Ehud Gruen

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.

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.

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.

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.

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.

Acoustic transmission in the dentate nucleus: I. Patterns of activity to click and hiss; II. Changes in activity and excitability after conditioning

Recordings were made from 95 units of the dentate nucleus of naive cats to determine if patterns of response to 70 dB clicks could be distinguished from those to another acoustic stimulus (a hiss) of approximately equal sound pressure level. Further studies of an additional 309 units were conducted to determine if unit excitability and the response to clicks changed after Pavlovian conditioning in which blink responses were elicited by the clicks as conditioned stimuli. Over 50% of units tested before conditioning responded to click or hiss with increased activity, and 8% responded in the first 4-8 ms after the onset of the rapidly rising click. Cross-correlation of the respective 160 ms poststimulus histogram averages of mean activity showed dissimilar patterns of response to clicks and hisses (Pearson product-moment correlation coefficient + 0.02). Thus the averaged population responses distinguished these stimuli. In addition, individual cells were found in each behavioral state that responded selectively to either click or hiss. After conditioning with click as the conditioned stimulus, the number of units responding in the first 4-8 ms to click increased to 23%. The mean magnitude of activity 4-8 ms after presenting the click increased after conditioning but not after sensitization produced by backward pairing of the stimuli used for conditioning. After backward pairing only 6% of the units responded in the first 4-8 ms to click. The changes in activity after conditioning were accompanied by increases in neural excitability to intracellularly applied depolarizing current. In contrast with the changes in activity, the increases in neural excitability were also found after backward pairing. We conclude that short as well as long latency acoustic transmissions to click change in the dentate nucleus after conditioning, that changes in response to click are expressed in 4-8 ms responsive cells, and that many of these cells have different patterns of spike activity in response to click and hiss. The findings support the hypothesis that the dentate nucleus can play a significant role in short as well as long latency, adaptive acoustic transmission that can enhance the response to an acoustic signal used as a Pavlovian conditioned stimulus.

Two different mechanisms control inhibition of spike discharge in neurons of cat motor cortex after stimulation of the pyramidal tract

Intracellular recordings were made from neurons of the motor cortex of awake cats while the pyramidal tract (PT) was stimulated at the level of the facial nucleus. In some neurons IPSPs of 35-120 ms peak latency were recorded that diminished in size or reversed with hyperpolarizing current. During these IPSPs a decrease in input resistance reflective of a conductance increase was measured. More often, however, PT stimulation produced IPSPs with comparable latencies that increased in size with hyperpolarizing current. These IPSPs diminished with depolarizing current, and in some instances they appeared to reverse with strong depolarization. During these IPSPs an increased input resistance reflective of a decreased conductance was measured. The results indicate that two different mechanisms control rapid inhibition of spike discharge in neurons of the motor cortex after PT stimulation.

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.