Neural substrates for species recognition in the time-coding electrosensory pathway of mormyrid electric fish

Mormyrid electric fish have species- and sex-typical electric organ discharges (EODs). One class of tuberous electroreceptors, the knollenorgans, plays a critical role in electric communication; one function is species recognition of EOD waveforms. In this paper, we describe cell types in the knollenorgan central pathway, which appear responsible for analysis of the temporal patterns of spikes encoded by the knollenorgans in response to EOD stimuli. Secondary sensory neurons in the nucleus of the electrosensory lateral line lobe (NELL) act as relays of peripheral responses. They fire a single phase-locked spike to an outside positive-going voltage step. Axons from the NELL project to the toral nucleus exterolateralis pars anterior (ELa). Immediately after they enter the ELa, they send collaterals to terminate on one to three ELa large cells and then continue in a lengthy neuronal pathway that traverses the ELa several times. After a path length of up to 5 mm, the NELL axon terminates on as many as 70 ELa small cells. Thus the large cells appear to be excited first, followed by the small cells, with the intervening length of the axon serving as a delay line. The large cells also respond with phase-locked spikes to voltage steps. Large cell axons extend for approximately 1 mm and terminate on several small cells within the ELa. The terminals are known to be GABAergic inputs and are presumed inhibitory. We propose that small cells receive direct inhibition from large cells and delayed excitation from NELL axons. The small cells may act as anti-co-incidence detectors to analyze the temporal structure of the EOD waveform.     

Antidromic responses of single units from the spiral ganglion

1. Antidromic responses of single units in the guinea pig spiral ganglion were recorded in response to shocks to the auditory nerve root. The orthodromic responses of these units were also recorded in response to sound. The aim of this study was 1) to classify units according to their response patterns to shocks and to sound and 2) to propose anatomic types that might correlate with these responses. The four classes of units were as follows: type I, olivocochlear (OC), long-latency: locked, and long-latency: jittering. 2. Type I units responded antidromically to shocks with little jitter and short latency. Their responses to sound were also of short latency and had irregular interspike intervals. Some of these units had complex spike waveforms. These units likely correspond to type I primary afferent neurons, the majority population of spiral ganglion cells. 3. One-third of the OC units responded to shocks, with little jitter and intermediate latency (2 ms). OC unit responses to sound were of long latency and had regular interspike intervals. These units likely correspond to efferent neurons that originate in the superior olivary complex of the brain and end on outer hair cells in the cochlea. 4. Long-latency: locked units responded to shocks with little jitter and long latency (4-11 ms). Many of these units had complex spike waveforms and most did not respond to high-level noise bursts. Long-latency: locked units may correspond to type II spiral ganglion neurons. 5. Long-latency: jittering units responded to shocks with a jitter of several milliseconds and long latency. Some of these units responded to sound in a pattern reminiscent of OC units. These units may constitute a subgroup of OC units that respond to shocks via activation of the reflex pathway from the cochlea to the superior olive and back out to the cochlea. 6. Further data were collected on the type I response to shocks. Antidromic spikes lacked the inflections seen on the waveforms that are typically seen on orthodromic spikes. Type I shock responses depended on shock level and duration and were reduced when a click preceded the shock by approximately 2 ms. Several type I characteristics depended on the rate of spontaneous discharge: for units of low and medium spontaneous rates (when compared with units of high rates), the shock thresholds were lower, shock latencies were longer, and the probability of firing repetitive spikes to a single shock was higher.(ABSTRACT TRUNCATED AT 400 WORDS)     

Visual evoked potentials elicited by circular grating

In the belief that it would prove a more effective stimulus for eliciting visual evoked potentials, a circular grating was designed so that the relationship between its bar width and visual acuity was held constant, and, therefore, the bars were equally well resolved across the visual field. Visual evoked potentials elicited by the onset presentation of the pattern were evaluated but found to be excessively variable owing to summation of different waveforms generated by equivalent stimulation of different parts of the visual field.     

Intracellular responses of the rat cochlear nucleus to sound and its role in temporal coding

The anteroventral cochlear nucleus (AVCN), the first centre of the central auditory pathway, contains globular bushy cells, which are unique in their ability to produce fast excitatory post-synaptic potentials (EPSPs). Using in vivo intracellular recordings in the rat AVCN we examined these fast EPSPs in relation to temporal coding. At frequencies up to 2.5 kHz, EPSPs were evoked on successive sine waves of the stimulus with EPSP summation limited. This one-to-one relationship between the EPSPs and the sound wave period was present at higher frequencies and over a greater intensity range than for action potentials. These results suggest that temporal coding is possible in globular bushy neurones by their ability to extract temporal information through fast processing of convergent presynaptic input.     

Recurrent inhibitory interneurons of the Rabbit's lateral posterior-pulvinar complex

We recorded from 118 neurons in the visual sector of the thalamic reticular nucleus (TRN) in anesthetized rabbits. Cells were identified by their location and characteristic burst responses to stimulation of the primary visual cortex (Cx) and optic chiasm (OX) and were classified into two groups. Type I cells had relatively short latencies from both OX and Cx stimulation, and the latency from OX was always longer than from Cx. In contrast, type II cells had much longer latencies after OX and Cx stimulation, and the latency from OX was always shorter than from Cx. Type I cells were located in the dorsal part of TRN, whereas type II cells were located in the ventral part of TRN. The physiological properties and location of type I TRN cells indicate that they are recurrent inhibitory interneurons of the dorsal lateral geniculate nucleus (LGN). Type II TRN cells most likely function as recurrent inhibitory interneurons for the lateral posterior nucleus-pulvinar complex (LP) because they could be activated antidromically by LP stimulation and orthodromically activated via axonal collaterals of LP cells. Type II TRN cells exhibited a prolonged depression after Cx or OX stimulation. Intracellular recordings showed that a prolonged inhibitory postsynaptic potential was evoked by Cx or OX stimulation. Therefore, these recurrent interneurons of LP, type II cells form mutual inhibitory connections just like those recurrent interneurons of LGN, type I cells. Our data suggest that the geniculocortical and extrageniculate visual pathways have similar recurrent inhibitory circuits.     

The response of cat visual cortex to flicker stimuli of variable frequency

We examined the possibility that neurons or groups of neurons along the retino-cortical transmission chain have properties of tuned oscillators: To this end, we studied the resonance properties of the retino-thalamo-cortical system of anaesthetized cats by entraining responses with flicker stimuli of variable frequency (2-50 Hz). Responses were assessed from multi-unit activity (MUA) and local field potentials (LFPs) with up to four spatially segregated electrodes placed in areas 17 and 18. MUA and LFP responses were closely related, units discharging with high preference during LFP negativity. About 300 ms after flicker onset, responses stabilized and exhibited a highly regular oscillatory patterning that was surprisingly similar at different recording sites due to precise stimulus locking. Fourier transforms of these steady state oscillations showed maximal power at the inducing frequency and consistently revealed additional peaks at harmonic frequencies. The frequency-dependent amplitude changes of the fundamental and harmonic response components suggest that the retino-cortical system is entrainable into steady state oscillations over a broad frequency range and exhibits preferences for distinct frequencies in the theta- or slow alpha-range, and in the beta- and gamma-band. Concomitant activation of the mesencephalic reticular formation increased the ability of cortical cells to follow high frequency stimulation, and enhanced dramatically the amplitude of first- and second-order harmonics in the gamma-frequency range between 30 and 50 Hz. Cross-correlations computed between responses recorded simultaneously from different sites revealed pronounced synchronicity due to precise stimulus locking. These results suggest that the retino-cortical system contains broadly tuned, strongly damped oscillators which altogether exhibit at least three ranges of preferred frequencies, the relative expression of the preferences depending on the central state. These properties agree with the characteristics of oscillatory responses evoked by non-temporally modulated stimuli, and they indicate that neuronal responses along the retino-cortical transmission chain can become synchronized with precision in the millisecond range not only by intrinsic interactions, but also by temporally structured stimuli.     

Nucleus preeminentialis of mormyrid fish, a center for recurrent electrosensory feedback. I. Electrosensory and corollary discharge responses

1. The nucleus preeminentialis (PE) is a large central structure that projects both directly and indirectly to the electrosensory lobe (ELL) where the primary afferents from electroreceptors terminate. PE receives electrosensory input directly from ELL and also from higher stages of the electrosensory pathway. PE is thus an important part of a central feedback loop that returns electrosensory information from higher stages of the system to the initial stage in ELL. 2. This study describes the field potentials and single-unit activity that are evoked in PE by electrosensory stimuli and by corollary discharge signals associated with the motor command that drives the electric organ to discharge. All recordings were extracellular in this study. 3. Two types of negative-going corollary discharge-evoked field potentials were found in PE: 1) a shallow, long-lasting negative wave with a latency at the peak of approximately 11 ms, and 2) a more sharply falling and larger negative wave with a shorter latency at the peak of approximately 9 ms. The long-latency wave was predominant in the dorsolateral and posterior parts of PE, whereas the short-latency wave was predominant in the medial and rostral regions. Both waves were only found in PE and thus can serve for its identification. 4. Electrosensory stimuli given either locally to a restricted skin region or symmetrically to the entire body evoked characteristic field potentials in both regions of PE. The mean latency between the stimulus and the peak of the response was 6.9 ms in the early negativity region and 12.2 ms in the late negative region. The responses to such stimuli were strongly facilitated by the electric organ corollary discharge. 5. Field potential responses to the electric organ corollary discharge were markedly plastic. Responses to the corollary discharge plus a paired electrosensory stimulus decreased over time and the response to the corollary discharge alone was markedly enhanced after a period of such pairing. 6. Local electrosensory stimulation of the skin showed that the caudal-rostral body axis is mapped from dorsal-medial to ventral-lateral in PE. The same somatotopy was found in the regions of the early and late negatives. The ventral and dorsal body appeared not to be separately mapped in PE. The areas representing the head and chin appendage ("Schnauzenorgan") are especially large in PE, due presumably to the high density of electroreceptors in these areas. 7. Two main types of units were recorded in PE: 1) inhibitory (I) cells with a corollary discharge response that was inhibited by an electrosensory stimulus to the center of their receptive fields; and 2) excitatory (E) cells with an excitatory response to electrosensory stimuli that was facilitated by the corollary discharge. Some of the E cells responded to the corollary discharge alone and some did not. Most cells appeared to be responding to input from mormyromast electroreceptors, but a few cells were driven by ampullary electroreceptors and a few by Knollenorgan electroreceptors. 8. The corollary discharge effects on I cells and E cells were plastic and depended on previous pairing with a sensory stimulus. The corollary discharge facilitation of E cells and inhibition of I cells decreased during pairing with a sensory stimulus, and the corollary discharge-driven excitation of I cells was much larger after pairing than before. 9. The results provide an initial overview of a major component in the control of electrosensory information processing by recurrent feedback from higher stages of the system.     

Sensory processing in the pallium of a mormyrid fish

To investigate the functional organization of higher brain levels in fish we test the hypothesis that the dorsal gray mantle of the telencephalon of a mormyrid fish has discrete receptive areas for several sensory modalities. Multiunit and compound field potentials evoked by auditory, visual, electrosensory, and water displacement stimuli in this weakly electric fish are recorded with multiple semimicroelectrodes placed in many tracks and depths in or near telencephalic area dorsalis pars medialis (Dm). Most responsive loci are unimodal; some respond to two or more modalities. Each modality dominates a circumscribed area, chiefly separate. Auditory and electrical responses cluster in the dorsal 500 micrometer of rostral and caudolateral Dm, respectively. Two auditory subdivisions underline specialization of this sense. Mechanoreception occupies a caudal area overlapping electroreception but centered 500 micrometer deeper. Visual responses scatter widely through ventral areas. Auditory, electrosensory, and mechanosensory responses are dominated by a negative wave within the first 50 msec, followed by 15-55 Hz oscillations and a slow positive wave with multiunit spikes lasting from 200 to 500 msec. Stimuli can induce shifts in coherence of certain frequency bands between neighboring loci. Every electric organ discharge command is followed within 3 msec by a large, mainly negative but generally biphasic, widespread corollary discharge. At certain loci large, slow ("deltaF") waves usually precede transient shifts in electric organ discharge rate. Sensory-evoked potentials in this fish pallium may be more segregated than in elasmobranchs and anurans and have some surprising similarities to those in mammals.     

ON and OFF channels of the frog optic tectum revealed by current source density analysis

The spatiotemporal patterns of excitatory synaptic activity in response to diffuse lightON and OFF stimuli were examined by means of current source density (CSD) analysis. The qualitative and quantitative analyses obtained from 24 depth profiles for each stimulus revealed obviously different distributions of synaptic activity in the laminar structure. Two or three dominant current sinks I, II, and III were evoked in response to diffuse light ON stimulation. Sink I was observed at the bottom of the retinorecipient layer. Both sinks II and III, showing an identical spatial pattern, were observed just above sink I. On the other hand, diffuse light OFF stimulation elicited up to six current sinks IV, V, VI, VII, VIII, and IX. Sink IV was observed at the bottom of the retinorecipient layer. Sink V was observed in the most superficial layer. Both sinks VI and VIII were located between the two preceding sinks. Finally, sinks VII and IX occurred below the retinorecipient layer. Five electrically evoked current sinks A, B, C, D, and E, characterized in our previous study, were also recognized in the present quantitative analysis. A statistical analysis revealed that, in visually evoked responses, statistical differences in the spatial distribution were not present between sinks I and IV, and sinks II and VIII (P < 0.05). The analysis also showed that, in electrically evoked responses, only a pair of sinks C and E exhibit virtually identical spatial distribution (P < 0.05). Based on well-known properties of the retinal ganglion cells, possible neuronal mechanisms underlying each of current sinks in the ON and OFF channels and their functional meanings were considered. Sink I reflects the excitatory monosynaptic activity derived from R3 retinal ganglion cells. Sink IV reflects the excitatory monosynaptic activity derived from both R3 and R4 cells. Sinks V, VI, VII, and IX may be composed of successive polysynaptic excitatory potentials derived from convergence of inputs from both R3 and R4 cells. We concluded that the early four sinks play in particular an important role in eliciting avoidance behavior. On the other hand, sinks II, III, and VIII reflect excitatory synaptic activities derived from - retinal fibers of another type having slow conduction velocity. These late current sinks were suggested to mediate prey catching and its facilitation.     

A ''neural'' response with 3-ms latency evoked by loud sound in profoundly deaf patients

A large negative deflection with a latency of 3 ms was observed in the auditory brainstem response (ABR) waveforms of some patients with peripheral profound deafness. This deflection was termed the N3 potential. In this paper, we review patients with the N3 potential and discuss the characteristics of abnormal ABR waveforms. The origin of the N3 potential was also discussed, especially with respect to vestibular evoked potentials. In most of the patients, audiograms showed no response to the maximum output of an audiometer in the high-frequency range and a residual response in the low-frequency range. The N3 potentials were noted at intensities of 80 dB nHL or greater. As the stimulus intensity increased, the amplitude of the potential increased and the latency decreased. A high repetition rate (83.3/s) of the click stimulus influenced the latency and amplitude of the N3 potential. The potential was replicated on retest within less than a month, and had a consistent latency and amplitude over the scalp. The results indicate that the N3 potential is not an electrical artifact but a physiological neural response evoked by a loud sound. The N3 potential is most likely not an auditory evoked response from cochlear or a response from a semicircular canal, because it has a 3-ms latency, a sharp waveform, and is unassociated with vertigo. The results suggest that the N3 potential may be a saccular acoustic response.     

Are frontal and parietal somatosensory evoked potentials functionally dissociated by changing stimulus rate?

The cortical somatosensory evoked potentials are known to be sensitive to relatively small changes in the stimulus repetition rate of the afferent nerve. However, conflicting reports exist as to whether frontally and parietally recorded potentials at a given latency show differential behaviors as a function of stimulus rate. Because such dissociations of frontal and parietal potentials can have significant implications for the SEP generation mechanisms, the present study was undertaken to further describe in detail these effects on frontal, central and parietal waveforms after median nerve stimulation. Increasing stimulus repetition rate from 1 Hz to 5 Hz had the following effects: (i) in 9 of 16 subjects, the frontal P20 diminished while parietal N20 clearly remained unaltered, (ii) the central P22 was reduced in all subjects, (iii) frontal N30 and parietal P27 were attenuated in all subjects, the average magnitude of the reductions being nearly equal for these deflections. The results support the view that changing stimulus rate can functionally dissociate frontal and parietal activity around 20 ms, indicating that several partially independent neural populations can contribute to the frontal P20. The results did not lend support for functional dissociation of frontal N30 from parietal P27.     

Waveform estimation techniques for event-related bioelectric signals: a study of performance

Many bioelectric signals result from the electrical response of a physiological system to an impulse that can be internal (ECG signals) or external (evoked potentials). A comparative study of performance of seven waveform estimation techniques used for event-related signals that are time-locked to a stimulus is presented in this paper. Computer generate 1 signals and noise for several signal-to-noise ratios (SNRs) are used to make ensembles of simulated noisy waveforms. The performance of each technique is numerically investigated using the root-mean-squared error and two well known SNR estimators. The results show that an adaptive impulse correlated filter performs the best. It is capable of estimating the deterministic component of the signal and removes the noise uncorrelated with stimulus even if this noise is colored and without the need for prealignment.     

A model for the early stages of motion processing based on spatial and temporal edge detection by X-cells

A model for the early stages of motion processing in the visual cortex is presented. The 'building block' for this model is the 'rebound response', which is the neuronal response evoked when a sufficient inhibitory stimulus is turned off. This response enables detection of temporal changes when the stimulus involves spatial changes. The model suggests that adjacent subunits in primary cortical cells have different weight functions for rebound responses, and thus a synergistic type of response is evoked in the preferred direction, which is predicted for both light and dark stimuli. Predictions of the model for different stimuli and receptive field structures are discussed. It appears to be more economical than previous motion models.     

Effect of flow on first-pass metabolism of drugs: single pass studies on 4-methylumbelliferone conjugation in the serially perfused rat intestine and liver preparations

Intracellular in vivo recordings of physiologically identified inferior colliculus central nucleus (ICc) auditory neurons (n = 71) were carried out in anesthetized guinea pigs. The neuronal membrane characteristics are described showing mainly quantitative differences with a previous report [Nelson, P.G. and Erulkar, S.D., J. Neurophysiol., 26 (1963) 908-923]. The spontaneous spike activity was consistent with the discharge pattern of most extracellularly recorded units. The action potentials showed different spike durations, short and long, and some of them exhibited hyperpolarizing post-potentials. There were also differences in firing rate. The ICc neurons exhibited irregular activity producing spike trains as well as long silent periods (without spikes). Intracellular current injection revealed membrane potential adaptation and shifts that outlasted the electrical stimuli by 20-30 ms. Both evoked synaptic potentials and the spike activity in response to click and tone-burst stimulation were analyzed. Depolarizing-hyperpolarizing synaptic potentials were found in response to contralateral and binaural sound stimulation that far outlasted the stimulus (up to 90 ms). When ipsilaterally stimulated, inhibitory responses and no-responses were also recorded. Although few cells were studied, a similar phenomenon was observed using tone-burst stimulation; moreover, a good correlation was obtained between membrane potential shifts and the triggered spikes (input-output relationship). These in vivo results demonstrate the synaptic activity underlying many of the extracellularly recorded discharge patterns. The data are consistent with the known multi-synaptic ascending pathway by which signals arrive at the ICc as well as the descending corticofugal input that may contribute to the generation of long duration post-synaptic potentials.     

Speech-evoked activity in primary auditory cortex: effects of voice onset time

Neural encoding of temporal speech features is a key component of acoustic and phonetic analyses. We examined the temporal encoding of the syllables /da/ and /ta/, which differ along the temporally based, phonetic parameter of voice onset time (VOT), in primary auditory cortex (A1) of awake monkeys using concurrent multilaminar recordings of auditory evoked potentials (AEP), the derived current source density, and multiunit activity. A general sequence of A1 activation consisting of a lamina-specific profile of parallel and sequential excitatory and inhibitory processes is described. VOT is encoded in the temporal response patterns of phase-locked activity to the periodic speech segments and by "on" responses to stimulus and voicing onset. A transformation occurs between responses in the thalamocortical (TC) fiber input and A1 cells. TC fibers are more likely to encode VOT with "on" responses to stimulus onset followed by phase-locked responses during the voiced segment, whereas A1 responses are more likely to exhibit transient responses both to stimulus and voicing onset. Relevance to subcortical speech processing, the human AEP and speech psychoacoustics are discussed. A mechanism for categorical differentiation of voiced and unvoiced consonants is proposed.     

Estrogen-sensitive neurons with preoptic projection in the lower brain stem of the female rat

Fifty-one neurons in the ventrolateral part of the medulla oblongata were antidromically activated by electrical stimulation of the suprachiasmatic part of the preoptic area in urethane-anestetized, ovariectomized and estrogen-primed female rats. Two types of antidromic responses were distinguished on the basis of their spike configurations and antidromic spike latencies. One type ("fast spikes") was characterized by a fast and smooth rising phase and a shorter duration of the initial positive deflection. The other type ("slow spikes") had a notch in the rising phase and took a longer time to complete the initial deflection. Mean antidromic spike latency for the fast spikes was 9.8 msec while the value for the slow spikes was 30.2 msec. Ionophoretic injection of estradiol was accomplished on 37 of the 51 antidromically identified cells, of which 21 showed slow responses and 16 responded with fast spikes. In cells with slow spikes, estradiol facilitated (n = 9) or suppressed (n = 3) their generation of action potentials. None of cells with fast responses changed their activity in response to estradiol. It is evident from the present experiment that neurons in the ventrolateral part of the medulla oblongata send their axons directly to the suprachiasmatic part of the preoptic area which plays an important role in the control of the ovulatory surge of LH and that some of these neurons themselves are the sensitive sites of estradiol.     

Bilateral projection of the canine tooth pulp to bulbar trigeminal neurons

Unit activity was recorded extracellulary from neurons of the cat medulla following electrical stimulation of the ipsilateral and/or contralateral cannine tooth pulps. The majority of the cells (67%) were only responsive to ipsilateral stimulation. However, many (28%) responded to stimulation of either canine pulp and a few (5%) responsed to contralateral stimulation alone. The neurons were localized histologically in the necleus proprius of the rostral trigeminal nucleus caudalis (NVCaud) and in dorsal portions of the ventromedially contiguous lateral reticular formation (LRF). Cells exclusively responsive to ipsilateral stimuli had a relatively wide dorsoventral distribution. In contrast, 'bilateral' and 'contralateral' cells were situated only in the deep NVCaud-LRF border zone or in immediately adjacent portions of the LRF. Generally, ipsilateral stimuli evoked response bursts with shorter latencies, more spike potentials and briefer interspike intervals than equivalent contralateral stimuli. In experiments designed to study afferent interactions, a conditioning stimulus, applied to either the ipsilateral or the contralateral canine, preceded a test stimulus applied to the other canine at predetermined interstimulus intervals. Responses to the test stimulus were either totally or partially suppressed when intervals of moderate duration (90-500 msec) were used. However, responses to the test stimulus frequently were enhanced when the intervals were breif (less than or equal to 60 msec) or when the teeth were stimulated simultaneously. The results reveal that bilateral afferents from the pulps of the canine teeth converge upon neurons of bulbar trigeminal structures, that the neurons are differentially responsive to the activation of ipsilateral and contralateral pulpal receptors and that bilateral afferent barrages originating in the canine pulps interact to modulate the firing patterns of the neurons.     

Inhibitory control of somatodendritic interactions underlying action potentials in neocortical pyramidal neurons in vivo: an intracellular and computational study

The effect of synaptic inputs on somatodendritic interactions during action potentials was investigated, in the cat, using in vivo intracellular recording and computational models of neocortical pyramidal cells. An array of 10 microelectrodes, each ending at a different cortical depth, was used to preferentially evoke synaptic inputs to different somatodendritic regions. Relative to action potentials evoked by current injection, spikes elicited by cortical microstimuli were reduced in amplitude and duration, with stimuli delivered at proximal (somatic) and distal (dendritic) levels evoking the largest and smallest decrements, respectively. When the inhibitory postsynaptic potential reversal was shifted to around -50 mV by recording with KCl pipettes, synaptically-evoked spikes were significantly less reduced than with potassium acetate or cesium acetate pipettes, suggesting that spike decrements are not only due to a shunt, but also to voltage-dependent effects. Computational models of neocortical pyramidal cells were built based on available data on the distribution of active currents and synaptic inputs in the soma and dendrites. The distribution of synapses activated by extracellular stimulation was estimated by matching the model to experimental recordings of postsynaptic potentials evoked at different depths. The model successfully reproduced the progressive spike amplitude reduction as a function of stimulation depth, as well as the effects of chloride and cesium. The model revealed that somatic spikes contain an important contribution from proximal dendritic sodium currents up to approximately 100 microm and approximately 300 microm from the soma under control and cesium conditions, respectively. Proximal inhibitory postsynaptic potentials can present this dendritic participation thus reducing the spike amplitude at the soma. The model suggests that the somatic spike amplitude and shape can be used as a "window" to infer the electrical participation of proximal dendrites. Thus, our results suggest that inhibitory postsynaptic potentials can control the participation of proximal dendrites in somatic sodium spikes.     

Hippocampal interneuron activity in unanesthetized rats: relationship to the sleep-wake cycle

Evoked population spikes and interneuronal discharges were recorded throughout the sleep-wake cycle in hippocampal regions CA1 and dentate gyrus (DG) of ten chronically implanted rats. During quiet wakefulness (QW) and slow-wave-sleep (SWS) (non-theta rhythm states), the primary shock of paired stimuli evoked in CA1 both high amplitude population spikes and multiple interneuron discharges when compared to active wakefulness (AW) and rapid-eye-movement (REM) sleep (theta rhythm states). A second shock was delivered to CA1 afferents 60 ms after the first shock. This second shock evoked a small population spikes during non-theta states, whereas it evoked higher amplitude population spikes in theta states. The second shock also evoked unit interneuron discharges in non-theta states but not in theta states. In the dentate gyrus, identical primary afferent stimulation evoked similar interneuron activity and uniform amplitude population spikes throughout the sleep-wake cycle. In contrast, the secondary shocks evoked a striking potentiation of the field population spike during sleep, SWS and REM sleep compared to AW and OW. Evoked DG interneuron spikes following the second population spike were greater in number during SWS compared to the other stages. Our findings suggest that hippocampal field potentials and interneuron activity recorded in vivo are regionally regulated, have unique state-dependent expression and are strongly influenced by inhibitory feed-forward mechanisms.