Scattered-light imaging in vivo tracks fast and slow processes of neurophysiological activation.

We imaged fast optical changes associated with evoked neural activation in the dorsal brainstem of anesthetized rats, using a novel imaging device. The imager consisted of a gradient-index (GRIN) lens, a microscope objective, and a miniature charged-coupled device (CCD) video camera. We placed the probe in contact with tissue above cardiorespiratory areas of the nucleus of the solitary tract and illuminated the tissue with 780-nm light through flexible fibers around the probe perimeter. The focus depth was adjusted by moving the camera and microscope objective relative to the fixed GRIN lens. Back-scattered light images were relayed through the GRIN lens to the CCD camera. Video frames were digitized at 100 frames per second, along with tracheal pressure, arterial blood pressure, and electrocardiogram signals recorded at 1 kHz per channel. A macroelectrode placed under the GRIN lens recorded field potentials from the imaged area. Aortic, vagal, and superior laryngeal nerves were dissected free of surrounding tissue within the neck. Separate shocks to each dissected nerve elicited evoked electrical responses and caused localized optical activity patterns. The optical response was modeled by four distinct temporal components corresponding to putative physical mechanisms underlying scattered light changes. Region-of-interest analysis revealed image areas which were dominated by one or more of the different time-course components, some of which were also optimally recorded at different tissue depths. Two slow optical components appear to correspond to hemodynamic responses to metabolic demand associated with activation. Two fast optical components paralleled electrical evoked responses.

Learning-related interpositus activity is conserved across species as studied during eyeblink conditioning in the rat.

Single-unit activity was monitored in the interpositus nucleus of the cerebellum during standard delay conditioning of the eyeblink response in freely-moving rats. The rats were implanted with recording electrodes in the interpositus nucleus then received paired presentations of a tone-conditioned stimulus (CS) and eye-shock unconditioned stimulus during acquisition training. The acquisition training was followed by CS-alone extinction training. Learning-related activity in the interpositus nucleus developed over the course of acquisition training and then activity returned to baseline levels during subsequent extinction training. These findings are consistent with rabbit studies that have demonstrated similar changes in neuronal activity in the interpositus nucleus over the course of acquisition and extinction of the eyeblink response, thus providing strong evidence for the generality of the neural substrates of eyeblink conditioning across species.

Information theoretic analysis of pulmonary stretch receptor spike trains.

Primary afferent neurons transduce physical, continuous stimuli into discrete spike trains. Investigators have long been interested in interpreting the meaning of the number or pattern of action potentials in attempts to decode the spike train back into stimulus parameters. Pulmonary stretch receptors (PSRs) are visceral mechanoreceptors that respond to deformation of the lungs and pulmonary tree. They provide the brain stem with feedback that is used by cardiorespiratory control circuits. In anesthetized, paralyzed, artificially ventilated rabbits, we recorded the action potential trains of individual PSRs while continuously manipulating ventilator rate and volume. We describe an information theoretic-based analytical method for evaluating continuous stimulus and spike train data that is of general applicability to any continuous, dynamic system. After adjusting spike times for conduction velocity, we used a sliding window to discretize the stimulus (average tracheal pressure) and response (number of spikes), and constructed co-occurrence matrices. We systematically varied the number of categories into which the stimulus and response were evenly divided at 26 different sliding window widths (5, 10, 20, 30,..., 230, 240, 250 ms). Using the probability distributions defined by the co-occurrence matrices, we estimated associated stimulus, response, joint, and conditional entropies, from which we calculated information transmitted as a fraction of the maximum possible, as well as encoding and decoding efficiencies. We found that, in general, information increases rapidly as the sliding window width increases from 5 to approximately 50 ms and then saturates as observation time increases. In addition, the information measures suggest that individual PSRs transmit more "when" than "what" type of information about the stimulus, based on the finding that the maximum information at a given window width was obtained when the stimulus was divided into just a few (usually <6) categories. Our results indicate that PSRs provide quite reliable information about tracheal pressure, with each PSR conveying about 31% of the maximum possible information about the dynamic stimulus, given our analytical parameters. When the stimulus and response are divided into more categories, slightly less information is transmitted, and this quantity also saturates as a function of observation time. We consider and discuss the importance of information contained in window widths on the time scales of an excitatory postsynaptic potential and Hering-Breuer reflex central delay.

Impairment in eyeblink classical conditioning in adult rats exposed to ethanol as neonates.

BACKGROUND: Eyeblink classical conditioning is a learning task that engages well-defined neural circuitry in the cerebellum and brainstem. Binge-like exposure to alcohol during the neonatal brain growth spurt in rats produces neurotoxic effects on both the cerebellum and the brainstem. The precise localization of the neural substrates of eyeblink conditioning makes it an ideal task to study functional disruptions in the cerebellum and brainstem caused by early exposure to alcohol. The purpose of this study was to determine whether impairments in eyeblink conditioning caused by neonatal binge exposure to alcohol persist into adulthood, indicative of long-lasting abnormalities in cerebellar and brainstem function. METHODS: Group Ethanol received alcohol doses of 5.25 g/kg/day via intragastric intubation on postnatal days 4-9. Group Sham Intubated underwent the intragastric intubation procedures on postnatal days 4-9 but did not receive any infusions. Group Nonintubated did not receive any intubations. When all rats were at least 3 months old, they were tested in either paired or unpaired eyeblink conditioning. RESULTS: Group Ethanol showed impaired eyeblink conditioning and some abnormalities in conditioned response timing. Control groups did not differ from each other. CONCLUSIONS: The present data indicate that early exposure to alcohol has long-term effects on eyeblink conditioning, perhaps through enduring effects associated with alcohol-induced loss of Purkinje cells of the cerebellum.

Computational modeling of the baroreflex arc: nucleus tractus solitarius.

In this study, we examine the utility of computational modeling in understanding nervous system function. We start by examining the reasons for, and major approaches to, computational modeling. We then chose a modeling approach and applied different variations to understanding nucleus tractus solitarius (NTS) neuronal responses to various baroreceptive stimuli. We examine the results in light of our objectives and with regard to the known parameters of the system under investigation. Our results demonstrate that modeling can be a useful tool in analysis of (and examination of underlying mechanisms for) NTS behavior on many levels.

A focusing image probe for assessing neural activity in vivo.

We describe a compact, focusing image probe to record rapid optical changes from neural tissue. A gradient index (GRIN) lens served as a relay lens from tissue to a microscope objective which projected an image onto a CCD camera. The microscope objective and camera assembly was adjusted independently from the GRIN lens, allowing focus changes without disturbing the probe/tissue interface; firm contact minimized movement and specular reflectance. Fiber optics around the probe perimeter provided diffuse illumination from a 780 nm laser, or 660 and 560 nm light emitting diodes. To characterize depth-of-field, we imaged a black suture through increasing tissue thicknesses. Light modulation by the suture remained detectable down to 900 microm using 780 nm illumination. We acquired images from cardiorespiratory areas of the rat dorsal medulla, at different depths and illumination wavelengths. Images illuminated at 560 nm were dominated by vasculature flow patterns, while 660 nm illumination revealed different spatial patterns which preceded vascular flow by 40 ms and may represent cardiac-related neural activity. Using 780 nm light, image sequences triggered by the cardiac R-wave showed vascular perfusion changes with delayed and broader responses at deeper levels. Electrical stimulation within the vagal bundle caused fast optical changes which track the electrical response, with a different spatial distribution from hemodynamic signals.

The cerebellum is necessary for rabbit classical eyeblink conditioning with a non-somatosensory (photic) unconditioned stimulus.

The present research investigated the acquisition of classically conditioned eyeblinks in rabbits using a light flash unconditioned stimulus (US), as well as the contribution of deep cerebellar nuclei to such an association. Two independent groups of animals experienced three phases of training: (1) pre-lesion delay conditioning using either a light- (Group 1) or an air puff-US (Group 2), (2) post-lesion testing of response performance, and (3) post-lesion acquisition to the opposite US. During the initial acquisition (720 trials), the groups did not differ with regard to their rate of learning or their overall level of responding. To assess the contribution of the cerebellum to the maintenance of responding, the interpositus nucleus was electrolytically lesioned and animals were given 8 days of additional training. Both groups exhibited a profound reduction in conditioned responding (CR) and showed no signs of recovery over the remainder of this phase (480 trials). Animals were then shifted to the opposite US (same eye) and given 12 days of training to assess the effect of interpositus lesions on the acquisition of CRs to a novel US. No learning was observed during this phase, regardless of whether animals experienced the light- or air puff-US. These results demonstrate: (1) the ability of a non-somatosensory stimulus to serve as a US during classical eyeblink conditioning; and (2) a common reliance on deep cerebellar nuclei for both somatosensory- and non-somatosensory-based reflexive motor learning. The findings are discussed in reference to the processing of conditioning stimuli within the brainstem-cerebellar circuitry that underlies eyeblink conditioning.

Ubiquitous molecular substrates for associative learning and activity-dependent neuronal facilitation.

Recent evidence suggests that many of the molecular cascades and substrates that contribute to learning-related forms of neuronal plasticity may be conserved across ostensibly disparate model systems. Notably, the facilitation of neuronal excitability and synaptic transmission that contribute to associative learning in Aplysia and Hermissenda, as well as associative LTP in hippocampal CA1 cells, all require (or are enhanced by) the convergence of a transient elevation in intracellular Ca2+ with transmitter binding to metabotropic cell-surface receptors. This temporal convergence of Ca2+ and G-protein-stimulated second-messenger cascades synergistically stimulates several classes of serine/threonine protein kinases, which in turn modulate receptor function or cell excitability through the phosphorylation of ion channels. We present a summary of the biophysical and molecular constituents of neuronal and synaptic facilitation in each of these three model systems. Although specific components of the underlying molecular cascades differ across these three systems, fundamental aspects of these cascades are widely conserved, leading to the conclusion that the conceptual semblance of these superficially disparate systems is far greater than is generally acknowledged. We suggest that the elucidation of mechanistic similarities between different systems will ultimately fulfill the goal of the model systems approach, that is, the description of critical and ubiquitous features of neuronal and synaptic events that contribute to memory induction.

Contextually based conditional discrimination of the rabbit eyeblink response.

Rabbits received conditional discrimination training using contextual stimuli to set the occasion for stimulus pairings during eyelid conditioning. Specifically, animals were exposed to either the presence or the absence of an oscillating chamber light throughout the intertrial interval (50 +/- 10 s). For half the animals, this light signaled paired presentations of a discrete tone conditioned stimulus (CS) and air puff unconditioned stimulus (US) while darkness signaled presentations of only the tone CS. The remaining animals experienced the opposite contextual relationship to the conditioning stimuli. These trial types occurred pseudo-randomly across a session, with all transitions between contextual settings (i.e., light or dark) taking place immediately at the CS-US offset. Under these conditions, animals successfully utilized the contextual stimuli as conditional cues for differential responding to the shared CS. Moreover, both light and dark were equally effective as discriminative stimuli. A subset of animals received further training in which the contextual contingency was removed by restricting all conditioning to the CS-alone context. Without the contingency in place, subsequent CS presentations (paired and CS-alone) evoked equivalent conditioned responding across three sessions of training. Following the reinstatement of the contextual contingencies, discriminatory responding was immediately observed and returned to previous levels within three sessions. Finally, animals appeared to use the static representation of the conditional cue, rather than the phasic transition between cues, for discriminatory responding. These findings are discussed in terms of current neurobiological models of eyelid conditioning.

Bidirectional regulation of neuronal potassium currents by the G-protein activator aluminum fluoride as a function of intracellular calcium concentration.

Hydrolysis-resistant activation of G-proteins by extracellular perfusion of fluoride ions was examined in Type B cells isolated from the cerebral ganglion of the marine mollusc Hermissenda. Under single-electrode voltage-clamp, modulation by aluminum fluoride ions of several classes of outward K+ currents as well as an inward Ca2+ current was observed. Following injection of the Ca2+ chelator EGTA, aluminum fluoride ions selectively increased a slow, voltage-dependent K+ current (IK) within 5 min of application, while in the absence of EGTA, aluminum fluoride ions induced a small, transient reduction of IK. Neither the magnitude nor steady-state inactivation of a fast, voltage-dependent K+ current (IA), nor a slow, Ca2+-dependent K+ current (IK-Ca), were affected by aluminum fluoride ions. In contrast, when perfusion of aluminum fluoride ions was accompanied by a repetitive depolarization and a concomitant increase in intracellular Ca2+, both IA and the combined late currents (IK and IK-Ca) were markedly reduced, a reduction which was not observed following depolarization alone or if the pairing of aluminum fluoride ions and depolarization was preceded by an injection of EGTA. The reduction of membrane conductance by the pairing of aluminum fluoride ions with depolarization could not be accounted for by an increased Ca2+ conductance, as aluminum fluoride ions produced only a small decrease in the voltage-dependent Ca2+ current. In total, these results indicate that regulatory G-proteins may bidirectionally modulate neuronal K+ currents, the direction of which is dependent on intracellular Ca2+ concentration. Such a dual regulatory mechanism may contribute to the modulation of membrane excitability observed when presynaptic activity is paired with postsynaptic depolarization, and thus may contribute to some forms of activity-dependent plasticity involving metabatropic receptors.

Simultaneous encoding of carotid sinus pressure and dP/dt by NTS target neurons of myelinated baroreceptors.

1. We seek to understand the baroreceptor signal processing that occurs centrally, beginning with the transformation of the signal at the first stage of processing. Because quantitative descriptions of the encoding of mean arterial pressure and its derivative with respect to time by baroreceptive second-order neurons have been unavailable, we characterized the responses of nucleus tractus solitarius (NTS) neurons that receive direct myelinated baroreceptor inputs to combinations of these two stimulus variables. 2. In anesthetized, paralyzed, artificially ventilated rabbits, the carotid sinus was vascularly isolated and the carotid sinus nerve was dissected free from surrounding tissue. Single-unit extracellular recordings were made from NTS neurons that received direct (with the use of physiological criteria) synaptic inputs from carotid sinus baroreceptors with myelinated axons. The vast majority of these neurons did not receive ipsilateral aortic nerve convergent inputs. With the use of a computer-controlled linear motor, a piecewise linear pressure waveform containing 32 combinations of pressure and its rate of change with respect to time (dP/dt) was delivered to the ipsilateral carotid sinus. 3. The average NTS firing frequency during the different stimulus combinations of pressure and dP/dt was a nonlinear and interdependent function of both variables. Most notable was the "extinctive" encoding of carotid sinus pressure by these neurons. This was characterized by an increase in firing frequency going from low to medium mean pressures (analyzed at certain positive dP/dt values) followed by a decrease in activity during high-pressure stimuli. All second-order neurons analyzed had their maximal firing rates when dP/dt was positive. 4. All neurons had their maximal firing frequency locations ("receptive field centers") at just 3 of 32 possible pressure-dP/dt coordinates. The responses of a small population of neurons were used to generate a composite description of the encoding of pressure and dP/dt. When combined as a composite of individually normalized values, the encoding of carotid sinus pressure and dP/dt may be approximated with the use of two-dimensional Gaussian functions. 5. We conclude that the population of NTS neurons recorded most faithfully encodes the rate and direction of (mean) pressure change, as opposed to providing the CNS with an unambiguous encoding of absolute pressure. Instead, the activity of these neurons, individually or as a population, serves as an estimate for the first derivative of the myelinated baroreceptor signal's encoding of mean pressure. We therefore speculate that the output of these individual neurons is useful in dynamic, rather than static, arterial pressure control.

Higher-order associative processing in Hermissenda suggests multiple sites of neuronal modulation.

Two important features of modern accounts of associative learning are (1) the capacity for contextual stimuli to serve as a signal for an unconditioned stimulus (US) and (2) the capacity for a previously conditioned (excitatory) stimulus to "block" learning about a redundant stimulus when both stimuli serve as a signal for the same US. Here, we examined the process of blocking, thought by some to reflect a cognitive aspect of classical conditioning, and its underlying mechanisms in the marine mollusc Hermissenda. In two behavioral experiments, a context defined by chemosensory stimuli was made excitatory by presenting unsignalled USs (rotation) in that context. The excitatory context subsequently blocked overt learning about a discrete conditioned stimulus (CS; light) paired with the US in that context. In a third experiment, the excitability of the B photoreceptors in the Hermissenda eye, which typically increases following light-rotation pairings, was examined in behaviorally blocked animals, as well as in animals that had acquired a normal CS-US association or animals that had been exposed to the CS and US unpaired. Both the behaviorally blocked and the "normal" learning groups exhibited increases in neuronal excitability relative to unpaired animals. However, light-induced multiunit activity in pedal nerves was suppressed following normal conditioning but not in blocked or unpaired control animals, suggesting that the expression of blocking is mediated by neuronal modifications not directly reflected in B-cell excitability, possibly within an extensive network of central light-responsive interneurons.

G-protein mediated responses to localized serotonin application in an invertebrate photoreceptor.

A G-protein dependent serotonin (5-HT) receptor on B photoreceptors of Hermissenda crassicornis was investigated. Microapplication of 5-HT to the soma region, but not to the terminal branches, resulted in a rapid, biphasic depolarization with a slow time course of dissipation. The 5-HT-induced depolarization increased at hyperpolarized potentials, and exhibited a strong and complex sensitivity to external K+, but not Na+ or Ca2+. The 5-HT response, but not a cholinergically mediated intraphotoreceptor interaction, was abolished by intracellular injection of the G protein antagonist, GDP[beta S], although the response was unaffected by pretreatment with pertussis toxin. These results are discussed in terms of known 5-HT receptor subtypes, and the potential role of this receptor in activity-dependent forms of plasticity exhibited by these cells.

Diverse current and voltage responses to baclofen in an identified molluscan photoreceptor.

1. gamma-Aminobuturic acid-B (GABAB) receptors play a role in the mediation of slow inhibitory postsynaptic potentials in mammalian as well as some nonmammalian species. In identified photoreceptors from the marine mollusc Hermissenda, recent evidence has suggested that GABA, as well as the GABAB receptor agonist baclofen, might simultaneously modulate multiple conductances on the postsynaptic membrane. Here, using intracellular current-clamp and single-electrode voltage-clamp techniques, we have characterized responses to baclofen in the B photoreceptors of the Hermissenda eye. 2. Microapplication of baclofen (12.5-62.5 microM) to the terminal branches of the B photoreceptors induced a slow, concentration-dependent hyperpolarization (approximately 3-8 mV) that was accompanied by a cessation of spontaneous action potentials and a positive shift in firing threshold. Both the hyperpolarization and the shift in spike threshold in response to baclofen were attenuated largely by the K+ channel blocker tetraethylammonium chloride (TEA; 50 mM). 3. Bath application of baclofen (100 microM) decreased the amplitude, duration, and the afterhyperpolarization (AHP) of evoked action potentials. Although baclofen's effect on spike duration and amplitude persisted in the absence of extracellular Ca2+, the reduction of the AHP by baclofen was eliminated, suggesting that multiple conductances mediated the baclofen-induced modification of the action potential. 4. Using a single-electrode voltage-clamp technique, microapplication of baclofen to the terminal branches of the B photoreceptor produced a slow, net outward current (< 0.5 nA) that reversed near the equilibrium potential for K+ and shifted to more positive potentials when extracellular K+ was increased, in approximate agreement with the Nernst equation for K+. 5. Baclofen induced an increase in amplitude of the nonvoltage dependent leak conductance (IL), and the increase was blocked by TEA. The baclofen-induced increase of IL was accompanied by an increase in amplitude and a negative shift in the voltage dependence of a slow, steeply voltage-dependent K+ current (IK), which displays selective sensitivity to TEA but does not normally contribute to leak conductance. The amplitude and steady-state inactivation of a fast, transient K+ current, as well as the amplitude of an inwardly rectifying K+ current were unaffected by baclofen. 6. Both the rate of activation as well as the amplitude of a voltage-dependent Ca2+ current (ICa) were reduced by baclofen. The reduction of ICa resulted in a concomitant suppression of a Ca(2+)-dependent K+ current (IK-Ca) that was sufficient to account for the reduction of the AHP after evoked action potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Trial-spacing effects in Hermissenda suggest contributions of associative and nonassociative cellular mechanisms.

In behaving Hermissenda, a preparatory conditioned response developed across repeated pairings of light (conditioned stimulus; CS) and rotation (unconditioned stimulus; US) with intertrial intervals (ITIs) of 60 and 120 s, but not 30 s. Likewise, contiguous in vitro stimulation of the visual and vestibular receptors, an analog of behavioral conditioning, resulted in an increase in the input resistance (i.e., excitability, a correlate of conditioning) of the B photoreceptors of the Hermissenda's eye, but only with ITIs greater than 60 s. Calcium signaling in the B cell, critical to the induction of this neuronal plasticity, was attenuated with shorter ITIs owing to (a) a reduction of the light-induced generator potential and hence voltage-dependent Ca2+ influx during the light CS, (b) a depression of the Ca2+ current that persisted throughout shorter ITIs, and (c) a steady-state inactivation of the Ca2+ current as a result of a sustained depolarization persisting from the previous trial. These results are consistent with a 2-process theory of associative learning in which a primary process (Ca2+ influx) may be opposed by a secondary process (depression of the Ca2+ current) during short ITIs.

Current, voltage and pharmacological substrates of a novel GABA receptor in the visual-vestibular system of Hermissenda.

In the marine mollusc, Hermissenda crassicornis, Type B photoreceptors exhibit an IPSP to both presynaptic hair cell stimulation and microapplication of gamma-aminobutyric acid (GABA) to the terminal branches. It was found that both the endogenous IPSP and the response to exogenously applied GABA were mediated to a large part by an outward current which reversed at approximately -80 mV. Additionally, these hyperpolarizing responses were found to mask a smaller depolarization that was mediated by the reduction of a basal outward current. Both the IPSP and the hyperpolarizing response to GABA, as well as the sublimated depolarizing response to GABA, were attenuated by the K+ channel blocker tetraethylammonium chloride (TEA) and displayed a strong sensitivity to [K+]o, while showing no sensitivity to [Cl-]o or the Cl- channel blocker picrotoxin. Moreover, iontophoretic injections of stable guanine analogues, GTP[gamma S] and GDP[beta S], into B photoreceptors eliminated both the IPSP and the GABA-induced hyperpolarization, while cholinergically mediated, interphotoreceptor interactions were unaffected. These results suggest that the endogenous receptor is at least partially homologous to the mammalian GABAB class receptor. Consistent with this classification, microapplication of selective GABAB receptor agonist baclofen onto the terminal region of the B photoreceptor resulted in a hyperpolarizing response that was qualitatively similar to that of GABA, although the GABAA agonist muscimol was also active, but less so than either GABA or baclofen. Attempts to block the endogenous IPSP or GABA-induced hyperpolarization by bath application of the GABAA receptor subtype antagonist bicuculline was ineffective and the GABAB receptor subtype antagonist saclofen was only weakly effective. These data demonstrate that the presynaptic hair cell's influence on postsynaptic B photoreceptors is in many respects similar to GABAB mediated responses in the mammalian CNS. This receptor is in some respects unique, however, in terms of its cross-sensitivity to both GABAA and GABAB agonists, its weak sensitivity to saclofen, and its apparent anomalous modulation of multiple K+ conductances.

NTS neuronal responses to arterial pressure and pressure changes in the rat.

Central representation of arterial pressure by baroreceptor target neurons in the nucleus of the solitary tract (NTS) has not been studied. The present experiments sought to characterize response patterns of NTS baroreceptive cells in anesthetized, paralyzed, and artificially ventilated rats to both resting pressures and induced blood pressure challenges. Single-unit extracellular recordings were made from 83 baroreceptive NTS cells that received a synaptic input after electrical stimulation of the aortic nerve [latency, 2.46 +/- 0.78 (+/- SD) ms] and were located at an anatomically defined region known to receive baroreceptor afferents. Seventy-one neurons were presumed second order, since they received a short (< 5 ms) and invariant (< +/- 0.5 ms) synaptic input from the ipsilateral aortic nerve. Thirty-five of these neurons were silent at resting blood pressures and produced few (1-4) spikes when presented with induced pressor responses. The remainder (n = 36) had ongoing activity that was not pulse rhythmic and that varied in rate nonuniformly with arterial blood pressure during an induced challenge. Ongoing activity was analyzed for active neurons, revealing both R wave-related and lung inflation-related phasic activity. The present data suggest that baroreceptive NTS neurons may be sensitive to many characteristics of the input signal, such as dP/dt, mean pressure, and cardiac frequency, as well as pulmonary afferent drive.

Postsynaptic calcium, but not cumulative depolarization, is necessary for the induction of associative plasticity in Hermissenda.

The neuronal modifications that underlie associative memory in Hermissenda have their origins in a synaptic interaction between the visual and vestibular systems, and can be mimicked by contiguous in vitro stimulation of these converging pathways. At the offset of vestibular stimulation (i.e., hair cell activity), the B photoreceptors are briefly released from synaptic inhibition resulting in a slight depolarization (2-4 mV). If contiguous pairings of light-induced depolarization and presynaptic vestibular activity occur in close temporal succession, this depolarization "accumulates" and has been hypothesized to culminate in a sustained rise in intracellular Ca2+ and a resultant Ca(2+)-mediated phosphorylation of K+ channels as well as an associated increase in input resistance. Here we demonstrate that this cumulative depolarization is neither necessary nor sufficient for the biophysical modifications of the B cell membrane indicative of memory formation. Consistent with several recent reports of one-trial learning in Hermissenda, one pairing of light with mechanical stimulation of the vestibular hair cells resulted in a rise in neuronal input resistance across the B cell membrane that was attenuated by a prepairing iontophoretic injection of the Ca2+ chelator EGTA (25 mM), indicating that this potentiation was Ca2+ dependent. However, the use of a single pairing negates the possibility of an accumulation of depolarization across trials. In a subsequent experiment, B photoreceptors underwent a cumulative depolarization, and a coincident rise in input resistance, during multiple pairings of light and hair cell stimulation. However, if the B photoreceptor was voltage clamped at its initial resting potential before and after each pairing, thus eliminating the cumulative depolarization, the rise in resistance not only persisted, but was enhanced. Moreover, if unpaired light presentations were followed by a current-induced depolarization (to mimic cumulative depolarization), no increase in input resistance was detected. To assess directly the effect of a cumulative depolarization on the voltage-dependent Ca2+ current, an analysis of the inward current on the B cell soma membrane was conducted. It was determined that (1) the inward current may undergo a partial inactivation during sustained depolarization, (2) the peak current was depressed during repetitive depolarizations, and (3) the peak current underwent a steady-state inactivation, such that it was reduced when elicited from holding potentials more positive than -60 mV. The analysis of this current suggests that pairings of light and presynaptic activity would reduce voltage-dependent Ca2+ influx when those pairings are conducted at depolarized membrane potentials, such as during cumulative depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Survey of 1977 crop year preharvest corn for vomitoxin.

Fifty-two preharvest corn samples were collected in mid-October 1977 from 26 farms in a four-county area of northwest Ohio. Vomitoxin ranging from 0.5 to 10 microgram/g was found in 24 of the samples. Analysis for vomitoxin was by gas-liquid chromatography. Preceding harvest in northwest Ohio, unusual wet conditions prevailed, making it favorable for Fusarium growth. Gibberella zeae-infected kernels ranged from 2 to 50% of the kernels analyzed for 44 corn samples, and 8 corn samples showed no infection.

Actinomycetales from corn.

Mesophilic Actinomycetales were isolated from whole corn, brewers grits, and break flour received from three different mills. In addition, strains were isolated from high-moisture (27 per cent) field corn; high-moisture, silo-stored corn (untreated); and high-moisture corn treated with ammonia, ammonium isobutyrate, or propionic-acetic acid. According to standard techniques, 139 strains were extensively characterized and 207 additional strains were partially characterized. On the basis of these characterizations, the streptomycete strains were identified by both the systems of Pridham et al. and Hütter because these systems are rapid and accurate. In general, only Streptomyces griseus (Krainsky) Waksman and Henrici was isolated from high-moisture whole corn (treated or untreated) except from grain exposed to ammonium isobutyrate. Strains isolated from high-moisture corn subjected to that treatment represented both S. griseus and S. albus (Rossi Doria) Waksman and Henrici. The strains isolated from corn and corn products from the three mills were identified with a number of streptomycete species. Of all Actinomycetales isolated, only three were not streptomycetes--two from brewer's grits and one from break flour.