Effect of thiamine on various types of synaptic junctions

Thiamine (1.10(-14) -- 1.10(-4) mol/l) reversibly increased the frequency of miniature excitatory postsynaptic potentials, amplitude and quantal content of excitatory postsynaptic potentials in crayfish glutaminergic synapse. Thiamine also increased spontaneous electrical activity and amplitude of synaptic potentials in guinea-pig taenia coli. In synaptosomes from the rat brain thiamine produced depolarization of nerve endings. The role of thiamine in the regulation of synaptic transmission and mechanism of its action are discussed.     

Identification of the nicotinic and muscarinic cholinoreceptors of the soma of the RPa4 neuron in the edible snail

Intracellular recording of potentials was employed to identify cholinoreceptors of the somatic membrane in Helix lucorum RPa4 neuron. A local application under pressure of specific agonists of nicotinic (nicotine, cytisine) and muscarinic (muscarine, arecoline) cholinoreceptors to the soma produced cell depolarization. Depolarization to acetylcholine application was short and was often followed by hyperpolarization. Selective desensitization of receptors by nicotine and muscarine as well as occupation of receptors by cytisine and arecoline reduced depolarization in response to acetylcholine. Nicotinic cholinergic blocker, d-tubocurarine, inhibited to a greater extent responses to nicotinic cholinomimetics, while atropine, muscarinic cholinergic blocker, inhibited responses to muscarinic cholinomimetics. Acetylcholine exhibited a mixed cholinomimetic effect: acetylcholine-induced responses were almost equally inhibited by d-tubocurarine and atropine. A hypothesis is suggested that somatic membrane of RPa4 neuron contains classical nicotinic and muscarinic acetylcholine receptors similar to those of the vertebrates.     

Effect of dopamine on the background and evoked activity of interneurons from isolated spinal cord segments of newborn rats

The effects of dopamine on the background and dorsal root stimulation-evoked activity of spinal interneurons and on the field potentials in the dorsal horn were studied in the isolated superfused spinal cord of 11-18 days old rats. It was established that application of dopamine in concentration 1.10(-6)-1.10(-3) mol/l caused exciting and depressing effects on the background and evoked activity of spinal cord cells. The postsynaptic wave of field potentials was depressed by dopamine. In some interneurons the effects of dopamine disappeared in the solution with 0.01 mmol/l Ca2+ and 1.5-2 mmol/l Mn2+ (trans-synaptic effects). In other cells exciting or inhibiting effects of dopamine were preserved in the medium blocking synaptic transmission, that evidenced for direct depolarization and hyperpolarization of the interneuron membrane. The effects of dopamine were reversible and dose-dependent. These results suggest that dopamine may have influence on the transmission of excitation in the spinal cord on the interneurons level. It is supposed to be possible that different types of interneurons responses are the result of the existence of two types of dopamine receptors in the spinal cord.     

Some Actions of 9-Amino-1,2,3,4-Tetrahydroacridine (THA) on Cholinergic Transmission and Membrane Currents in Rat Sympathetic Ganglia

THA (Tacrine) is an anticholinesterase drug reported to alleviate cognitive deficit in Alzheimer's disease. We have used rat isolated superior cervical sympathetic ganglia as a model mammalian cholinergic neural system to study effects of THA on cholinergic synaptic transmission and postsynaptic membrane currents. At 0.1 - 3 microM, THA augmented the postsynaptic depolarizations and inward clamp currents produced by acetylcholine but not by the cholinesterase-resistant analogue, DMPP. Higher concentrations depressed these responses to both acetylcholine and DMPP, and reduced the acetylcholine-induced increase in membrane current noise. At 1 microM, THA did not affect the amplitude or time-course of fast (nicotinic) excitatory postsynaptic currents (epscs) evoked by single orthodromic volleys, but higher concentrations induced a biphasic epsc decay. In contrast, low concentrations of THA (1 - 3 microM) greatly augmented and prolonged the muscarinic slow epsc evoked by repetitive orthodromic volleys: this effect was blocked by 1 microM atropine. Concentrations above 0.1 mM produced a membrane depolarization and inhibited a variety of membrane ionic currents, including voltage-gated Ca current and subsequent Ca-activated K currents, and voltage-gated M- and A-type K currents. It is concluded that the principal effect of THA is to inhibit cholinesterase, and that the main consequence of this is to augment and prolong the muscarinic slow epsc. In contrast, the nicotinic fast epsc is not increased but instead may be reduced through a nicotinic channel-blocking action. Although THA could also block several other ion channels the concentrations required were too high to contribute significantly to its principal pharmacological actions on ganglionic transmission.     

Effects of Pb2+ on muscarinic modulation of glutamatergic synaptic transmission in rat hippocampal CA1 area

Lead (Pb(2+)) is a pollutant commonly found in the environment. It causes a wide variety of detrimental effects on developing central nervous system. However, the mechanisms of its neurotoxicity remained to be elucidated. In hippocampus, the muscarinic cholinergic system modulates certain forms of synaptic transmission and plasticity, and plays an important role in learning and memory. In this study, the effects of Pb(2+) on muscarinic modulation of glutamatergic synaptic transmission in hippocampal CA1 area were investigated using the conventional whole-cell patch-clamp technique in rat hippocampal slices. In the presence of nicotinic antagonist mecamylamine, carbachol (CCh), a cholinergic agonist, concentration-dependently inhibited glutamatergic excitatory postsynaptic currents (EPSCs), enhanced paired-pulse facilitation (PPF) and the response to 10-Hz pulse-trains. The analysis of the spontaneous excitatory postsynaptic currents (sEPSCs) showed the activation of muscarinic receptors by CCh decreased the frequency, amplitude and decay time of sEPSCs. The 10 microM Pb(2+) depressed the inhibition of EPSCs by CCh, reduced the CCh-induced enhancement of PPF and the response to 10-Hz pulse-trains, and also affected the modulation of sEPSCs by CCh. The results suggested that the activation of muscarinic acetylcholine (ACh) receptors in hippocampus could modulate glutamatergic synaptic transmission, while Pb(2+) exposure would lead to an alteration of muscarinic modulation, which might be involved in the Pb(2+)-induced impairment of synaptic transmission and plasticity during learning and memory.     

Tetrahydro-9-aminoacridine presynaptically inhibits glutamatergic transmission in the rat amygdala

The effect of the centrally active anticholinesterase inhibitor Terahydro-9-aminoacridine (THA) on synaptic transmission was studied in rat amygdala neurons in the in vitro slice preparation. THA reversibly suppressed the excitatory postsynaptic potential (EPSP) in a concentration-dependent manner. Postsynaptic depolarization induced by α-amino-5-methyl-4-isoxazole propionate (AMPA) was not decreased by THA. These results demonstrate that THA has a presynaptic inhibitory action on the physiology of synaptic transmission in the amygdala. Pretreating the slices with atropine did not affect THA's effect, indicating that the presynaptic muscarinic receptors are not involved.     

Presynaptic inhibition of Schaffer collateral synapses by stimulation of hippocampal cholinergic afferent fibres

It has been known for decades that muscarinic agonists presynaptically inhibit Schaffer collateral synapses contacting hippocampal CA1 pyramidal neurons. However, a demonstration of the inhibition of Schaffer collateral synapses induced by acetylcholine released by cholinergic hippocampal afferents is lacking. We present original results showing that electrical stimulation at the stratum oriens/alveus with brief stimulus trains inhibited excitatory postsynaptic currents evoked by stimulation of Schaffer collaterals in CA1 pyramidal neurons of rat hippocampal slices. The increased paired-pulse facilitation and the changes in the variance of excitatory postsynaptic current amplitude that paralleled the inhibition suggest that it was mediated presynaptically. The effects of oriens/alveus stimulation were inhibited by atropine, and blocking nicotinic receptors with methyllycaconitine was ineffective, suggesting that the inhibition was mediated via the activation of presynaptic muscarinic receptors. The results provide a novel demonstration of the presynaptic inhibition of glutamatergic neurotransmission by cholinergic fibres in the hippocampus, implying that afferent cholinergic fibres regulate the strength of excitatory synaptic transmission.     

Spontaneous activity in rat vestibular nuclei in brain slices and effects of acetylcholine agonists and antagonists

Extracellular recording was used to investigate spontaneously active neurons in all four major nuclei of the rat vestibular nuclear complex (VNC) in brainstem slices. The density of spontaneously active neurons was highest in the medial vestibular nucleus (MVN), slightly lower in the superior (SuVN) and spinal (SpVN) nuclei, and lowest in the lateral vestibular nucleus (LVN). We compared the effects of acetylcholine agonists and antagonists on spontaneously discharging neurons in MVN, SuVN, and SpVN with those in the nearby dorsal cochlear nucleus (DCN). The proportion of neurons responding to carbachol was greatest in DCN and smallest in SpVN. Unlike in DCN, some neurons in MVN, SuVN, and SpVN showed decreased firing during carbachol or muscarine. Magnitudes of responses to carbachol and muscarine were closely correlated (P<0.01). MVN neurons possessed nicotinic as well as muscarinic receptors. Activation of either type was unaffected by blocking synaptic transmission. The IC(50) values for the muscarinic subtype-preferential antagonists were compared, and tropicamide, preferential for M(4), was the most potent. Our results suggest that: (1) the relative numbers of spontaneously active neurons in rat VNC differ among nuclei; (2) acetylcholine agonists elicit changes in mean firing rates of neurons in MVN, SuVN and SpVN, but fewer neurons respond, and responses are smaller than in DCN; (3) both muscarinic and nicotinic acetylcholine receptors are present on MVN neurons, but muscarinic receptors may be more prominent.     

Direct muscarinic and nicotinic receptor-mediated excitation of rat medial vestibular nucleus neurons in vitro

We have utilized intracellular recording techniques to investigate the cholinoceptivity of rat medial vestibular nucleus (MVN) neurons in a submerged brain slice preparation. Exogenous application of the mixed cholinergic agonists, acetylcholine (ACh) or carbachol (CCh), produced predominantly membrane depolarization, induction of action potential firing, and decreased input resistance. Application of the selective muscarinic receptor agonist muscarine (MUSC), or the selective nicotinic receptor agonists nicotine (NIC) or 1,1-dimethyl-4-phenylpiperazinium (DMPP) also produced membrane depolarizations. The MUSC-induced depolarization was accompanied by decreased conductance, while an increase in conductance appeared to underlie the NIC- and DMPP-induced depolarizations. The muscarinic and nicotinic receptor mediated depolarizations persisted in tetrodotoxin and/or low Ca²⁺/high Mg²⁺ containing media, suggesting direct postsynaptic receptor activation. The MUSC-induced depolarization could be reversibly blocked by the selective muscarinic-receptor antagonist, atropine, while the DMPP-induced depolarization could be reversibly suppressed by the selective ganglionic nicotinicreceptor antagonist, mecamylamine. Some neurons exhibited a transient membrane hyperpolarization during the depolarizing response to CCh or MUSC application. This transient inhibition could be reversibly blocked by the γ-aminobutyric acid (GABA) antagonist, bicuculline, suggesting that the underlying hyperpolarization results indirectly from the endogenous release of GABA acting at GABA receptors. This study confirms the cholinoceptivity of MVN neurons and establishes that individual MVN cells possess muscarinic as well as nicotinic receptors. The data provide support for a prominent role of cholinergic mechanisms in the direct and indirect regulation of the excitability of MVN neurons.     

Exogenous and endogenous cannabinoids suppress inhibitory neurotransmission in the human neocortex

Activation of CB(1) receptors on axon terminals by exogenous cannabinoids (eg, Δ(9)-tetrahydrocannabinol) and by endogenous cannabinoids (endocannabinoids) released by postsynaptic neurons leads to presynaptic inhibition of neurotransmission. The aim of this study was to characterize the effect of cannabinoids on GABAergic synaptic transmission in the human neocortex. Brain slices were prepared from neocortical tissues surgically removed to eliminate epileptogenic foci. Spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were recorded in putative pyramidal neurons using patch-clamp techniques. To enhance the activity of cannabinoid-sensitive presynaptic axons, muscarinic receptors were continuously stimulated by carbachol. The synthetic cannabinoid receptor agonist WIN55212-2 decreased the cumulative amplitude of sIPSCs. The CB(1) antagonist rimonabant prevented this effect, verifying the involvement of CB(1) receptors. WIN55212-2 decreased the frequency of miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin, but did not change their amplitude, indicating that the neurotransmission was inhibited presynaptically. Depolarization of postsynaptic pyramidal neurons induced a suppression of sIPSCs. As rimonabant prevented this suppression, it is very likely that it was due to endocannabinods acting on CB(1) receptors. This is the first demonstration that an exogenous cannabinoid inhibits synaptic transmission in the human neocortex and that endocannabinoids released by postsynaptic neurons suppress synaptic transmission in the human brain. Interferences of cannabinoid agonists and antagonists with synaptic transmission in the cortex may explain the cognitive and memory deficits elicited by these drugs.     

Overview of nicotinic receptors and their roles in the central nervous system.

Alzheimer's disease is a complex disorder affecting multiple neurotransmitters. In particular, the degenerative progression is associated with loss within the cholinergic systems. It should be anticipated that both muscarinic and nicotinic mechanisms are affected as cholinergic neurons are lost. This review focuses on the basic roles of neuronal nicotinic receptors, some subtypes of which decrease during Alzheimer's disease. Nicotinic acetylcholine receptors belong to a superfamily of ligand-gated ion channels that play key roles in synaptic transmission throughout the central nervous system. Neuronal nicotinic receptors, however, are not a single entity, but rather there are many different subtypes constructed from a variety of nicotinic subunit combinations. This structural diversity and the presynaptic, axonal, and postsynaptic locations of nicotinic receptors contribute to the varied roles these receptors play in the central nervous system. Presynaptic and preterminal nicotinic receptors enhance neurotransmitter release, and postsynaptic nicotinic receptors mediate a small minority of fast excitatory transmission. In addition, some nicotinic receptor subtypes have roles in synaptic plasticity and development. Nicotinic receptors are distributed to influence many neurotransmitter systems at more than one location, and the broad, but sparse, cholinergic innervation throughout the brain ensures that nicotinic acetylcholine receptors are important modulators of neuronal excitability.     

l-Carnitine: therapeutic strategy for metabolic encephalopathy

The effects of 4-aminopyridine (4-AP) on membrane electrical properties and synaptic transmission in neurons of the isolated rabbit superior cervical ganglion were investigated. 4-AP (0.03-0.1 mM) increased the amplitude of the fast excitatory postsynaptic potential (f-EPSP) without affecting appreciably either the acetylcholine (ACh) depolarization induced by iontophoresis of ACh or the passive and active membrane properties of the neurons. At concentrations of 1-5 mM, 4-AP reversibly depressed the amplitude of the f-EPSP as well as the ACh depolarization; a slight to moderate prolongation of the action potential duration was observed. In addition to the effects on evoked synaptic potentials, 4-AP induced spontaneous discharges which were abolished reversibly by curare, low Ca solution or Co. The results indicate that 4-AP at low concentrations facilitated evoked as well as spontaneous release of ACh by a presynaptic mechanism, whereas at higher concentrations it exerted a curare-like effect on the postsynaptic membrane.     

Cholinergic Dysfunction Alters Synaptic Integration between Thalamostriatal and Corticostriatal Inputs in DYT1 Dystonia

Projections from thalamic intralaminar nuclei convey sensory signals to striatal cholinergic interneurons. These neurons respond with a pause in their pacemaking activity, enabling synaptic integration with cortical inputs to medium spiny neurons (MSNs), thus playing a crucial role in motor function. In mice with the DYT1 dystonia mutation, stimulation of thalamostriatal axons, mimicking a response to salient events, evoked a shortened pause and triggered an abnormal spiking activity in interneurons. This altered pattern caused a significant rearrangement of the temporal sequence of synaptic activity mediated by M(1) and M(2) muscarinic receptors in MSNs, consisting of an increase in postsynaptic currents and a decrease of presynaptic inhibition, respectively. Consistent with a major role of acetylcholine, either lowering cholinergic tone or antagonizing postsynaptic M(1) muscarinic receptors normalized synaptic activity. Our data demonstrate an abnormal time window for synaptic integration between thalamostriatal and corticostriatal inputs, which might alter the action selection process, thereby predisposing DYT1 gene mutation carriers to develop dystonic movements.     

Postsynaptic block of frog neuromuscular transmission by conotoxin GI

Conotoxin GI, a peptide neurotoxin contained in the venom of the marine snail Conus geographus, was applied to the cutaneous pectoris muscle of the frog, and the effects on the postsynaptic response to acetylcholine were examined. Conotoxin GI reversibly blocked nerve-evoked muscle contractions at concentrations greater than or equal to 3 to 4 microM. Micromolar concentrations of conotoxin GI significantly reduced the amplitude of miniature endplate potentials and membrane depolarizations produced by ionophoretic application of acetylcholine, suggesting that the toxin reduced the postsynaptic sensitivity to acetylcholine. The reduction in the sensitivity of the muscle to acetylcholine was not due to changes in muscle fiber resting membrane potential or input resistance. Conotoxin GI reduced the amplitudes but did not affect the rates of decay of focal, extracellularly recorded endplate currents or miniature endplate currents, suggesting that the toxin did not affect the lifetime of ion channels opened by acetylcholine. Miniature endplate currents decay five to six times more slowly than normal when acetylcholinesterase is blocked with neostigmine methyl sulfate due to repeated binding of acetylcholine to receptors as it diffuses from the synaptic cleft. Conotoxin GI reduced the amplitude and increased the rate of decay of miniature endplate currents recorded in the presence of neostigmine methyl sulfate, suggesting that the toxin reduced the binding of acetylcholine to endplate receptors. These results are consistent with the hypothesis that conotoxin GI blocks neuromuscular transmission at the frog endplate by reducing the binding of acetylcholine to receptors.     

Interaction of Noradrenergic and Cholinergic Agonists with Ligands Increasing K-conductance of Guinea Pig Hippocampal Neurons,in vitro

Single electrode current clamp and voltage clamp recordings were employed to study the effects of noradrenergic agonists and a cholinergic agonist (carbachol, Cch) on the resting membrane potential of CA3 neurons in guinea pig hippocampal slices. Stimulation of muscarinic and beta-adrenergic receptors depolarized, and stimulation of alpha1-adrenergic receptor hyperpolarized, CA3 neurons but the membrane potential changes were small. Hyperpolarizations or outward currents induced by baclofen, adenosine or serotonin (5-HT) were strongly potentiated by alpha-noradrenergic agonists and suppressed by Cch at concentrations ten times lower than those having any direct effects on membrane potential. Both the enhancement of the baclofen-induced hyperpolarization by phenylephrine and its suppression by Cch were pronounced at low concentrations of baclofen, but diminished at higher concentrations. The modulatory effects persisted after blockade of sodium spikes by tetrodotoxin and after blockade of fast inhibitory and excitatory synaptic transmission by picrotoxin and 6-cyano-7-nitroquinoxaline-2,3-dione. Our data suggest that, through the postsynaptic interaction with ligands activating potassium conductance, noradrenergic and muscarinic receptor stimulation can exert a stronger inhibitory and excitatory effect on CA3 pyramidal neurons at their resting membrane potential than would be expected from the changes in membrane potential induced by these neuromodulators on their own.     

Induction of activity-dependent LTD requires muscarinic receptor activation in medial prefrontal cortex

The medial prefrontal cortex (mPFC) forms part of a neural circuit involved in the formation of lasting associations between objects and places. Cholinergic inputs from the basal forebrain innervate the mPFC and may modulate synaptic processes required for the formation of object-in-place memories. To investigate whether acetylcholine regulates synaptic function in the rat mPFC, whole-cell voltage-clamp recordings were made from pyramidal neurons in layer V. Bath application of the cholinergic agonist carbachol caused a potent and long-term depression (LTD) of synaptic responses that was blocked by the muscarinic receptor antagonist scopolamine and was mimicked, in part, by the M(1) receptor agonists McN-A-343 or AF102B. Furthermore, inhibition of PKC blocked carbachol-mediated LTD. We next determined the requirements for activity-dependent LTD in the prefrontal cortex. Synaptic stimulation that was subthreshold for producing LTD did, however, result in LTD when acetylcholine levels were enhanced by inhibition of acetylcholinesterase or when delivered in the presence of the M(1)-selective positive allosteric modulator BQCA. Increasing the levels of synaptic stimulation resulted in M(1) receptor-dependent LTD without the need for pharmacological manipulation of acetylcholine levels. These results show that synaptic stimulation of muscarinic receptors alone can be critical for plastic changes in excitatory synaptic transmission in the mPFC. In turn, these muscarinic mediated events may be important in the formation of object-in-place memories. A loss of basal forebrain cholinergic neurons is a classic hallmark of Alzheimer's dementia and our results provide a potential explanation for the loss of memory associated with the disease.     

Sucrose-gap recordings of nerve-evoked potentials in mammalian parasympathetic ganglia

The sucrose-gap recording technique was used to study mammalian parasympathetic ganglionic transmission. Both a fast nicotinic depolarization potential and a slow muscarinic hyperpolarizing potential were recorded in vesical pelvic ganglia (VPG). A long afterhyperpolarization caused by an electrical response of through-fibers was also recorded. However, a slow excitatory postsynaptic potential (S-EPSP) was not readily observed and may be masked by the long afterhyperpolarization. In addition, the slow inhibitory postsynaptic potential (S-IPSP) of the VPG was due to a direct effect of acetylcholine (ACh). Thus, sucrose-gap recordings of VPG potentials are similar to those obtained in sympathetic ganglia, but the mechanism for transmission of the S-IPSP may be different in the respective ganglia.     

Postsynaptic M1 and M3 receptors are responsible for the muscarinic enhancement of retrograde endocannabinoid signalling in the hippocampus

The cholinergic system is crucial for higher brain functions including learning and memory. These functions are mediated primarily by muscarinic acetylcholine receptors (mAChRs) that consist of five subtypes (M(1)-M(5)). A recent study suggested a novel role of acetylcholine as a potent enhancer of endocannabinoid signalling that acts retrogradely from postsynaptic to presynaptic neurons. In the present study, we further investigated the mechanisms of this cholinergic effect on endocannabinoid signalling. We made paired whole-cell recordings from cultured hippocampal neurons, and monitored inhibitory postsynaptic currents (IPSCs). The postsynaptic depolarization induced a transient suppression of IPSCs (DSI), a phenomenon known to involve retrograde signalling by endocannabinoids. The cholinergic agonist carbachol (CCh) markedly enhanced DSI at 0.01-0.3 microM without changing the presynaptic cannabinoid sensitivity. The facilitating effect of CCh on DSI was mimicked by the muscarinic agonist oxotremorine-M, whereas it was eliminated by the muscarinic antagonist atropine. It was also blocked by a non-hydrolizable analogue of GDP (GDP-beta-S) that was applied intracellularly to postsynaptic neurons. The muscarinic enhancement of DSI persisted to a substantial degree in the neurons prepared from M1-knockout and M3-knockout mice, but was virtually eliminated in the neurons from M1/M3-compound-knockout mice. CCh still enhanced DSI significantly under the blockade of postsynatpic K(+) conductance, and did not significantly influence the depolarization-induced Ca(2+) transients. These results indicate that the activation of postsynaptic M1 and M3 receptors facilitates the depolarization-induced release of endocannabinoids.     

Muscarinic Modulation of Intrinsic Burst Firing in Rat Hippocampal Neurons

Intracellular recordings in rat hippocampal slices were used to examine how exogenous and endogenous cholinergic agonists modulate the firing pattern of intrinsically burst-firing pyramidal cells. About 24% of CA1 pyramidal cells generated all-or-none, high-frequency bursts of fast action potentials in response to intracellular injection of long positive current pulses. Application of carbachol (5 μM) converted burst firing in these neurons into regular trains of independent spikes. Acetylcholine (5 μM) exerted a similar effect, provided acetylcholine esterase activity was blocked with neostigmine (2 μM). Atropine (1 μM) reversed this cholinergic effect, indicating its mediation by muscarinic receptors. Cholinergic agonists also caused mild neuronal depolarization but the block of intrinsic burst firing was independent of this effect. Repetitive stimulation of cholinergic fibres in the presence of neostigmine (2 μM) evoked a slow cholinergic excitatory postsynaptic potential (EPSP) lasting about a minute. During the slow EPSP, burst firing could not be evoked by depolarizing pulses and the neurons fired in regular mode. These effects were prevented by pretreatment with atropine (1 μM). Exogenously applied cholinergic agonists and endogenously released acetylcholine also reduced spike frequency accommodation and suppressed the long-duration afterhyperpolarization in burst-firing pyramidal cells in an atropine-sensitive manner. A membrane-permeable cAMP analogue (8-bromo-cAMP; 1 mM) also reduced frequency accommodation and blocked the long-duration afterhyperpolarization, but did not affect intrinsic burst firing at all. Taken together, the data show that muscarinic receptor stimulation transforms the stereotyped, phasic response of burst-firing neurons into stimulus-graded, tonic discharge.