Passive signal propagation and membrane properties in median photoreceptors of the giant barnacle

Optimized Signal Flow through Photoreceptors Supports the High-Acuity Vision of Primates

The fovea is a neural specialization that endows humans and other primates with the sharpest vision among mammals. This performance originates in the foveal cones, which are extremely narrow and long to form a high-resolution pixel array. Puzzlingly, this form is predicted to impede electrical conduction to an extent that appears incompatible with vision. We observe the opposite: signal flow through even the longest cones (0.4-mm axons) is essentially lossless. Unlike in most neurons, amplification and impulse generation by voltage-gated channels are dispensable. Rather, sparse channel activity preserves intracellular current, which flows as if unobstructed by organelles. Despite these optimizations, signaling would degrade if cones were lengthier. Because cellular packing requires that cone elongation accompanies foveal expansion, this degradation helps explain why the fovea is a constant, miniscule size despite multiplicative changes in eye size through evolution. These observations reveal how biophysical mechanisms tailor form-function relationships for primate behavioral performance.

The dynamics of signaling at the histaminergic photoreceptor synapse of arthropods

Histamine, a ubiquitous aminergic messenger throughout the body, also serves as a neurotransmitter in both vertebrates and invertebrates. In particular, the photoreceptors of adult arthropods use histamine, modulating its release to signal increases and decreases in light intensity. Strong evidence from various arthropod species indicates that histamine is synthesized and stored in photoreceptors, undergoes Ca-dependent release, inhibits postsynaptic interneurons by gating Cl channels, and is then recycled. In Drosophila, the synthetic enzyme, histidine decarboxylase, and the subunits of the histamine-gated chloride channel have been cloned. Possible histamine transporters at synaptic vesicles and for reuptake remain elusive. Indeed, the mechanisms that remove histamine from the synaptic cleft, and that help terminate histamine's action, are unexpectedly complex, their details remaining unresolved. A major pathway in Drosophila, and possibly other arthropod species, is by conjugation of histamine to beta-alanine to form carcinine in adjacent glia. This conjugate then returns to the photoreceptors where it is hydrolysed to liberate histamine, which is then loaded into synaptic vesicles. Evidence from other species suggests that direct reuptake of histamine into the photoreceptors may also occur. Light depolarizes the photoreceptors, causing histamine release and postsynaptic inhibition; dimming hyperpolarizes the photoreceptors, causing a decrease in histamine release and an "off" response in the postsynaptic cell. Further pursuit of histamine's action at these highly specialized synapses should lead to an understanding of how they signal minute changes in presynaptic membrane potential, how they reliably extract signals from noise, and how they adapt to a wide range of presynaptic membrane potentials.

Uptake of precursor and synthesis of transmitter in a histaminergic photoreceptor

As a first step in understanding how the supply of the neurotransmitter histamine is maintained in a photoreceptor, we followed the uptake and metabolism of the immediate precursor of histamine, histidine. [3H]Histidine taken up into photoreceptors and glia was detected using autoradiography, and synthesis of [3H]histamine from [3H]histidine was assayed with thin-layer chromatography. Photoreceptors from barnacles were pulsed (15 min) with [3H]histidine (0.2-200 microM), then maintained in normal saline for up to 24 hr. Autoradiography showed that photoreceptor somata, axons, and presynaptic arbors were labeled, but only weakly, like (nonhistaminergic) ganglion cells. Label instead was concentrated over surrounding glia. Stimulating preparations with light did not increase photoreceptor labeling. Grain counts from photoreceptor axons showed uptake of [3H]histidine into these neurons by a Na+-dependent mechanism with a Km of approximately 50 microM. Over 24 hr only 1% of the [3H]histidine taken up by preparations was converted to [3H]histamine either in the dark or in the light. Injections of [3H]histidine directly into photoreceptors established that synthesis takes place within the photoreceptors and confirmed that stimulation with light did not measurably affect the rate of conversion of [3H]histidine to [3H]histamine. These results suggest that de novo synthesis of transmitter is unlikely to be as important as its reuptake in maintaining neurotransmitter supply in these photoreceptor terminals. In support of this conclusion, photoreceptors accumulated more label when transmitter release was stimulated with high K+ and histamine uptake was antagonized with chlorpromazine.

Uptake of precursor and synthesis of transmitter in a histaminergic photoreceptor

As a first step in understanding how the supply of the neurotransmitter histamine is maintained in a photoreceptor, we followed the uptake and metabolism of the immediate precursor of histamine, histidine. [3H]Histidine taken up into photoreceptors and glia was detected using autoradiography, and synthesis of [3H]histamine from [3H]histidine was assayed with thin-layer chromatography. Photoreceptors from barnacles were pulsed (15 min) with [3H]histidine (0.2-200 microM), then maintained in normal saline for up to 24 hr. Autoradiography showed that photoreceptor somata, axons, and presynaptic arbors were labeled, but only weakly, like (nonhistaminergic) ganglion cells. Label instead was concentrated over surrounding glia. Stimulating preparations with light did not increase photoreceptor labeling. Grain counts from photoreceptor axons showed uptake of [3H]histidine into these neurons by a Na+-dependent mechanism with a Km of approximately 50 microM. Over 24 hr only 1% of the [3H]histidine taken up by preparations was converted to [3H]histamine either in the dark or in the light. Injections of [3H]histidine directly into photoreceptors established that synthesis takes place within the photoreceptors and confirmed that stimulation with light did not measurably affect the rate of conversion of [3H]histidine to [3H]histamine. These results suggest that de novo synthesis of transmitter is unlikely to be as important as its reuptake in maintaining neurotransmitter supply in these photoreceptor terminals. In support of this conclusion, photoreceptors accumulated more label when transmitter release was stimulated with high K+ and histamine uptake was antagonized with chlorpromazine.

The distribution of histamine and serotonin in the barnacle's nervous system

The use of antisera directed against conjugates of histamine and serotonin has revealed the locations of neurons labeling for these transmitters in the nervous system of barnacles. Photoreceptors label for histamine but not serotonin and also satisfy a number of other criteria indicating that histamine is their neurotransmitter. Photoreceptors also take up radioactively labeled histamine but not serotonin. Within the barnacle's brain no somata are consistently found that label with antiserum against histamine, but one to three pairs of small cells, depending on species, label with antiserum against serotonin. The most impressive serotonin-like immunoreactivity in the brain, however, is in a pair of large fibers ascending through the circumesophageal connectives and ramifying extensively. Within the ventral ganglion, the only other ganglion in the barnacle, ten pairs of cells label with antiserum against histamine. These neurons are confined to the posterior portion of the ganglion but ramify extensively throughout the ganglion. Antiserum against serotonin labels about 15 cell pairs, depending on species, located throughout the ganglion. The positions of the arbors of many of these cells suggest that these amines have a role in modulating either the motor pathways underlying feeding or the visual pathways responsible for the detection of shadows.

Active currents regulate sensitivity and dynamic range in C. elegans neurons

Little is known about the physiology of neurons in Caenorhabditis elegans. Using new techniques for in situ patch-clamp recording in C. elegans, we analyzed the electrical properties of an identified sensory neuron (ASER) across four developmental stages and 42 unidentified neurons at one stage. We find that ASER is nearly isopotential and fails to generate classical Na+ action potentials. Rather, ASER displays a high sensitivity to input currents coupled to a depolarization-dependent reduction in sensitivity that may endow ASER with a wide dynamic range. Voltage clamp revealed depolarization-activated K+ and Ca2+ currents that contribute to high sensitivity near the zero-current potential. The depolarization-dependent reduction in sensitivity can be attributed to activation of K+ current at voltages where it dominates the net membrane current. The voltage dependence of membrane current was similar in all neurons examined, suggesting that C. elegans neurons share a common mechanism of sensitivity and dynamic range.

Sensory characteristics of the P afferent neurone of the crab thoracic-coxal muscle receptor organ

Intracellular recordings were made from the P fibre, the smallest of the three afferent neurones innervating the thoracic-coxal muscle receptor organ of the crab (Carcinus maenas). While the two larger afferents are nonspiking, the response of the P fibre to a trapezoidal change in receptor muscle length consists of a single action potential signalling the onset of stretch superimposed on a graded amplitude receptor potential. The P fibre is sensitive to the velocity of the applied stretch, but is insensitive to static joint position, stretch amplitude and the velocity of the release phase. The presence and amplitude of the action potential depends on the initial length of the receptor muscle, the tension caused by efferent activation of the receptor muscle prior to receptor stretch, and on the velocity of stretch. Length constant (1.9 mm) and specific membrane resistance (76 K omega x cm2) values obtained for the P fibre, together with its small diameter (7 microns) suggest that this neurone is less well adapted to conveying passive signals to the thoracic ganglion than are the S and T fibres. It is likely that the P fibre complements the length sensitivity of the S fibre and the tension and velocity sensitivity of the T fibre by signalling the onset of receptor stretch via single action potentials.

Selective, activity-dependent uptake of histamine into an arthropod photoreceptor

The synapses made by many arthropod photoreceptors are disinhibitory and use histamine as their transmitter. Because decreases and not increases in the cleft concentration of transmitter constitute the important event at these synapses, a transporter to clear the cleft of histamine would seem particularly crucial to signal transfer. We report here that 3H-histamine is taken up selectively into barnacle photoreceptors by a Na+-dependent mechanism, presumably a transporter. Using light microscopic autoradiography, we observe heavy label over axons and presynaptic terminals of these neurons when they are stimulated during uptake. The radioactivity taken up was identified as 3H-histamine by thin layer chromatography; no metabolites were detected, even after 5 hr. Radiolabeled 5-hydroxytryptamine and GABA are not taken up by the photoreceptor. 3H-histamine uptake into photoreceptors is decreased markedly by an excess of unlabeled histamine and by chlorpromazine and phenoxybenzamine. Unexpectedly for uptake dependent on the NA+ gradient, photoreceptor terminals label more intensely in the light (when depolarized) than in the dark (when hyperpolarized). Glia label more strongly than photoreceptors in dark-incubated preparations. The presence of presynaptic uptake strengthens the evidence that histamine is the neurotransmitter of arthropod photoreceptors and provides a mechanism by which this synapse could recycle transmitter, control its steady-state cleft concentration, and clear it from the cleft in response to decreases in its release from the photoreceptors.

Selective, activity-dependent uptake of histamine into an arthropod photoreceptor

The synapses made by many arthropod photoreceptors are disinhibitory and use histamine as their transmitter. Because decreases and not increases in the cleft concentration of transmitter constitute the important event at these synapses, a transporter to clear the cleft of histamine would seem particularly crucial to signal transfer. We report here that 3H-histamine is taken up selectively into barnacle photoreceptors by a Na+-dependent mechanism, presumably a transporter. Using light microscopic autoradiography, we observe heavy label over axons and presynaptic terminals of these neurons when they are stimulated during uptake. The radioactivity taken up was identified as 3H-histamine by thin layer chromatography; no metabolites were detected, even after 5 hr. Radiolabeled 5-hydroxytryptamine and GABA are not taken up by the photoreceptor. 3H-histamine uptake into photoreceptors is decreased markedly by an excess of unlabeled histamine and by chlorpromazine and phenoxybenzamine. Unexpectedly for uptake dependent on the NA+ gradient, photoreceptor terminals label more intensely in the light (when depolarized) than in the dark (when hyperpolarized). Glia label more strongly than photoreceptors in dark-incubated preparations. The presence of presynaptic uptake strengthens the evidence that histamine is the neurotransmitter of arthropod photoreceptors and provides a mechanism by which this synapse could recycle transmitter, control its steady-state cleft concentration, and clear it from the cleft in response to decreases in its release from the photoreceptors.

The motornervous system of Ascaris: electrophysiology and anatomy of the neurons and their control by neuromodulators

Analysis of the electrical properties of neurons in the motornervous system of Ascaris sutom suggests that it is largely an analogue system. The motorneurons do not conduct action potentials and they release transmitter tonically at their normal resting potential; transmitter release is increased or decreased as a continuous function of membrane potential. Despite extensive physiological descriptions of the electrical properties of the neurons and their synapses, as well as morphological descriptions of the synaptic circuitry of the system, the predicted activities of the neurons in the circuit differ from those observed by direct recording in semi-intact behaving animals. We conclude that the description of the circuit is incomplete. There are several possibilities for the missing elements, including chemical, proprioceptive, and additional neuronal components. Recently, attention has been focussed most heavily on the intercellular chemical signalling systems; in addition to those mediated by classical neurotransmitters, a surprisingly complex array of neuropeptides has been identified. One family of these peptides, the AF peptides, has been analyzed in detail. It comprises at least 20 peptides, and they fall into sequence-related subfamilies. One of these subfamilies, containing 6 peptides, is encoded by a single transcript, suggesting that the AF peptides are under multiple genetic control. All AF peptides tested have potent activity on the motornervous system and/or on muscle. There are multiple physiological activities, and cellular localization studies show multiple patterns of cellular expression. Studies on Panagrellus and Caenorhabditis emphasize the diversity of this family and its genetic control.

Currents in the presynaptic terminal arbors of barnacle photoreceptors

We have described the currents flowing across the presynaptic membranes of the four median photoreceptors of the giant barnacle, Balanus nubilus, using a quasi-voltage clamp arrangement. Membrane potential, measured in the terminal region of one photoreceptor, was controlled in all four terminals by feedback current supplied through the nerve containing the photoreceptors' axons. The [Ca2+]o in the saline was reduced to decrease the Ca2+ current, enabling better voltage control, and tetraethylammonium ion (TEA, 20 mM) was added to block a fast voltage-dependent K+ conductance. Depolarizing voltage steps from the resting potential in the dark (-60 mV) evoked slow, inward Ca(2+)-dependent currents which could be blocked by Co2+, Mg2+, or Cd2+. The Ca2+ currents were followed by large outward currents that persisted for many seconds after the offset of moderate or large pulses. These tail currents increased in magnitude and duration with pulse duration and reversed at about -80 mV, consistent with previous evidence for a Ca(2+)-activated K+ conductance in this membrane. When the Ca(2+)-activated outward current was reduced to zero by increasing the [K+]o so as to set EK at -20 mV, and then stepping the voltage to this value, the step evoked a steady inward Ca2+ current. Thus, the Ca2+ current did not show voltage- or Ca(2+)-dependent inactivation. When Ba2+ was substituted for Ca2+, 500-ms depolarizing steps evoked steady inward currents but no outward currents. In any given experiment, the activation voltage of the Ca2+ or Ba2+ current did not depend on holding potential. At the barnacle photoreceptor's synapse, the postsynaptic cell adapts to maintained presynaptic voltage by a mechanism that is not understood. We conclude that neither Ca2+ current inactivation nor a shift in activation voltage with holding potential can account for this adaptation.

Membrane parameters, signal transmission, and the design of a graded potential neuron

1. The large monopolar cells (LMCs) of the fly, Calliphora vicina, visual system transmit graded potentials over distances of up to 1.0 mm. An electrical model was constructed to investigate the design principles relating their membrane parameters to signal transmission and filtering. 2. Using existing anatomical measurements, a cable model (van Hateren 1986) was fitted to the measured intracellular responses of the cells to injected current. The LMC has three functional components: a distal synaptic zone of low impedance, an axon with high specific membrane resistance (greater than 50.10(5) M omega.micron 2), and a high impedance proximal terminal. These components interact to transmit information efficiently. The low input impedance synaptic zone charges and discharges the axon rapidly, ensuring a good frequency response. The high resistance axon conducts signals with little decrement. The model shows that graded potential transmission in LMCs selectively filters synaptic noise and predicts the changes in response waveform that occur during transmission. 3. The parameters of the model were adjusted to determine the relative costs and benefits of alternative cable designs. The design used in LMCs is the most expensive and the most effective. It requires the largest currents to generate responses but transmits signals with least decrement. Parallel neurons in the fly visual system have fewer input synapses and this could low-pass filter their graded response.

Properties of barnacle photoreceptor cells in culture

1. Properties of median photoreceptor cells in cultured ocelli from the giant barnacle (Balanus nubilus) were compared in isolated ocelli, ocelli maintained with the supraesophageal ganglion, and fresh ocelli. 2. Cultured photoreceptor cells exhibited slight deterioration after 2-4 weeks. Cell bodies maintained their structure but apparently lost some dendrites. Electron micrographs revealed fewer rhabdomeres. Axons did not degenerate. 3. Intracellularly recorded responses to light in both cultured preparations were qualitatively normal with a small decrease in sensitivity and increase in input resistance. The waveforms of the light responses were normal. 4. The characteristic shadow reflex was maintained after 6 weeks.

Adaptation in the input-output relation of the synapse made by the barnacle's photoreceptor

A study was made of synaptic transmission between the four median photoreceptors of the giant barnacle (Balanus nubilus) and their post-synaptic cells (I-cells). Simultaneous intracellular recordings were made from the presynaptic terminal region of a photoreceptor and from the soma of an I-cell. The photoreceptor's membrane potential provided feed-back to bath electrodes that passed current into the receptors' axons, permitting the voltage to be controlled at the point of arborization of their presynaptic terminals. Simultaneous recordings from a second photoreceptor showed that its voltage tracked the first. Step depolarizations of the receptors from their dark resting potential (about -60 mV) caused hyperpolarizations of the I-cell that reached a peak, then decayed to a plateau value. The amplitude of the I-cell's response grew with presynaptic depolarizations, saturating at presynaptic values 10-20 mV depolarized from dark rest. Step hyperpolarizations of the receptors from dark rest evoked depolarizations of the I-cell consisting of an initial peak, which varied greatly in amplitude and wave form from preparation to preparation, followed by a plateau. The presence of this post-synaptic response indicates that transmitter is released continuously from the receptors at their dark resting potential. An input-output relation of the synapse was obtained by presenting step depolarizations from a holding potential of -80 mV, where steady-state transmitter release is shut off. The relation is sigmoidal; in the exponentially rising phase of the curve, a 5-11 mV presynaptic change produces a 10-fold change in post-synaptic response. When the presynaptic holding potential was set at values ranging from -80 to -40 mV, the relation between the I-cell's response and the absolute potential to which the receptor was stepped shifted along the presynaptic voltage axis. The slopes of the input-output relations were roughly parallel or increased as the photoreceptors were held more depolarized. This observation limits the possible mechanisms of the shift.

Electrophysiology of identified neurosecretory and non-neurosecretory cells in the cockroach pars intercerebralis

Two cell types can be distinguished with intracellular recording from the pars intercerebralis of the American cockroach (Periplaneta americana). The first type, which corresponds morphologically to the medial neurosecretory cell, always had spontaneously occurring, overshooting action potentials. These action potentials are probably endogenously produced. Tetrodotoxin experiments revealed that sodium is the dominant ion of the action potential. The action potentials are followed by a relatively long after-hyperpolarization. The input resistance of these cells ranged from 120 to 390 M omega. A mathematical model, based on cellular morphology and response to current pulses, revealed a membrane time constant of about 100 msec and an axonal:somatic conductance ratio of approximately 13. Area-specific membrane resistance was estimated at 33 k omega cm2. These cells also often had reversible and spontaneous inhibitory postsynaptic potentials. The second cell type, which is non-neurosecretory, never produced spontaneous action potentials and rarely had synaptic potentials. Action potentials could be evoked by current injection into the cell body or by extracellular stimulation of their axons in the posteroventral portion of the the protocerebrum. These action potentials also depend on sodium ions. Their input resistance ranged from 16 to 35 M omega. They had a membrane time constant of approximately 15 msec and an axonal:somatic conductance ratio of about 9. Their area specific membrane resistance was estimated at 14 k omega cm2.

Photoreceptors and visual interneurons in the medicinal leech

The medicinal leech has five pairs of eyes, each with about 50 photoreceptors. Receptors produce propagating impulses which constitute their output to second order neurons in the CNS. Within the eye, receptors have diverse thresholds, and thus the aggregate output of the eye is graded with light intensity. By having many receptors in parallel, the eye may achieve better intensity discrimination and temporal response than would be predicted from the relatively poor characteristics of individual receptors. Receptors in eyes 3-5 on one side of the animal excite the ipsilateral LV (lateral visual) cell, an interneuron in the first segmental ganglion. By physiological tests the receptor axons are electrically coupled to the LV cell. Moreover, the LV cell is Lucifer Yellow dye-coupled to many fine fibers that appear to be receptor axons of the ipsilateral eyes 3-5. The receptors of the contralateral eyes 3-5, and those of the photosensitive sensilla lining the body inhibit the LV cell via polysynaptic pathways. Thus, the LV cells are central elements of the neural circuit processing input from the leech's spatially distributed visual system.

Application of the Goldman-Hodgkin-Katz current equation to membrane current-voltage data

A new approach for the analysis of current-voltage (IV) data is presented and applied to a variety of published data collected from various systems. Our analysis of published results shows that our method of analysis can account for the observed IV data. The calculated permeability coefficients are in reasonable agreement with those calculated from ion fluxes. In those cases where two ions are assumed to carry the current, the ratios of the calculated permeability coefficients are in agreement with those ratios determined from the Goldman-Hodgkin-Katz voltage equation. In most cases, the entire IV curve can be accounted for by using our method of analysis. In several examples, only a portion of the IV curve is in agreement with the predictions. We attribute the failure to account for the IV data to reflect the failure of one or more of the assumptions used by the GHK current equation. In other cases, assuming that an additional ion carries the current, the treatment can account for the IV data. However, the identity of the extra ion cannot be established from these published data without additional studies (e.g., ionic replacement studies).

Cochlear transduction: an integrative model and review

A model for cochlear transduction is presented that is based on considerations of the cell biology of its receptor cells, particularly the mechanisms of transmitter release at recepto-neural synapses. Two new interrelated hypotheses on the functional organization of the organ of Corti result from these considerations, one dealing with the possibility of electrotonic interaction between inner and outer hair cells and the other with a possible contributing source to acoustic emissions of cochlear origin that results from vesicular membrane turnover.

Effects of rectification on synaptic efficacy

We have investigated the effects of postsynaptic membrane properties on the shape of synaptic potentials generated by time-varying synaptic conductances. We used numerical simulation techniques to model cells of several different geometrical forms, from an isopotential sphere to a neuron with a soma and a dendritic tree. A variety of postsynaptic membrane properties were tested: (a) a passive resistance-capacitance membrane, (b) a membrane represented by the Hodgkin and Huxley (HH) equations, and (c) a membrane that was passive except for a delayed rectification represented by a voltage- and time-dependent increase in GK. In all cases we investigated the effects of these postsynaptic membrane properties on synaptic potentials produced by synaptic conductances that were fast or slow compared with the membrane time constant. In all cases the effects of postsynaptic rectification occurred on postsynaptic potentials of amplitudes as low as 1 mV. The HH model (compared with the passive model) produced an increased peak amplitude (from the increase in GNa) but a decreased half-width and a decreased time integral (from the increase in GK). These effects of the HH GK change were duplicated by a simple analytical rectifier model.

Intracellular recording from vertebrate myelinated axons: mechanism of the depolarizing afterpotential

1. Electrophysiological techniques are described which allow intracellular recording from peripheral myelinated axons of lizards and frogs for up to several hours. The sciatic and intramuscular axons studied here have resting potentials of -60 to -80 mV and action potentials (evoked by stimulation of the proximal nerve trunk) of 50-90 mV. They show a prominent depolarizing afterpotential (d.a.p.), which is present both in isolated axons and in axons still attached to their peripheral terminals. This d.a.p. has a peak amplitude of 5-20 mV at the resting potential, and decays with a half-time of 20-100 msec.2. The peak amplitude of the d.a.p. is voltage-sensitive, increasing to up to 26 mV with membrane hyperpolarization. The d.a.p. disappears as the axon is depolarized to -60 to -45 mV, and does not appear to reverse with further depolarization.3. The d.a.p. is not reduced when bath Ca is replaced by 2-10 mm divalent Mn or Ni. The d.a.p. is not reversed when axons depleted of Cl (by prolonged exposure to Cl-deficient, SO(4)-enriched solutions) are bathed in Cl-rich solutions. These results suggest that the d.a.p. is not mediated by a conductance change specific for Ca or Cl ions. Partial substitution of tetramethylammonium for bath Na, or addition of 10(-5)m-tetrodotoxin to the normal bathing solution, reduces the amplitude of both the action potential and the d.a.p.4. The amplitude of the d.a.p. is not sensitive to bath [K] over the range 1-7.5 mm, provided that all measurements are made at the same holding potential. This result argues that the d.a.p. is not mediated by accumulation of K outside the active axon.5. Treatments expected to inhibit the Na-K exchange pump (cooling from 25 to 10 degrees C, or 0.15 mm-ouabain) do not enlarge or prolong the d.a.p., although they do abolish a slower hyperpolarizing afterpotential seen following repetitive stimulation.6. The passive voltage response of the axon to small injected pulses of depolarizing or hyperpolarizing current shows a prominent, slowly decaying component with a time course similar to that of the d.a.p. Depolarizing current reduces the input resistance of the axon, and increases the rate of decay of both the passive voltage response and the d.a.p. There is a slight conductance increase during the peak of the d.a.p., but the same conductance increase can be produced by a comparable passive depolarization.7. We conclude that the d.a.p. is due mainly to a passive capacitative current, probably resulting from discharge of the internodal axonal membrane capacitance through a resistive current pathway beneath or through the myelin sheath. We suggest that this slow capacitative discharge becomes evident as soon as most of the nodal ionic channels activated during the action potential have closed. An electrical model of the myelinated axon that incorporates the postulated internodal leakage pathway can account both for the prolonged d.a.p. recorded inside the axon, and for the potential profile recorded extra-axonally in or near the internodal periaxonal space.

Properties of tetraethylammonium ion-resistant K+ channels in the photoreceptor membrane of the giant barnacle

After the offset of illumination, barnacle photoreceptors undergo a large hyperpolarization that lasts seconds or minutes. We studied the mechanisms that generate this afterpotential by recording afterpotentials intracellularly from the medial photoreceptors of the giant barnacle Balanus nubilus. The afterpotential has two components with different time-courses: (a) an earlier component due to an increase in conductance to K+ that is not blocked by extracellular tetraethylammonium ion (TEA+) or 3-aminopyridine (3-AP) and (b) a later component that is sensitive to cardiac glycosides and that requires extracellular K+, suggesting that it is due to an electrogenic Na+ pump. The K+ conductance component increases in amplitude with increasing CA++ concentration and is inhibited by extracellular Co++; the Co++ inhibition can be overcome by increasing the Ca++ concentration. Thus, the K+ conductance component is Ca++ dependent. An afterpotential similar to that evoked by a brief flash of light is generated by depolarization with current in the dark and by eliciting Ca++ action potentials in the presence of TEA+ in the soma, axon, or terminal regions of the photoreceptor. The action potential undershoot is generated by an increase in conductance to K+ that is resistant to TEA+ and 3-AP and inhibited by Co++. The similarity in time-course and pharmacology of the hyperpolarization afterpotentials elicited by (a) a brief flash of light, (b) depolarization with current, and (c) an action potential indicates that Ca++-dependent K+ channels throughout the photoreceptor membrane are responsible for all three hyperpolarizing events.

Cellular synthesis and axonal transport of gamma-aminobutyric acid in a photoreceptor cell of the barnacle

1. [3H]glutamate or [3H]gamma-aminobutyric acid (GABA) was injected into the photoreceptor cell of the lateral ocellus of Balanus eburneus, in order to study the transmitter substance of the cell. 2. The photoreceptor cell synthesized [3H]GABA from injected [3H]glutamate. 3. The newly formed [3H]GABA moved inside the photoreceptor axon towards the axon terminal with a velocity of about 0.9 mm/hr. Injected [3H]GABA moved at 0.9 mm/hr and also at 0.4 mm/hr. 4. Axonally transported [3H]GABA reached the axon terminal within several hours following the injection. It did not accumulate at the terminal, but gradually disappeared. 5. Light-microscope and electron-microscope autoradiography following the injection of [3H]GABA revealed that [3H]-reacted silver grains were present in a certain type of axon terminal. The terminal thus identified as that of a photoreceptor cell contains many clear, polymorphic synaptic vesicles about 300-500 A in diameter, some dense-cored vesicles 700-1300 A in diameter, and glycogen granules. The terminal forms many synapses, and each synapse has a synaptic dense body. The terminal always faces two post-synaptic elements at the synapse, forming a triad with a gap distance of about 160-200 A. 6. A GABA analogue, [3H]di-aminobutyric acid, was selectively taken up into the terminals previously identified as those of photoreceptors. 7. These results support the notion that the transmitter substance of the photoreceptor cell of the barnacle is GABA.

Isopotentiality and an optical determination of series resistance in Limulus ventral photoreceptors

1. Photoreceptor somas in the ventral rudimentary eye of Limulus polyphemus were impaled with three micropipettes. Two micropipettes were connected in a voltage-clamp circuit and the cells were stimulated by brief flashes. The third micropipette did not measure any significant deviations from the 'clamped' voltage during responses to the flashes, in several geometries of electrode placement, even for very bright flashes. Therefore using the described techniques there is no evidence for spatial non-uniformity of intracellular voltage in the soma of these photoreceptors. 2. A voltage-sensitive dye was used to monitor light-induced changes in membrane voltage while intracellular voltage was held clamped by a feed-back circuit. With a known series resistance connected between the bath and ground the dye recorded a light-induced change in membrane voltage. When there was no added series resistance, the light-induced change was smaller and often undetectable. From these data the naturally occurring series resistance was calculated to be less than or equal to 30 k omega. 3. From these measurements, as well as from calculations for a model spherical cell, we conclude that membrane potential can be controlled to within 2 mV using our micropipette 'point clamp' methods, for all but the brightest stimuli.

Hypothalamic sensitivity to leukocytic pyrogen of adult and new-born guinea-pigs

1. Experiments were conducted to localize the hypothalamic site of action of microinjected leucocytic pyrogen and to compare the pyrogenic sensitivity of this locus in adult and new-born guinea-pigs.2. To identify the site reactive to leucocytic pyrogen, bilateral (0.8-1.0 mm from the mid line) injections of 1 microliter were made into conscious adult guinea-pigs via cannulas stereotaxically palced at 0.5 mm intervals and varying depths from the olfactory tegmentum to the mammillary bodies. Injections into the preoptic area produced sharp monophasic fevers with short latencies, whereas injections into circumjacent sites evoked smaller fevers with longer latencies. 3. To assess the ontogeny of the pyrogenic sensitivity of this locus, the febrile response to 1.00, 0.50, and 0.25 microliter leucocytic pyrogen injected bilaterally was compared to 0 to 5-, 6 to 12-, and 13 to 16-day old and in adult guinea-pigs. The minimum pyrogenic dose in both new-born and adult guinea-pigs was 0.25 microliter, but the 0 to 5-day old animals which responded with a fever to this dose were few in number and large in weight; 'small-for age' neonates became hypothermic. 4. The number of febrile animals increased with age; it also could be increased by increasing the dose of leucocytic pyrogen at any age. 5. These results suggest that febrile responsiveness may depend on the stage of development of, presumably, the pyrogen-receptive mechanism. They further imply that the preoptic sites where leucocytic pyrogen acts and thermoafferents are integrated may not be the same, since thermoregulatory capability is fully competent from birth.

Calcium channels in the high resistivity axonal membrane of photoreceptors of the giant barnacle

1. The distribution of calcium channels in the cell membrane of the photoreceptor neurone of the giant barnacle, Balanus nubilus, was studied by recording intracellularly in or near the soma, in the axon, and near the presynaptic terminals. The membrane properties of these different regions of the cell could be studied by separately superfusing each region with test salines or by cutting the axon between two regions. 2. In the presence of tetraethylammonium (TEA) or 3-aminopyridine (3-AP), but not in their absence, Ca dependent action potentials could be evoked with depolarizing current pulses in the somatic, axonal, and terminal regions. Consequently, voltage-sensitive Ca channels and TEA-sensitive channels are present in all three regions of the cell. 3. The action potentials recorded from the three regions were similar in their slow times-to-peak (30-300 msec), long durations (0.2-2 sec in 100 mM-TEA), and long-lasting (0.2-10 sec) undershoots. The action potentials were inhibited by extracellular Co. 4. Clear differences were consistently observed between terminal action potentials and axonal or somatic action potentials in TEA. Terminal action potentials displayed a lower voltage threshold, faster rate of rise, and were less sensitive to inhibition by extracellular cobalt, suggesting that the Ca current is greater in the terminal region. 5. Bathing the receptor axon in low Ca or Co solutions led to a greater attenuation of large depolarizing components of the visual signal as they spread to the presynaptic terminals.

Neuronal properties underlying processing of visual information in the barnacle

Generation of a transient, amplified response to the dimming of light in the visual system of the barnacle involves two synaptic stages. It is accomplished primarily by decrementally conducting neurones that are similar to bipolar cells of the vertebrate retina.

Voltage sensitive calcium channels in the presynaptic terminals of a decrementally conducting photoreceptor

1. Intracellular recordings were made from the presynaptic regions of the photoreceptors of the median ocellus of the giant barnacle, Balanus nubilus.2. Millivolt changes in membrane potential near the dark resting level in the terminals elicit post-synaptic activity and consequently must be sufficient to modulate transmitter release from these endings.3. In normal saline the terminal voltage usually changes in a graded manner to increasing intensities of illumination of the cell. When the terminal region is superfused with saline containing TEA, 3-AP or high concentrations of K, an all-or-none action potential can be elicited consistently by light or injected current.4. The peak value of this action potential depends on the Ca concentration in the saline. The action potential can be generated if Sr or Ba ions replace Ca, but is reduced or blocked if Mg, Co, or Mn ions are added to the saline. It is virtually unaffected by TTX or replacement of Na with TMA ions in the saline. These results suggest that Ca carries most or all of the inward current during the action potential.5. The action potential is followed by a large undershoot which can last several seconds. The amplitude and duration of the action potential and the duration of the undershoot all grow in increasing concentrations of TEA up to 400 mM, the highest concentration tested. The threshold for the action potential decreases as the concentration of TEA is increased to 10 mM; increasing the concentration further has no effect on the threshold. These observations suggest that TEA blocks a voltage-sensitive potassium conductance at low concentrations but has less effect on the current responsible for the undershoot.6. Electrophysiological and pharmacological evidence suggests that the Ca channels are concentrated in the presynaptic terminals of this photoreceptor.

Morphology and responses to light of the somata, axons, and terminal regions of individual photoreceptors of the giant barnacle

1. The median eye of the giant barnacle, B. nubilus, comprises four large photoreceptor neurones which are visible under the dissecting microscope for almost their entire length. We have studied the structure of, and the responses to light recorded in, the somata, axons, and terminal regions of these neurones.2. The photoreceptor somata, each 40-70 mum in diameter, extend numerous light-sensitive dendritic processes whose membranes form rhabdomeric microvilli. Recordings from the soma show that dim light evokes a steady, noisy depolarization; brighter light elicits a transient depolarization which decays to a maintained plateau, followed by a hyperpolarization when the light is turned off.3. Light-induced voltage changes spread decrementally along the photoreceptor axons, which average 10 mm in length and 25 mum in diameter. In distal parts of the axon, near the presynaptic terminals, depolarizations and hyperpolarizations can be as large as 50% or more of their values in the soma.4. There is no demonstrable electrical coupling between photoreceptor neurones as shown by simultaneous recordings from two receptor somata or axons.5. Each photoreceptor axon enters the mid line commissure of the supraoesophageal ganglion, bifurcates, and arborizes in a restricted zone of neuropil in each hemiganglion. The large size of the terminal processes of these neurones and their characteristic cytoplasmic inclusions enable one to trace them with the electron microscope as they branch in the neuropil.6. The terminal processes subdivide and end in 1-3 mum diameter branches which are the sites of apparently chemical synapses. Vesicle-containing, presynaptic loci on these processes of the receptor cell are invariably apposed to two post-synaptic processes from cells as yet unidentified.