Effects of tacrine on brain muscarinic-receptor-mediated second-messenger signals

The purpose of this study was to investigate the effects of 9-amino-1,2,3,4-tetrahydroacridine (THA; Tacrine) on muscarinic-receptor-linked second-messenger systems in rat brain and to determine the selectivity and mechanisms of these effects. Both competitive and noncompetitive antagonism was revealed in saturation radioligand binding studies performed in cortical and striatal tissue, depending on THA concentration. Micromolar THA concentrations blocked muscarinic-receptor-mediated inhibition of cAMP formation and stimulation of phosphoinositide (PI) hydrolysis with poor selectivity between the two responses. While both responses were blocked in the same concentration range (4-60 mumol/l), non-competitive antagonism of PI hydrolysis occurred at THA concentrations greater than 10 mumol/l while competitive antagonism was displayed for the cAMP response at concentrations of THA up to 40 mumol/l. THA was equally effective at inhibiting PI hydrolysis stimulated by histamine, phenylephrine or oxotremorine-M, when these agonists were employed in concentrations equal to their EC50s for the response. THA did not antagonize PI hydrolysis mediated by the quisqualate receptor at any agonist concentration used. Furthermore, THA blocked carbachol- but not morphine-induced inhibition of forskolin-stimulated cAMP formation in the striatum.

Application of transfected cell lines in studies of functional receptor subtype selectivity of muscarinic agonists

Chinese hamster ovary cell lines which stably express individual subtypes of muscarinic acetylcholine receptors (m1-m5) were used to assess the potential selectivity of known muscarinic agonists in effecting coupling of different receptor subtypes to signal transduction mechanisms. Phosphoinositide hydrolysis was measured in m1-, m3- and m5-Chinese hamster ovary cells, whereas inhibition of forskolin-mediated cyclic AMP formation was measured upon the activation of m2 and m4 muscarinic receptors. The two muscarinic agonists pilocarpine and McN-A-343 were notably subtype selective on a functional basis. Pilocarpine was more efficacious in stimulating Phosphoinositide hydrolysis linked to m1 as compared to either m3 or m5 muscarinic receptors. On the other hand, McN-A-343 produced marked inhibition of cyclic AMP formation in m4-Chinese hamster ovary cells but only a small response at m2 receptors. The subtype selectivity of pilocarpine and McN-A-343 in these cases was not due to differences in the level of expression of muscarinic receptors in the various cell lines. In contrast, equalizing receptor number in pairs of cell lines masked apparent selectivities in other cases. Our data highlight the functional discrimination of pilocarpine and McN-A-343 among muscarinic receptor subtypes and emphasize the importance of using cell lines which express an equal number of receptors in the process of searching for subtype selective agonists.

beta-Amyloid 25-35 activates nitric oxide synthase in a neuronal clone

The role played by the beta-amyloid protein in the neuropathology which accompanies Alzheimer's disease remains unclear. In an effort to unravel some of the cellular actions of beta-amyloid, we investigated its effects on nitric oxide (NO) release in cultured neuron. The putative neurotoxic fragment 25-35 of beta-amyloid stimulated release of NO in a neuronal cell line, as measured by an increase in cyclic GMP formation which is attenuated by NO synthase inhibitors and NO scavengers. These results suggest that NO might mediate intercellular communication effected by beta-amyloid. Our results provide the first piece of evidence that beta-amyloid directly activates a putative neurotoxic second messenger transduction mechanism. These findings might be of potential value in understanding the molecular basis of Alzheimer's disease pathology and in targeting new effective therapeutic approaches.

The calmodulin antagonist calmidazolium stimulates release of nitric oxide in neuroblastoma N1E-115 cells

Effects of the calmodulin inhibitor calmidazolium on stimulation of nitric oxide (NO) release were investigated in neuroblastoma N1E-115 cells. NO release was determined indirectly by measuring cyclic GMP formation. Instead of the expected decrease in NO generation based on the calmodulin dependence of neuronal NO synthase, calmidazoline paradoxically increased cyclic GMP formation. Maximal activation occurred at 3 min and the effects were concentration dependent. This calmidazolium-stimulated NO release was markedly blocked by hemoglobin and N-monomethyl-L-arginine.

Comparison of the concentration of messenger RNA encoding four muscarinic receptor subtypes in control and Alzheimer brains

We determined the concentration of the messenger RNA species which encode four (m1-m4) of the five cloned muscarinic receptors in brains of Alzheimer's disease patients as compared to age-matched controls. Assays were performed using the quantitative method of DNA-excess solution hybridization in the cerebral cortex (frontal, temporal and occipital), hippocampus, nucleus basalis of Meynert and brainstem. The results suggest a statistically significant decrease in the m1 muscarinic receptor message in the temporal and occipital cortex, with no change in other regions. There was no change in the level of mRNA encoding the m2, m3 or m4 receptors in any of the brain regions studied.

Comparison of the level of mRNA encoding m1 and m2 muscarinic receptors in brains of young and aged rats

We compared the concentration of mRNA encoding the m1 and m2 muscarinic receptors in several brain regions obtained from young (5-8 months) and aged (24-28 months) male Fischer 344 rats. DNA-excess solution hybridization was employed as a quantitative measure of mRNA concentration. The results indicate the absence of changes in the m1 receptor message with aging in the cerebral cortex, hippocampus and striatum. While there was no statistically significant aging-associated alteration in the concentration of the message encoding the m2 receptor in the thalamus, midbrain, cerebellum and brainstem, there was a decrease in the message level in the hypothalamus.

Lack of intrinsic activity and significant subtype selectivity of SR 95639A at muscarinic receptors

We assessed the intrinsic activity of the purported selective muscarinic M1 receptor agonist SR 95639A (morpholinoethylamino-3-benzocyclohepta-(5,6-c)-pyridazine) in inducing several receptor-mediated signals. Our results indicate that SR 95639A lacks the ability to activate phosphoinositide hydrolysis in rat cerebral cortex or in Chinese hamster ovary cells transfected with the genes of the muscarinic m1 and m3 receptors. Similarly, this compound did not exhibit intrinsic activity in stimulating muscarinic receptors which inhibit cyclic AMP synthesis and did not suppress acetylcholine release in rat striatum. In addition, SR 95639A did not show a marked selectivity at the level of the ligand recognition site at the muscarinic M1, M2 and M3 receptors, since it bound to these receptor subtypes with equilibrium dissociation constants of 4, 6 and 11 microM, respectively.

Complex allosteric modulation of cardiac muscarinic receptors by protamine: potential model for putative endogenous ligands

A large number of diverse pharmacological agents bind to a secondary domain on the muscarinic receptor, to influence allosterically the interaction of ligands at the primary binding site. Based on common structural features of these antagonists, we examined the interaction of protamine, an endogenous polycationic peptide, and of polyamines with muscarinic receptors in rat heart. Our results provide several lines of qualitative evidence that protamine allosterically modulates the conformation of muscarinic receptors, in a marked negatively cooperative manner. It decelerated the dissociation of N-[3H]methylscopolamine ([3H] NMS) initiated by atropine, in a concentration-dependent fashion. Inhibition by protamine of [3H]NMS binding at equilibrium showed a distinct plateau, which increased in magnitude at higher ligand concentrations. Scatchard analysis of saturation isotherms of [3H]NMS binding in the absence and presence of protamine indicated that protamine did not alter Bmax in a statistically significant fashion, although there was a trend of a concentration-dependent increase in this parameter. On the other hand, it caused a marked concentration-dependent decrease in the affinity of [3H]NMS, and this effect reached a ceiling limit. However, there were marked quantitative deviations of the interaction of protamine from a simple ternary allosteric model. Some of these discrepancies could be explained by the tendency of protamine to increase Bmax. The allosteric actions of protamine demonstrated in kinetic and equilibrium experiments were selective for m1 and m2 muscarinic receptors, compared with m3, m4, and m5 receptors, as studied in Chinese hamster ovary cells transfected with the genes of the different muscarinic receptors. Arginine residues play an important role in the allosteric interaction of protamine, inasmuch as poly-L-arginine qualitatively mimicked the effects of protamine. In contrast, no effects of the polyamines spermine, spermidine, and putrescine were observed on [3H]NMS binding. This is the first report on the allosteric modulation of muscarinic receptors by an endogenous peptide.

Modulation by certain conserved aspartate residues of the allosteric interaction of gallamine at the m1 muscarinic receptor

Muscarinic acetylcholine receptors belong to a superfamily of G-protein coupled receptors and contain within their structure several conserved aspartate residues. These residues have been implicated to play important roles in the interaction of agonists and their competitive antagonists with the receptor. In the present work, we investigated whether the same residues might also serve as important contact points for allosteric antagonists of muscarinic receptors, because the majority of these compounds are cationic in nature, or if such residues are involved in modification of receptor conformation by these antagonists. Gallamine was used as a prototype for these antagonists. Site-directed mutagenesis of the m1 muscarinic receptor subtype was utilized to define some of the molecular determinants involved in cooperative allosteric interactions. We report that substitution of the aspartate residue at position 71, but not at positions 99 and 122 with asparagine, affected the affinity of gallamine for the unliganded m1 receptor. A similar substitution at positions 71 and 99 decreased the magnitude of its cooperative effects on the binding of [3H]N-methylscopolamine. Our data suggest that these residues are implicated in cooperative interactions. At present, however, we cannot discount a more pivotal role of other residues on the receptor sequence in allosteric interactions. The data also support the notion that different molecular entities are required for the binding of allosteric antagonists as compared to the interaction of agonists and competitive antagonists at the receptor.

Muscarinic receptor function and acetylcholinesterase activity after chronic administration of tacrine to mice at therapeutic drug concentrations

We administered 9-amino-1,2,3,4-tetrahydroacridine (THA, Tacrine) to mice in doses (0.3-3 mg/kg) which have been shown to enhance cognition. Animals were sacrificed at various time points and several markers of cholinergic function were measured. Following 3 mg/kg THA, drug levels in brain were sufficient to inhibit 78-80% of brain acetylcholinesterase activity, regardless of treatment duration. However, repeated administration of THA did not alter the number of muscarinic receptors or the phosphoinositide response to muscarinic receptor agonists. Thus, at therapeutically relevant doses, THA inhibits the activity of brain acetylcholinesterase substantially, but does not affect the density of muscarinic receptors on their ability to activate second messenger systems. These results are in contrast to those obtained by other investigators who found significant decreases in muscarinic receptor number following chronic administration of higher doses of THA.

Muscarinic subtype selectivity of tetrahydroaminoacridine: possible relationship to its capricious efficacy

Tetrahydroaminoacridine discriminated slightly in its potency to displace [3H]N-methylscopolamine ([3H]NMS) binding from different muscarinic receptor subtypes (M2 greater than M1 greater than M3) and to allosterically decelerate ligand binding (M2 greater than or equal to M1 greater than M3). The steep displacement curves suggest that marked changes in receptor occupancy may occur within a relatively narrow dose range. Thus, individual inter-patient variability and inconsistent results in clinical studies may be related to blockade of muscarinic receptors, which would oppose the beneficial effects resulting from acetylcholinesterase inhibition.

The presence of an M4 subtype muscarinic receptor in the bovine adrenal medulla revealed by mRNA and receptor binding analyses

The muscarinic receptor subtype present in the bovine adrenal medulla was characterized. Hybridization of RNA to highly specific m1-m5 muscarinic receptor cDNA probes detected the presence of only m4 subtype mRNA in this tissue. Muscarinic receptor binding studies using the non-selective ligand [3H]N-methyl-scopolamine showed a single class of binding sites with a maximum density of 19.8 fmol/mg protein and a dissociation constant (KD) of 220 pM in the adrenal medulla, while the M1 selective ligand [3H]telenzepine did not bind detectably. Competition of specific antagonists with [3H]N-methyl-scopolamine for binding to the membranes produced a rank order of potencies with a profile that fitted either the cloned m3 or m4 receptor. In further comparative studies, the adrenal gland of the rat showed the presence of m4 subtype mRNA in addition to the m3 subtype.

Effects of agonist efficacy on desensitization of phosphoinositide hydrolysis mediated by m1 and m3 muscarinic receptors expressed in Chinese hamster ovary cells

Muscarinic receptor agonist-induced desensitization of phosphoinositide (PI) hydrolysis and loss of receptors were studied in Chinese hamster ovary (CHO) cells transfected with the m1 and m3 muscarinic receptor genes. Long-term exposure to the full agonist carbamylcholine (CBC) resulted in a time-dependent attenuation of the maximal PI response and a decrease in agonist potency. This desensitization was accompanied by a parallel loss of maximal ligand binding without an alteration of the binding affinity. The time course of both receptor desensitization and down-regulation was similar in m1 and m3 CHO cells. The PI response to the partial agonist McN-A-343 (McN) in m1 cells was more sensitive to desensitization by CBC than the response to the latter agonist, and this desensitization was faster than receptor down-regulation. Desensitization of the PI response to McN was reflected as a decrease in the maximal response without a marked change in potency. McN induced slow desensitization of the PI response to CBC but a much faster desensitization of its own response. Our data provide evidence that although muscarinic agonist-induced desensitization of PI hydrolysis in CHO cells is due mainly to loss of receptors, there are other important factors which play a role in this process, e.g., receptor-effector uncoupling. The relative contribution of these different mechanisms depends on the efficacy of the agonists used for the receptor desensitization and activation steps.

N-hydroxylamine is not an intermediate in the conversion of L-arginine to an activator of soluble guanylate cyclase in neuroblastoma N1E-115 cells

This study evaluates the role of N-hydroxylamine (NH2OH) in activating soluble guanylate cyclase in the mouse neuroblastoma clone N1E-115. It has been proposed that NH2OH is a putative intermediate in the biochemical pathway for the generation of nitric oxide (NO)/endothelium-derived relaxing factor (EDRF) from L-arginine. NH2OH caused a time- and concentration-dependent increase in cyclic GMP formation in intact cells. This response was not dependent on Ca2+. In cytosol preparations the activation of guanylate cyclase by L-arginine was dose-dependent and required Ca2+ and NADPH. In contrast, NH2OH itself did not activate cytosolic guanylate cyclase but it inhibited the basal activity of this enzyme in a concentration-dependent manner. The formation of cyclic GMP in the cytosolic fractions in response to NH2OH required the addition of catalase and H2O2. On the other hand, catalase and/or H2O2 lead to a decrease in L-arginine-induced cyclic GMP formation. Furthermore, NH2OH inhibited L-arginine- and sodium nitroprusside-induced cyclic GMP formation in the cytosol. The inhibition of L-arginine-induced cyclic GMP formation in the cytosol by NH2OH was not reversed by the addition of superoxide dismutase. These data strongly suggest that NH2OH is not a putative intermediate in the metabolism of L-arginine to an activator of guanylate cyclase.

Allosteric interactions at the m1, m2 and m3 muscarinic receptor subtypes

The purpose of our study was to investigate the interactions of allosteric antagonists at the individual m1, m2 and m3 muscarinic receptor subtypes. This was achieved through the use of transformed Chinese hamster ovary cells stably expressing the rat m1 or m3 receptor genes. A homogeneous population of the m2 subtype was obtained from rat heart tissue. Our data indicate that the cardioselective antagonists (gallamine, methoctramine, AF-DX 116 and himbacine) display the following rank order of potency for both displacing ligand binding to the primary site on the receptor and allosterically decelerating ligand dissociation: m2 greater than m1 greater than m3. Schild analysis showed the following rank order of the magnitude of gallamine's cooperative interactions with the three receptor subtypes: m3 greater than m1 greater than m2. By comparison, the ion-channel blockers (verapamil, phencyclidine and quinidine) exhibited a rank order of potency for cooperative effects similar to that of cardioselective antagonists; however, these blockers did not show appreciable specificity in their interaction with the receptor primary binding site. There was a lack of correlation between the displacement of ligand binding and the allosteric potencies of the allosteric antagonists at each of the three muscarinic receptor subtypes, thus revealing the complex nature of interaction (both competitive and allosteric) between many of these compounds with the muscarinic receptor. Despite the fact that the majority of allosteric muscarinic antagonists are also K+ channel blockers, the use of pertussis toxin did not support the notion that this channel represents the allosteric site coupled to the receptor.

DFP-induced regulation of cardiac muscarinic receptor mRNA in vivo measured by DNA-excess solution hybridization

Relationship between in vivo down-regulation of cardiac muscarinic receptors and changes in their encoding mRNA was investigated. Rats were treated either once or for ten days with an irreversible inhibitor of acetylcholinesterase, followed by measurements of cardiac acetylcholinesterase, the density and affinity of muscarinic receptors, and the concentration of mRNA coding for these receptors. mRNA was quantitated using the sensitive method of DNA-excess solution hybridization. Our data indicate that while short-term treatment resulted in a marked decrease in the density of cardiac muscarinic receptors by 34%, there was no accompanying significant change in the concentration of their mRNA. In contrast, long-term inhibition of acetylcholinesterase significantly decreased the concentration of both receptors and mRNA by 40% and 29%, respectively. These results are indicative of multiple mechanisms of down-regulation of cardiac muscarinic receptors, some of which might involve alterations at the transcriptional level.

Lack of desensitization of muscarinic receptor-mediated second messenger signals in rat brain upon acute and chronic inhibition of acetylcholinesterase

We studied the effects of acute and chronic in vivo inhibition of acetylcholinesterase on both the density and function of brain muscarinic cholinergic receptors. Adult male rats were treated either once or multiple times over a period of 10 days with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (DFP). The concentration and affinity of muscarinic receptors in various brain regions were determined using radioligand binding techniques. Acute DFP treatment resulted in a significant reduction in receptor number only in the brain stem, while chronic treatment caused receptor down-regulation in the brain stem, cerebral cortex, and striatum. There was no change in ligand affinity in any of the brain regions. In sharp contrast, muscarinic receptor function was fully preserved, in terms of coupling of the receptors to increased phosphoinositide hydrolysis in the cerebral cortex, hippocampus, and striatum, or inhibition of cyclic AMP formation in the cerebral cortex or striatum. Therefore, there is a marked lack or correlation between DFP-induced muscarinic receptor down-regulation and receptor desensitization.

Agonist-induced down-regulation of m1 muscarinic receptors and reduction of their mRNA level in a transfected cell line

Agonist-induced reduction in both the number of m1 muscarinic receptors and the mRNA coding for the receptor protein was investigated in Chinese hamster ovary (CHO) cells which were transfected with the m1 muscarinic receptor gene. Receptor concentration was measured by the specific binding of the muscarinic ligand, [3H]quinuclidinyl benzilate ([3H]QNB), and Northern blot hybridization analysis was used to evaluate the levels of receptor mRNA. Incubation of cells with 1 mM of the muscarinic agonist, carbamylcholine (CBC), for 24 h decreased receptor density and mRNA levels in cells by 65% and 73%, respectively. These results indicate that agonist-induced down-regulation of m1 muscarinic receptors might be due to, at least in part, a decrease in receptor synthesis resulting from a reduction in the steady-state level of their mRNA.

Effects of canine endotoxin shock on lymphocytic beta-adrenergic receptors

To determine whether beta-adrenergic receptors on circulating lymphocytes are impaired during endotoxemia and the precise role of catecholamines in this process, we allocated 16 dogs to three groups: I) control-saline vehicle (n = 5), II) endotoxin--Escherichia coli endotoxin 1.0 mg/kg iv bolus (n = 6), and III) endotoxin + propranolol--E. coli endotoxin 1.0 mg/kg after pretreatment with propranolol, 1.5 mg/kg iv bolus followed by a continuous infusion, 30 micrograms/kg per min, (n = 5). Five hours after endotoxin injection, lymphocytic beta-adrenergic receptor number and sodium fluoride (NaF)-stimulated cyclic AMP accumulation were reduced by 41 +/- 6% and 25 +/- 7% of baseline values, respectively, which were significantly different from those observed in the control group (both P less than .01). Propranolol pretreatment prevented the endotoxin-induced reduction in lymphocytic beta-adrenergic receptor number (P less than .02 compared with the endotoxin group), but not the decrease in NaF-stimulated cyclic AMP accumulation (P less than .01 compared with the control group). Myocardial beta-adrenergic receptor number was reduced in the endotoxin group compared with that observed in the control group (P less than .06). These changes were associated with a decreased chronotropic response to isoproterenol in the endotoxin group compared with the control group (P less than .05). We conclude that decreased lymphocytic beta-adrenergic receptor number in endotoxin shock is caused by circulating catecholamines, whereas alterations distal to the receptors may be due to other mechanisms.

Selectivity of McN-A-343 in stimulating phosphoinositide hydrolysis mediated by M1 muscarinic receptors

The potency and efficacy of McN-A-343 (McN) in stimulating phosphoinositide (PI) hydrolysis were investigated in Chinese hamster ovary cells transfected with the m1 and m3 muscarinic receptor genes, in comparison with carbamylcholine (CBC). In m1 cells, CBC and McN increased PI hydrolysis by 17- and 9-fold over basal, respectively, with corresponding EC50 values of 4.2 and 4.3 microM. Whereas the maximal stimulatory response to CBC was slightly less in m3 cells (11-fold over basal), McN elicited only up to a 2-fold increase in PI hydrolysis in these cells. Competition binding data with N-[3H]methylscopolamine showed that McN had a higher affinity in m1 than in m3 cells, whereas CBC did not differentiate well between the two receptor subtypes. The partial agonistic activity of McN was demonstrated by its ability to suppress the stimulation by CBC to its own maximal response in both cell lines in a dose-dependent manner and by its low efficacy and the absence of receptor spareness. The PI response to the full agonist CBC in m3 cells was associated with a larger receptor reserve than in m1 cells. Thus, differences in receptor spareness cannot account for the apparent selectivity of McN in activating m1 muscarinic receptors. Differences in the sensitivity of m1 and m3 cells to McN were not due to differences in receptor concentration, despite the fact that the receptor density in m1 cells was 2-fold higher than in m3 cells. Our results suggest an actual selectivity (but not necessarily specificity) of the effects of McN in increasing Pl hydrolysis mediated by M1 receptors.

Aging does not alter muscarinic receptor-mediated inhibition of cyclic AMP formation in the striatum and hippocampus

The effects of aging on the inhibition of forskolin-stimulated cyclic AMP formation by muscarinic receptors were investigated. There were no detectable changes in the magnitude of maximal inhibition by carbamylcholine or the potency of the agonist in inducing this response in either the striatum or hippocampus obtained from young or old Fisher 344 rats.

p-Fluoro-hexahydro-sila-difenidol exhibits poor selectivity between M3 and M1 muscarinic receptors

We investigated the potential ability of p-fluoro-hexahydro-sila-difenidol (p-F-HHSiD) to discriminate between M1 and M3 muscarinic receptor subtypes using Chinese hamster ovary cells stably transfected with the genes encoding the two receptors. Both radioligand binding and functional assays were utilized for this purpose. In contrast to initial reports of a 14-fold selectivity of this antagonist for M3 versus M1 receptors, we have detected a qualitatively similar selectivity that was markedly smaller in magnitude.

Receptor-mediated generation of an EDRF-like intermediate in a neuronal cell line detected by spin trapping techniques

We have studied receptor-mediated generation of an activator of soluble guanylate cyclase in cultured mouse neuroblastoma cells (clone N1E-115) by ESR/spin trapping spectroscopy. A spin adduct was detected during the activation of muscarinic receptors by carbamylcholine in the presence of the spin trap 3,5-dibromo 4-nitrosobenzene sulphonate (DBNBS). The spin adduct does not correspond to that originating from the free radical nitric oxide or hydroxylamine. The same adduct was generated in cytosol preparations from N1E-115 cells incubated with L-arginine, NADPH, in the presence of calcium. The use of isotopically labelled guanidino-N15-L-arginine supported the generation of a DBNBS spin trapped adduct originating from the guanidino moiety of L-arginine. Superoxide dismutase (SOD) stabilized the precursor of the spin adduct as well as the activator of soluble guanylate cyclase derived from L-arginine. Our results provide direct evidence for the receptor-mediated formation of a diffusible precursor of NO. derived from L-arginine.

Relationship between the partial inhibition of muscarinic receptor-mediated phosphoinositide hydrolysis by phorbol esters and tetrodotoxin in rat cerebral cortex

Our results demonstrate that phorbol esters and tetrodotoxin (TTX) partially inhibit muscarinic receptor-mediated increase in phosphoinositide (PI) hydrolysis in rat cerebral cortex cell aggregates; this inhibition was observed using several muscarinic agonists. While these effects were not accompanied by major changes in the total muscarinic receptor population, phorbol esters, but not TTX, reduced the relative concentration of the high affinity binding sites of the M1-selective ligands pirenzepine and telenzepine. In contrast, the binding of a muscarinic agonist to multiple receptor conformations was not influenced by either phorbol esters or TTX. Our data also show that the partial inhibition of the PI response by these agents is not due to a selective effect on the response mediated by a certain muscarinic receptor subtype or a receptor population which is more sensitive to agonist-induced desensitization. Evidence is provided that the effects of both phorbol esters and TTX might be mediated largely, although not entirely, by a common mechanism.