Modulation of monosynaptic transmission by presynaptic inhibition during fictive locomotion in the cat

The effect of multisensory inputs onto the presynaptic inhibitory pathways affecting IA terminals was studied during fictive locomotion in decerebrated cats. The effect was evaluated from changes in amplitude of the monosynaptic excitatory postsynaptic potential (EPSP) measured in lumbosacral motoneurones. Responses were grouped and averaged according to their timing within the step cycle divided into five bins. Presynaptic inhibition was evoked by stimulating group I afferents from the posterior biceps-semitendinosus (PBSt) muscles and one of three cutaneous nerves: superficial peroneal (SP), sural and saphenous. Statistical analysis was applied to compare (1) EPSPs conditioned by PBSt input alone and those conditioned by the combined PBSt and cutaneous inputs, and (2) each bin dividing the step cycle to disclose phase-dependent changes. Results from 19 motoneurones showed that: (1) there was a significant phase-dependent modulation in EPSP amplitude (by 25%) with the maximum usually occurring during the depolarized phase; (2) PBSt alone reduced the EPSP amplitude (by 21%) in 3.2 bins on average; (3) combined PBSt and cutaneous stimuli further modified (up or down) the EPSP amplitude in half the trials but only in one to two bins; and (4) the most efficient cutaneous nerve (SP) usually decreased the PBSt-evoked reduction in EPSP size. Minimal changes in membrane input resistance suggest that the EPSP modifications were mostly due to presynaptic inhibition. Results indicate that muscle afferents can induce an important phase-dependent presynaptic inhibition of monosynaptic transmission and that concomitant activation of cutaneous afferents can alter this inhibition but only for a restricted part of the step cycle.     

Attention and arousal related modulation of spontaneous gamma-activity in the auditory cortex of the cat

Sensory information processing in neocortex is associated with rhythmic synchronized gamma frequency firing of sensory cortical units and similar frequency oscillations of the field potentials. Different aspects of the gamma activity (20-80 Hz) have been suggested as correlates of attention, arousal and sensory binding. It is clear that attention has a modality selective influence, while arousal has a more general effect on the sensory systems. We used an experimental conditioning paradigm to separate these differential effects of attention and arousal on spontaneous neocortical gamma activity. We recorded field potentials with epidural electrodes placed above the auditory cortical areas of cats. The animals performed a simple instrumental alimentary conditioning task with different modality (visual and auditory) conditioned stimuli. When they attended to the auditory conditioned stimulus, both frequency and power increase of spontaneous gamma activity were detected. However when they attended visual, we found no power increase of gamma activity recorded above auditory areas, while the frequency increase was the same as in the "attend auditory" condition. We conclude that the power modulation of gamma activity is modality specific and thus can be attributed to selective attention, whereas the frequency modulation of gamma activity shows no modality specificity, it is influenced by the arousal level.     

Long-latency auditory-evoked potentials: Role of polysensory association cortex in the cat

The objective of this study has been to define the role of polysensory association cortex in the generation of "wave NA" and of "wave C," long-latency auditory-evoked potentials recorded from the vertex of conscious cats as, respectively, a marked negative potential of latency 30-48 msec followed by a broad positive wave of latency 50-75 msec. Wave C may represent the feline analogue of the longer latency human auditory-evoked potential wave P2, insofar as both waveforms are very large amplitude, long duration positivities characterized by long recovery cycles. Based on previous studies of wave C and the generators of other middle-latency evoked potentials, we hypothesized that both wave NA and wave C might reflect, at least in part, the cortical culmination of a nonlemniscal line auditory association system arising in reticulothalamic projections to intralaminar and associated ventral thalamic regions. Relays from these thalamic areas are known to project to polysensory association cortex, including pericruciate gyrus, anterolateral gyrus, and medial suprasylvian gyrus. Therefore we implemented a series of lesion experiments to characterize the role of each of these cortical areas in the production of wave NA and wave C. Our results indicate that all three polysensory association areas contribute significantly to both waves NA and C, although the largest effects followed ablation of the pericruciate area alone. Thus, the generator substrates of waves NA and C appear to involve a long-recovery cycle system which functionally incorporates activation of association cortex.     

Inhibition in association cortex neurons upon direct and transcallosal stimulation in the cat

The experiments were performed on cats immobilized with myorelaxants. Depression of background activity or IPSP was recorded in neurons of the associative cortex in response to direct cortical and transcallosal stimulation. With the intensification of stimulation the duration of inhibition increased up to a certain limit for each studied cell. After this it still could increase if the number of the stimuli increased. Prolonged inhibition (up to several seconds) in response to a series of stimuli had the same nature as short inhibition about 20 ms evoked by a single stimulus. It was due to a prolonged hyperpolarizing postsynaptic potential. A suggestion is made that neurons producing monosynaptical inhibition in the associative cortex cells can be situated in the contralateral hemisphere.     

Spatially opponent excitation and inhibition in simple cells of the cat visual cortex

The receptive fields of simple cells in the cat visual cortex are, by definition, divided into ON and OFF subfields. There is little doubt that each subfield is generated by excitatory input from geniculate neurons of the appropriate center type: ON subfields by ON-center cells, and OFF subfields by OFF-center cells. In intracellular records, ON subfields can be detected as regions in which light elicits a barrage of EPSPs, while in OFF subfields, turning a light off does the same. In addition, visual stimuli can evoke strong IPSPs, but these IPSPs have a receptive field spatially opponent to that of the EPSPs: Inhibition is evoked by turning a light off in an ON region and turning a light on in an OFF region. This inhibition probably arises from other cortical simple cells, and may contribute to such receptive-field properties as antagonism between subfields, binocular disparity sensitivity, and orientation selectivity.     

Modulation of neuronal firing mode in cat and guinea pig LGNd by histamine: possible cellular mechanisms of histaminergic control of arousal.

The thalamus is innervated by histaminergic fibers presumably arising from neurons in the tuberomammillary nucleus of the hypothalamus. The possible function of this histaminergic projection was addressed through investigation of the cellular actions of histamine on guinea pig and cat dorsal lateral geniculate (LGNd) relay neurons maintained as a slice in vitro. Local application of histamine to LGNd relay neurons resulted in a slow depolarization that was associated with a decrease in membrane conductance and was blocked by the H1-antagonists pyrilamine, triprolidine, or diphenhydramine. Current versus voltage relationships revealed that the slow depolarization was associated with an inward current that reversed near EK, indicating that it was due to a decrease in a potassium current. The slow depolarizing response to histamine was occluded by maximal activation of the slow depolarizing responses resulting from stimulation of alpha 1-adrenergic or muscarinic receptors, suggesting that they are all mediated by reduction in the same potassium current and/or alteration of a common second messenger. In the presence of H1-receptor antagonists, application of histamine resulted in a small depolarization that was associated with a marked increase in apparent membrane conductance. Voltage-clamp recordings revealed that this response was associated with enhancement of the hyperpolarization-activated cation current Ih. This response to histamine was blocked by local or bath application of the H2-antagonists cimetidine or tiotidine. The functional consequences of these actions of histamine were addressed with extracellular and intracellular recordings in guinea pig and cat LGNd relay neurons. Extracellular recordings in cat LGNd revealed the occurrence of highly regular 1-4 Hz rhythmic burst discharges. Application of histamine halted rhythmic bursting and replaced it with a prolonged period of single-spike activity. Intracellular recordings indicate that the histamine-induced switch in firing mode is due largely to the slow depolarizing response mediated by H1-receptors, but is also facilitated by the enhancement of Ih mediated by H2-receptors. These postsynaptic actions indicate that increased activity in the tuberomammillary histaminergic system may result in a switch of thalamic neuronal activity from rhythmic burst firing to single-spike activity and thereby promote the accurate transmission and processing of sensory information and cognition.     

Responses of neurons of the parietal association cortex of the cat to acoustic stimulation before and after suppression of the auditory cortex in the cat

The aim of this work was to study the influence of auditory cortex suppression in cat on response patterns of the parietal associative cortex neurons responding to different frequency tones. Suppression was performed by two methods: bilateral isolation and application of 6% nembutal solution on the cortex surface. Frequency-thresholds curves were plotted for all neurons studied. Prior to suppression the majority (84%) of studied neurons had one or two characteristic frequencies. After suppression the percentage of such cells fell to 63% of all responding neurons. Frequency range to which neurons could respond was altered as well. Normally almost all neurons tested could respond to a wide spectrum of presented frequencies. After suppression 69% of neurons did not respond to tones above 8-10 kHz. This may indicate that mainly information about high frequency tones is transmitted via the auditory cortex. The possibility that associative thalamic nuclei are the main source of acoustic information for parietal associative cortex neurons is discussed.     

Modulation of cat monosynaptic reflexes by substance P

Substance P (SP) applied by iontophoresis at different current strengths to single motoneurons of cat spinal cord did not cause these units to discharge. SP produced a gradual and prolonged change in synaptic excitability as measured by response to dorsal root stimulation. The effect outlasted application of SP. The lowest effective dose of SP diminished motoneuron response to dorsal root stimulation (inhibitory modulation). Doses 2-4 times as great enhanced the response to dorsal root stimulation (facilitatory modulation) without causing the motoneuron to discharge spontaneously. These observations suggest that one physiological role of SP is modulation of synaptic transmission, i.e. alteration of efficacy of transmission without acting as a primary transmitter at the postsynaptic membrane.     

Modulation of cat monosynaptic reflexes by substance P

Substance P (SP) applied by iontophoresis at different current strengths to single motoneurons of cat spinal cord did not cause these units to discharge. SP produced a gradual and prolonged change in synaptic excitability as measured by response to dorsal root stimulation. The effect outlasted application of SP. The lowest effective dose of SP diminished motoneuron response to dorsal root stimulation (inhibitory modulation). Doses 2–4 times as great enhanced the response to dorsal root stimulation (facilitatory modulation) without causing the motoneuron to discharge spontaneously. These observations suggest that one physiological role of SP is modulation of synaptic transmission, i.e. alteration of efficacy of transmission without acting as a primary transmitter at the postsynaptic membrane.     

Acetylcholine-induced inhibition in the cat visual cortex is mediated b7 a GABAergic mechanism

The influence of iontophoretically applied acetylcholine (ACh) on single-unit activity in the visual cortex was studied in anesthetized cats. The dominant effect consisted of a slow facilitation of neuronal responses to moving light bars. This cholinergic action was sometimes paralleled by a decrease of the cells' selectivity to the direction of stimulus movement. In about one-third of the neurons studied ACh-iontophoresis suppressed maintained and visually driven activity within a few hundred milliseconds from the onset of its application. This effect was antagonized by concurrent iontophoresis of the muscarinic ACh-receptor antagonist scopolamine and the GABAA-receptor antagonist bicuculline methiodide. In 7% of the units studied a fast excitation was elicited by ACh application showing a similar time course as the rapid suppressive effect. It is concluded that ACh-induced inhibition is mediated by an activation of GABAergic interneurons. The role of cholinergic depression and facilitation in cortical information processing is discussed.     

Modulation of olfactory perception by visual cortex stimulation

When attempting to identify an object based on smell alone, people often visualize the perceived source of the odorant. This close association between olfactory and visual functions is supported by neuroimaging studies demonstrating activation of visual cortex during performance of purely olfactory tasks. Such activation might simply reflect the correlation between olfactory percepts and the corresponding visual images, or it might reflect a causal contribution of visual processing to olfactory perception. Here we provide evidence in support of the latter possibility. Using repetitive transcranial magnetic stimulation, we show that stimulating human visual cortex improves performance on a task requiring discrimination among different odor qualities. No significant improvement is found for tasks involving discrimination between intensities of the same odor, from stimulation of auditory cortex, or from "sham" stimulation. These results are thus consistent with a specific visual cortical influence on high-level olfactory perception. They also demonstrate that unimodal perceptual tasks are influenced by processing within cortical areas of other, seemingly unrelated, sensory systems.