Rhythmic activities in the rat solitary complex in vitro

Spatiotemporal Structure and Dynamics of Spontaneous Oscillatory Synchrony in the Vagal Complex

Fundamental structure and dynamics of spontaneous neuronal activities without apparent peripheral inputs were analyzed in the vagal complex (VC), whose activities had been generally thought to be produced almost passively to peripheral cues. The analysis included the caudal nucleus of the tractus solitarius—a main gateway for viscerosensory peripheral afferents and involved dynamically and critically in cardiorespiratory brainstem networks. In the present study, a possibility of self-organized brain activity was addressed in the VC. While VC neurons exhibited sparse firing in anesthetized rats and in in vitro preparations, we identified peculiar features of the emergent electrical population activity: (1) Spontaneous neuronal activity, in most cases, comprised both respiration and cardiac cycle components. (2) Population potentials of polyphasic high amplitudes reaching several millivolts emerged in synchrony with the inspiratory phase of respiratory cycles and exhibited several other characteristic temporal dynamics. (3) The spatiotemporal dynamics of local field potentials (LFPs), recorded simultaneously over multiple sites, were characterized by a stochastic emergence of high-amplitude synchrony. By adjusting amplitude and frequency (phase) over both space and time, the traveling synchrony exhibited varied degrees of coherence and power with a fluctuating balance between mutual oscillators of respiratory and cardiac frequency ranges. Full-fledged large-scale oscillatory synchrony over a wide region of the VC emerged after achieving a maximal stable balance between the two oscillators. Distinct somatic (respiratory; ~1 Hz) and visceral (autonomic; ~5 Hz) oscillators seemed to exist and communicate co-operatively in the brainstem network. Fluctuating oscillatory coupling may reflect varied degrees of synchrony influenced by the varied amplitude and frequency of neuronal activity in the VC. Intranuclear micro-, intrabulbar meso-, and wide-ranging macro-circuits involving the VC are likely to form nested networks and strategically interact to maintain a malleable whole-body homeostasis. These two brainstem oscillators could orchestrate neuronal activities of the VC, and other neuronal groups, through a phase-phase coupling mechanism to perform specific physiological functions.

Sonic hedgehog signalling in neurons of adult ventrolateral nucleus tractus solitarius

The transmembrane receptor Patched (Ptc) mediates the action of the diffusing factor Sonic hedgehog (Shh), which is implicated in establishing morphogenetic gradients during embryonic development. Whereas alteration of Ptc function is associated with developmental abnormalities and brain tumors, its functional activity and roles in the adult brain have yet to be elucidated. Here we describe the complementary pattern of Shh and Ptc expression in the rat dorsal vagal motor nucleus and the ventrolateral nucleus tractus solitarius (vNTS), respectively. Those two interconnected structures regulate the cardiorespiratory function during hypoxia. Bath application of a subnanomolar concentration of aminoterminal Shh protein (ShhN) to a slice preparation of the vNTS induces a rapid decrease in neuronal firing followed by a bursting activity that propagates in the neuronal network. Intracellular current injections show that bursts result from an action on the neuronal membrane electro-responsiveness. Both inhibiting and bursting effects are blocked by the monoclonal Shh antibody 5E1 and may require the Ptc binding site of ShhN. Thus, ShhN acting on specific neuronal sites controls electrophysiological properties of differentiated neurons of the vNTS. We speculate on a retrocontrol of cardiorespiratory signals in the vNTS, by Shh generated in dorsal vagal motoneurons.

Serotonin induces an increase in D-myo-inositol (1,4,5)-trisphosphate 3-kinase activity in rat brainstem slices

Serotonin robustly potentiated the activity of the InsP3 3-kinase in rat brainstem slices. This potentiation was mediated through activation of 5-HT2 receptors since it was only retrieved with the selective 5-HT2 agonist DOI but not with the 5-HT1A agonist 8OHDPAT. The enhancement of the InsP3 3-kinase activity by serotonin is positively modulated by pretreatment of the slices with the phosphatase inhibitor okadaic acid. Moreover, the specific CaMKII antagonists KN-62 and KN-93 dramatically reduced the serotonin-evoked increase in the InsP3 3-kinase activity. It is thus concluded that InsP3 3-kinase up-regulation occurs through activation of PLC-coupled serotoninergic receptors and requires the phosphorylation of the enzyme by the ubiquitous multimeric protein kinase CaMKII.

Plasticity of tyrosine hydroxylase gene expression in the rat nucleus tractus solitarius after ventilatory acclimatization to hypoxia

The aim of this study was to define the influence of long-term hypoxia on gene expression of tyrosine hydroxylase (TH) in the rat nucleus tractus solitarius (NTS). Animals were exposed to normobaric hypoxia (10% O2 in nitrogen) for 2 weeks. At this time, the hypoxia-induced hyperventilation reached a plateau, indicating ventilatory acclimatization. In horizontal brainstem sections, hypoxia-induced changes in TH protein and TH mRNA were assessed by immunocytochemistry and in-situ hybridization, respectively. Long-term hypoxia increased TH mRNA levels seen as both an increase in the number of grains per cell and an extension of the labeled area. The highest degree of labeling was found selectively located in caudal NTS. Hypoxia also enhanced TH immunoreactivity in the caudal NTS but this labeling extended more rostrally than that of TH mRNA. The data suggest that there is an hypoxia-induced plasticity of gene expression at the gene level in the NTS, which is associated with ventilatory acclimatization. The hypoxia model described in this study may serve as a framework for future regulatory studies.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.

Ventilatory acclimatization to chronic hypoxia: relationship to noradrenaline metabolism in the rat solitary complex

1. The relationship between ventilatory acclimatization to chronic hypoxia (10% O2-90% N2) and noradrenaline metabolism was examined in two regions located immediately caudal and rostral to the obex within the rat solitary complex. 2. Three experimental protocols were established. In protocol 1, the percentage changes in respiratory tidal volume, frequency and minute ventilation elicited by 4, 7, 10 and 14 days of hypoxia were assessed by flow plethysmography in awake rats, and then the content of tyrosine hydroxylase was measured in the solitary complex. In protocol 2, the time course response of tyrosine hydroxylase protein level was determined after 3, 7, 14 and 22 days of hypoxia by using a quantitative immunoblotting method for the protein assay. In protocol 3, the turnover of noradrenaline was estimated in the solitary complex after 14 days of hypoxia. 3. A progressive increase in ventilation was observed to reach a maximum (+105 +/- 15%, mean +/- S.E.M.) above normoxic control after 10 days of hypoxia, at which time it stabilized. Furthermore, tyrosine hydroxylase protein increased progressively and reached a maximal level at 14 days of hypoxia (+36 +/- 4%, mean +/- S.E.M.). Return to the basal level of tyrosine hydroxylase was observed after 22 days of hypoxia. 4. Tyrosine hydroxylase content (+36 +/- 4%) and noradrenaline turnover (+394 +/- 3%) increased exclusively in the caudal part of the solitary complex. 5. The ventilatory acclimatization to chronic hypoxia preceded the increase in tyrosine hydroxylase and these two parameters were significantly correlated. 6. These data suggest that ventilatory acclimatization to chronic hypoxia is associated with topical modifications of the brainstem catecholamine metabolism.

Spontaneous synaptic activities in rat nucleus tractus solitarius neurons in vitro: evidence for re-excitatory processing

The pattern of synaptic interactions between neurons of the nucleus tractus solitarius (NTS) has been analyzed using whole cell recording in rat brainstem slices. Following tractus solitarius (TS) stimulation 15/55 neurons presented a prolonged (up to 300 ms) increased excitability (PIE neurons) and 40/55 neurons presented a prolonged (up to 200 ms) reduced excitability (PRE neurons). In the absence of afferent sensory input all neurons showed spontaneous synaptic activity. Ongoing synaptic activity in PIE cells was glutamatergic and characterized by the absence of detectable inhibitory potentials while in PRE cells it was 90% GABAergic and 10% glutamatergic. Glutamatergic synaptic currents in PIE cells and GABAergic synaptic currents in PRE were studied using probability density and intensity functions. Distribution of time intervals between synaptic events indicated the latter were generated, in both PIE and PRE cells, by two simultaneous processes: (1) a close to Poisson process generating independent events; and (2) a subsidiary re-excitatory process generating synaptic events separated by intervals shorter than 20 ms. Blockade of glutamatergic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) or blockade of action potentials by tetrodotoxin (TTX; 1 microM) suppressed the subsidiary process. In conclusion, we propose that PIE cells (1) form a re-excitatory network contributing to generation of excitatory activity in the NTS and (2) are located presynaptically with respect to PRE cells.

Medial vestibular neurons are endogenous pacemakers whose discharge is modulated by neurotransmitters

1. Neurons in the medial vestibular nucleus (MVN), recorded in a rat brain slice preparation, exhibit a highly regular, high-frequency (5- to 35-Hz) spontaneous discharge. The rhythmic firing rate was constant (< 5% variation) and sustained for a long time (maximum observation, 4 hr). 2. The rhythmic firing was evident even in neurons (n = 15) completely isolated from exogenous input fibers, suggesting that it is due to an endogenous pacemaker property. When recorded intracellularly, the discharge was found to be associated with a smooth, concave pacemaker prepotential, and the rate of firing was reduced in proportion to applied hyperpolarizing current, indicating that these are pacemaker discharges. 3. This conclusion is supported by the observation that perfusion with a low-calcium/high-magnesium Krebs-Ringer solution, which completely and reversibly blocks all synaptic transmission, did not abolish the spontaneous discharge. The low-calcium/high-magnesium solution increased spontaneous firing in some neurons and decreased in others, suggesting that the firing is synaptically modulated and the synaptic influence is tonically active. 4. Application of kynurenate (10 mM), an antagonist of the excitatory amino acid receptors, gradually reduced neuronal discharges in most neurons (22 of 25), while the addition of 10 mM sucrose as an osmotic control had no effect. Depression of neuronal discharges reached its minimum (an average of 60% of the control level) and was maintained at that level until gradually washed out. The rhythmic firing pattern persisted in all neurons even after the excitatory receptors were blocked. 5. When the GABAA receptor antagonist, bicuculline (20 microM), was applied, elevation of neuronal discharges was evident in most neurons (30 of 32) tested. The firing increased gradually, with a final control level of 130% (121-160%). In contrast, the GABAB receptor antagonist, phaclofen (20 microM and 100 microM), had no effect in most neurons (19 of 23) tested. Further, the excitatory and inhibitory action could be detected on the same neuron when bicuculline and kynurenate were both evaluated (n = 10). 6. These results indicate that the spontaneous discharge of MVN neurons is due to an endogenous pacemaker under the tonic influence of both inhibitory and excitatory transmitter actions. The bicuculline-sensitive GABAA receptors and the kynurenate-sensitive glutamate receptors both mediate the tonic modulation.

Mechanisms of respiratory rhythm generation

In mammals, a three-phasic respiratory rhythm is generated by a network of various types of neurons in the lower brainstem. The cellular mechanisms of rhythmogenesis involve cooperative interactions between synaptic processes and specific membrane properties. The network seems to be driven by extrinsic sources in mature animals, whereas in the immature network pacemaker neurons might be involved.