amygdala, but had no effect on the duration of time to kindling. We interpret these findings as indicating that the cholinergic system is affected by the kindling process, but does not seem to be crucial to the phenomenon.

Reduced kainic acid binding in rat hippocampal formation after limbic kindling

The specific binding of [3H]kainic acid to hippocampal membranes was examined autoradiographically in rats kindled by tetanic stimulation of the amygdala or angular bundle. One day after the last of 3 class 4-5 kindled seizures, the specific binding of [3H]kainic acid in stratum lucidum of area CA3 was 47-61% less than in electrode-implanted unstimulated controls. Specific binding in the inner third of the dentate molecular layer was reduced to a lesser degree. These observations demonstrate that kainic acid receptors are down-regulated by kindling stimulation.

Antidepressant withdrawal-induced activation (hypomania and mania): mechanism and theoretical significance

Electrocortical and behavioral arousal are separate phenomena subserved by different neural substrata operating in parallel. A comprehensive theory of 'activation' must take into account the relationships between the electrical and behavioral activating systems. In pathological or experimentally induced states paradoxes, resolvable by a theory positing functional interaction between these systems, arise. EEG arousal is directly mediated, in both the waking and sleeping state, by cholinergic mechanisms. Antidepressant withdrawal precipitates cholinergic overdrive; this would account for the apparent disturbances of REM sleep occurring when antidepressants are stopped. Generally, cholinergic overdrive would produce behavioral inhibition but in particular instances it triggers marked psychomotor arousal by mobilizing a 'limbic activating system'. The existence of a monoaminergic 'limbic activating system', system 'A', with the properties attributed to it in this paper, is supported by both clinical and laboratory observations. System 'A' theory provides a parsimonious means of adequately explaining many phenomena. This theory also has in its favor explanatory power and scope. The Cholinergic-Monoaminergic Interaction Theory of antidepressant withdrawal induced activation and of rapidly-cycling manic-depressive illness maintains that system 'A' and a cholinergic inhibitory system interact dynamically, and that excessive monoaminergic function can precipitate excessive cholinergic function and a dearth of monoaminergic function (due to autoregulation) and hence depression. Likewise, excessive cholinergic function is posited to activate monoaminergic systems and hence to secondarily cause behavioral activation. Rapidly-cycling manic-depressive patients, according to the model, develop alternating cholinergic and monoaminergic overdrive states because the homeostatic mechanisms which should serve to maintain, within normal limits, the composite of cholinergic inhibitory and monoaminergic activating influences are defective. Consequently, rather than reaching a reasonable balance compatible with adaptive function there is oscillation between extremes. Each oscillatory movement is actually a move towards the 'golden mean' and is induced by deviation from this ideal but the defective homeostatic mechanisms promote ' perpetual ' overshooting. Lithium and ECT may be useful in the treatment of rapidly-cycling patients as both treatments may down-regulate muscarinic receptors, and otherwise modify cholinergic and monoaminergic systems in ways promoting homeostasis.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Long-term reduction in beta-adrenergic receptor binding after amygdala kindling in rats

The long-term effect of amygdala kindling on beta-adrenergic receptor binding was examined in three groups of Wistar rats. The animals in one of two kindled groups received six stage-5 generalized convulsions, one each day, and then were not stimulated for 23 days before being killed (23-day group). Animals in the other kindled group received only five generalized convulsions prior to 22 days of no stimulation but then received their sixth convulsion 1 day before being killed (1-day group). All animals including the operated controls experienced similar handling. A significant reduction in [3H]dihydroalprenolol binding in the anterior cortex and kindled amygdala was observed in the 23-day animals compared with control animals, whereas the 1-day animals exhibited intermediate values which were not different from either of the other two groups. Thus, amygdala kindling results in a long-lasting reduction in beta-adrenergic binding which, paradoxically, can be partially ameliorated by a single convulsion 1 day before killing.

Interaction between spontaneous and electrically induced convulsions and their short- and long-term effects in the abstinence after chronic barbital treatment in the rat

Male rats were treated with barbital supplied in their drinking water (daily dose around 200 mg/kg) for 50 weeks. When the treatment was stopped (day 0) spontaneous convulsions were monitored for the first 3 days of the abstinence. On day 3 a convulsion was induced by electricity in half of the rats (controls and barbital-treated) and 1 h later the sensitivity to hexobarbital was determined with a threshold test. Sensitivity to hexobarbital was then tested in the same manner at approximately weekly intervals for the first 110 days of the abstinence. On day 3 of the abstinence a tolerance to hexobarbital (48% increase in threshold above controls) and a reduced threshold to induce convulsions with electricity (-27% compared with controls) was seen in previously barbital-treated animals. Spontaneous or induced convulsions occurring prior to the hexobarbital threshold determination decreased the tolerance to the same extent (-22 to -28%). On day 28 rats with no convulsions up to day 3 had a marked renewal of tolerance to hexobarbital (29% increase above controls), while rats with convulsions recorded up to day 3 had less or no such tolerance. There was a positive correlation (r = 0.63) between the hexobarbital thresholds in barbital-treated rats recorded on day 3 and on day 28. Later in the abstinence, barbital-treated rats with convulsions prior to day 3 tended to have a hexobarbital threshold slightly but significantly elevated compared with the controls (10-15%). This change could be a sign of a long-lasting increased excitation.

Neurochemical correlates of the kindling model of epilepsy

Repeated electrical stimulation of the brain can produce many epileptogenic effects including those which characterize the kindling model. Kindling stimulation, by definition, changes the brain is such a way that formerly subconvulsive stimuli can elicit electrographic and convulsive seizure activity. In addition, the kindled animal becomes more susceptible to many, but not all, other types of seizures. These facts suggest that kindling produces brain changes which may selectively model some types of epileptiform excitability. In order to understand the basis for such changes numerous neurochemical studies have been attempted in the last few years. Although many changes have been demonstrated to be produced by kindling, few studies have been designed to specifically examine the long-lasting (permanent) neurochemical correlates of kindling stimulation. In this review, neurochemical data relevant to kindling are presented and discussed in terms of their possible significance to the seizure susceptibility changes produced by kindling.

Seizures down-regulate muscarinic cholinergic receptors in hippocampal formation

Muscarinic cholinergic receptors (MCR) have been previously shown to decline in the hippocampal formation (HPF) of amygdala-kindled rats. Seizures have been proposed as the process responsible for this down-regulation. We now demonstrate similar down-regulation of MCR within HPF in 3 additional methods of inducing seizures: electroconvulsive shock, entorhinal kindling and entorhinal lesion. Two key parameters which causally link the MCR declines with seizures are their time course and reversal with anticonvulsants. The transient decline of MCR induced by entorhinal lesion-induced seizures parallels the time course established in amygdala kindling. Further, phenobarbital could block both these seizures and the MCR declines. Together, this supports the relationship of seizures causing the declines. We postulate that the MCR down-regulation represents an endogenous inhibitory response of neurons that are intensely and repeatedly depolarized during the seizures.

The effect of entorhinal cortical ablation on the distribution of muscarinic cholinergic receptors in the rat hippocampus

Removal of the entorhinal cortical projection to the hippocampus in adult rats decreased the density of muscarinic cholinergic receptors in the denervated dentate gyrus outer molecular layer at two days postlesion. Thirty days following the lesion (in adults and neonates) there is a small receptor density increase in the outer molecular layer (may be due to tissue shrinkage), and a larger increase in the lacunosum-moleculare. The receptor density decrease seen two days postlesion suggests the presence of presynaptic muscarinic receptors on the lost entorhinal cortical fibers. The distribution and extent of the receptor changes seen at 30 days postlesion are inconsistent with the cholinergic fiber reorganization which follows an entorhinal cortical lesion, but are consistent with a proposed model of non-cholinergic afferent mediated control of muscarinic receptor density in the rat hippocampus.

Electrolytic entorhinal lesions cause seizures

The entorhinal cortex is a key site of interneuronal communication between a variety of cortical and subcortical areas and hippocampal formation. Lesioning the entorhinal cortex is commonly used in studies of the hippocampal formation, animal behavior and neuronal plasticity. We have found that electrolytic destruction of the entorhinal cortex consistently produces limbic seizure activity in rats. The propensity of lesions in this area for producing seizure activity may facilitate insights into the normal function of this network of neural connections. This unexpected phenomenon represents a potential confounding variable for all researchers using this method for making brain lesions.

Specificity of muscarinic acetylcholine receptor regulation by receptor activity

Regulation of muscarinic acetylcholine receptor concentration by receptor activity in neuron-like NG108-15 hybrid cells is a highly specific process. Receptor levels, monitored by binding of [3H]quinuclidinyl benzilate ([3H]QNB), decreased 50--75% following 24-h incubation of cells with muscarinic agonists, but none of the following cellular processes was altered by this chronic receptor stimulation: (1) glycolytic energy metabolism, measured by [3H]deoxy-D-glucose ([3H]DG) uptake and retention; (2) rate of cell division; (3) transport, measured by [3H]valine and [3H]uridine uptake; (4) RNA biosynthesis, measured by [3H]uridine incorporation; (5) protein biosynthesis, measured by [3H]valine and [35S]methionine incorporation into total protein and into protein fractions obtained by polyacrylamide gel electrophoresis. In contrast, chronic stimulation did cause a threefold decrease in the capacity of carbachol to stimulate phosphatidylinositol (PI) turnover, a receptor-mediated response. In addition to cholinomimetics, the neuroeffector adenosine (1 mM for 24 h) also caused a decrease in [3H]QNB binding levels, but chronic stimulation of alpha-adrenergic, opiate, prostaglandin E1, and prostaglandin F2 alpha receptors found on NG108-15 cells caused no changes. The data indicate that loss of muscarinic receptors caused by receptor stimulation is not a consequence of fundamental changes evoked in overall cellular physiology but reflects a specific regulation of cholinoceptive cell responsiveness.