Biomedical subjects

S Firestein

Publications and source records attributed to S Firestein.

42 records · Page 3Linked to original sources

PubMed1991

Electrotonic structure of olfactory sensory neurons analyzed by intracellular and whole cell patch techniques.

1. Experimental studies employing whole cell patch recordings from freshly isolated olfactory sensory neurons of the salamander (Ambystoma tigrinum) yield much higher estimates of specific membrane resistance (Rm) than studies using conventional intracellular recordings from in situ neurons. Because Rm is critical for understanding information transfer in these cells, we have used computational methods to analyze the possible reasons for this difference. 2. Compartmental models were constructed for both the in situ and isolated neurons, using SABER, a general-purpose simulation program. For Rm in the in situ cell, we used a high value of 100,000 omega.cm2, as estimated in the whole cell recordings from isolated cells. A shunt across the cell membrane caused by the penetrating microelectrode was simulated by several types of shunt mechanisms, and its effects on lowering the apparent value of resting membrane potential (MP), input resistance (RN), and membrane time constant (tau m) and increasing the electrotonic length (L) were analyzed. 3. A good approximation of the electrotonic properties recorded intracellularly was obtained in the in situ model with high Rm combined with an electrode shunt consisting of Na and K conductances. A raised K conductance (1-5 nS) helps to maintain the resting MP while contributing to the increased conductance, which lowers RN and shortens the apparent tau m toward the experimental values. 4. Combined shunt resistances of 0.1-0.2 G omega (5-10 nS) gave the best fits with the experimental data. These shunts were two to three orders of magnitude smaller than the values reported from intracellular penetrations in muscle cells and motoneurons. This may be correlated with the smaller electrode tips used in the recordings from these small neurons. We thus confirm the prediction that even small values of electrode shunt have relatively large effects on the recorded electrotonic properties of small neurons, because of their high RN (2-5 G omega). 5. We have further explored the effects on electrotonic structure of a nonuniform Rm by giving higher Rm values to the distally located cilia compared with the proximal soma-dendritic region, as indicated by recent experiments. For the same RN, large increases in ciliary Rm above 100,000 omega.cm2 can be balanced by relatively small decreases below that value in soma-dendritic Rm. A high ciliary Rm appears to be a specialization for transduction of the sensory input, as reported also in photoreceptors and hair cells.

Ambystoma↗

PubMed1991

Single odor-sensitive channels in olfactory receptor neurons are also gated by cyclic nucleotides.

Olfactory transduction is thought to occur by processes that are mainly restricted to the specialized cilia emanating from the distal end of the receptor neuron's single dendrite. The involvement of a cAMP-based second messenger system seems likely, and a cyclic nucleotide-sensitive current has been recorded in patches of membrane from the cilia. However, the small diameter of the cilia and the high density of channels within the membrane limit the application of the patch recording technique in the cilia. We have found that the cAMP-sensitive channels also exist at a much lower density within the far more accessible dendritic membrane. Recording from on-cell patches, we have observed single-channel activity in response to extracellularly applied odor substances. The channels have a single-channel conductance of 40 pS and a reversal potential near 0 mV. These same channels are activated by treatments that elevate intracellular cyclic nucleotide concentrations. The results provide a direct demonstration that the cyclic nucleotide-gated channel is the conductance pathway for the odor-elicited current.

1-Methyl-3-isobutylxanthine↗

PubMed1991

Analysis of single cyclic nucleotide-gated channels in olfactory receptor cells.

In the accompanying article (Firestein et al., 1991b), we have demonstrated that odor- and cyclic nucleotide-sensitive channels exist at a low density in the dendritic membranes of isolated salamander olfactory receptor neurons. Here, we analyze the cyclic nucleotide sensitivity of these channels using the inside-out patch recording technique. Both cAMP and cGMP, at micromolar concentrations, are capable of inducing channel openings. The biophysical parameters of channel activity are nearly the same in response to either ligand. The unitary conductance is about 45 pS, the reversal potential of single-channel currents is +5 mV, and the I/V relation is linear over the range -80 to +80 mV. The channel activity shows no obvious voltage dependence in divalent cation-free symmetrical solutions. The channel shows no desensitization, even to agonist exposures lasting 15 sec. Mean open time is about 1.5 msec; the closed time distribution is best fit by two exponentials with a fast time constant in the submillisecond range (ca. 0.15 msec) and a slower time constant in the millisecond range (ca. 1.5 msec). The only clear difference in the activity of the two ligands is in their affinity constants. The K1/2 for cAMP is 20 microM; that for cGMP is 4 microM. In both cases, the Hill coefficient is greater than 2, suggesting that channel opening requires the cooperative action of three ligand molecules.

Animals↗

PubMed1990

Time course of the membrane current underlying sensory transduction in salamander olfactory receptor neurones.

1. Odour elicited currents in freshly isolated olfactory receptor neurones were analysed using the whole-cell patch-clamp technique. Brief pulses (35-50 ms) and steps (100 ms-5 s) of odour solution were delivered by pressure ejection from a nearby micropipette. 2. Pulses of odour solution directed at the cell induced an inward depolarizing current of 50-750 pA leading to the generation of action potentials. The I-V relation for this current was linear over the range -60-(+)20 mV and showed a reversal potential of +5 mV. The magnitude of the current increased with stimulus strength, for a given pulse duration, over approximately one decade of concentration change. 3. Pulses of odour solution focally delivered to the cilia elicited a large response, but those directed toward the soma did not. Conversely pulses of K+ solution at the cilia failed to evoke any response while those directed at the dendrite and soma elicited an inward clamp current. This provides direct evidence that odour sensitivity is localized mainly to the cilia and possibly the distal dendrite. 4. The odour elicited current activated with a long latency of 150-600 ms after the odour solution arrived at the cell. This latency, as well as the time-to-peak and the rise half-time, were relatively independent of stimulus concentration, changing less than 25% over the entire concentration range of stimulus sensitivity. These observations are consistent with the participation of a second messenger system in olfactory transduction. 5. For brief stimulus pulses less than 100 ms, the stimulus diffused away before the odour response current reached its peak value, so that the peak and decay of the odour response occurred in the absence of significant odour stimulus. The time course of the current decay was fitted by a single exponential with a time constant that was concentration dependent, varying from 0.8 to 1.3 s. 6. For longer steps of stimulus presentation, up to 1 s, the magnitude of the response current became a function of the duration of the pulse as well as the stimulus concentration, indicating that the transduction process involved an integrating step. This is consistent with the idea that the odour elicited current is the result of the summation of many smaller unitary events. From responses to weak stimulation an integration period of 700-1000 ms was calculated. 7. During prolonged steps of maintained stimulus presentation (greater than 5 s) the odour elicited current was transient.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1989

Odor-induced membrane currents in vertebrate-olfactory receptor neurons.

In olfactory receptor neurons, odor molecules cause a depolarization that leads to action potential generation. Underlying the depolarization is an ionic current that is the earliest electrical event in the transduction process. In two preparations, olfactory receptor neurons were voltage-clamped and stimulated with odors and this generator current was measured. In addition, a method was developed to estimate the time course and absolute concentration of odorants delivered to the receptor sites. With this method, olfactory neurons were found to have relatively high stimulus thresholds, steep dose-response relations, long latencies, and an apparent requirement for cooperativity at one or more steps in the pathway from odorant binding to activation of the generator current.

Ambystoma↗

PubMed1987

Gated currents in isolated olfactory receptor neurons of the larval tiger salamander.

The electrical properties of enzymatically isolated olfactory receptor cells were studied with whole-cell patch clamp. Voltage-dependent currents could be separated into three ionic components: a transient inward sodium current, a sustained inward calcium current, and an outward potassium current. Three components of the outward current could be identified by their gating and kinetics: a calcium-dependent potassium current [IK(Ca)], a voltage-dependent potassium current [IK(V)], and a transient potassium current (Ia). Typical resting potentials were near -54 mV, and typical input resistance was 3-6 G omega. Thus, only 3 pA of injected current was required to depolarize the cell to spike threshold near -45 mV. The response to a current step consisted of either a single spike regardless of stimulus strength, or a train of less than 8 spikes, decrementing in amplitude and frequency over approximately equal to 250 msec. Thus, the receptor response cannot be finely graded with stimulus intensity.

Animals↗