Flux and composition of interstellar dust at Saturn from Cassini's Cosmic Dust Analyzer

Interstellar dust (ISD) is the condensed phase of the interstellar medium. In situ data from the Cosmic Dust Analyzer on board the Cassini spacecraft reveal that the Saturnian system is passed by ISD grains from our immediate interstellar neighborhood, the local interstellar cloud. We determine the mass distribution of 36 interstellar grains, their elemental composition, and a lower limit for the ISD flux at Saturn. Mass spectra and grain dynamics suggest the presence of magnesium-rich grains of silicate and oxide composition, partly with iron inclusions. Major rock-forming elements (magnesium, silicon, iron, and calcium) are present in cosmic abundances, with only small grain-to-grain variations, but sulfur and carbon are depleted. The ISD grains in the solar neighborhood appear to be homogenized, likely by repeated processing in the interstellar medium.

Electrical properties affecting discharge of units of the mid and posterolateral thalamus of conscious cats

Discharge properties in response to intracellularly applied, rectangular currents were measured in units of the mid (lateralis dorsalis and centrolateral nuclei) and posterolateral (lateralis posterior and pulvinar nuclei) thalamus of conscious cats. A separate aim was to determine if neuronal excitability changed in association with changes in stimulus-evoked activity after the animals were trained to discriminate between two acoustic stimuli when performing a conditioned motor response. Low threshold spike (l.t.s.) discharges were observed in three of 272 cells given 1 nA intracellular, hyperpolarizing current pulses of 40 ms duration. This finding supports the view that thalamic neurons of conscious animals operate mainly in the relay as opposed to the oscillatory mode. Application of larger and longer hyperpolarizing currents in the cells produced rebound l.t.s. discharges, supporting the expectation that most thalamic neurons are capable of producing this type of discharge. Decrements of spike afterhyperpolarizations (AHP) and broadening of spike bases upon repeated discharge also were observed in each area of the thalamus studied. After conditioning, changes were found in the posterolateral thalamus (but not in the mid-thalamus) in the proportions of cells with spontaneous, rapid (>/=50 Hz), repetitive, discharges (RRD) and rapid, sustained discharges at rates >/=100 Hz during application of depolarizing current (RSD). In the posterolateral thalamus the percentage of units responding to 1 nA depolarization with RSD fell from 71% before conditioning to 45% after conditioning. The percentage of cells with RRD decreased from 69% to 46%. The changes were accompanied by a 3 mV hyperpolarization of the membrane potentials of the cells and a decrease in baseline activity. After conditioning, increases in excitability were found in cells of the mid thalamus that responded selectively to the click conditioned stimulus (CS) that elicited the conditioned response, and decreases in excitability were found in cells of the posterolateral thalamus that responded to the discriminative acoustic stimulus (DS) to which the animals were trained not to respond. An earlier study showed a potentiation of discharge in response to the CS in units of the midthalamus after similar conditioning and a reduction of the proportion of DS responsive units and peak discharge to the DS in units of the posterolateral thalamus. We conclude that the discharge properties of units of the mid and posterolateral thalamus can change to support discrimination between acoustic stimuli of different functional significance after conditioning.

Multiple representations of information in the primary auditory cortex of cats. I. Stability and change in slow components of unit activity after conditioning with a click conditioned stimulus

Recordings of activity were made from 647 single units of the A(I) cortex of awake cats to evaluate behavioral state-dependent changes in the population response to a 70-dB click. Averages of PST histograms of unit activity were used to assess the changes in response. This report focuses on slow components of the responses disclosed by averages employing bin widths of 16 ms. Responses were compared before and after a Pavlovian blink CR was produced by forward pairing of click conditioned stimuli (CSs) with USs. A backward-paired 70-dB hiss was presented as a discriminative stimulus. Studies were also done after backward pairing of the click CSs (backward conditioning) that produced weak sensitization instead of a conditioned response. There were four main findings. First, components of activity elicited 32-160 ms after presenting the hiss decreased significantly after conditioning and after backward conditioning. The decreases after conditioning represented the most pronounced changes in activity evoked by either clicks or hisses in this behavioral state. Second, baseline firing decreased after both conditioning and backward conditioning. The direction of baseline change was opposite that found in adjacent cortical regions and in A(I) cortex after operant conditioning employing an acoustic cue. Third, prior to conditioning, unit activity in response to the hiss declined before the sound of the hiss reached its peak or terminated. This decrease was thought to represent a habituatory adaptation of response to a prolonged acoustic stimulus. This type of habituation to a lengthy stimulus has been recognized, behaviorally, but has not been observed previously in the activity of units of the auditory receptive cortex. Fourth, the percentage of click responsive units did not change significantly after the click was used as a CS for conditioning, and despite the accompanying changes in baseline activity, the absolute levels of activity summed in the first 16 ms after click delivery remained stable across behavioral states in which the motor response to the click was altered profoundly. The onset of the conditioned motor response began 20 ms after the click, and was shown earlier to depend on rapid, potentiated transmission through the cochlear nucleus and motor cortex for its generation. Thus the stability of the response to the click in the primary auditory receptive cortex was unexpected. This led us to make further analyses of the data with 2- and 4-ms bin widths (see companion report) that eventually disclosed a potentiated response to the click. The findings show stability and change in the response to the click as a CS, depending on the band pass (bin width) used for analysis of spike activity. In the representation disclosed by low pass filtering in this study, the response was stable. This representation provided information suitable for identifying commonalties of the click signals across varying behavioral states. The representations of the click and hiss contained in the slow components of the population response in the A(I) cortex were uncorrelated with the selective potentiation of activity in motor cortex and behavioral performance in response to click as a CS after conditioning. Although changes in the activity evoked by hisses occurred after conditioning, the changes also occurred after backward conditioning when only small, sensitized behavioral responses to clicks and hisses were observed. Basic theoretical considerations about information transmission in complex neural networks plus clinical observations comparing derangements of linguistic and non-linguistic cortical functions in humans suggest that multiple representations of conditioned stimulus inputs may exist in local populations of cortical neurons. Together, our studies provide evidence for two different, concurrent representations of information about a click CS encoded in the spike activity of the A(I) cortex.

Multiple representations of information in the primary auditory cortex of cats. II. Stability and change in early (<32 ms), rapid components of activity after conditioning with a click conditioned stimulus

Activity was recorded from single units of the A(I) cortex of awake animals to identify early (<32 ms) components of the population response to a 70 dB click and establish if they changed after using the click as a CS for conditioning. A 70 dB hiss was used as a discriminative stimulus. Responses to these stimuli were compared before and after a forward order of pairing that produced conditioning and a backward order of pairing that produced weak sensitization (backward conditioning). Averages of discharges in 2 and 4 ms bins distinguished primary (8-12 ms) from secondary (12-16 ms) temporal components of response to the click, and confirmed that the onset of the response was shorter in A(I) (8 ms, mean of 647 units) than in the adjacent, A(II) cortex (16 ms, mean of 95 units). (All times include a 1.6 ms transmission delay in sound arrival.) Primary and secondary components of A(I) responses to click did not change uniformly after changes in behavioral state, and were affected differently by both conditioning and backward conditioning. The percentage of cells with onsets of response to the click at secondary latencies (and to the hiss at tertiary latencies) increased after backward conditioning but not after conditioning, as did the magnitude of activity in response to the click. (The latter had a lesser degree of increase after conditioning.) The primary response to the click did not show these increases. The non-uniform changes suggested that temporal processing of the click was conducted differently in the 8-12 ms post stimulus period than in the 12-16 ms period. Within the total population of cells, it was possible to identify a small subgroup (13%) of highly auditory-responsive units that showed an increased primary response to the click as a CS selectively after conditioning and not after backward conditioning. The secondary component of response in these cells increased after both conditioning and backward conditioning. The percentages of cells responding to the click and hiss at primary latencies did not change significantly after conditioning, even in the subgroup of highly responsive cells. The results characterize differently timed components of rapid responses to acoustic stimuli in the A(I) cortex, disclose significant temporal differences in primary, secondary and tertiary information processing that affect the representations of the transmitted acoustic message across different behavioral states, and find one representation in a small subgroup of cells that supports the hypothesis that cells of the A(I) cortex have a selectively potentiated response to the CS after conditioning.

Enhanced sensitivity to N-methyl-D-aspartate receptor activation in transgenic and knockin mouse models of Huntington's disease

We used two mouse models of Huntington's disease (HD) to examine changes in glutamate receptor sensitivity and striatal electrophysiology. One model, a transgenic, consisted of mice expressing exon 1 of the human HD gene and carrying 141-157 CAG repeat sequences (R6/2 line). The second model, a CAG repeat "knockin," consisted of mice with different lengths of CAG repeats (CAG71 and CAG94 repeats). The effects of glutamate receptor activation were examined by visualizing neurons in brain slices with infrared videomicroscopy and differential interference contrast optics to determine changes in somatic area (cell swelling). Striatal and cortical neurons in both models (R6/2 and CAG94) displayed more rapid and increased swelling to N-methyl-D-aspartate (NMDA) than those in controls. This effect was specific as there were no consistent group differences after exposure to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) or kainate (KA). Intracellular recordings revealed that resting membrane potentials (RMPs) in the R6/2 transgenics were significantly more depolarized than those in their respective controls. RMPs in CAG94 mice also were more depolarized than those in CAG71 mice or their controls in a subset of striatal neurons. Confirming previous results, R6/2 mice expressed behavioral abnormalities and nuclear inclusions. However, CAG71 and CAG94 knockins did not, suggesting that increased sensitivity to NMDA may occur early in the disease process. These findings imply that NMDA antagonists or compounds that alter sensitivity of NMDA receptors may be useful in the treatment of HD.

Differences in responses to 70 dB clicks of cerebellar units with simple versus complex spike activity: (i) in medial and lateral ansiform lobes and flocculus; and (ii) before and after conditioning blink conditioned responses with clicks as conditioned stimuli

Activity was recorded from 554 cerebellar units in eleven conscious cats to determine if responses to 70 dB clicks differed in units with simple and complex spike discharges. Effects of region of recording and behavioral state (with click used as a conditioned stimulus for conditioning) were also assessed. Cells with only simple spikes were distinguished from cells that had the following types of complex spike events: Type I-simple or initial spike followed > 1 ms by multiple spikes with baseline displacement (classical complex spikes), Type II--followed < or = 1 ms by spikes with or without baseline displacement (spikes in the absolute refractory period should arise from a separate site of initiation), and Type III-followed by spikes and displacement too close to the baseline noise to distinguish as Type I or II. Among the groups mean baseline activity was greatest in cells with Type I complex spikes, least in cells with Type III complex spikes, and greater in Type II cells than simple cells. Significant increases in activity within 32 ms of presenting clicks were found in the groups of Type II cells and simple cells. These appear to be the main auditory responsive cells of the cerebellar regions studied. Activity of Type II cells best reflected the temporal properties of the click; responses of simple cells had slower onsets (except in flocculus) and longer durations. Responses to click in Type II and simple cells differed in recordings from: (i) lateral ansiform lobe (lateral crus I and portions of crus II), (ii) medial ansiform lobe (medial crus I), and (iii) flocculus. The largest mean responses above baseline in the first 32 ms after click were found in Type II cells of the lateral ansiform lobe with onsets of 8-16 ms. Magnitudes of response differed before and after conditioning and backward conditioning. In the lateral ansiform lobe, the < 32 ms response to click was greater in Type II than simple cells in each state, but showed a greater increase above baseline after backward conditioning when conditioned responses were not produced than after conditioning. The onset of increased activity to click conditioned stimuli in Type II cells of the lateral ansiform region preceded the onset of the blink conditioned response after conditioning, consisted almost entirely of simple spikes, and reflected an increase in magnitude of response as opposed to an increased number of responsive units. After conditioning, an increased number of units in the flocculus responded to click conditioned stimuli in the 16-24 ms post stimulus period. Of the 16 cells with an onset of increased activity at this time, eight showed only simple spike activity. Seven of the remaining eight cells (all Type II) showed a significant increase in conditioned stimulus-evoked complex spiking above the low (usually < 1/s) baseline level of complex spike discharges. The findings support the conclusions that cerebellar units can respond rapidly enough to acoustic stimuli to play a role in auditory as well as motor processing and that the responses to 70 dB clicks differ among cells with simple and complex spike discharges. The differences are influenced substantially by the region of cerebellar recording and the behavioral state. The findings in cells of the flocculus offer the first evidence that complex as well as simple spike activity can contribute to an increased probability of discharge to click as a conditioned stimulus after conditioning.

Dopaminergic modulation of early signs of excitotoxicity in visualized rat neostriatal neurons

Cell swelling induced by activation of excitatory amino acid receptors is presumably the first step in a toxic cascade that may ultimately lead to cell death. Previously we showed that bath application of N-methyl-D-aspartate (NMDA) or kainate (KA) produces swelling of neostriatal cells. The present experiments examined modulation of NMDA and KA-induced cell swelling by dopamine (DA) and its receptor agonists. Nomarski optics and infra-red videomicroscopy were utilized to visualize neostriatal medium-sized neurons in thick slices from rat pups (12-18 postnatal days). Increase in somatic cross-sectional area served as the indicator of swelling induced by bath application of glutamate receptor agonists. NMDA induced cell swelling in a dose-dependent manner. Activation of DA receptors in the absence of NMDA did not produce swelling. DA and the D1 receptor agonist SKF 38393, increased the magnitude of swelling produced by NMDA. This effect was reduced in the presence of the D1 receptor antagonist, SCH 23390. In contrast, activation of D2 receptors by quinpirole decreased the magnitude of NMDA-induced cell swelling. DA slightly attenuated cell swelling induced by activation of KA receptors. Quinpirole produced a significant concentration-dependent reduction in KA-induced swelling while SKF38393 increased KA-induced swelling, but only at a low concentration of KA. Together, these results provide additional support for the hypothesis that the direction of DA modulation depends on the glutamate receptor subtype, as well as the DA receptor subtype activated. One possible consequence of these observations is that endogenous DA may be an important contributing factor in the mechanisms of cell death in Huntington's disease.

Facilitation of acoustic responses of cartwheel neurons of the cat dorsal cochlear nucleus

Responses to clicks were increased in cartwheel cells of the dorsal cochlear nucleus of cats after pairing presentations of the clicks with local iontophoretic delivery of glutamate. The cells were identified by bursting discharges, and were recorded intracellularly in vivo. The findings indicate that inhibitory interneurons such as cartwheel cells can participate in complex adaptive acoustic signal processing. Each cell displayed doublet discharges of > 800 Hz. In 70% of the cells, some of the doublet discharges reached rates > 1000 Hz.

Activation of cells of cochlear nucleus by electrical stimulation of lateral hypothalamus

Effects of electrical stimulation of the lateral hypothalamus (HS) were examined in 67 cells of the dorsal or ventral cochlear nucleus. Both short latency activity in the 10-20 ms post-stimulus period and late activity in the > 20 ms post-stimulus period were elicited in response to HS. A greater percentage of units exhibited the short latency response in dorsal (89%) than ventral (68%) cochlear nucleus. It was not previously recognized that stimulation of the hypothalamus could elicit increases in spike activity in this auditory relay nucleus. The hypothalamus is known to play a role in visceral-emotional functions, including feeding, fleeing, fighting and reproductive behavior. These results suggest a means by which neural activities supporting these functions could influence acoustic relay transmissions.

Inhibition of discharge in inferior colliculus, AII cortex and Ep cortex after presentations of click stimuli

A temporally related reduction of discharge in response to 70-dB clicks was identified in secondary auditory (AII) cortex (48-56 ms after click), posterior ectosylvian (Ep) cortex (40-56 ms after click) and inferior colliculus (IC) (56-76 ms after click). Units in primary auditory (AI) cortex, dorsal cochlear nucleus (DCN) and ventral cochlear nucleus (VCN) did not demonstrate a significant reduction of discharge at comparable periods. Neurons of AI cortex showed increased activity 36-40 ms after click. The timing of the periods of inhibited discharge in AII, Ep and IC, taken with the earlier activation of AI, supported the hypothesis of an inhibitory auditory pathway emanating from AI, affecting secondary auditory cortical regions and IC.

Identification of short latency auditory responsive neurons in the cat dentate nucleus

Intracellular recordings of activity in response to acoustic stimuli were obtained from units of the dentate nucleus of conscious cats. Twelve units with short latency responses to 70 dB clicks or hisses were injected intracellularly with biocytin and identified morphologically. The identified cells were small, relatively aspinous, multipolar cells with diameters < 20 microns. Most had beaded dendritic varicosities. Six were located centrally, and five were on the border of the nucleus. One appeared to be an axonal process. The results provide direct evidence that small cells of the dentate nucleus can respond with short latencies of 4-14 ms to acoustic stimuli. We suggest that these cells are part of a primary ascending auditory transmission pathway between cochlear nuclei and the motor cortex.

Response to acoustic stimuli increases in the ventral cochlear nucleus after stimulus pairing

Recordings of activity in response to click and hiss were made from 364 units of the ventral cochlear nucleus of cats. The unit response to acoustic stimuli increased after forward or backward pairing of the stimuli with glabella tap and hypothalamic electrical stimulation. The results provide evidence against the widely held view that transmission through this initial brain stem relay of the auditory system is invariant, and suggest, instead, that the activity of the ventral cochlear nucleus changes to support increased attentiveness to acoustic signals after variably ordered pairing of conditioned and unconditioned stimuli.

Unit activity to click CS changes in dorsal cochlear nucleus after conditioning

Recordings were made of single unit activity (n = 360 units) from the dorsal cochlear nucleus of cats. Different patterns of activity were elicited by acoustic stimuli before and after Pavlovian conditioning. The peak response to a forward paired click conditioned stimulus (CS) increased whereas that to a backward paired hiss discriminative stimulus (DS) did not. The percentage of units responding to the CS increased from 34% to 46% after conditioning. The findings do not support the widely accepted hypothesis that learning has no effect on transmission through the first brain stem relay of the auditory system and indicate, instead, that the cochlear nucleus can participate in complex adaptive acoustic signal processing.

The dentate nucleus is a short-latency relay of a primary auditory transmission pathway

Recordings of unit activity showing 4-6 ms latency responses to a click stimulus provided evidence that the dentate nucleus could function as a short-latency auditory relay. On the basis of these findings, plus fiber fillings from injections of phaseolus leucoagglutinin into the dentate, a new auditory pathway between dorsal and ventral cochlear nuclei, dentate nucleus, and rostral thalamus is proposed. The pathway could provide direct, short-latency transmissions to the motor cortex that bypass the classical auditory receptive cortex.

Changes in membrane currents during Pavlovian conditioning of single cortical neurons

Single electrode voltage clamp recordings were made during Pavlovian conditioning of single units of the motor cortex of cats. Units that developed a conditioned spike discharge in response to a click conditioned stimulus (CS) after pairing the click with glabella tap and local ionophoretic application of glutamate showed increases in input resistance and reductions of an early outward current induced by depolarizing commands and by return to holding potentials after hyperpolarizing commands. Changes in later currents were also found in some cells. Units that failed to develop a conditioned response did not show these changes. The decreases in membrane currents could contribute to an increased spike discharge in response to the CS as could the increased input resistance observed after conditioning. Conductance changes of this type may serve as engrams by which some forms of memory and learning are expressed across both vertebrate and invertebrate species.

Patterns of unit activity in the rostral thalamus of cats related to short-latency discrimination between different auditory stimuli

Short-latency auditory-responsive units were found in the rostral thalamus of cats during performance of conditioned eyeblink responses (CRs) elicited discriminatively by a forward-paired, 70-dB-click conditioned stimulus (CS) as opposed to a backward-paired, 70-dB-hiss discriminative stimulus (DS). Discharges in response to the CS or DS were found in 57% of 138 units tested. Forty-one percent of units responding to the CS did so at latencies of less than 40 msec. After conditioning a discriminative CR to click CS, an increase in the ratio of CS-evoked activity to baseline activity was found relative to that before conditioning. This increase was attributable, in part, to a decrease in baseline activity and, in part, to an increase in the magnitude of response to the CS. These responses preceded early components of the electromyographically measured motor responses with latencies sufficient to contribute to initiation of the movement. After acquisition of the CR, the proportion of CS responsive units also increased. We conclude that this area of the thalamus, a region thought to support thalamocortical reverberatory activity, also functions to transmit short-latency auditory signals. Our evidence further suggests that this region may participate in the elicitation of conditioned responses by specific auditory stimuli and in discrimination between auditory stimuli of different significance.

Oversized, auditory responsive units of rostral, mid, and posterolateral thalamus

Activity was recorded from 343 units of rostral, mid, and posterolateral thalamus following a conditioned click stimulus (CS). Over 40% of units responded with increased discharge in cats conditioned to blink to the CS. Twenty-nine units with short latency (less than 40 ms) responses were injected intracellularly with phaseolus lectin and identified morphologically; 83% had long, thick primary dendrites with smaller secondary branches. Almost half (46%) had larger somata than the largest previously described thalamic neurons of this morphologic classification. The results suggest that a previously unidentified class of oversized cells is likely to contain many short latency, auditory responsive units. A substantial number of these cells (36%) projected extrathalamically into the internal capsule, and thus may constitute a new auditory pathway between thalamus and cortex.

An aminopyridine-sensitive, early outward current recorded in vivo in neurons of the precruciate cortex of cats using single-electrode voltage-clamp techniques

Studies were performed in cortical neurons to determine if voltage- and time-dependent membrane currents could be recognized and characterized in the dynamic, in vivo state. Intracellular measurements made in neurons of the precruciate cortex of awake cats with single-electrode voltage-clamp (SEVC) techniques disclosed an early outward current to depolarizing command steps in 124 of 137 cells studied. The voltage-dependent properties of the early outward current closely resembled those of A-currents studied in vitro in vertebrate and invertebrate neurons. The current was activated rapidly at onset latencies of less than two ms, fell to flat plateau levels within 60-120 ms during sustained depolarization, and was reduced or eliminated in 22 of 23 cells following intracellular administration of 3- or 4-aminopyridine. The magnitude of outward current in response to depolarizing commands was increased by preceding steady hyperpolarization and reduced by preceding steady depolarization. (The steady potentials were of 9.8 s duration and +/- 40 mV apart from the holding potentials.) Since return to the holding potentials occurred 80 ms before the onset of the command steps, the changes in membrane properties that were induced lasted beyond cessation of the steady polarizing stimuli themselves. Spiking did not prevent recognition of the early outward current as judged from its appearance before and after intracellular application of QX-314 to reduce spike activity. Apart from fast inward currents associated with spike potentials, the early outward current was the most conspicuous and characteristic membrane current noted in these recordings. An additional current component that was noted but not characterized in these studies was a slow, depolarization-induced inward current that could be reduced by intracellular injection of QX-314.

Acetylcholine reduces net outward currents measured in vivo with single electrode voltage clamp techniques in neurons of the motor cortex of cats

Effects of acetylcholine (ACh) on membrane currents of cells of the motor cortex were measured directly, in vivo, in awake cats using single electrode voltage clamp (SEVC) techniques. Extracellular applications (90-95 nA) of 2 M ACh for periods of 30 s or less produced significant decreases in net outward currents elicited by depolarizing commands whereas applications of saline did not. Reductions of net outward currents were also obtained after intracellular pressure injections of cyclic guanosine monophosphate (GMP)-dependent protein kinase (cGPK) mixed with 10 microM cyclic GMP.

Intracellular injection of cGMP-dependent protein kinase results in increased input resistance in neurons of the mammalian motor cortex

Purified, cyclic GMP-dependent protein kinase (cGPK) was pressure-injected into neurons of the precruciate cortex of awake cats. Input resistances increased within seconds after injection and remained elevated for 2 min or longer. The increases were larger when cGPK was injected in a mixture with 10 microM cGMP than when injected alone. Injections of heat-inactivated cGPK, with or without 10 microM cGMP, failed to produce increases in input resistance. The present results indicate that injection of activated cGPK into neurons of the mammalian motor cortex can mimic actions of extracellularly applied acetylcholine and intracellularly applied cGMP, the latter in 100-fold higher concentrations than those used here, in neurons of the same cortical areas.

Responses of morphologically identified cortical neurons to intracellularly injected cyclic GMP

Cyclic nucleotides are thought to act as second messengers of neurotransmission inside central neurons, and cyclic guanosine monophosphate (cGMP) has been postulated to act as a messenger for muscarinic, cholinergic transmission. Nonetheless, the action of cGMP has not yet been established in identified cortical neurons. We injected cGMP and horseradish peroxidase (HRP) intracellularly in neurons of the motor cortex of awake cats. Fifty-four percent of injected cells responded to cGMP and HRP with an increase in input resistance within 30 s after injection. None of a control group of cells injected with HRP without cGMP so responded. In cells receiving intracellular depolarizing current sufficient to produce repeated spike discharge at the time of injection, the increase in input resistance after cGMP persisted for as long as the cells could be held. There was no significant increase in firing rate after injection of cGMP. Cells responding to cGMP with an increased input resistance were identified as pyramidal cells of layer V. One inverted pyramidal cell of layer VI also showed an increase in input resistance in response to cGMP. Previous studies have suggested that muscarinic cholinergic agents produce an increased input resistance (thought to reflect a decreased potassium conductance) underlying an increased rate of discharge in neocortical neurons. Our results favor a dual action of muscarinic cholinergic transmission in mammalian cortical neurons--the increase in input resistance in layer V pyramidal cells mediated by cGMP, and the increase in rate of discharge mediated by other means.

Responses of morphologically identified cortical neurons to intracellularly injected cyclic AMP

Intracellular injections of cyclic adenosine monophosphate (cAMP) and horseradish peroxidase (HRP) were made in neurons of the motor cortex of awake cats. Eighty-six percent of injected cells responded to cAMP and HRP with a rapid decrease in input resistance. The decreases in input resistance occurred immediately after injection and began to return toward baseline 2 to 3 min later. The decreases were significantly greater than the small decreases in input resistance normally seen in uninjected cells held for 2 min or more after penetration and exceeded comparably small decreases in input resistance seen after control injections of 5'-AMP plus HRP. Pyramidal cells of layer V were identified as responding to cAMP with a decreased input resistance. A spiny stellate cell of layer III and a pyramidal cell of layer VI were also identified that showed similar responses. Increased rates of discharge were also observed after penetration with electrodes containing cAMP, but significant changes in input resistance were not found in association with the increased rates of discharge. After pressure injection of cAMP, the rates of discharge decreased toward more normative values. Our findings indicate that cAMP has an effect on cortical neurons similar to that found in some types of invertebrate (molluscan) neurons and dissimilar to the effect of cyclic guanosine monophosphate.

Intradendritic recordings from neurons of motor cortex of cats

The properties of neuronal spike potentials ranging from 20 to 60 mV in size were studied in stable recordings made from the motor cortex of unanesthetized, unparalyzed cats. Histologic evidence that injection of horseradish peroxidase (HRP) was confined to single neurons and appropriately high levels of potassium measured with K+ ion-sensitive electrodes indicated that these recordings were obtained from single, intracellularly penetrated neurons. Pressure injection of small volumes of 4% HRP in 1 M KC1, intracellularly, was characteristically associated with transient increases in spike size and decreases in input resistance. Recoveries of HRP-filled dendritic processes without filling of somata were obtained from penetrations giving spikes smaller than the recorded resting potential (spike-undershoot recordings). Recordings with dendritic recoveries had higher input resistances and showed greater increases in spike size during pressure injection than did recordings with both somata and dendrites recovered. The activity in response to a weak click elicited in cells with spike potentials between 20 and 40 mV was as great or greater than that elicited in cells with larger spike potentials or in extracellularly recorded units. We conclude that many stable spike-undershoot recordings of neurons of the motor cortex of awake cats reflect penetrations of dendritic processes as opposed to injury because they show a) a normal functional response to weak auditory stimuli delivered over periods of many minutes, b) a reversible increase in spike size during pressure injection of small volumes of HRP in KC1 consistent with penetration of a cable remote from the site of spike generation, and c) the frequent recovery of HRP-marked dendrites without somata.

Effects of acetylcholine and cyclic GMP on input resistance of cortical neurons in awake cats

The effects on input resistance (Rm) of extracellular iontophoresis of acetylcholine (ACh) and of intracellular iontophoresis of cyclic GMP (cGMP) were studied in neurons of the coronal-pericruciate cortex of awake cats. Control studies were also conducted including iontophoresis of saline extracellularly and 5'-GMP intracellularly. (1) Substantial increases in Rm occurred in approximately half the neurons given ACh or cGMP. (2) In the absence of associated repetitive spike discharge induced by intracellular injection of depolarizing current pulses during iontophoretic applications, the increases in Rm were transient occurring in less than 30 sec and lasting 4--5 min. (3) With associated current-induced spike discharge, the increases in Rm persisted for as long as the neurons could be held -- up to 1.5 h maximally. (4) Rm was not increased if saline was substituted for ACh, if 5'-GMP was substituted for cGMP, or if the neuron was only discharged repeatedly. (5) The magnitude and time course of both transient and persistent increases in Rm were comparable between cells given ACh or cGMP and whether action potentials and resting potentials were greater than or equal to 40 mV (average 47 mV)* or were less.

Characterization of electrophysiological properties of intracellularly recorded neurons in the neocortex of awake cats: a comparison of the response to injected current in spike overshoot and undershoot neurons

Intracellular recordings were obtained from 212 neurons of the coronal pericruciate cortex of 7 awake, untrained cats. Glass microelectrodes, filled with K+ citrate alone or K+ citrate with either cyclic GMP or 5'-GMP were used for recording and for injecting steady depolarizing and hyperpolarizing currents intracellularly. The effects of rectangular linearly rising (ramp) current pulses were also studied. Results were compared in spike overshoot* versus undershoot recordings. Spike overshoot recordings had action potentials (APs) larger than associated baseline shifts on penetration; undershoot recordings had APs smaller than associated baseline shifts on penetration. Undershoot recordings are more commonly encountered in mammalian neocortex than are overshoot recordings. (1) Except for sizes and slopes of APs and other effects consistent with the penetration of passive dendritic cables remote from regions of active spike initiation or propagation, no differences in response to current injection or in other electrophysiological properties were found between overshoot and undershoot recordings. (2) Injection of depolarizing currents produced de-reases in the amplitudes of APs, decreased rates of rise and fall of APs and increased frequencies of AP discharge. Injection of hyperpolarizing current produced slowing or cessation of AP discharge with little or only slight increases in AP amplitude when the resting potential was greater than 47 mV. (3) An effectively linear relationship was found between changes in AP size and the magnitude of weak injection depolarizing currents. This relationship provides a basis for measuring changes in cortical neuronal input resistance by the differential spike height method. (4) Most neurons showed little or no accommodative response to the injection of linearly rising, depolarizing currents. Simple or ceiling threshold-latency curves rather than minimal gradient curves were obtained in 83% of the cells in which ramp currents were injected. (5) Modal values of resting potentials between 47 and 53 mV, without increased rates of spontaneous discharge, indicate that most cells have a critical firing threshold near that reported for somatodendritic (SD) rather than initial segment (IS) generated spikes. The evidence suggests that undershoot recordings primarily reflect penetrations of passive dendritic regions rather than functional modification of neurocellular properties as a consequence of impalement.