Brainstem and spinal projections of augmenting expiratory neurons in the rat

Neurokinin-1 receptor activation in Botzinger complex evokes bradypnoea

In the present study, we examined the role of the neurokinin-1 receptor (NK1R) in the modulation of respiratory rhythm in a functionally identified bradypnoeic region of the ventral respiratory group (VRG) in the in situ arterially perfused juvenile rat preparation. In electrophysiologically and functionally identified bradypnoeic sites corresponding to the Bötzinger complex (BötC), microinjection of the selective NK1R agonist [Sar(9)-Met(O(2))(11)]-substance P (SSP) produced a significant reduction in phrenic frequency mediated exclusively by an increase in expiratory duration (T(E)). The reduction was characterized by a significant increase in postinspiratory (post-I) duration with no effect on either late-expiratory duration (E2) or inspiratory duration (T(I)). In contrast, in a functionally identified tachypnoeic region, corresponding to the preBötzinger complex (Pre-BötC), control microinjection of SSP elicited tachypnoea. Pretreatment with the NK1R antagonist CP99994 in the BötC significantly attenuated the bradypnoeic response to SSP injection and blunted the increase in T(E) duration. This effect of SSP mimicked the extension of T(E) produced by activation of the Hering-Breuer reflex. Therefore, we hypothesized that activation of NK1Rs in the BötC is requisite for the expiratory-lengthening effect of the Hering-Breuer reflex. Unilateral electrical stimulation of the cervical vagus nerve produced bradypnoea by exclusively extending T(E). Ipsilateral blockade of NK1Rs by CP99994 following blockade of the contralateral BötC by the GABA(A) receptor agonist muscimol significantly reduced the extension of T(E) produced by vagal stimulation. Results from the present study demonstrate that selective activation of NK1Rs in a functionally identified bradypnoeic region of the VRG can depress respiratory frequency by selectively lengthening post-I duration and provide evidence that endogenous activation of NK1Rs in the BötC appears to be involved in the expiratory-lengthening effect of the Hering-Breuer reflex. In conclusion, our findings demonstrate that selective activation of NK1Rs in discrete regions of the VRG can exert functionally diverse effects on breathing.

Distribution and medullary projection of respiratory neurons in the dorsolateral pons of the rat

The dorsolateral pons around the parabrachial nucleus including the Kölliker-Fuse nucleus is closely linked with the medullary respiratory center and plays an important role in respiratory control. We aimed to elucidate the firing properties, detailed distributions, and medullary projections of pontine respiratory neurons in pentobarbitone-anesthetized, paralyzed, and artificially ventilated rats with intact vagi. A total of 235 respiratory neurons were recorded from the dorsolateral pons in and around the Kölliker-Fuse nucleus. Six types of firing patterns were identified: inspiratory, expiratory-inspiratory phase spanning, inspiratory-expiratory phase spanning, decrementing expiratory, augmenting expiratory, and whole-phase expiratory patterns. Of these, the inspiratory neurons and the expiratory-inspiratory phase spanning neurons, which constituted the largest population (61%), were characterized most carefully by changing lung inflation levels, since under some conditions both showed similar firing patterns. Many (58%) of the 133 respiratory neurons examined were antidromically activated by electrical stimulation of the medulla. They were activated from the ventrolateral medulla around the ventral respiratory group and the Bötzinger complex and from the dorsomedial medulla around the nucleus tractus solitarii and the hypoglossal nucleus. The projections to the dorsomedial medulla were bilateral in many cases, and those to the ventrolateral medulla were unilateral. Of these medullary projections, two specific projections could be characterized in detail. First, many expiratory-inspiratory phase spanning neurons projected to the hypoglossal nucleus, suggesting that these pontine neurons are important premotor neurons of the hypoglossal motoneurons. This projection explains well the hypoglossal inspiratory activity, which is often dissociated from the phrenic inspiratory activity. Second, most whole-phase expiratory neurons that were distributed medially to the KF nucleus sent their axons toward the spinal cord via the midline medulla. These findings provide a new insight into the pontine control of medullary and spinal respiratory function.

Respiratory rhythm entrainment by somatic afferent stimulation

Respiratory and locomotor patterns are coupled during locomotion. The objectives of this study were to (1) demonstrate that respiratory rhythms are entrained by sensory input from somatic afferents, (2) establish whether the parabrachial nucleus mediates entrainment, (3) examine responses of single respiratory neurons in the ventral respiratory group (VRG) to somatic afferent stimulation, and (4) use a computational model of the pontomedullary respiratory network (Rybak et al., 2004a,b) to suggest neuronal mechanisms for entrainment. We used an in situ preparation in young rats that retained pontomedullary respiratory circuits and spinal pathways transmitting somatosensory input. We demonstrate that rhythmic stimulation of somatic afferents entrains respiratory rhythm on a 1:1 basis (1:1), increasing breathing frequency up to approximately 1.4-2.2 times greater than spontaneous frequency. Stable entrainment occurred only when stimuli were delivered during expiration. Reversible blockade of the lateral parabrachial nucleus eliminated entrainment. Somatic afferent stimulation produced significant increases in the firing rate of augmenting expiratory (E2) neurons but shortened the firing duration of postinspiratory (post-I) neurons. A computational model reproduced 1:1 entrainment and other experimental findings based on the assumption that the somatic afferents initiate early onset of inspiration via activation of medullary E2 neurons. The model also predicted that afferent stimulation evoked transient hyperpolarization of ramp-inspiratory (ramp-I) neurons. This was confirmed experimentally by intracellular recording from ramp-I neurons. Our experimental and modeling results demonstrate that an entrainment pathway from somatic afferents to the VRG via the lateral parabrachial nucleus causes resetting of respiratory rhythm through excitation of E2 and consequent inhibition of post-I neurons.

Respiration-related afferents to parabrachial pontine regions

The dorsolateral pons around the parabrachial nucleus is an important participant in respiratory control. This area involves various respiration-related neurons, and their respiratory modulation is thought to arise from afferents from medullary respiratory neurons. Today, however, only a limited number of afferent sources have been identified. First, relatively well-characterized afferents to the pons are those originating from two types of the lung stretch receptors, slowly adapting and rapidly adapting receptors. That is, the majority of the second-order relay neurons of these receptors in the nucleus tractus solitarii project to the pons. Second, certain types of respiratory neurons of the medullary respiratory groups are either known to or presumed to project to the pons. For instance, major inhibitory neurons of the Botzinger complex, augmenting and decrementing expiratory neurons, send afferents to the pons. This article overviews such afferents and discusses their connectivity with pontine neurons.

Functional and structural models of pontine modulation of mechanoreceptor and chemoreceptor reflexes

The dorsolateral and ventrolateral pons (dl-pons, vl-pons) are critical brainstem structures mediating the plasticity of the Hering-Breuer mechanoreflex (HBR) and carotid chemoreflex (CCR). Review of anatomical evidence indicates that dl-pons and vl-pons are connected reciprocally with one another and with medullary nucleus tractus solitarius (NTS) and ventral respiratory group (VRG). With this structural map, functional models of HBR and CCR are proposed in which the respiratory rhythm is modulated by short-term depression (STD) or potentiation (STP) of corresponding primary NTS-VRG and auxiliary pons-VRG excitatory or inhibitory pathways. Behaviorally, STD and STP of respiratory reflexes are akin to non-associative learning such as habituation, sensitization or desensitization to afferent inputs. Computationally, the STD and STP effects amount to signal differentiation and integration in the time domain, or high-pass and low-pass filtering in the frequency domain, respectively. These functional and structural models of pontomedullary signal processing provide a novel conceptual framework that unifies a wealth of experimental observations regarding mechanoreceptor and chemoreceptor reflex control of breathing.

Functional organization of respiratory neurones: a brief review of current questions and speculations

This article presents a short overview of current knowledge about the medullary respiratory neurones and the generation of breathing rhythm. The background respiratory neurophysiology of the medulla and pons is briefly reviewed, with some current ideas about the organization of the pontine-medullary respiratory control system and its development. Questions and speculations about the organization and generation of respiratory rhythm are included, with a view to stimulating experiments to provide answers.

Distribution of glycine transporter 2 mRNA-containing neurons in relation to glutamic acid decarboxylase mRNA-containing neurons in rat medulla

We studied the distribution of medullary glycinergic neurons in relation to GABAergic neurons, by using in situ hybridization method for mRNA encoding either glycine transporter 2 (GLYT2) or glutamic acid decarboxylase isoform 67 (GAD67). GLYT2 mRNA-positive (GLYT2+) neurons were distributed widely and clustered in (1). the respiration-related area of the ventrolateral medulla called the Bötzinger complex, (2). the nucleus retroambiguus caudal to the obex or the caudal ventral respiratory group, (3). the spinal trigeminal nucleus, (4). a small area immediately dorsal to the inferior olivary nucleus, and (5). the border zone between the hypoglossal nucleus and the surrounding reticular formation. It was characteristic that in the dorsomedial medulla, GLYT2+ neurons were distributed only sparsely in contrast to dense GAD67+ neurons. Only few GLYT2+ neurons were distributed in the medial and interstitial subnuclei of the nucleus tractus solitarii. In particular virtually no GLYT2+ neurons were found in the area postrema. Furthermore, in the reticular formation and the spinal trigeminal nucleus, GAG67+ neurons tended to be distributed in the area where GLYT2+ neurons were sparse, and vice versa. These results provide useful information for the effort of determining neurotransmitters involved in the medullary neurons.