A cyclic nucleotide-gated conductance in olfactory receptor cilia

Adaptation in chemoreceptor cells. I. Self-adapting backgrounds determine threshold and cause parallel shift of response function

1. The self-adapting effects of chemical backgrounds on the response of primary chemoreceptor cells to superimposed stimuli were studied using lobster (Homarus americanus) NH4 receptor cells. 2. These receptors responded for several seconds to the onset of the backgrounds, and then returned to their initial level of spontaneous activity (usually zero). The strongest response always occurred only during the steepest concentration change; the response then decayed back to zero or to the earlier spontaneous firing level, while the background concentration was still rising, and remained silent during the entire time that the background was maintained constant (20-30 min) 3. Exposure to constant self-adapting backgrounds eliminated the response of NH4 receptor cells to stimuli of concentration lower than the background, and reduced the responses to all higher stimulus concentrations tested by a nearly equal amount. This resulted in a parallel shift of the stimulus-response function to the right along the abscissa. 4. Since the response threshold was completely re-set by adaptation to backgrounds, NH4 receptors seem to function mostly as detectors of relative rather than absolute stimulus intensity across their entire dynamic range: the response to a given stimulus-to-background ratio remained the same over 3 log step increases of background concentration. 5. As in other sensory modalities, a parallel shift of response functions appears to be an important property of chemoreceptor cells, allowing for this sensory system to function over a wider stimulus intensity range than the instantaneous dynamic range of individual receptor cells.

Amiloride-blockable sodium currents in isolated taste receptor cells

Isolated taste receptor cells from the frog tongue were investigated under whole-cell patch-clamp conditions. With the cytosolic potential held at -80 mV, more than 50% of the cells had a stationary inward Na current of 10 to 700 pA in Ringer's solution. This current was in some cells partially, in others completely, blockable by low concentrations of amiloride. With 110 mM Na in the external and 10 mM Na in the internal solution, the inhibition constant of amiloride was (at -80 mV) near 0.3 microM. In some cells the amiloride-sensitive conductance was Na specific; in others it passed both Na and K. The Na/K selectivity (estimated from reversal potentials) varied between 1 and 100. The blockability by small concentrations of amiloride resembled that of channels found in some Na-absorbing epithelia, but the channels of taste cells showed a surprisingly large range of ionic specificities. Receptor cells, which in situ express these channels in their apical membrane, may be competent to detect the taste quality "salty." The same cells also express TTX-blockable voltage-gated Na channels.

Odorant response of isolated olfactory receptor cells is blocked by amiloride

Olfactory receptor cells were isolated from the nasal mucosa of Rana esculenta and patch clamped. Best results were obtained with free-floating cells showing ciliary movement. 1) On-cell mode: Current records were obtained for up to 50 min. Under control conditions they showed only occasional action potentials. The odorants cineole, amyl acetate and isobutyl methoxypyrazine were applied in saline by prolonged superfusion. At 500 nanomolar they elicited periodic bursts of current transients arising from cellular action potentials. The response was rapidly, fully and reversibly blocked by 50 microM amiloride added to the odorant solution. With 10 microM amiloride, the response to odorants was only partially abolished. 2) Whole-cell mode: Following breakage of the patch, the odorant response was lost within 5 to 15 min. Prior to this, odorants evoked a series of slow transient depolarizations (0.1/sec, 45 mV peak to peak) which reached threshold and thus elicited the periodic discharge of action potentials. These slow depolarizing waves were reversibly blocked by amiloride, which stabilized the membrane voltage between -80 and -90 mV. We conclude that amiloride inhibits chemosensory transduction of olfactory receptor cells, probably by blocking inward current pathways which open in response to odorants.

Ion channels from chemosensory olfactory neurons

The olfactory epithelium has the ability to respond to a large number of volatile compounds of small molecular weight. Ultimately, such a property lies on a specialized type of neuron, the olfactory receptor cell. In the presence of odorants, the olfactory receptor neuron responds with action potentials whose frequency depends on odorant concentration. The primary events in the process of olfactory transduction are thought to occur at the cilia of olfactory receptor neurons and involve the binding of odorants to receptor molecules followed by the opening of ion channels. A crucial step in understanding olfactory transduction requires identifying the mechanisms that regulate the electrical activity of olfactory cells. In the last couple of years, patch-clamp recording from isolated olfactory cells and reconstitution of olfactory membranes in planar lipid bilayers have begun to shed light on some of these mechanisms. Although the information emerging from such studies is still preliminary, there are already well-defined hypotheses on the molecular events that might underlie the primary events in olfactory transduction. Currently, attention is being focused on the notions that second messengers might be involved in the activation of ion channels in olfactory cilia, and that odorant binding to a receptor molecule might lead directly to the gating of ion channels in chemosensory olfactory membranes. The coming years promise to be exciting ones in the field of olfactory transduction. We have now the necessary tools to be able to confront hypotheses and experimental facts.

Ion transport across the frog olfactory mucosa: the action of cyclic nucleotides on the basal and odorant-stimulated states

The action of cyclic nucleotides on the short-circuit current across the isolated bullfrog olfactory mucosa was studied both in the absence and presence of odorants. 8-Bromo-cAMP applied to the ciliated side of the mucosa caused a concentration-dependent, reversible increase in the basal short-circuit current, but not when it was applied to the submucosal side. The current had a sigmoidal concentration dependence described by the Hill equation. The magnitude of the odorant-evoked current was enhanced after bathing the ciliated side with cAMP analogs or modulators of intracellular cAMP. GTP gamma S added to the ciliated side increased the odorant-evoked current, while GDP beta S caused a decrease. Current transients induced by stimulating the ciliated side with either pulses of odorant or 8-bromo-cAMP were partially suppressed by amiloride, but only when amiloride and stimulant were presented simultaneously. Pulses of 8-bromo-cAMP and odorant presented simultaneously resulted in currents that added nonlinearly. In the absence of odorant, 8-bromo-cGMP caused a concentration-dependent decrease in net inward current that was reversed by 8-bromo-cAMP. Odorant-evoked currents were also reduced by 8-bromo-cGMP, and these could not be reversed by 8-bromo-cAMP. The results indicate that one type of olfactory transduction process involves the activation by cAMP of an inward current through an amiloride-sensitive apical ion channel and that this mechanism is mediated by a stimulatory G-protein.

Functional reconstitution of the olfactory membrane: incorporation of the olfactory adenylate cyclase in liposomes

Chemosensory cilia isolated from the olfactory epithelium of Rana catesbeiana were solubilized with Lubrol PX in the presence of supplementary lipid, forskolin, and sodium fluoride. Subsequent removal of the detergent by adsorption onto Biobeads SM2 results in the formation of proteoliposomes that display forskolin- and GTP gamma S-sensitive adenylate cyclase activity. Sucrose gradient centrifugation of liposomes formed in the presence of fluorescently labeled phosphatidylcholine demonstrates association between the olfactory adenylate cyclase and the exogenously added lipid. Forskolin stimulates the enzyme in reconstituted membranes with the same potency as in native membranes (EC50 = 1-2 microM). However, GTP gamma S is 350-fold more potent in native membranes (EC50 = 4.0 +/- 0.5 nM) than in reconstituted membranes (EC50 = 1.4 +/- 0.3 microM). These studies represent a first step toward the functional reconstitution and molecular dissection of the olfactory membrane.

Large olfactory responses of the carp after complete removal of olfactory cilia

To study the role of olfactory cilia on olfactory reception, the carp olfactory cilia were removed by modified "ethanol-calcium shock" and the bulbar responses were recorded before and after deciliation. Large olfactory responses to various amino acids were observed after complete deciliation. The relation between magnitude of olfactory response and alanine concentration before and after deciliation was essentially unchanged. The present results suggests that the olfactory cilia may not be necessary for receptor neuron function in the carp.

[Functional guanine nucleotide-binding proteins in receptor-mediated modulation of voltage-dependent ion channels]

G-proteins act as transducers between cell surface receptors activated by extracellular signals and enzymatic effectors which control the concentrations of cytosolic signal molecules such as cAMP, cGMP, inositol phosphates and calcium. The receptor/G-protein-induced changes of the intracellular concentration of such signal molecules correlates with activity changes of various voltage-dependent ion channels. In some instances, cytosolic signal molecules appear to interact directly with ion channels, thereby causing an alteration of ion channel activity. In other instances, signal molecules affect the function of ion channels by activating protein kinases which, in turn, phosphorylate either proteins constituting extracellular signal- and voltage-dependent ion channels or non-identified membranous regulatory components. Recent findings suggest a third, membrane-confined mechanism which does not involve cytosolic signal molecules but a close control of voltage-dependent ion channels by G-proteins. Ion channels that are modulated by extracellular signals according to this newly discovered principle include those for calcium and potassium in neuronal, cardiac and endocrine cells. G-proteins involved in the hormonal stimulation of potassium and calcium channels belong to the family of Gi-type G-proteins which are functionally uncoupled from activating receptors by pertussis toxin. In addition, the cholera toxin-sensitive G-protein, Gs, may directly stimulate cardiac calcium channels. Hormonal inhibition of calcium channels is possibly mediated by Go which, like G-proteins of the Gi-family, is functionally impaired by pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Microinjection of catalytic subunit of cyclic AMP-dependent protein kinase triggers acute morphological changes in thyroid epithelial cells

In dog thyroid epithelial cells in primary culture, thyrotropin acting through cyclic AMP induced rapid morphological changes associated with complete disruption of actin containing stress fibers. This modification preceded cell retraction and rounding up. These morphological effects were also induced by glass capillary microinjection of purified catalytic subunit of cAMP-dependent protein kinase. This provides the first direct evidence in intact cells that catalytic subunit, which is released upon activation of cAMP-dependent protein kinases, is responsible for cAMP-dependent morphological transformation.

Vasoactive intestinal peptide activates Ca2(+)-dependent K+ channels through a cAMP pathway in mouse lacrimal cells

The action of vasoactive intestinal peptide (VIP) on Ca2(+)-dependent K+ currents, in dissociated mouse lacrimal cells, was investigated using patch clamp techniques. In whole cell recordings, VIP (10-100 pM) increased the magnitude of the Ca2(+)-dependent K+ current. In single channel recordings, VIP increased the fraction of time the large charybdotoxin-sensitive Ca2(+)-activated K+ channel spent in the open state. The activity of this channel was also increased by adding forskolin or 8-bromo cAMP to the bath. Additionally, application of either cAMP or catalytic subunit of cAMP-dependent protein kinase directly to the cytoplasmic surface of excised inside out patches reversibly lengthened the time Ca2(+)-activated K+ channels spent in the open state. These data suggest that VIP stimulates Ca2(+)-activated K+ channels by a cAMP-dependent pathway in mouse lacrimal acinar cells.

Activation by odorants of a multistate cation channel from olfactory cilia

Single-channel records were obtained after fusion of ciliary membranes from the olfactory epithelium of Rana catesbeiana to planar lipid bilayers, and odorant-activated cation-selective channels were identified. In addition, a 190-pS potassium-selective channel and a 40-pS cation-selective channel were found in a 0.2 M salt-containing buffer. Odorant-sensitive channels were directly and reversibly activated by nanomolar concentrations of the bell pepper odorant 3-isobutyl-2-methoxypyrazine and the citrus odorant 3,7,-dimethyl-2,6-octadienenitrile. These channels display burst kinetics, multiple conductance levels between 35 and 420 pS, and open times in the millisecond range. With increasing concentrations of odorant, the probability of populating the higher conductance levels increases. These results show that direct activation of channels by odorants may mediate excitation of the olfactory receptor cell.

Transduction in taste receptor cells requires cAMP-dependent protein kinase

In taste chemoreception, cyclic adenosine monophosphate (cAMP) appears to be one of the intracellular messengers coupling reception of stimulus to the generation of the response. The recent finding that sweet agents cause a GTP-dependent generation of cAMP poses the question of how this cytosolic messenger acts at the membrane of taste receptor cells. We have shown that cAMP causes a substantial depolarization in these cells. Here we show with whole-cell recordings and inside-out membrane patches that the depolarization caused by cAMP is accounted for by the action of cAMP-dependent protein kinase, which inactivates potassium channels predominantly of 44 pS conductance. Thus, intracellular signalling of the gustatory cells differs from that of olfactory and photoreceptor cells, where cyclic nucleotides control unspecific channels by binding to them rather than by inducing their phosphorylation.

Cyclic nucleotides may mediate taste transduction

Taste stimulus adsorption is believed to occur at the taste cell microvillous membrane. But due to technical difficulties of inserting glass electrodes into the mammalian taste cell, little is known about the mechanisms of taste transduction. Reliable intracellular recordings are necessary to determine the characteristics of taste cells. This has been accomplished previously in the mouse and is reported here. Recent experiments indicated that cyclic nucleotides can act on the inner surface of the membranes of a variety of cells to alter their ion-channel activity, and these substances might act as intracellular transmitters in taste cells. But tight junctions found at the apical membrane of mammalian taste cells do not allow stimuli to enter the taste bud, making it difficult to alter the environment of the taste cell by perfusing with chemical solutions. Here we report that cyclic AMP, cyclic GMP, EGTA or tetraethyl-ammonium electrophoretically injected into the mouse taste cell induce membrane depolarization and increased membrane resistance. These results suggest that a cyclic nucleotide enzymatic cascade, modulated by calcium ions, may mediate the potassium permeability that controls taste, in a way analogous to visual and olfactory transduction.

Pre-natal development of rat nasal epithelia. IV. Freeze-fracturing on apices, microvilli and primary and secondary cilia of olfactory and respiratory epithelial cells, and on olfactory axons

Olfactory axons and apical structures of olfactory epithelia and of nasal respiratory epithelia of rat embryos were studied with the freeze-fracture technique; adult tissue samples of the same sources were used for comparison. At the onset of epithelial differentiation (14th gestational day) intramembranous particle densities are the same for all structures in both epithelial types. During further development, particle densities in membranes of primary cilia remain lower than those in membranes of other apical structures. Otherwise, I found the following from the 14th to the 19th day of gestation. a. Olfactory receptor cells of embryos of all age groups have axons wherein the membrane particle densities are about half those of adults. These densities are always lower than those of dendritic ending structures. Dendritic endings with primary cilia have lower densities than endings with secondary cilia; densities mainly increase when the endings sprout secondary cilia. Adult values are reached at the 18th day of gestation. b. Olfactory supporting cells with only globular particles in their apices gradually transform into, or are replaced by, supporting cells which also have dumbbell-shaped particles in their apices. Particle densities are always higher in apical structures of supporting cells than in apical structures of receptor cells. Adult values are reached at the 17th day of gestation. c. Putative ciliated and ciliated respiratory epithelial cells have considerably lower particle densities in membranes of their apical structures than do olfactory epithelial cells. Of special interest is that this is also true for secondary respiratory and olfactory cilia; as soon as genesis of secondary cilia in either epithelial type begins, their membrane features differ. Also, in contrast to apical structures of the olfactory epithelium, particle densities in apical structures of the respiratory epithelium do not systematically change during pre-natal development, and resemble the density values of adults. An exception are the microvilli of the respiratory cells with secondary cilia, membranes of which have considerably higher particle densities in adults than in embryos.

IN CONCLUSION

Transformations of olfactory receptor cell dendritic endings with primary cilia into endings with secondary cilia, and of olfactory supporting cells with globular particles in their apices into cells with dumbbell-shaped particles in their apices are accompanied by increases in the densities of their intramembranous particles. These developmental changes parallel the electrophysiological onset of olfactory receptor cell specificity.

Olfaction by melanophores: what does it mean?

Hypotheses on general olfaction can be divided into two broad groups: those that predict the existence of olfactory-specific olfactory receptor proteins and those that do not. Recently, much attention has been paid to the discovery of an odorant-stimulated adenylate cyclase in purified olfactory cilia. This finding has, for the most part, been accepted as evidence that the former hypotheses are correct. Here we report that frog melanophores, which are nonolfactory in nature, disperse their melanosomes in response to the same types and concentrations of odorants used in the investigations of olfactory cilia and that pigment dispersion is accompanied by rises in intracellular cAMP levels. The effects show that the existence of a cAMP-based second messenger system in olfactory cilia is not in itself proof of the existence of olfactory-specific olfactory receptor proteins. Also they explain the basis of Ottoson's pioneering work of 30 years ago on the electrical responses of frog olfactory epithelium to stimulation with alcohols. The results suggest that there could be two mechanisms that are important for the detection of odorants: one based on specific receptors, the other nonspecific, but both working through activation of cAMP.

Depolarization induced by injection of cyclic nucleotides into frog taste cell

In order to identify the intracellular transmitter involved in the taste transduction process, cyclic nucleotides were iontophoretically injected into the frog taste cells while membrane potentials were recorded intracellularly. Injection of either cyclic GMP or cyclic AMP induced a depolarization response of about 5 mV in the taste cells, but injection of Cl- had no effect. The rate of a repolarization after the depolarization elicited by cyclic GMP was larger than that after cyclic AMP. The possible role of cyclic nucleotide in the taste transduction was discussed.

Cyclic AMP functions as a primary sexual signal in gametes of Chlamydomonas reinhardtii

When Chlamydomonas reinhardtii gametes of opposite mating type are mixed together, they adhere by a flagella-mediated agglutination that triggers three rapid mating responses: flagellar tip activation, cell wall loss, and mating structure activation accompanied by actin polymerization. Here we show that a transient 10-fold elevation of intracellular cAMP levels is also triggered by sexual agglutination. We further show that gametes of a single mating type can be induced to undergo all three mating responses when presented with exogenous dibutyryl-cAMP (db-cAMP). These events are also induced by cyclic nucleotide phosphodiesterase inhibitors, which elevate endogenous cAMP levels and act synergistically with db-cAMP. Non-agglutinating mutants of opposite mating type will fuse efficiently in the presence of db-cAMP. No activation of mating events is induced by calcium plus ionophores, 8-bromo-cGMP, dibutyryl-cGMP, nigericin at alkaline pH, phorbol esters, or forskolin. H-8, an inhibitor of cyclic nucleotide-dependent protein kinase, inhibits mating events in agglutinating cells and antagonizes the effects of cAMP on non-agglutinating cells. Adenylate cyclase activity was detected in both the gamete cell body and flagella, with the highest specific activity displayed in flagellar membrane fractions. The flagellar membrane adenylate cyclase is preferentially stimulated by Mn++, unresponsive to NaF, GTP, GTP gamma S, AlF4-, and forskolin, and is inhibited by trifluoperazine. Cyclic nucleotide phosphodiesterase activity is also present in flagella. Our observations indicate that cAMP is a sufficient initial signal for all of the known mating reaction events in C. reinhardtii, and suggest that the flagellar cyclase and/or phosphodiesterase may be important loci of control for the agglutination-stimulated production of this signal.

Cyclic AMP regulates the proportion of functional acetylcholine receptors on chicken ciliary ganglion neurons

Previous studies have shown that the number of functional acetylcholine receptors (AcChoRs) on chicken ciliary ganglion neurons in culture is considerably smaller than the total number of AcChoRs detected on the neurons by labeled receptor probes. Here we use patch-clamp recording to show that a cAMP-dependent process enhances the AcCho response of the neurons by a mechanism likely to involve an increase in the number of functional AcChoRs. The increase occurs without requiring protein synthesis and without involving a detectable increase in the total number of AcChoRs on the cell surface measured with a labeled receptor probe. The results imply that the neurons have functional and nonfunctional pools of AcChoRs and that functional receptors can be recruited from intracellular receptors or from nonfunctional receptors on the cell surface by a cAMP-dependent process. A cAMP-dependent regulation of the number of functional neurotransmitter receptors would provide a reversible mechanism by which cell-cell interactions could modulate synaptic transmission in the nervous system.

Patch-clamp study of isolated taste receptor cells of the frog

Taste discs were dissected from the tongue of R. ridibunda and their cells dissociated by a collagenase/low Ca/mechanical agitation protocol. The resulting cell suspension contained globular epithelial cells and, in smaller number, taste receptor cells. These were identified by staining properties and by their preserved apical process, the tip of which often remained attached to an epithelial (associated) cell. When the patch pipette contained 110 mM KCl and the cells were superfused with NaCl Ringer's during whole-cell recording, the mean zero-current potential of 22 taste receptor cells was -65.2 mV and the slope resistance 150 to 750 M omega. Pulse-depolarization from a holding voltage of -80 mV activated a transient TTX-blockable inward Na current. Activation became noticeable at -25 mV and was half-maximal at -8 mV. Steady-state inactivation was half-maximal at -67 mV and complete at -50 mV. Peak Na current averaged -0.5 nA/cell. The Ca-ionophore A23187 shifted the activation and inactivation curve to more negative voltages. Similar shifts occurred when the pipette Ca was raised. External Ni (5 mM) shifted the activation curve towards positive voltages by 10 mV. Pulse depolarization also activated outward K currents. Activation was slower than that of Na current and inactivation slower still. External TEA (7.5 mM) and 4-amino-pyridine (1 mM) did not block, but 5 mM Ba blocked the K currents. K-tail currents were seen on termination of depolarizing voltage pulses. A23187 shifted the IK(V)-curve to more negative voltages. Action potentials were recorded when passing pulses of depolarizing outward current. Of the frog gustatory stimulants, 10 mM Ca caused a reversible 5- to 10-mV depolarization in the current-clamp mode. Quinine (0.1 mM, bitter) produced a reversible depolarization accompanied by a full block of Na current and, with slower time-course, a partial block of K currents. Cyclic AMP (5 mM in the external solution or 0.5 microM in the pipette) caused reversible depolarization (to -40 to -20 mV) due to partial blockage of K currents, but only if ATP was added to the pipette solution. Similar responses were elicited by stimulating the adenylate cyclase with forskolin. Blockage of cAMP-phosphodiesterase enhanced the response to cAMP. These results suggest that cAMP may be one of the cytosolic messengers in taste receptor cells. Replacement of ATP by AMP-PNP in the pipette abolished the depolarizing response to cAMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct ion channel gating: a new function for intracellular messengers

1. There is widespread belief that intracellular messengers [e.g., Ca2+, cyclic AMP, cyclic GMP, inositol-1,4,5-triphosphate (IP3)] assert their actions primarily through activation of protein kinases. 2. In studies of excitable cells protein kinase activation has been shown to alter membrane ionic conductance, presumably through phosphorylation of ion channels (see Levitan, 1985). However, recent reports from several laboratories indicate that intracellular messengers can also affect membrane ionic conductances directly without invoking protein kinase activation. 3. In this article we examine those examples of direct activation of ionic conductances by intracellular messengers which are supported by single-channel studies of isolated membrane patches. The list of cell types displaying this kind of response is growing and includes cells of neuronal as well as nonneuronal origin.

Ionic regulation of adenylate cyclase from the cilia of Paramecium tetraurelia

The kinetics of the ionic regulation of an adenylate cyclase associated with the excitable ciliary membrane from Paramecium tetraurelia was examined. Glycerol (30%, v/v) stabilized the enzyme, and activated by an increase in Vmax. (3-fold) and a decrease in the apparent Km for MgATP (6-fold). Kinetic analysis of Mg2+ effects showed a stimulation via a single metal-binding site separate from the substrate site, with a dissociation constant, Ks, of 0.27 mM. Analysis of Ca2+ effects showed (i) an uncompetitive inhibition with respect to substrate MgATP, and (ii) dependence of the extent of inhibition on the free Mg2+ concentration. Ki values ranged from 4 to 130 microM-Ca2+ in the presence of 0.55-2 mM-Mg2+ respectively. This indicates competition between Mg2+ and Ca2+ at the metal-binding site. The Ca2+ effect was specific; Sr2+ and Ba2+ were almost without effect, and 100 microM-Ba2+ did not interfere with the Ca2+ inhibition. The actions of Ca2+ were readily reversible after addition of EGTA. K+ activated the adenylate cyclase at concentrations around 20 mM. The stimulatory potency of K+ was dependent on the free Mg2+ concentration. At 1 mM free Mg2+, 20 mM-K+ doubled the adenylate cyclase activity. The inhibitory Ca2+ and stimulatory K+ inputs were independent of each other.

Patch-clamp studies of isolated mouse olfactory receptor neurons

Olfactory receptor neurons isolated from embryonic, neonatal, and adult mice were studied using the patch-clamp technique. Several distinct types of ion channels were characterized in patches of membrane from the neuronal soma and the dendritic knob of receptor neurons, including a 130-pS Ca++-activated K+ channel with voltage-dependent kinetics, an 80-pS Ca++-activated K+ channel with voltage-insensitive kinetics, a 25-pS K+ channel with properties similar to inward rectifiers, and a 40-pS K+ channel that was activated and then inactivated by rapid depolarization. Evidence of large-conductance (greater than 200 pS) Cl- channels, which were Ca++ insensitive and increasingly active at depolarizing membrane potentials, and voltage-activated Ca++ channels (16 pS) was also obtained. From K+ channel activity recorded from cell-attached patches, the intracellular [Ca++] was inferred to be below 0.1 microM, and the membrane potential was inferred to be approximately -50 mV. The receptor neurons had high input resistances, and action potentials could be elicited by picoampere amounts of depolarizing current. The receptor neurons responded to applied odorant molecules and to forskolin with increases in membrane conductance. These results provide a description of the membrane properties of olfactory receptor neurons and a basis for understanding their electrical activity and response to odorants.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.