Lobster Olfactory Receptor Cells Research Articles

Organizational Complexity in Lobster Olfactory Receptor Cellsa

The current working model of transduction in lobster olfactory receptor cells suggests that: (1) inositol-1,4,5-triphosphate (IP3) is the excitatory olfactory second messenger in these cells; (2) activation of the cell also involves a secondary, current-carrying channel; and (3) the phosphoinositol pathway works in parallel to a second, cyclic nucleotide-mediated signaling pathway that provides input of opposite polarity into the cell. The complexity of intracellular signaling in lobster olfactory receptor cells renders the cells capable of fine tuning, and even integrating, the signal they send to the brain.

Presynaptic afferent inhibition of lobster olfactory receptor cells: reduced action-potential propagation into axon terminals.

Action-potential propagation into the axon terminals of olfactory receptor cells was measured with the use of voltage-sensitive dye imaging in the isolated spiny lobster brain. Conditioning shocks to the olfactory nerve, known to cause long-lasting suppression of olfactory lobe neurons, allowed the selective imaging of activity in receptor cell axon terminals. In normal saline the optical signal from axon terminals evoked by a test stimulus was brief (40 ms) and small in amplitude. In the presence of low-Ca2+/high-Mg2+ saline designed to reduce synaptic transmission, the test response was unchanged in time course but increased significantly in amplitude (57 +/- 16%, means +/- SE). This increase suggests that propagation into receptor cell axon terminals is normally suppressed after a conditioning shock; this suppression is presumably synaptically mediated. Thus our results show that presynaptic inhibition occurs at the first synapse in the olfactory pathway and that the inhibition is mediated, at least in part, via suppression of action-potential propagation into the presynaptic terminal.

Temporal resolution in olfaction: stimulus integration time of lobster chemoreceptor cells

The stimulus integration time of lobster olfactory receptor cells in situ was determined using extracellularly recorded spiking responses from receptor cells and on-line high-resolution measurement of odor square pulses. At a fixed odor concentration, odor steps of 200 ms duration elicited maximum responses; shorter odor steps did not drive the cells to their maximum response and longer odor steps added spikes but did not result in higher firing rates. Excitatory processes peaked within 220 ms of stimulus onset. At 160­300 ms, stimulus intensity discrimination was most evident. Adaptation processes reduced response magnitude to near-zero levels within 1000 ms of stimulus onset. Olfactory receptor cells thus resolve odor peak onsets within the first few hundred milliseconds: this time window corresponds to the 4­5 Hz frequency of olfactory sampling (i.e. 'sniffing') as well as the rapid fluctuations in odor concentration that are common in natural odor plumes. The stimulus integration time of 200 ms may play a role in the filtering of information used by lobsters to orient to distant odor sources.

Single odors differentially stimulate dual second messenger pathways in lobster olfactory receptor cells

Quench-flow measurements are used to determine the subsecond kinetics of odor-induced changes in second messenger concentrations in lobster olfactory receptor neurons. Individual odors transiently and differentially increase the production of both adenosine cAMP and inositol 1,4,5-trisphosphate (IP3) within 50 msec of odor stimulation. The ability of two different odors to stimulate cAMP and IP3 correlates with the odors' ability to excite and inhibit receptor cells physiologically. These results strengthen the proposition, heretofore based largely on evidence from cultured cells, that dual second messenger pathways mediate excitatory and inhibitory input to lobster olfactory receptor cells.

Odor-evoked inhibition in primary olfactory receptor neurons.

Odors can inhibit as well as excite lobster olfactory receptor cells. Inhibitory components of an odor mixture act within the normal, first 500 ms odor sampling interval of the animal to reduce the peak magnitude and increase the latency of the net excitatory receptor potential in a concentration-dependent manner. The intracellular effects are reflected in the propagated output of the cell. The results argue that inhibitory odor input is functional in olfaction by potentially serving to increase the diversity of the neuronal patterns that are thought to be the basis of odor discrimination.

Signal transmission in lobster olfactory receptor cells: Functional significance of electrotonic structure analysed by a compartmental model

The electrotonic structure of lobster olfactory receptor cells was evaluated using general purpose simulation software in a compartmental model derived from electron-microscopic reconstruction. The model with non-uniform membrane resistance (R m) was used to (i) simulate current spread and (ii) determine if the electronic structure of the cell improves signal recognition in the soma. The odor-evoked conductance change in dendrites was calculated according to the Michaelis-Menten equation with the assumption that the outer dendritic segments function as independent stimulus detectors. The inflection point of the concentration-response function measured in the soma was shifted to lower concentrations relative to that measured in the ciliary (outer dendritic) arbor. The shift, which was greater for inputs with lower efficacy (represented in the model by smaller Hill coefficients) and for the dynamic phase of the response than for the steady-state phase, effectively increased the selectivity of the somatic response. Randomized input distributed uniformly to progressively more restricted areas of the ciliary arbor showed that stimulation of larger areas (presumably the entire ciliary arbor) decreased the statistical variability of the somatic response.

Cyclic nucleotides mediate an odor-evoked potassium conductance in lobster olfactory receptor cells

Odors activate at least two distinct transduction pathways in lobster olfactory receptor cells that, respectively, excite and inhibit the cell. Data presented suggest that odors selectively activate the inhibitory conductance through the second messenger cAMP. Not all cells support both odor-evoked excitatory and inhibitory conductances; in the current investigation, about 50% of the cells tested were inhibited by odors. In the majority of cells that, as a group, support an inhibitory response to odor stimulation, activation of adenylate cyclase with forskolin or inhibition of phosphodiesterase activity with 3-isobutyl-1-methylxanthine (IBMX) elicits an outward current with a time course similar to that of odor-evoked outward currents. The membrane-permeant cyclic nucleotide analogs 8-Br-cAMP and 8-Br-cGMP have a similar effect. Forskolin and IBMX enhance the magnitude of odor-evoked outward currents when the drug and the odor are copresented to the cell. In contrast, these same drugs have little or no effect on cells that, as a group, fail to support an inhibitory response to odor stimulation. This study provides the first direct evidence implicating cAMP in olfactory transduction in an invertebrate and contrasts with similar studies in vertebrates that have implicated cAMP as a second messenger mediating excitation.

Physiological and pharmacological evidence for histamine as a neurotransmitter in the olfactory CNS of the spiny lobster

Perfusing histamine (HA, 0.1 μM) into the brain of the spiny lobster reversibly altered the spontaneous activity in 24 (86%) of 28 morphologically unidentified, odor-responsive interneurons. The effects of HA were dose-dependent and could be selectively and reversibly antagonized by cimetidine, a vertebrate H2 antagonist, suggesting that the action of HA in the central nervous system (CNS) was mediated by a receptor pharmacologically similar to an HA receptor expressed by lobster olfactory receptor cells. Perfusing HA into the brain also reversibly altered the spontaneous and/or odor-evoked activity of 6 (67%) of 9 morphologically identified, odor responsive interneurons that arborized in the olfactory lobe (OL). These results extend previous evidence from our lab that the OL contains HA-immunoreactive interneurons and that OL tissue can synthesize HA from its precursor and further implicate HA as a putative neurotransmitter in the olfactory CNS of the spiny lobster.

Inhibition of lobster olfactory receptor cells by an odor-activated potassium conductance

1. Whole cell current-clamp recordings show that odors not only depolarize but may also hyperpolarize lobster olfactory receptor cells. Odor-evoked hyperpolarizations occurred in 36% of 178 receptor cells examined. Cell-attached recordings of action potentials followed by current-clamp recordings in the same cell indicate that depolarizing and hyperpolarizing responses were associated with increases (excitation) and decreases (inhibition) in action potential frequency, respectively. Since odorants that hyperpolarized one receptor cell depolarized other cells and since individual cells may be both excited and inhibited, the inhibitory and excitatory nature of the response must be conferred by the odorant-receptor and transduction processes expressed by the receptor cell. 2. The input resistance dropped from 1.73 G omega at rest to 1.45 G omega during odor-evoked hyperpolarization, and the membrane time constant correspondingly decreased from 114 to 61 ms. The increased conductance persisted throughout the stimulation period (5 s). 3. Shifting the K+ reversal to a more negative potential by lowering the [K+]o from 14 to 2.8 mM increased the magnitude of hyperpolarization. The hyperpolarization could be reversibly blocked by dendritic treatment with 5-10 mM 4-aminopyridine (4-AP) or 10 mM cesium ion, but not by 10 mM tetraethylammonium (TEA). 4. Substituting 80% of the [Cl-]o with NO3- increased the amplitude of the hyperpolarization. Based on a calculated equilibrium potential of -32 mV for chloride, an increase in chloride conductance in a low [Cl-]o environment should have decreased the magnitude of the response. Presumably the change in [Cl-]o acts through the dendritic steady-state chloride conductance to shift the membrane potential further from the reversal potential for K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Whole cell recording from lobster olfactory receptor cells: multiple ionic bases for the receptor potential

Depolarizing receptor potentials in lobster olfactory receptor cells evoked by complex stimuli are associated with an increase in membrane conductance, but do not share the same ionic requirements. These findings are consistent with the hypothesis that odors activate more than one type of excitatory conductance in primary chemosensory neurons.

Ionic currents and ion channels of lobster olfactory receptor neurons.

The role of the soma of spiny lobster olfactory receptor cells in generating odor-evoked electrical signals was investigated by studying the ion channels and macroscopic currents of the soma. Four ionic currents; a tetrodotoxin-sensitive Na+ current, a Ca++ current, a Ca(++)-activated K+ current, and a delayed rectifier K+ current, were isolated by application of specific blocking agents. The Na+ and Ca++ currents began to activate at -40 to -30 mV, while the K+ currents began to activate at -30 to -20 mV. The size of the Na+ current was related to the presence of a remnant of a neurite, presumably an axon, and not to the size of the soma. No voltage-dependent inward currents were observed at potentials below those activating the Na+ current, suggesting that receptor potentials spread passively through the soma to generate action potentials in the axon of this cell. Steady-state inactivation of the Na+ current was half-maximal at -40 mV. Recovery from inactivation was a single exponential function that was half-maximal at 1.7 ms at room temperature. The K+ currents were much larger than the inward currents and probably underlie the outward rectification observed in this cell. The delayed rectifier K+ current was reduced by GTP-gamma-S and AIF-4, agents which activate GTP-binding proteins. The channels described were a 215-pS Ca(++)-activated K+ channel, a 9.7-pS delayed rectifier K+ channel, and a 35-pS voltage-independent Cl- channel. The Cl- channel provides a constant leak conductance that may be important in stabilizing the membrane potential of the cell.

Histamine-induced modulation of olfactory receptor neurones in two species of lobster, Panulirus argus and Homarus americanus.

In two species of lobster, application of the biogenic amine, histamine (HA), to the soma of olfactory receptor cells suppressed both spontaneous and odour-evoked activity, as shown by electrophysiological recording from single cells. The action of HA was graded, reversible, specific to HA, and had a threshold between 0.1 and 1 mumol l-1. HA increased the conductance of the membrane, primarily to chloride ions. The vertebrate HA receptor antagonist, cimetidine, and the nicotinic receptor antagonist, d-tubocurarine, but not other known vertebrate HA receptor antagonists, reversibly blocked the action of HA. These results suggest that a histaminergic mechanism modulates stimulus-response coupling in lobster olfactory receptor cells and potentially implicate a novel HA receptor, pharmacologically similar to the one recently described in the visual system of flies.

Whole cell recording from lobster olfactory receptor cells: responses to current and odor stimulation.

1. The basic electrical properties of olfactory (antennule) receptor cells were studied in an in situ preparation of the spiny lobster using whole cell patch-clamp recording. 2. The current-voltage relationship of the cells was linear for membrane potentials between -150 and -40 mV and rectified at more positive membrane potentials. The input resistance at rest averaged 508 M omega. The cells displayed two time constants, with mean values of 29.8 and 8.2 ms. 3. Depolarizing current steps elicited fast, overshooting action potentials at a mean threshold of -32 mV from an imposed resting membrane potential of -65 mV. The action potentials were tetrodotoxin (TTX) and tetraethylammonium (TEA) sensitive, suggesting they are typical sodium/potassium action potentials. 4. Odor stimulation evoked slow, dose-dependent, depolarizing receptor potentials up to 50 mV in amplitude. In approximately 30% of cells tested, these led to repetitive spiking when the cells were depolarized beyond -45 to -30 mV. The amplitude of the receptor potential was graded as a linear function of the logarithm of the odor concentration. 5. The amplitude of the receptor potential varied linearly with the membrane potential between -70 and -30 mV. Extrapolated reversal potentials appeared to be normally distributed around a mean value of -3.6 mV. 6. The results collectively indicate that lobster olfactory receptor cells have electrical properties similar to, but not necessarily identical with, those currently envisaged for olfactory receptor cells in other species.

Failure to implicate cAMP in transduction in lobster olfactory receptor cells

The possible role of CAMP in olfactory transduction in the spiny lobster was investigates using radioimmunoassay of cAMP and intracellular recording. Application of forskolin or 1-isobutyl-3-methylxanthine increased cAMP levels in intact sensilla thereby demonstrating adenylate cyclase and phosphodiesterase activity in the sensilla. cAMP-dependent transduction mechanisms are unlikely to mediate most odor responses in lobsters, in contrast to transduction mechanisms in amphibians and rats

Hyperpolarizing receptor potentials in lobster olfactory receptor cells: implications for transduction and mixture suppression

Intracellular recording from the olfactory cells of two species of lobsters revealed that small but concentration-dependent and repeatable hyperpolarizing receptor potentials could be evoked by a mixture of L-arginine, L-cysteine and L-proline, as well as by histamine. Large, depolarizing receptor potentials were evoked in the same cells by a complex odor mixture. Simultaneous application of depolarizing and hyperpolarizing stimuli reduced the magnitude of the evoked depolarization