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Respiratory rhythm plays an important role in cognitive functions in rodents, as well as in humans. Respiratory related oscillation (RRO), generated in the olfactory bulb (OB), is an extrinsic rhythm imposed on brain networks. In rats, RRO can couple with intrinsic brain oscillations at theta frequency during sniffing and in the delta range outside of such episodes. Disruption of gamma synchronization in cortical networks by ketamine is well established whereas its effects on slow rhythms are poorly understood. We found in this study, that RRO in prefrontal cortex (PFC) and hippocampus (HC) remains present after ketamine injection, even on the background of highly unstable respiratory rate, co-incident with "psychotic-like" behavior and abnormal cortical gamma activity. Guided by the timing of ketamine-induced gamma reaction, pairwise coherences between structures exhibiting RRO and their correlation structure was statistically tested in 5-min segments post-injection (0-25 min) and during recovery (1, 5, 10 h). As in control, RRO in the OB was firmly followed by cortical-bound OB exits directed toward PFC but not to HC. RRO between these structures, however, significantly correlated with OB-HC but not with OB-PFC. The only exception to this general observation was observed during a short transitional period, immediately after injection. Ketamine has a remarkable history in psychiatric research. Modeling chronic NMDA-hypofunction using acute NMDA-receptor blockade shifted the primary focus of schizophrenia research to dysfunctional cortical microcircuitry and the recent discovery of ketamine's antidepressant actions extended investigations to neurophysiology of anxiety and depression. Cortical oscillations are relevant for understanding their pathomechanism.

The supramammillary nucleus (SUM) is part of an ascending pathway conveying behavior-dependent drive to the septal generator of limbic theta rhythm. The SUM is, however, reciprocally connected to the septohippocampal system and there is strong evidence that both septum and SUM are capable of generating theta rhythmic activity. The present study examined the possible role of a descending rhythmic input to the SUM using simultaneously recorded hippocampal EEG and SUM neuronal activity in anesthetized rats. Fourier based phase analysis was performed on recordings in which fast theta rhythmic activity was elicited by tail pinch and in which a slower theta rhythm persisted after cessation of the sensory stimulus. It was found that the firing of a subpopulation of SUM neurons followed the hippocampal theta waves with a constant time delay, rather than a constant phase, suggesting that during deceleration associated with a shift from sensory-elicited theta to spontaneous theta rhythm they followed a descending rhythmic input, most likely from the medial septum. Neurons of a second group, which fired at the hippocampal theta peaks, did not show such relationship demonstrating heterogeneity in the population of rhythmic SUM neurons and their possible roles in theta generation. Combined with previous studies focusing on the role of the ascending theta drive from the SUM, these results demonstrate dynamic bidirectional coupling between subcortical theta generators. Thus, during certain states, rhythmically firing SUM neurons lead the septal theta oscillator, in others the direction may reverse and SUM follows a theta drive of septal origin.

Using the techniques of time domain correlation (mid-signal spike-triggered averaging) and frequency domain correlation (neuron-to-nerve coherence), 24% (54) of a sample of 229 neurons of the rostral para-ambiguual field have shown to have activity correlated to the 8- to 13-Hz rhythm in the inferior cardiac sympathetic nerve discharge (SND) of urethane-anesthetized cat. This correlation existed in both baroreceptor-innervated and -denervated animals. Of the correlated neurons, 37% (20) were non-rhythmically firing and displayed flat autospectra, while 63% (34) fired rhythmically and contained well-defined peaks in their autospectra. The group firing rate of these neurons was 4.3±0.4 spikes/s, indicating that they are not pacemaker neurons for the 10-Hz SND rhythm. The group time of firing of these neurons to the next peak of the SND slow wave was 52±4 ms. Correlation of the activity of medullary neurons with the 8–13-Hz rhythm of the SND was previously claimed only for rostral ventrolateral medulla, caudal raphe, and rostral caudal ventrolateral medulla. This present finding suggests that this behavior may be more widely spread throughout the medulla.

The spectral composition of the inspiratory nerve discharge was studied in spontaneously breathing Sprague-Dawley rats under urethane (n = 7) or barbiturate (n = 10). Left phrenic nerve activity was recorded simultaneously with right phrenic or left recurrent laryngeal nerves. We found that all neurograms showed prominent fast oscillators at common frequencies in the high frequency (HFO) range. Concurrent medium frequency oscillations (MFO) were present in inspiratory nerve discharges of four rats anesthetized with Nembutal. Significant coherences between nerves were uncommon (n = 2) and were only found between HFOs. Thus although nerve autospectra were dominated by HFO, weak correlations indicated a relatively weak system HFO in th central pattern generator, in the anesthetized rat.

In order to separate the effect of different theta generators in the hippocampus and to characterize the pattern of relationships between them, in this study, we calculated the coherence that remains between EEG signals, recorded (1) in the stratum oriens of the CA1 region and (2) close to the hippocampal fissure in the dentate gyrus of the right or left hippocampus, after the variations, common also for a third recording site is eliminated (partialization). We found that in both anesthetized and freely moving rats, there is a selective high correlation (coherence) between theta rhythmic activities of contralateral homonymous sites of the hippocampus. The coherence between field potentials recorded in ipsilateral superficial and deep layers was eliminated when allowance was made for any of the contralateral hippocampal recordings. On the other hand, coherence between contralateral homonymous theta dipoles did not decrease when partialized by a heteronymous hippocampal EEG signal. The present results support earlier findings on multiple hippocampal theta dipoles and indicate that they can be separated using partial coherence analysis. The left and right superficial and deep dipoles oscillate as if they formed two separate systems one extending over the superficial CA1 layers on both sides and the other consisting of the left and right deep hippocampal theta dipoles. The results also suggest an important role of the commissural projections in interhemispheric theta synchronization.

The median raphe nucleus (MR) exerts a pronounced desynchronizing influence on the hippocampal EEG. MR stimulation disrupts theta, while MR lesions produce constant uninterrupted theta. The MR receives pronounced excitatory amino acid (EAA)-containing afferents that have been implicated in several MR-mediated behaviors. The present study examined the effects on the hippocampal EEG of MR injections of the following EAA antagonists in the urethane-anesthetized rat: 2-amino-7-phosphonoheptanoate (AP-7), dizocilpine maleate (MK-801), and gamma-glutamyl-aminomethylsulfonic acid (GAMS). MR injections of the competitive (AP-7) and non-competitive (MK-801) N-methyl-D-aspartic acid (NMDA) receptor antagonists produced theta at short latencies (2.86 min; 4.02 min, respectively) and for long durations (116.1 min; 66.8 min, respectively). It was further shown that the theta-eliciting effects of AP-7 injections could be reliably and temporarily reversed with MR injections of NMDA. MR injections of the kainate/quisqualate receptor antagonist (GAMS) also produced theta at relatively short latencies (6.5 min) and for long durations (60.5 min) indicating that EAA effects on the MR are not NMDA receptor specific. Injections of each of the foregoing EAA antagonists into regions of the brainstem adjacent to the MR including the dorsal raphe nucleus and the medullary or pontine reticular formation generated theta at very long latencies or were without effect. The present findings indicate EAA afferents to the MR normally exert an excitatory influence on the MR in its desynchronization of the hippocampal EEG, whereas the removal of EAA inputs to MR produces the opposite: a reduction of MR activity and hence the elicitation of theta.(ABSTRACT TRUNCATED AT 250 WORDS)

In this study we examined the synchronization of the discharge of phrenic and recurrent laryngeal motoneurons in anesthetized rat pups 14 to 36 days of age and kittens, 14–15 days old. We found that the inspiratory nerve activity consisted of synchronized bursts separated by 20–35 ms, corresponding to medium-frequency oscillations (MFO). Accordingly, the autospectra of the neurograms had two peaks, one at the respiratory rate and the other between 22.8–43.0 Hz. No significant coherence was found between MFOs in the discharges of different nerves. High-frequency oscillations (HFO) characteristic for the adult inspiratory nerve activity were not present in the newborn rats. These findings demonstrate that phrenic nerve discharge of rat pups, like that of kittens and piglets, is in the MFO range, and suggest that MFO activity is an index of an early developmental stage of the respiratory system.

Extracellular single-unit recording techniques were used to examine the firing characteristics of neurons in the medial septum/diagnol band of Broca complex (MS/DB) following injections of the 5-HT1A agonist, 8-OH-DPAT, into the median raphe nucleus (MRN) of urethane-anesthetized rats. It had previously been shown that MRN injections of 8-OH-DPAT produce hippocampal theta rhythm. Injections of 8-OH-DPAT into the MRN produced a change in firing characteristics of MS/DB neurons from an irregular discharge to a pattern of rhythmical bursting in synchrony with hippocampal theta rhythm. Cross-correlational and coherence analyses demonstrated that the rhythmical firing pattern of MS/DB neurons strongly correlated with rhythmical fluctuations in the hippocampal EEG during periods of hippocampal theta produced by 8-OH-DPAT injections, but not during baseline conditions (i.e. hippocampal desynchronization). The results suggest that MRN control of the hippocampal EEG is modulated by the MS/DB. Serotonergic projections from the MRN to the MS/DB may normally act to inhibit the rhythmical bursting of MS/DB neurons, thereby producing hippocampal desynchronization. Suppression of MRN 5-HT neurons by MRN injections of 8-OH-DPAT would disinhibit MS/DB neurons, allowing them to burst rhythmically and thereby produce hippocampal theta rhythm.

In unanaesthetized mid-collicular decerebrate cats, stimulation of low-threshold afferents running in the inferior cardiac nerve can evoke in renal nerve a long-latency reflex response, which is able to reduce or overcome the inhibition developing after the late response. The spatial and temporal summation have great importance in generating this very late response (VLR). Increasing the stimulus intensity and the frequency of the impulses, using short trains or a previous A + C tetanic stimulation of the segmental inputs considerably increased the chance of the VLR overcoming the late inhibition.