Effect of norepinephrine receptors on trigeminal rhythm generation in newborn rats

Central pattern generators in the brainstem and spinal cord: an overview of basic principles, similarities and differences

Central pattern generators (CPGs) are generally defined as networks of neurons capable of enabling the production of central commands, specifically controlling stereotyped, rhythmic motor behaviors. Several CPGs localized in brainstem and spinal cord areas have been shown to underlie the expression of complex behaviors such as deglutition, mastication, respiration, defecation, micturition, ejaculation, and locomotion. Their pivotal roles have clearly been demonstrated although their organization and cellular properties remain incompletely characterized. In recent years, insightful findings about CPGs have been made mainly because (1) several complementary animal models were developed; (2) these models enabled a wide variety of techniques to be used and, hence, a plethora of characteristics to be discovered; and (3) organizations, functions, and cell properties across all models and species studied thus far were generally found to be well-preserved phylogenetically. This article aims at providing an overview for non-experts of the most important findings made on CPGs in in vivo animal models, in vitro preparations from invertebrate and vertebrate species as well as in primates. Data about CPG functions, adaptation, organization, and cellular properties will be summarized with a special attention paid to the network for locomotion given its advanced level of characterization compared with some of the other CPGs. Similarities and differences between these networks will also be highlighted.

Inhibition of Trigeminal Respiratory Activity by Suckling

The trigeminal motor system is involved in many rhythmic oral-motor behaviors, such as suckling, mastication, swallowing, and breathing. Despite the obvious importance of functional coordination among these rhythmic activities, the system is not well-understood. In the present study, we examined the hypothesis that an interaction between suckling and breathing exists in the brainstem, by studying the respiratory activity in trigeminal motoneurons (TMNs) during fictive suckling using a neonatal rat in vitro brainstem preparation. The results showed that fictive suckling, which was neurochemically induced by bath application of N-methyl-D,L-aspartate and bicuculline-methiodide, or by local micro-injection of the same drugs to the trigeminal motor nucleus, inhibited the inspiratory activities in both respiration TMNs and respiratory rhythm-generating neurons. Under patch-clamp recording, fictive suckling caused membrane potential hyperpolarization of respiration TMNs. We conclude that the brainstem preparation contains an inhibitory circuit for respiratory activity in the trigeminal motor system via the rhythm-generating network for suckling. Abbreviations: BIC, bicuculline methiodide; GABA, gamma aminobutyric acid; NMA, N-methyl-D,L-aspartate; NMDA, N-methyl-D-aspartate; and TMN, trigeminal motoneuron.

Ventrolateral lesions at the ponto-medullary junction and the effects of noradrenaline on respiratory rhythm in rat brainstem-spinal cord preparations

AIMS

We examined whether responses of respiratory frequency (fR) to noradrenaline (NA) were eliminated by mechanical lesions in the ventrolateral area at the ponto-medullary junction in preparations of newborn rat pons-medulla-spinal cord (PMS).

MAIN METHODS

Preparations obtained from 2- to 4-day-old rats were superfused with artificial cerebrospinal fluid that was equilibrated with oxygenated (95% O2 plus 5% CO2 gas, and fR was monitored at the C4 ventral root at 24 degrees C. Bilateral lesions were made in the ventrolateral area between the VIth cranial nerve root and the anterior inferior cerebellar artery in PMS (n=11). The resting fR and response to exogenous NA (7 microM) were compared with those of medulla-spinal cord (MS) preparations (n=6). Immunohistochemistry of PMS preparations was performed to detect tyrosine hydroxylase (TH)-positive neurons at the ponto-medullary junction.

KEY FINDINGS

PMS preparations with the lesions had (1) a significantly higher resting fR but 2 significantly less fR facilitation after NA application than those of intact PMS preparations, and (3) significantly lower resting fR and (4) significantly less fR reduction after NA application than those of MS preparations. TH-positive neurons were detected in the region from the rostral dorsolateral to the caudal ventrolateral pons (the A5 area), as well as in the ventral area near the facial nucleus.

SIGNIFICANCE

Results suggest that ventrolateral area at ponto-medullary junction plays a significant role in exogenous NA-induced fR changes under the influence of pons-induced tonic fR inhibition in newborn rat brainstem-spinal cord preparations.

Effect of 2,4-dinitrophenol under non-oxygenated condition in pons–medulla–spinal cord preparations in newborn rats: Comparison with medulla–spinal cord preparations

We tested whether depression of respiratory frequency (fR) under non-oxygenated artificial cerebrospinal fluid (aCSF) in pons-medulla-spinal cord (PMS) and medulla-spinal cord (MS) preparations is significantly influenced by the mitochondrial uncoupler 2,4-dinitrophenol (2,4-DNP) in newborn rats. Preparations were obtained from 0- to 4-day-old rats, and fR was monitored at the C4 ventral root in environmental temperature (24 degrees C). 2,4-DNP was dissolved in aCSF (1, 10 or 30 microM; pH 7.4), and we measured fR in PMS (n=19) and MS (n=16), both of which were superfused with aCSF equilibrated with oxygenated (95% O2-5% CO2) or non-oxygenated (10% O2-5% CO2, balanced with pure N2) gas. Our results showed that: (1) fR was significantly lower in PMS than MS, (2) fR was significantly decreased under non-oxygenated aCSF in both PMS and MS and (3) fR under non-oxygenated aCSF was significantly increased by 2,4-DNP applications at 10 and 30 microM in PMS but not in MS. Our results suggest that depression in fR under non-oxygenated aCSF in PMS and MS may not be due simply to O2 limitation, and 2,4-DNP has a stimulant effect on the medullary respiratory rhythm generator (RRG) through pontine RRG regulatory mechanisms under non-oxygenated aCSF.

Pre-/post-otic rhombomeric interactions control the emergence of a fetal-like respiratory rhythm in the mouse embryo

How regional patterning of the neural tube in vertebrate embryos may influence the emergence and the function of neural networks remains elusive. We have begun to address this issue in the embryonic mouse hindbrain by studying rhythmogenic properties of different neural tube segments. We have isolated pre- and post-otic hindbrain segments and spinal segments of the mouse neural tube, when they form at embryonic day (E) 9, and grafted them into the same positions in stage-matched chick hosts. Three days after grafting, in vitro recordings of the activity in the cranial nerves exiting the grafts indicate that a high frequency (HF) rhythm (order: 10 bursts/min) is generated in post-otic segments while more anterior pre-otic and more posterior spinal territories generate a low frequency (LF) rhythm (order: 1 burst/min). Comparison with homo-specific grafting of corresponding chick segments points to conservation in mouse and chick of the link between the patterning of activities and the axial origin of the hindbrain segment. This HF rhythm is reminiscent of the respiratory rhythm known to appear at E15 in mice. We also report on pre-/post-otic interactions. The pre-otic rhombomere 5 prevents the emergence of the HF rhythm at E12. Although the nature of the interaction with r5 remains obscure, we propose that ontogeny of fetal-like respiratory circuits relies on: (i) a selective developmental program enforcing HF rhythm generation, already set at E9 in post-otic segments, and (ii) trans-segmental interactions with pre-otic territories that may control the time when this rhythm appears.

Alpha-1 adrenergic input to solitary nucleus neurones: calcium oscillations, excitation and gastric reflex control

The nucleus of the solitary tract (NST) processes substantial visceral afferent input and sends divergent projections to a wide array of CNS targets. The NST is essential to the maintenance of behavioural and autonomic homeostasis and is the source, as well as the recipient, of considerable noradrenergic (NE) projections. The significance of NE projections from the NST to other CNS regions has long been appreciated, but the nature of NE action on NST neurones themselves, especially on the alpha-1 receptor subtype, is controversial. We used a combination of methodologies to establish, systematically, the effects and cellular basis of action of the alpha-1 agonist, phenylephrine (PHE), to control NST neurones responsible for vago-vagal reflex regulation of the stomach. Immunocytochemical and retrograde tracing studies verified that the area postrema, A2, A5, ventrolateral medulla and locus coeruleus regions are sources of catecholaminergic input to the NST. In vivo electrophysiological recordings showed that PHE activates physiologically identified, second-order gastric sensory NST neurones. In vivo microinjection of PHE onto NST neurones caused a significant reduction in gastric tone. Finally, in vitro calcium imaging studies revealed that PHE caused dramatic cytosolic calcium oscillations in NST neurones. These oscillations are probably the result of an interplay between agonist-induced and inositol 1,4,5-trisphosphate (IP(3))-mediated intracellular calcium release and Ca(2+)-ATPase control of intracellular calcium storage pumps. The oscillations persisted even in perfusions of zero calcium-EGTA Krebs solution suggesting that the calcium oscillation is mediated principally by intracellular calcium release-reuptake mechanisms. Cyclical activation of the NST may function to increase the responsiveness of these neurones to incoming afferent input (i.e., elevate the "gain"). An increase in gain of afferent input may cause an amplification of the response part of the reflex and help explain the powerful effects that alpha-1 agonists have in suppressing gastric motility and producing anorexia.

Localization of premotoneurons for an NMDA-induced repetitive rhythmical activity to TMNs

The localization or characteristics of the premotoneurons for trigeminal rhythmical activity have not been clarified. We investigated the localization of premotoneurons generating an NMDA-induced repetitive rhythmical activity to trigeminal motoneurons (TMNs). The minimal circuitry for this rhythmical activity was determined using a fragmented slice preparation of the isolated brain stem from neonatal rats (0-3 days old). We recorded rhythmical neural activities from TMNs using whole and fragmented brainstem slices preparation including the trigeminal motor nucleus in the presence of the excitatory amino acid agonist NMA and the GABAA receptor antagonist, bicuculline methiodide (BIC). TMNs receive projections from premotoneurons for an NMDA-induced rhythmical activity, which can be located in the area 300 microm surrounding the trigeminal motor nucleus. NMA (20 microM) and BIC (10 microM) induced repetitive rhythmical activities on TMNs.

Swallowing-like activity elicited in vitro in neonatal rat organ attached brainstem block preparation

The purpose of this study was to induce swallowing in an in vitro neonatal rat brainstem preparation and to analyze the circuit. When we applied GABA(A) receptor antagonist (bicuculine methiodide, BIC) into the the nucleus tractus solitarius (NTS) in the organ attached brainstem preparation of neonatal (0-3 days after birth) rats, jaw closing movement, palatal lifting, and tongue peristalsis-like movement were seen, subsequent to elevation of the tip of the tongue and anterior movement of the larynx (closure of the trachea). The NTS has been proposed to be a critical locus for swallowing pattern generation in mammals. Electrical stimulation into the NTS or the vagal afferent nerve (X) following an application of BIC (10 microM) to the recording chamber initiated the same organ movement. This movement caused temporary inhibition of respiratory activity that was simultaneously recorded from the fourth cervical ventral nerve (C4). We were also able to elicit this activity in a whole organ (from lip to stomach, midline intact) preparation, whose oral cavity was filled with dye (pontamine sky-blue 3 mM, 50 microl), using each of the three types of stimulation. The esophagus, which was never stained by spontaneous respiratory movements, was stained only after the experimental stimulation. We concluded that the activity elicited was swallowing-like activity and the smallest circuit for swallowing pattern generation exists in this preparation.

Differential discharge patterns of rhythmical activity in trigeminal motoneurons during fictive mastication and respiration in vitro

Rhythmical activity in trigeminal motoneurons (TMNs) was studied in an in vitro neonatal rat brainstem preparation that retains functionally active circuits for oral-motor behaviors. Whole-cell current-clamp recording from TMNs demonstrated rhythmical activities during both spontaneously generated respiratory activity and neurochemically induced rhythmical oral-motor activity. TMNs showed spontaneous rhythmical (0.08 +/- 0.04 Hz) activities of burst-firing pattern during inspiration synchronized with inspiratory activities recorded in hypoglossal nerves. During rhythmical oral-motor activity induced by bath application of N-methyl-d,l-aspartic acid and the GABA(A) receptor antagonist bicuculline methiodide, TMNs showed only a rhythmical (5.6 +/- 0.8 Hz) pattern of single-spike discharge. TMNs never showed a burst-firing pattern during rhythmical oral-motor activity even when membrane potentials were shifted either to depolarized or hyperpolarized levels. Rhythmical activity in TMNs exhibited different discharge patterns between rhythmical oral-motor activity and respiratory activity generated in vitro.