Activation of phosphoinositide turnover and protein kinase C by neurotransmitters that modulate calcium channels in embryonic chick sensory neurons

Gamma aminobutyric acid (GABA) and norepinephrine modulate the excitability of primary chick sensory neurons by decreasing the voltage dependent Ca current. Although previous electrophysiological studies indicate that neurotransmitter modulation of the Ca current in these neurons involves protein kinase C, the biochemical aspects of this mechanism have not been examined directly. We find that both norepinephrine (via a unique alpha receptor subtype) and GABA (via GABAb receptors) linked to pertussis toxin sensitive pathways, stimulate the metabolism of membrane phosphatidylinositol phospholipids in primary chick sensory neurons. In addition, norepinephrine causes the rapid translocation of C kinase activity from cytosolic to membrane associated distribution, consistent with its rapid activation in response to applied neurotransmitter. The pharmacology, pertussis toxin sensitivity and time course of the biochemical changes due to neurotransmitter treatment parallel the effects of these transmitters on calcium current modulation. These biochemical studies confirm the hypothesis that activation of protein kinase C is critically involved in calcium channel modulation in embryonic chick sensory neurons.

Long-lasting enhancement of glutamatergic synaptic transmission by acetylcholine contrasts with response adaptation after exposure to low-level nicotine

Attempts to mimic synaptic delivery of acetylcholine (ACh) with brief, repetitive pulses of high concentration ACh at synapses of medial habenula (MHN) and interpeduncular nucleus (IPN) neurons in vitro elicited temporally distinct facilitation and inhibition of glutamate secretion via nicotinic and muscarinic ACh receptor-mediated pathways, respectively. ACh-induced nicotinic facilitation was sustained for up to 2 hr, whereas muscarinic inhibition was transient. Prolonged exposure to nicotine inactivated nicotinic receptors selectively, thus decreasing the relative contribution of the facilitatory versus inhibitory influences of ACh. The net effect of ACh in modulating glutamatergic transmission at MHN-IPN synapses may be determined by pre-exposure to nicotine, because the drug appears to switch the balance between the facilitatory and inhibitory actions of ACh.

Nicotine-induced enhancement of glutamatergic and GABAergic synaptic transmission in the mouse amygdala

Presynaptic nicotinic acetylcholine receptors (nAChRs) are thought to mediate some of the cognitive and behavioral effects of nicotine. The olfactory projection to the amygdala, and intra-amygdaloid projections, are limbic relays involved in behavioral reinforcement, a property influenced by nicotine. Co-cultures consisting of murine olfactory bulb (OB) explants and dispersed amygdala neurons were developed to reconstruct this pathway in vitro. Whole cell patch-clamp recordings were obtained from amygdala neurons contacted by OB explant neurites, and spontaneous and evoked synaptic currents were characterized. The majority of the 108 innervated amygdala neurons exhibited glutamatergic spontaneous postsynaptic currents (PSCs), 20% exhibited GABAergic spontaneous PSCs, and 17% exhibited both. Direct extracellular stimulation of OB explants elicited glutamatergic synaptic currents in amygdala neurons. Antibodies to nAChR subunits co-localized with an antibody to synapsin I, a presynaptic marker, along OB explant processes, consistent with the targeting of nAChR protein to presynaptic sites of the mitral cell projections. Hence, we examined the role of presynaptic nAChRs in modulating synaptic transmission in the OB-amygdala co-cultures. Focal application of 500 nM to 1 microM nicotine for 5-60 s markedly increased the frequency of spontaneous PSCs, without a change in the amplitude, in 39% of neurons that exhibited glutamatergic spontaneous PSCs (average peak fold increase = 125.2 +/- 33.3). Nicotine also enhanced evoked glutamatergic currents elicited by direct stimulation of OB explant fibers. Nicotine increased the frequency of spontaneous PSCs, without a change in the amplitude, in 35% of neurons that exhibited GABAergic spontaneous PSCs (average peak fold increase = 63.9 +/- 34.3). Thus activation of presynaptic nAChRs can modulate glutamatergic as well as GABAergic synaptic transmission in the amygdala. These results suggest that behaviors mediated by olfactory projections may be modulated by presynaptic nAChRs in the amygdala, where integration of olfactory and pheromonal input is thought to occur.

Differential modulation of nicotinic acetylcholine receptor subtypes and synaptic transmission in chick sympathetic ganglia by PGE(2)

The diversity of neuronal nicotinic acetylcholine receptors (nAChRs) is likely an important factor in the modulation of synaptic transmission by acetylcholine and nicotine. We have tested whether postsynaptic nAChRs are modulated in a subtype-specific manner by prostaglandin E(2) (PGE(2)), a regulator of neuronal excitability in both the central and peripheral nervous systems, and examined the effects of PGE(2) on nicotinic transmission. Somatodendritic nAChRs in chick lumbar sympathetic ganglia include four nAChR subtypes distinguished on the basis of conductance and kinetic profile. Nanomolar PGE(2) applied to the extrapatch membrane differentially regulates opening probability (Po), frequency and the opening duration of each nAChR channel subtype in cell-attached patches. PGE(2) decreases the Po of the predominant nAChR subtype (36 pS) and significantly increases Po and open duration of the 23 pS subtype. The 23 pS subtype is gated by the alpha 7-selective agonist choline, and choline-gated currents are inhibited by alpha-bungarotoxin. To examine whether PGE(2) modulates nAChRs at synaptic sites, we studied the effects of PGE(2) on amplitude and decay of synaptic currents in visceral motoneuron-sympathetic neuron co-cultures. PGE(2) significantly decreases the amplitude of miniature excitatory postsynaptic currents (mEPSCs), consistent with the predominant inhibition by PGE(2) of all but the 23 pS subtype. The time constant of mEPSCs at PGE(2)-treated synapses is prolonged, which is also consistent with an increased contribution of the longer open duration of the 23 pS nAChR subtype with PGE(2) treatment. To examine the presynaptic effect of PGE(2), nanomolar nicotine was used. Nicotine induces facilitation of synaptic transmission by increasing mEPSC frequency, an action thought to involve presynaptic, alpha 7-containing nAChRs. In the presence of PGE(2), nicotine-induced synaptic facilitation persists. Thus the net effect of PGE(2) is to alter the profile of nAChRs contributing to synaptic transmission from larger conductance, briefer opening channels to smaller conductance, longer opening events. This subtype-specific modulation of nAChRs by PGE(2) may provide a mechanism for selective activation and suppression of synaptic pathways mediated by different nAChR subtype(s) at both pre- and postsynaptic sites.

Facilitation of glutamatergic neurotransmission by presynaptic nicotinic acetylcholine receptors

The profiles of presynaptic facilitation of glutamate release as elicited by nicotine and acetylcholine were compared in two limbic pathways recapitulated in vitro. At synapses of medial habenula (MHN) and interpeduncular nucleus (IPN) neurons, application of nicotine increased the frequency of TTX-resistant, spontaneous postsynaptic currents (SSCs) by an average of 5-fold. In contrast, the average increase in SSC frequency elicited by nicotine was more than 120 fold at synapses of olfactory bulb (OB) and amygdala neurons. At both preparations, pulses of ACh caused presynaptic facilitation that lasted longer than that elicited by nicotine. The subunit composition of presynaptic nAChRs may contribute to the different profiles of facilitation observed. The large magnitude, fast kinetics, and alpha-bungarotoxin sensitivity of facilitation observed at OB-amygdala synapses is consistent with participation of alpha7-type nAChRs. As subunit-selective deletion of alpha5 or alpha7 altered the profile of nicotine-elicited facilitation at MHN-IPN synapses, presynaptic nAChRs at MHN-IPN synapses appear to be more complex. Such heteromeric combinations of nAChRs may contribute to the lower magnitude and slower kinetics of presynaptic facilitation at MHN-IPN synapses. Calcium influx through either voltage-gated calcium channels or directly through presynaptic alpha7-containing nAChRs is sufficient to support nicotine-elicited facilitation of glutamate release. Resultant increases in intracellular calcium may further modulate presynaptic nAChR activity in a subunit-composition dependent manner.

Regulation of neurogenesis by interactions between HEN1 and neuronal LMO proteins

Basic-helix-loop-helix transcription factors regulate neurogenesis and neuronal differentiation by as yet unknown mechanisms. We show that an embryonic neuronal-specific basic-helix-loop-helix protein, HEN1 (also known as NSCL1 or NHLH), interacts with ‘LIM only’ proteins. Examination of the expression patterns of XHEN1 and XLMO-3, the Xenopus homologues of these human genes, reveals extensive overlap during early neurogenesis: at the onset of gastrulation on the dorsal side of the blastopore lip and, subsequently, in the prospective neural plate. Binding of XLMO-3 increases the transcriptional activity of XHEN1 in vivo. Co-expression of these two genes in Xenopus embryos induces a cascade of expression of neuronal-specific basic-helix-loop-helix proteins that leads to neuronal differentiation. We propose that XHEN1, in concert with XLMO-3, is a critical regulator of neurogenesis.

Cysteine-rich domain isoforms of the neuregulin-1 gene are required for maintenance of peripheral synapses

Neuregulin-1 (NRG-1) signaling has been implicated in inductive interactions between pre- and postsynaptic partners during synaptogenesis. We used gene targeting to selectively disrupt cysteine-rich domain-(CRD-) containing NRG-1 isoforms. In CRD-NRG-1-/-mice, peripheral projections defasciculated and displayed aberrant branching patterns within their targets. Motor nerve terminals were transiently associated with broad bands of postsynaptic ACh receptor (AChR) clusters. Initially, Schwann cell precursors accompanied peripheral projections, but later, Schwann cells were absent from axons in the periphery. Following initial stages of synapse formation, sensory and motor nerves withdrew and degenerated. Our data demonstrate the essential role of CRD-NRG-1-mediated signaling for coordinating nerve, target, and Schwann cell interactions in the normal maintenance of peripheral synapses, and ultimately in the survival of CRD-NRG-1-expressing neurons.

Multiorgan autonomic dysfunction in mice lacking the beta2 and the beta4 subunits of neuronal nicotinic acetylcholine receptors

Transcripts for the beta2 and the beta4 nicotinic acetylcholine receptor (nAChR) subunits are found throughout the CNS and the peripheral nervous system. These two beta subunits can form heteromultimeric channels with any of the alpha2, alpha3, alpha4, or alpha5 subunits in heterologous expression systems. Nonetheless, the subunit composition of native nAChRs and the role of different nAChR subtypes in vivo remain unclear. We prepared null mutations for the beta2 and the beta4 genes and bred beta2-/-beta4-/- mice by mating mice of identical beta2-/-beta4+/- or beta2+/-beta4-/- genotype. The beta2-/- and the beta4-/- single-mutant mice grow to adulthood with no visible phenotypic abnormalities. The beta2-/-beta4-/- double mutants survive to birth but have impaired growth and increased perinatal mortality. They also present enlarged bladders with dribbling urination and develop urinary infection and bladder stones. The ocular pupils are widely dilated and do not constrict in response to light. Histological studies revealed no significant abnormalities of brain or peripheral tissues except for hyperplasia in the bladder mucosa of beta4-/- and beta2-/-beta4-/- mutants. Bladder strips from beta2-/-beta4-/- mice did not respond to nicotine but contracted when stimulated with a muscarinic agonist or electric field stimulation. Bladder strips from beta4 mutants did not respond to nicotine despite the absence of major bladder dysfunction in vivo. Acetylcholine-activated whole-cell currents were absent in superior cervical ganglion neurons from beta2-/-beta4-/- mice and reduced in neurons from beta4-/- mice. Although there is apparent redundancy and a superficially normal phenotype in beta2-/- and beta4-/- mice, physiological studies indicate major deficits in the beta4-/- mice. Our previous description of a similar phenotype in alpha3-/- mice and the current data suggest that the alpha3 and the beta4 subunits are major components in autonomic nAChRs. The phenotype of the beta2-/-beta4-/- and alpha3-/- mice resembles the autosomal recessive megacystis-microcolon-hypoperistalsis syndrome in humans.

Target-specific control of nicotinic receptor expression at developing interneuronal synapses in chick

Neuronal differentiation and development of synaptic specializations are strongly influenced by cellular interactions. We compared the effects of interaction with distinct autonomic targets on the molecular and biophysical differentiation of 'upstream' neuron-neuron synapses. Contact with cardiac tissue induced expression of nicotinic receptor channels (nAChRs) distinct from those induced by renal tissue in presynaptic autonomic neurons. The kinetics of cholinergic currents at interneuronal synapses are dictated by the peripheral target contacted. Analysis of the nAChR channel subtypes and subunits in individual neurons demonstrated that the profile of transmitter receptors expressed at mature neuron-neuron synapses develops from the convergent influences of input-derived (anterograde) and target-specific (retrograde) signals.

lynx1, an endogenous toxin-like modulator of nicotinic acetylcholine receptors in the mammalian CNS

Elapid snake venom neurotoxins exert their effects through high-affinity interactions with specific neurotransmitter receptors. A novel murine gene, lynx1, is highly expressed in the brain and contains the cysteine-rich motif characteristic of this class of neurotoxins. Primary sequence and gene structure analyses reveal an evolutionary relationship between lynx1 and the Ly-6/neurotoxin gene family. lynx1 is expressed in large projection neurons in the hippocampus, cortex, and cerebellum. In cerebellar neurons, lynx1 protein is localized to a specific subdomain including the soma and proximal dendrites. lynx1 binding to brain sections correlates with the distribution of nAChRs, and application of lynx1 to Xenopus oocytes expressing nAChRs results in an increase in acetylcholine-evoked macroscopic currents. These results identify lynx1 as a novel protein modulator for nAChRs in vitro, which could have important implications in the regulation of cholinergic function in vivo.

Presynaptic ionotropic receptors and the control of transmitter release

The quantity of neurotransmitter released into the synaptic cleft, the reliability with which it is released, and the response of the postsynaptic cell to that transmitter all contribute to the strength of a synaptic connection. The presynaptic nerve terminal is a major regulatory site for activity-dependent changes in synaptic function. Ionotropic receptors for the inhibitory amino acid GABA, expressed on the presynaptic terminals of crustacean motor axons and vertebrate sensory neurons, were the first well-defined mechanism for the heterosynaptic transmitter-mediated regulation of transmitter release. Recently, presynaptic ionotropic receptors for a large range of transmitters have been found to be widespread throughout the central and peripheral nervous systems. In this review, we first consider some general theoretical issues regarding whether and how presynaptic ionotropic receptors are important regulators of presynaptic function. We consider the criteria that should be met to identify a presynaptic ionotropic receptor and its regulatory function and review several examples of presynaptic receptors that meet at least some of those criteria. We summarize the classic studies of presynaptic inhibition mediated by GABA-gated Cl channels and then focus on presynaptic nicotinic ACh receptors and presynaptic glutamate receptors. Finally, we briefly discuss evidence for other types of presynaptic ionotropic receptors.

Neuronal nicotinic acetylcholine receptor modulation by general anesthetics

1. General anesthetics have been shown to inhibit synaptic transmission in multiple areas of the central and peripheral nervous systems. 2. The mechanism of inhibition is not well understood. 3. It has become clear that general anesthetics modulate the function of members of the ligand gated ion channel superfamily, including receptors for GABA(A), glycine (Harrison et al., Mol. Pharmacol. 44(3), 1993, 628-632) and 5HT3 (Zhou and Lovinger, J. Pharmacol. Exp. Therap. 278(2), 1996, 732-740). 4. Studies of the activity of general anesthetics on recombinant neuronal nicotinic acetylcholine receptors have added this receptor family to those potently inhibited by general anesthetics (Flood et al., Anesthesiology 86(4), 1997, 859-865; Violet et al., Anesthesiology 86(4), 1997, 866-874). 5. Studies of neuronal nicotinic receptors in native neurons suggest that the inhibition of these receptors by general anesthetics at low clinical concentrations may be biologically significant (Nicoll, Science 199(4327), 1978, 451-452). 6. Recent work on neuronal nicotinic acetylcholine receptors in the central nervous system suggests that their primary role may be to modulate synaptic transmission (Role and Berg, Neuron 16(6), 1996, 1077-1085). 7. Thus, inhibition of nicotinic modulation in the central nervous system may result in inhibition of synaptic transmission and some of the behavioral consequences of general anesthesia.

Functional contribution of the alpha7 subunit to multiple subtypes of nicotinic receptors in embryonic chick sympathetic neurones

1. Many studies of the alpha7 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) family have demonstrated that this alpha-bungarotoxin (alpha-BgTx)-binding neuronal receptor can participate in ACh-gated channels. Heterologous expression studies reveal that alpha7 subunits form homomeric channels of unusually high Ca2+ permeability. However, the physiological role of the alpha7 subunit in native neuronal nAChR channels is less clear. 2. We present evidence that the alpha7 subunit contributes to the function of at least three subtypes of native nAChR expressed by embryonic chick sympathetic neurones. These subtypes are functionally distinct from heterologously expressed homomeric alpha7 nAChRs as well as homomeric-like currents described in studies of hippocampal and parasympathetic neurones. 3. The proposed nAChRs differ from one another and from homomeric alpha7 nAChRs in their sensitivity to block by alpha7 subunit-specific antagonists: alpha-BgTx and methyllycaconitine (MLA). While MLA blocks 60 % of the macroscopic ACh response, alpha-BgTx inhibits a small component of the macroscopic current described by slow-on and slow-off kinetics. 4. Functional deletion of the alpha7 subunit by antisense oligonucleotide treatment eliminates the susceptibility of the nAChRs to block by both MLA and alpha-BgTx. 5. Single channel recordings combined with pharmacological and antisense-mediated 'deletion' techniques reveal that alpha-BgTx-sensitive alpha7-containing nAChRs have a small unitary conductance (18 pS), brief open time kinetics and relatively low open probability (Po). MLA-sensitive alpha7 nAChRs are characterized by a conductance of approximately 35 pS, intermediate burst duration, and a relatively high Po. 6. The third nAChR subtype deleted by alpha7 antisense treatment is characterized by a unitary conductance of 50 pS and prolonged opening duration. 7. We propose that these three populations of native alpha7-containing nAChRs are distinct heteromeric complexes that include other alpha and/or beta nAChR subunits.

Functional contribution of the alpha5 subunit to neuronal nicotinic channels expressed by chick sympathetic ganglion neurones

1. Heterologous expression studies of the alpha5 subunit of the neuronal acetylcholine receptor (nAChR) gene family have demonstrated that it can participate in the function of ACh-gated channels if co-expressed with another alpha- and a beta-subunit. Previous studies also indicate prominent expression of alpha5 in both central and peripheral nervous systems. The participation of alpha5 in native nAChRs and its functional role in these channels is, however, unknown. 2. In this study, we present evidence that alpha5 has a role in at least two distinct subtypes of nAChR complexes expressed by embryonic chick sympathetic neurones. 3. alpha5 contributes not only to agonist but also to antagonist sensitivity of natively expressed nAChR channels. Functional deletion of the alpha5 subunit by antisense oligonucleotide treatment removes the nAChRs with relatively low affinity to ACh and cytisine. Deletion of alpha5 also eliminates channels that are blocked by the alpha7-specific antagonist methyllycaconitine (MLA) while increasing the percentage of current carried by nAChRs that are sensitive to alpha-bungarotoxin (alpha-BgTx). 4. Single channel analyses indicate that functional deletion of alpha5 results in the deletion of both the 'brief' and 'long' open duration, 50 pS subtypes of nAChR channels while increasing the expression of the 18 pS, alpha-BgTx-sensitive native nAChRs normally detected in sympathetic neurones at later developmental stages. 5. The biophysical and pharmacological profiles of native nAChRs revealed by this study and previous work are discussed in the context of a proposed model of the nAChR channels expressed by chick sympathetic neurones throughout development.

Modulation of nicotinic AChR channels by prostaglandin E2 in chick sympathetic ganglion neurons

The effects of prostaglandin E2 (PGE2), an important metabolite of arachidonic acid, were studied on the activity of nicotinic AChR channels in cultured chick sympathetic ganglion neurons. In whole cell recordings, PGE2 (25 nM) inhibited significantly the ACh-evoked macroscopic current. In cell-attached patch recordings, PGE2 significantly inhibited single AChR channel currents as a result of a decrease in the frequency of channel opening, with no change in open time and conductance. PGE2 did not alter the extent or rate of agonist-induced desensitization of the AChR channels. These effects are specific since the related compound PGD2 had no effect on AChR channel function. Because there is an abundant endogenous production of PGE2 within sympathetic ganglia in response to certain stimuli, the inhibition of AChR channel function by PGE2 could serve an important role to modulate synaptic transmission in the sympathetic nervous system.

Presynaptic ionotropic receptors

Recent studies have provided new insights into the role of presynaptic ligand-gated ion channels in modifying synaptic transmission. Along with a growing list of different types of presynaptic ionotropic receptors and the cell types that express them, there have been advances in characterizing the molecular components of the receptors as well as the signaling processes that link receptor activation to changes in neurotransmitter release. Perhaps most striking is the recent convergence of data from biochemical, molecular and electrophysiological studies, implicating presynaptic ionotropic receptors in the effects of psychoactive and addictive drugs.

Nicotine enhancement of fast excitatory synaptic transmission in CNS by presynaptic receptors

The behavioral and cognitive effects of nicotine suggest that nicotinic acetylcholine receptors (nAChRs) participate in central nervous system (CNS) function. Although nAChR subunit messenger RNA (mRNA) and nicotine binding sites are common in the brain, there is little evidence for synapses mediated by nAChRs in the CNS. To test whether, CNS nAChRs might modify rather than mediate transmission, the regulation of excitatory synaptic transmission by these receptors was examined. Nanomolar concentrations of nicotine enhanced both glutamatergic and cholinergic synaptic transmission by activation of presynaptic nAChRs that increased presynaptic [Ca2]i. Pharmacological and subunit deletion experiments reveal that these presynaptic nAChRs include the alpha 7 subunit. These findings reveal that CNS nAChRs enhance fast excitatory transmission, providing a likely mechanism for the complex behavioral effects of nicotine.

Developmental regulation of multiple nicotinic AChR channel subtypes in embryonic chick habenula neurons: contributions of both the alpha 2 and alpha 4 subunit genes

Habenula neurons from both early and late stage embryonic chickens express multiple subtypes of nicotinic acetylcholine receptor channels (nAChRs). The channel subtypes expressed by habenula neurons are similar in functional properties, but apparently distinct in subunit composition, from their peripheral counterparts in autonomic ganglia. Early in development, nicotine activates four classes of neuronal bungarotoxin (nBGT)-sensitive channels (approx. conductance = 15, 30, 50, 60pS) that are intermingled on the surface of habenula neuronal somata. In neurons removed from older animals, nAChR channel activity has increased 4- to 40-fold and channel subtypes have become spatially segregated from one another. Analysis of the profile of nAChR subunit gene expression by polymerase chain reaction indicates that several of the alpha-type subunit genes, including alpha 2,3,4,5,7, and alpha 8, as well as both beta 2 and beta 4, are expressed. Treatment of the neurons with subunit specific antisense oligonucleotides reveals that the alpha 2 and alpha 4 (but not alpha 3) subunits contribute to the functional profile of native nAChRs expressed by habenula neurons. Consideration of the functional properties and apparent subunit composition of autonomic ganglion nAChRs in the chick suggests that habenula neurons may utilize a very distinct set of subunit combinations to produce an array of nAChR channel subtypes similar in both conductance and pharmacological profile to those expressed by sympathetic neurons.

Protein kinase C blocks somatostatin-induced modulation of calcium current in chick sympathetic neurons

1. Somatostatin produces a voltage-dependent inhibition of N-type Ca2+ current in chick sympathetic neurons. Pretreatment of chick sympathetic ganglion neurons with protein kinase C (PKC) activators has no effect on calcium current (ICa) but reduces the inhibition of ICa by somatostatin. 2. The effects of the alkaloid PKC activator (-)-indolactam V were indistinguishable from those of 4 beta-phorbol-12-myristate-13-acetate (4 beta-PMA). The inactive isomers (+)-indolactam V and 4 alpha-PMA did not alter the modulation of ICa by somatostatin. 3. Modulation of ICa by somatostatin desensitizes, with a time for half desensitization of approximately 3 min. PKC activation mimics the normal desensitization process in that responses to 30 nM somatostatin are inhibited to a greater extent than are responses to 1 microM somatostatin. 4. PKC appears to act at the level of the somatostatin receptor or receptor-G protein interaction because PKC activation does not alter Ca2+ current inhibition in response to a nonhydrolyzable analog of GTP, GTP-gamma-S, which directly activates G proteins. 5. The specific PKC inhibitor calphostin C largely reverses the effects of phorbol esters, but does not slow the normal rate of desensitization of somatostatin responses. This indicates that PKC is not involved in the homologous desensitization of the somatostatin receptor. 6. Neither substance P, which activates PKC in these cells, nor arachidonic acid, another PKC activator, altered the action of somatostatin on ICa.

Peptide modulation of ACh receptor desensitization controls neurotransmitter release from chicken sympathetic neurons

1. The distribution and release of substance P (SP) in embryonic chicken lumbar sympathetic ganglia was examined with the use of immunohistochemistry and radioimmunoassay, respectively. SP immunoreactivity was detected in nerve fibers surrounding individual sympathetic neurons and was released by ganglionic depolarization. 2. Effects of SP on nicotinic acetylcholine receptor (AChR) function was assayed in embryonic sympathetic neurons in vitro by whole-cell patch clamp. SP (0.1-20 microM) accelerated the rate of decay (desensitization) of ACh-induced currents. The AChR desensitization time course is biphasic and described by the sum of two exponential functions dependent on agonist concentration (time constant of the faster component, tau f = 1-2 s, and the slower time constant, tau s = 10-25 s). SP selectively decreased tau s and the contribution of the slow component to the overall rate of current decay. The effects of SP on desensitization were concentration dependent and reversible. SP slowed recovery from desensitization by 2.5-fold. 3. SP shifted the dose-response curve for ACh-induced desensitization, reducing the concentration of ACh required to produce half-maximal desensitization by approximately twofold. 4. Preapplication of SP was equivalent to SP applied together with ACh in accelerating AChR desensitization. SP did not alter the time course of currents elicited by nondesensitizing concentrations of ACh, carbamylcholine (CARB), or dimethylphenylpiperazinium (DMPP). These data suggest that AChR activation is neither necessary nor sufficient for the peptide to modulate receptor function. A kinetic model of the effects of SP on specific steps in AChR desensitization is presented. 5. SP enhanced the rate of decay of synaptic currents in sympathetic neurons innervated in vitro, decreasing the synaptic current duration by up to 80%. 6. Effects of SP on neurotransmitter release from sympathetic neurons were evaluated by measuring the release of [3H]-norepinephrine (NE). ACh and CARB stimulated NE release in a concentration- and calcium-dependent manner. SP alone had no effect on NE secretion, but the peptide inhibited NE release induced by ACh or CARB by 40-50%. 7. Although agonists specific for either nicotinic or muscarinic receptors stimulated release of NE, SP selectively inhibited the nicotinic component of transmitter secretion. Thus SP suppressed NE release induced by DMPP by up to 80% but had no effect on muscarine or depolarization-induced NE secretion. 8. Parallel studies of the modulatory effects of SP on whole-cell currents and NE secretion revealed that SP inhibition of transmitter release from sympathetic neurons is directly proportional to the extent of potentiation of AChR desensitization.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced ACh sensitivity is accompanied by changes in ACh receptor channel properties and segregation of ACh receptor subtypes on sympathetic neurons during innervation in vivo

Although presynaptic input can influence the number and distribution of ACh receptors (AChRs) on muscle, the role of cellular interactions in the development of transmitter sensitivity in neurons is less clear. To determine whether presynaptic input modifies neuronal AChR channel function and distribution, we must first ascertain the profile of changes in receptor properties relative to the timing of synapse formation. We have examined the temporal aspects of synaptogenesis in the lumbar sympathetic ganglia of the embryonic chick in anatomical experiments with anterograde 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate labeling of presynaptic inputs and cytochrome oxidase histochemistry. Biophysical studies of sympathetic neurons, within hours of removal from animals at different stages relative to synapse formation, show that both the properties and distribution of AChR channels are modified concurrent with a significant increase in presynaptic input to the neurons. The most striking change in AChR channel distribution is revealed by patching multiple sites on the surface of individual neurons. Following innervation in vivo, many neurons express only one of the four AChR channel subtypes and the AChRs are clustered in discrete, high-activity patches. Furthermore, when neurons at this stage express more than one AChR channel subtype, the different classes are often spatially segregated from one another on the cell surface. This contrasts with patches from neurons removed earlier on, which have lower overall activity, often comprised of multiple channel subtypes. Comparison of the AChR properties of acutely dispersed neurons to those of neurons maintained in vitro indicates that most features of AChR channels are conserved despite their removal from presynaptic and other in vivo influences. These findings are consistent with inductive interactions between pre- and postsynaptic neurons playing an important regulatory role in transmitter receptor expression.

Uptake of antisense oligonucleotides and functional block of acetylcholine receptor subunit gene expression in primary embryonic neurons

Several recent studies have used antisense oligonucleotides in the nervous system to probe the functional role of particular gene products. Since antisense oligonucleotide-mediated block of gene expression typically involves uptake of the oligonucleotides, we have characterized the mechanism of this uptake into developing neurons from embryonic chickens. Antisense oligonucleotides (15 mers) added to the bathing media are taken up into the embryonic chicken sympathetic neurons maintained in vitro. A portion of the oligonucleotide uptake is temperature dependent and saturates at extracellular oligonucleotide concentrations > or = 20 microM. This temperature sensitive, saturable component is effectively completed by single nucleotides of ATP and AMP and is reminiscent of receptor-mediated endocytosis of oligonucleotides described in non-neuronal cells. The efficiency of the oligonucleotide uptake system is dependent on the developmental stage of the animal but independent of the number of days that the neurons are maintained in vitro. Following the uptake of antisense oligonucleotides directed against ion channel subunit genes expressed by these neurons (nicotinic acetylcholine receptor subunit alpha 3; nAChR alpha 3), biophysical assays reveal that the functional expression of the target gene is largely blocked. Thus the number of wild type nAChR channels expressed is decreased by approximately 80%-90%. Furthermore, following antisense deletion of alpha 3, "mutant" nAChRs with distinct functional characteristics are expressed. In sum, these studies characterize the uptake of antisense oligonucleotide and demonstrate the functional block of specific gene expression in primary developing neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Diversity in primary structure and function of neuronal nicotinic acetylcholine receptor channels

Neuronal nicotinic acetylcholine receptors are oligomeric protein complexes whose component subunits are each encoded by a family of homologous genes. The current challenge is to determine the functional contributions of the related subunits to the receptor-linked ion channels they compose and to uncover the physiological impact of the distinct channel classes expressed in vivo. In the past year, new approaches to the analysis of these receptors have yielded important insights into their stoichiometry, pharmacology and functional properties.

Functional contribution of neuronal AChR subunits revealed by antisense oligonucleotides

Although multiple related genes encoding nicotinic acetylcholine receptor (AChR) subunits have been identified, how each of these subunits contributes to AChRs in neurons is not known. Sympathetic neurons express four classes of AChR channels and six AChR subunit genes (alpha 3, alpha 4, alpha 5, alpha 7, beta 2, and beta 4). The contribution of individual subunits to AChR channel subtypes in these neurons was examined by selective deletion with antisense oligonucleotides. An alpha 3 antisense oligonucleotide decreased the number and altered the properties of the normally expressed ACh-activated channels. The remaining AChR channels have distinct biophysical and pharmacological properties that indicate an important functional contribution of the alpha 7 subunit.

Developmental changes in transmitter sensitivity and synaptic transmission in embryonic chicken sympathetic neurons innervated in vitro

Dispersed neurons from embryonic chicken sympathetic ganglia were innervated in vitro by explants of spinal cord containing the autonomic preganglionic nucleus or somatic motor nucleus. The maturation of postsynaptic acetylcholine (ACh) sensitivity and synaptic activity was evaluated from ACh and synaptically evoked currents in voltage-clamped neurons at several stages of innervation. All innervated cells are more sensitive to ACh than uninnervated neurons regardless of the source of cholinergic input. Similarly, medium conditioned by either dorsal or ventral explants mimics innervation by enhancing neuronal ACh sensitivity. This increase is due to changes in the rate of appearance of ACh receptors on the cell surface. There are also several changes in the nature of synaptic transmission with development in vitro, including an increased frequency of synaptic events and the appearance of larger amplitude synaptic currents. In addition, the mean amplitude of the unit synaptic current mode increases, as predicted from the observed changes in postsynaptic sensitivity. Although spontaneous synaptic current amplitude histograms with multimodal distributions are seen at all stages of development, histograms from early synapses are typically unimodal. Changes in the synaptic currents and ACh sensitivity between 1 and 4 days of innervation were paralleled by an increase in the number of synaptic events that evoked suprathreshold activity in the postsynaptic neurons. The early pre- and postsynaptic differentiation described here for interneuronal synapses formed in vitro may be responsible for increased efficacy of synaptic transmission during development in vivo.

Development of synaptic transmission at autonomic synapses in vitro revealed by cytochrome oxidase histochemistry

We have studied the development of synaptic transmission by innervating sympathetic neurons in vitro and monitoring synaptic activity with both physiological recording and cytochrome oxidase histochemistry. The onset of synaptic transmission was reflected in increased cytochrome oxidase reaction product within individual neurons. Within 24 hours of co-culture, relatively low frequency suprathreshold potentials were recorded in approximately 20% of the innervated neurons. At this stage the cytochrome oxidase activity of innervated neurons, as assessed by optical density of the histochemical reaction product, was increased twofold compared with uninnervated neurons. Over the next 2-4 days of innervation, changes in the pattern and extent of synaptic activity and superthreshold events were accompanied by a net fourfold increase in cytochrome oxidase activity levels compared with noninnervated neurons. The increase in density of cytochrome oxidase reaction product observed after innervation was reversed completely by blockade of synaptic transmission. Differences in the efficacy of synaptic input provided to the sympathetic neurons by appropriate versus inappropriate presynaptic sources was determined by co-culturing sympathetic neurons with explants that contained either preganglionic neurons or somatic motor neurons. Although sympathetic neurons innervated by motor neuron explants had increased levels of cytochrome oxidase activity compared with noninnervated controls, the density of cytochrome oxidase reaction product was even greater in sympathetic neurons innervated by preganglionic explants. We conclude that both the onset of innervation of sympathetic neurons as well as the subsequent maturation of synaptic function is directly reflected in graded increases in cytochrome oxidase reaction product.

Substance P modulates single-channel properties of neuronal nicotinic acetylcholine receptors

Substance P (SP) is present in avian sympathetic ganglia and accelerates the decay rate of acetylcholine (ACh)-evoked macroscopic currents in sympathetic neurons. We demonstrate here that SP modulates ACh-elicited single channels in a manner consistent with an enhancement of ACh receptor (AChR) desensitization. Furthermore, since AChR channel function was monitored in cell-attached patches with SP applied to the extra-patch membrane, the peptide must act via a second messenger mechanism. SP specifically decreases the net ACh-activated single-channel current across the patch membrane by decreasing both channel opening frequency and mean open time kinetics. These experiments demonstrate that a peptide can modulate neuronal AChR function by a second messenger mechanism.

Functional properties and developmental regulation of nicotinic acetylcholine receptors on embryonic chicken sympathetic neurons

Measurement of acetylcholine (ACh)-induced currents indicates that the sensitivity of embryonic sympathetic neurons increases following innervation in vivo and in vitro. We have used single-channel recording to assess the contribution of changes in ACh receptor properties to this increase. Early in development (before synaptogenesis), we detect three classes of ACh-activated channels that differ in their conductance and kinetics. Molecular studies indicating a variety of neuronal receptor subunit clones suggest a similar diversity. Later in development (after innervation), changes in functional properties include increases in conductance and apparent mean open time, the addition of a new conductance class, as well as apparent clustering and segregation of channel types. These changes in channel function are compatible with the developmental increase in ACh sensitivity.

Neural regulation of acetylcholine sensitivity in embryonic sympathetic neurons

The development of transmitter sensitivity is an important component of synaptic differentiation. Despite a wealth of information about the appearance of acetylcholine (AcCho) sensitivity at the neuromuscular junction, the onset and regulation of this critical aspect of synaptogenesis has not previously been examined for synapse formation between neurons. To determine whether there is a role of presynaptic input in the induction of AcCho sensitivity at interneuronal synapses, AcCho-induced currents were measured in embryonic sympathetic neurons before and after synapse formation in vitro. The total AcCho sensitivity of postsynaptic neurons was increased nearly 10-fold after innervation. The effects of innervation are mimicked by medium conditioned by preganglionic neurons, suggesting that presynaptic neurons regulate postsynaptic AcCho sensitivity by release of a soluble factor. These observations provide evidence that presynaptic input regulates neuronal sensitivity to an identified synaptic transmitter.

Activators of protein kinase C enhance acetylcholine receptor desensitization in sympathetic ganglion neurons

Recent studies suggest that phosphorylation may regulate the rate of desensitization of nicotinic acetylcholine (AcCho) receptors (AcChoR) in vertebrate muscle and Torpedo. It is not known if phosphorylation is involved in regulation of the neuronal AcChoR, however. In this study we examine the possibility that protein kinase C might regulate nicotinic AcChoR function in neurons. Several activators of protein kinase C (1-oleoyl-2-acetylglycerol, phorbol 12,13-diacetate, and phorbol 12,13-dibutyrate) were tested for their ability to modulate AcChoR function in embryonic chicken sympathetic ganglion neurons. Neurons were voltage-clamped at the resting potential, and the response to AcCho was tested before and after treatment with activators of protein kinase C. We find that all of these agents enhance the rate of decay of AcCho-induced current without affecting peak current amplitude or cellular input resistance. The drugs were ineffective if applied concurrently with AcCho: significant effects could be detected after 60 sec of pretreatment. A phorbol that does not increase protein kinase C activity (4 beta-phorbol) was ineffective in enhancing the decay of AcCho-induced current. Thus, the effects of these agents on AcChoR function are likely to be mediated by their interaction with C kinase, rather than by direct interaction with the AcChoR channel. Our data suggest that kinase C may regulate agonist-induced desensitization of the neuronal AcChoR channel.

Changes in the number of chick ciliary ganglion neuron processes with time in cell culture

The purpose of this study was to describe the shape of chick ciliary ganglion neurons dissociated from embryonic day 8 or 9 ganglia and maintained in vitro. Most of the neurons were multipolar during the first three days after plating, with an average of 6.0 processes extending directly from the cell body. The neurons became unipolar with time. The remaining primary process accounted for greater than 90% of the total neuritic arbor. This striking change in morphology was not due to the selective loss of multipolar cells, or to an obvious decline in the health of apparently intact cells. The retraction of processes was neither prevented nor promoted by the presence of embryonic muscle cells. Process pruning occurred to the same extent and over the same time course whether the cells were plated on a monolayer of embryonic myotubes or on a layer of lysed fibroblasts. Process retraction is not an inevitable consequence of our culture conditions. Motoneurons dissociated from embryonic spinal cords remained multipolar over the same period of time. We conclude that ciliary ganglion neurons breed true in dissociated cell culture in that the multipolar-unipolar transition reflects their normal, in vivo, developmental program.

The distribution of acetylcholine receptor clusters and sites of transmitter release along chick ciliary ganglion neurite-myotube contacts in culture

Acetylcholine receptors accumulate along the length of cholinergic neuron-skeletal muscle contacts in vitro. The main purpose of this study was to describe, in a quantitative way, the distribution of acetylcholine receptor clusters induced by ciliary ganglion neurons over a period of time extending from hours to weeks after contacts are established. Neurites were filled with Lucifer Yellow and receptor clusters were identified with rhodamine-bungarotoxin. A cluster located within 5 micron of a nerve process or 10 micron of the base of a growth cone was considered to be a neurite-associated receptor patch (NARP). The first synaptic potentials were evoked 20 min after growth cone-myotube contact, and, after 24 h of co-culture, greater than 60% of the nerve-muscle pairs tested were functionally connected. NARPs appear rapidly; the first clusters were detected approximately 6 h after the neurons were plated. They were composed of several small subclusters or speckles of rhodamine-bungarotoxin fluorescence. The initial accumulation of receptors may occur at the advancing tips of nerve processes because NARPs were found at greater than 80% of the growth cone-muscle contacts examined between 12 and 24 h of co-culture. Over the 3-wk period examined, the mean incidence of NARPs ranged between 1.0 and 2.6 per 100 micron of neurite-myotube contact, with the peak observed on the second day of co-culture. During the first 3 d in culture, when the neurons were multipolar, nearly all of the primary processes induced one or more clusters. With time, as the neurons become unipolar (Role and Fischbach, 1987) NARPs persisted along the remaining dominant process. Measurements made during the third day of co-culture suggest that NARPs disappear along shorter neurites before they retract. Synaptic currents were detected by focal extracellular recording at 55% of the NARPs. The fact that spontaneous or evoked responses were not recorded at 45% suggests that contacts with clusters exhibit two functional states. Two types of presynaptic specialization at identified NARPs observed by scanning electron microscopy appear to be correlated with the functional state.

On the mechanism of acetylcholine receptor accumulation at newly formed synapses on chick myotubes

We have examined the specificity and the mechanism of acetylcholine receptor (AChR) accumulation at embryonic chick nerve-muscle contacts that form in culture. Spinal cord motoneurons were identified in vitro after labeling them in vivo with Lucifer Yellow-wheat germ agglutinin conjugates. All of their processes induced receptor clusters on contacted myotubes; after 24 to 48 hr of co-culture, the incidence of neurite-associated receptor patches (NARPs) was approximately 1.2/100 microns of contact. In contrast, NARPs were rarely associated with spinal cord interneurons (approximately 0.1/100 microns of contact). Neurons dissociated from ciliary ganglia induce NARPS to the same extent as motoneurons. The relative contribution to NARPs of AChRs present in the membrane prior to plating ciliary ganglion neurons and of "new" AChRs inserted 8, 11, or 17 hr after addition of neurons was assessed with two fluorescent receptor probes. Rhodamine-conjugated alpha-bungarotoxin was used to label either old or new receptors; a monoclonal, anti-receptor antibody visualized with fluorescein-second antibody was used to label all (new and old) receptors. Analysis of digitized fluorescence images showed that NARPs contained both new and old receptors but that within the first 24 hr of co-culture the majority (60 to 80%) were new. We estimate that cholinergic neurites increase the rate of receptor insertion 4- to 5-fold during the first 8 hr of NARP formation. The contribution of new receptors to NARPs declines with time. After 3 days of co-culture, receptors inserted over an 8-hr interval comprised only 20% of the total NARP complement.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P modulation of acetylcholine-induced currents in embryonic chicken sympathetic and ciliary ganglion neurons

Substance P has been identified by combined immunohistochemical and radioimmunological techniques to be present in preganglionic cholinergic and sensory nerve fibers of amphibian, mammalian, and avian autonomic ganglia. The peptide has been shown to depolarize sympathetic neurons of frog and guinea pig and to decrease the cholinergic activation of Na+ influx and catecholamine release from chromaffin cells. The aim of this study was to examine the interaction of acetylcholine and substance P on autonomic neurons. This report demonstrates a direct effect of substance P on acetylcholine-induced inward currents in both sympathetic and parasympathetic neurons clamped near resting membrane potential. Under these conditions, substance P dramatically enhances the rate of decay of the inward current in the continued presence of agonist without substantially affecting peak inward current. This effect is consistent with an enhancement of acetylcholine-receptor desensitization. Since substance P-containing cell bodies have been demonstrated in the avian (preganglionic) column of Terni as well as in fibers from the nucleus of Edinger-Westphal, the observed peptide inhibition of cholinergic activation of the neurons may function physiologically to modulate synaptic function in autonomic ganglia.

Both nicotinic and muscarinic receptors mediate catecholamine secretion by isolated guinea-pig chromaffin cells

We have studied the roles of nicotinic and muscarinic receptors in the acetylcholine-evoked secretion of catecholamine from guinea-pig chromaffin cells. Isolated guinea-pig chromaffin cells secrete catecholamine in response to acetylcholine, nicotine, and a variety of muscarinic agonists. Optimal concentrations of acetylcholine (50-200 microM) induce the release of 10-25% of the catecholamine content of the cells in 10 min. Maximal secretion evoked by nicotine or by muscarinic agonists is 5-12% of the catecholamine content of the cells. Secretion evoked by optimal concentrations of nicotine (50 microM) and muscarine (200 microM) are additive, and together these agonists cause catecholamine release equivalent to that produced by optimal concentrations of acetylcholine. Atropine causes a biphasic inhibition of acetylcholine-induced catecholamine secretion; low concentrations of atropine (0.02-0.01 microM) inhibit by 35-45% the catecholamine secretion evoked by 100 microM acetylcholine. Increasing the atropine concentration from 0.1 to 5 microM causes no further decrease in acetylcholine-evoked release, but at concentrations above 5 microM, a second distinct phase of inhibition appears. At 100 microM, atropine reduces acetylcholine-evoked secretion by 85%. At 0.1 microM, atropine significantly inhibits secretion induced by muscarinic, but not nicotinic, agonists. Tubocurarine (50 microM) does not block muscarinic stimulation of release, but inhibits acetylcholine- and nicotine-evoked release by 70 and 80%, respectively. Our experiments indicate that nicotinic and muscarinic stimulation represent distinct mechanisms for the activation of catecholamine release from guinea-pig chromaffin cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Catecholamine uptake into isolated adrenal chromaffin cells: inhibition of uptake by acetylcholine

We have investigated the process of catecholamine uptake in guinea-pig chromaffin cells. Isolated guinea-pig chromaffin cells accumulate [3H]norepinephrine and [3H]epinephrine by a saturable transport system. Catecholamine uptake is dependent upon temperature, energy, and extracellular Na+. The apparent KmS for norepinephrine and epinephrine transport are approximately 1 and 3.5 microM, respectively; the transport maximum (Vmax) for both compounds is about 100 pmol/min/mg protein. The uptake of norepinephrine into chromaffin cells is inhibited by imipramine (Ki = 50 nM) and by desmethylimipramine (IC50 = 20 nM). In both its substrate specificity and its sensitivity to pharmacological inhibition, the catecholamine uptake system in chromaffin cells is similar to the catecholamine transport system previously described in sympathetic neurons. Decreasing external Na+ from 130 to 19 mM increases the apparent Km for norepinephrine to 2.8 microM. Decreasing external norepinephrine increases the Na+ concentration required for half-maximal transport. Agents that depolarize chromaffin cells, such as acetylcholine and veratridine, significantly inhibit [3H]norepinephrine uptake. This decrease in uptake is due to an increase in the apparent Km for norepinephrine. The inhibition of [3H]norepinephrine uptake by depolarizing agents cannot be accounted for by the preferential release of newly-accumulated [3H]norepinephrine, or by the competitive inhibition of [3H]norepinephrine uptake by secreted catecholamines. The inhibition of catecholamine uptake by depolarizing agents suggests that the transport system may be regulated by the membrane potential. Norepinephrine and epinephrine that are spontaneously released from the adrenal medulla may be recaptured in vivo. The inhibition of transport by acetylcholine may prevent the re-uptake of catecholamine released during the physiological stimulation of secretion.

Acetylcholine release from growth cones detected with patches of acetylcholine receptor-rich membranes

Synaptic potentials can be evoked at nerve-muscle junctions in vitro within minutes after an exploring growth cone contacts a receptive myotube. Functional transmission is also evident in vivo on the time scale of minutes after motor axons enter adjacent myotomes. The ability to release acetylcholine (ACh) may be induced in motor nerve terminals after they contact receptive target cells. Alternately, growth cones may be capable of releasing ACh before contact. To examine the development of ACh release we have used isolated patches of acetylcholine receptor(AChR)-rich membrane as sensitive detectors of ACh. We report here that the growth cones of embryonic chick ciliary ganglion neurones can release ACh, even when the cells are grown in the absence of target myotubes.

Purification of adrenal medullary chromaffin cells by density gradient centrifugation

A method has been developed for the isolation of chromaffin cells from guinea pig adrenal glands. Crude suspensions of adrenal cells are prepared by the digestion of adrenal with collagenase. Chromaffin cells are then purified from these crude suspensions by isopycnic centrifugation through a 5-25% (w/v) gradient of metrizamide. More than 90% of the cells in the preparation are viable chromaffin cells, as judged by trypan blue exclusion and fluorescence histochemistry. A more convenient method for monitoring the purity of chromaffin cell preparations, using neutral red staining, is also described. Chromaffin cells are the primary neutral red staining cell in the adrenal gland, and more than 90% of the purified chromaffin cells stain with this dye. Purified chromaffin cells contain 400 +/- 50 nmol of epinephrine/mg protein (133 +/- 16 nmol/10(6) cells), and secrete epinephrine in response to acetylcholine. At a concentration of 100 microM, acetylcholine causes a 10- to 20-fold increase in the secretion of epinephrine from the cells. The method described in this paper is a useful procedure for the preparation of pure, functional chromaffin cells.

Mechanisms of ionophore-induced catecholamine secretion

A number of carboxylic ionophores stimulate the secretion of norepinephrine from cell suspensions prepared from a transplantable rat pheochromocytoma. The divalent-cation ionophore ionomycin stimulates catecholamine secretion by a mechanism that is dependent upon the presence of extracellular Ca++. It is likely that ionomycin-induced catecholamine secretion results from the ionophore-mediated entry of Ca++ into the cells. The monovalent-cation ionophore monensin stimulates catecholamine secretion by a mechanism that is independent of extracellular Ca++, but is markedly dependent upon extracellular Na+. Monensin probably transports Na+ into the pheochromocytoma cells and increases the intracellular concentration of Na+ in these cells. This rise in intracellular Na+ may cause the release of Ca++ from some intracellular store. Lasalocid stimulates catecholamine secretion by a mechanism that is independent of extracellular Ca++ and is only slightly dependent upon extracellular Na+. The action of lasalocid, in contrast to the actions of ionomycin and monensin, is potentiated by decreased pH. It is likely that lasalocid enters the cells in its uncharged, protonated form. Once inside the cells, lasalocid may promote the release of intracellular Ca++. Alternatively, lasalocid and monensin may stimulate catecholamine secretion by the process which is independent of Ca++. These experiments show that ionophores can stimulate catecholamine secretion by at least three distinct ionic mechanisms.