Carotid and cardiopulmonary chemoreceptor activity increases hippocampal theta rhythm in conscious rabbits

Interregional phase-amplitude coupling between theta rhythm in the nucleus tractus solitarius and high-frequency oscillations in the hippocampus during REM sleep in rats

Cross-frequency coupling (CFC) between theta and high-frequency oscillations (HFOs) is predominant during active wakefulness, REM sleep and behavioral and learning tasks in rodent hippocampus. Evidence suggests that these state-dependent CFCs are linked to spatial navigation and memory consolidation processes. CFC studies currently include only the cortical and subcortical structures. To our knowledge, the study of nucleus tractus solitarius (NTS)-cortical structure CFC is still lacking. Here we investigate CFC in simultaneous local field potential recordings from hippocampal CA1 and the NTS during behavioral states in freely moving rats. We found a significant increase in theta (6-8 Hz)-HFO (120-160 Hz) coupling both within the hippocampus and between NTS theta and hippocampal HFOs during REM sleep. Also, the hippocampal HFOs were modulated by different but consistent phases of hippocampal and NTS theta oscillations. These findings support the idea that phase-amplitude coupling is both state- and frequency-specific and CFC analysis may serve as a tool to help understand the selective functions of neuronal network interactions in state-dependent information processing. Importantly, the increased NTS theta-hippocampal HFO coupling during REM sleep may represent the functional connectivity between these two structures which reflects the function of the hippocampus in visceral learning with the sensory information provided by the NTS. This gives a possible insight into an association between the sensory activity and REM-sleep dependent memory consolidation.

Displaying the autonomic processing network in humans - a global tractography approach

Regulation of the internal homeostasis is modulated by the central autonomic system. So far, the view of this system is determined by animal and human research focusing on cortical and subcortical grey substance regions. To provide an overview based on white matter architecture, we used a global tractography approach to reconstruct a network of tracts interconnecting brain regions that are known to be involved in autonomic processing. Diffusion weighted imaging data were obtained from subjects of the human connectome project (HCP) database. Resulting tracts are in good agreement with previous studies assuming a division of the central autonomic system into a cortical (CAN) and a subcortical network (SAN): the CAN consist of three subsystems that encompass all cerebral lobes and overlap within the insular cortex: a parieto-anterior-temporal pathway (PATP), an occipito-posterior-temporo-frontal pathway (OPTFP) and a limbic pathway. The SAN on the other hand connects the hypothalamus to the periaqueductal grey and locus coeruleus, before it branches into a dorsal and a lateral part that target autonomic nuclei in the rostral medulla oblongata. Our approach furthermore reveals how the CAN and SAN are interconnected: the hypothalamus can be considered as the interface-structure of the SAN, whereas the insula is the central hub of the CAN. The hypothalamus receives input from prefrontal cortical fields but is also connected to the ventral apex of the insular cortex. Thus, a holistic view of the central autonomic system could be created that may promote the understanding of autonomic signaling under physiological and pathophysiological conditions.

The Logic of Carotid Body Connectivity to the Brain

The carotid body has emerged as a therapeutic target for cardio-respiratory-metabolic diseases. With the expansive functions of the chemoreflex, we sought mechanisms to explain differential control of individual responses. We purport a remarkable correlation between phenotype of a chemosensory unit (glomus cell-sensory afferent) with a distinct component of the reflex response. This logic could permit differential modulation of distinct chemoreflex responses, a strategy ideal for therapeutic exploitation.

Arousal from sleep in response to intermittent hypoxia in rat pups is modulated by medullary raphe GABAergic mechanisms

Arousal is an important defense against hypoxia during sleep. Rat pups exhibit progressive arousal impairment (habituation) with multiple hypoxia exposures. The mechanisms are unknown. The medullary raphe (MR) is involved in autonomic functions, including sleep, and receives abundant GABAergic inputs. We hypothesized that inhibiting MR neurons with muscimol, a GABA(A) receptor agonist, or preventing GABA reuptake with nipecotic acid, would impair arousal and enhance arousal habituation and that blocking GABA(A) receptors with bicuculline would enhance arousal and attenuate habituation. Postnatal day 15 (P15) to P25 rat pups were briefly anesthetized, and microinjections with aCSF, muscimol, bicuculline, or nipecotic acid were made into the MR. After a ∼30-min recovery, pups were exposed to four 3-min episodes of hypoxia separated by 6 min of normoxia. The time to arousal from the onset of hypoxia (latency) was determined for each trial. Latency progressively increased across trials (habituation) in all groups. The overall latency was greater after muscimol and nipecotic acid compared with aCSF, bicuculline, or noninjected controls. Arousal habituation was reduced after bicuculline compared with aCSF, muscimol, nipecotic acid, or noninjected pups. Increases in latency were mirrored by decreases in chamber [O2] and oxyhemoglobin saturation. Heart rate increased during hypoxia and was greatest in muscimol-injected pups. Our results indicate that the MR plays an important, not previously described, role in arousal and arousal habituation during hypoxia and that these phenomena are modulated by GABAergic mechanisms. Arousal habituation may contribute to sudden infant death syndrome, which is associated with MR serotonergic and GABAergic receptor dysfunction.

Electrocortical functional connectivity in infancy: response to body tilt

To test the hypothesis that infant cortical regions activated by a head-up tilt also exhibit increased functional electrocortical connectivity, prone sleeping newborn and 2- to 4-month-old infants were tilted head-up to 30 degrees. Electroencephalogram (EEG) data were collected with 128 electrodes and coherence calculated to quantify electrocortical synchrony. Local coherence, defined as the average of coherence measurements between the EEG at each electrode site and neighboring sites (approximately 1 cm electrode spacing), was found in activated cortical regions that had previously shown increased high-frequency power with tilt. Long-distance coherence was computed between the regions. Newborn infants had significant increases in local coherence in the activated left frontal, right frontal-temporal, and occipital cortical regions; long-distance coherence increased between the right frontal-temporal and occipital regions. In contrast, infants at 2 to 4 months old, the age of maximum risk for sudden infant death syndrome, had no significant changes in coherence. Newborn and 2- to 4-month-old infants thus have different electrocortical responses to a classic cardiovascular challenge.

Characterisation of c-Fos expression in the central nervous system of mice following right atrial injections of the 5-HT3 receptor agonist phenylbiguanide

Cardiopulmonary receptors relay signals to the central nervous system via vagal and spinal visceral afferents. To date there are no detailed topographical studies in mice indicating the distribution of central neurones activated following stimulation of cardiopulmonary afferents. In anaesthetised mice, we injected the 5-HT(3) receptor agonist phenylbiguanide (PBG), a drug that is known to stimulate cardiopulmonary afferent C-fibres, into the right atrium of the heart and mapped c-Fos expression within specific regions of the central nervous system. Intra-atrial injection of PBG produced a reflex cardiorespiratory response including a pronounced bradycardia and a respiratory depression. Using immunohistochemical detection of the protein product of the immediate-early gene c-fos, we mapped the brain regions affected by cardiopulmonary 5-HT(3) receptor stimulation. Within the nucleus of the solitary tract (nTS) of PBG-injected mice, we detected an increased number of c-Fos-positive nuclei in the dorsolateral and gelatinous parts at the level of the area postrema (-7.48 mm bregma) but not at more rostral or caudal levels (-7.76, -7.20, -6.84 and -6.36 mm bregma) relative to vehicle-injected control mice. In addition, c-Fos expression in the crescent part of the lateral parabrachial nucleus was decreased in PBG-injected mice whereas no significant differences were detected between PBG-injected and control mice in the number of c-Fos-positive nuclei in the dorsal part of the lateral parabrachial nucleus. PBG injections had no significant effects on the number of c-Fos-positive catecholaminergic neurones within the C1/A1, C2/A2, A5, A6 and A7 cell groups. Likewise, PBG injections had no significant effects on c-Fos expression in other central regions involved in cardiorespiratory control or cardiorespiratory reflexes (selected non-catecholaminergic nuclei in the medulla and midbrain periaqueductal gray, the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala). Identification of specific regions of the nTS complex involved in relaying signals from afferent cardiopulmonary C-fibres to the central nervous system will be useful for future studies aimed at understanding neural mechanisms underlying cardiopulmonary reflexes and physiological responses to cardiopulmonary disease.

Topographic localization of electrocortical activation in newborn and two- to four-month-old infants in response to head-up tilting

AIMS

(1) To confirm that head-up tilting causes sustained increases in the heart rate (HR) of newborn infants but not during the period of maximum vulnerability to SIDS at 2-4 mo of age, and (2) to determine whether electrocortical activation (changes in high-frequency EEG power) also shows topographic and age-dependent effects of tilting.

METHODS

HR and electrocortical activity were recorded in 15 newborn and 12 2- to 4-mo-old infants during head-up tilting. Infants were tilted, three times, to a 30 degrees head-up position. Electrocortical activity was acquired using a 128-lead EEG system. Changes in HR and high-frequency (12-50 Hz) power in the electrocortical signal were computed from the flat to the head-up position.

RESULTS

Newborn infants had significant increases in HR and robust increases in high-frequency power in the left frontal, right frontal-temporal, and occipital regions following head-up tilt. At 2 to 4 mo of age, HR did not change significantly and tilt-related increases in high-frequency power were smaller.

CONCLUSION

The patterns of HR change and electrocortical activation with tilting of newborn infants are different from infants at the age of highest risk for SIDS.

Autonomic brainstem nuclei are linked to the hippocampus

Vagal nerve stimulation has been reported to enhance memory in both rats and humans, and to be an effective treatment for epilepsy in some patients, but the underlying neuroanatomical substrate(s) responsible for these effects remains unknown. Since there is no direct anatomical projection from the nucleus tractus solitarius, the main vagal relay site of the brain, to the hippocampus, we tested whether a multisynaptic pathway exists. Pseudorabies virus, a pig herpesvirus that can be used as a retrograde transneuronal tracer, was injected into the ventral CA1 hippocampus of rats, and after 4 days, pseudorabies virus infected neurons were identified in the general visceral portion of the nucleus tractus solitarius, with the majority being localized in the A2 noradrenergic cell group. Other autonomic brainstem nuclei, including the parabrachial nucleus, locus coeruleus, A1 and A5 noradrenergic cell groups, and C1 adrenergic cell group, were labeled. In order to identify some of the potential relay sites of the nucleus tractus solitarius-->hippocampal pathway, immunotoxin lesions of the ventral CA1 region were made that selectively destroyed either the noradrenergic or cholinergic fibers. After 2 weeks' recovery, pseudorabies virus was injected in this same CA1 area, and 4 days later, the transneuronal labeling in the nucleus tractus solitarius was reduced by approximately 65%. These findings suggest that the noradrenergic neurons of the locus coeruleus and cholinergic neurons of the medial septum/diagonal band are likely to be relay sites for this pathway. Other potential linkages are discussed. In summary, this is the first anatomical report to show that the general visceral region of nucleus tractus solitarius is linked via multisynaptic relays to the hippocampus.