Normal breathing requires preBötzinger complex neurokinin-1 receptor-expressing neurons.

The normal breathing rhythm in mammals is hypothesized to be generated by neurokinin-1 receptor (NK1R)-expressing neurons in the preBötzinger complex (preBötC), a medullary region proposed to contain the kernel of the circuits generating respiration. If this hypothesis is correct, then complete destruction of preBötC NK1R neurons should severely perturb and perhaps even fatally arrest breathing. Here we show that specific and near complete bilateral (but not unilateral) destruction of preBötC NK1R neurons results in both an ataxic breathing pattern with markedly altered blood gases and pH, and pathological responses to challenges such as hyperoxia, hypoxia and anesthesia. Thus, these approximately 600 neurons seem necessary for the generation of normal breathing in rats.

Serotonin and the control of ventilation in awake rats.

In awake, unrestrained, intact rats, reserpine, para-chlorophenylalanine, 6-fluorotryptophan, and para-chloroamphetamine depleted whole brain serotonin and produced a substantial and sustained hyperventilation as evidenced by a 5--9 torr drop in PaCO2. Administration of 5-hydroxytryptophan to rats treated with para-chlorophenylalanine partially alleviated the hyperventilation. No change in ventilation was observed after alpha-methyltyrosine. 5,7-Dihydroxytryptamine produced contradictory results. On the basis of these pharmacological studies, we propose that some serotonin-mediated nerve transmissions might function under physiological conditions to inhibit the central nervous system output which controls normal breathing.

Short-term plasticity of descending synaptic input to phrenic motoneurons in rats.

Respiratory afferent stimulation can elicit increases in respiratory motor output that outlast the period of stimulation by seconds to minutes [short-term potentiation (STP)]. This study examined the potential contribution of spinal mechanisms to STP in anesthetized, vagotomized, paralyzed rats. After C(1) spinal cord transection, stimulus trains (100 Hz, 5-60 s) of the C(1)-C(2) lateral funiculus elicited STP of phrenic nerve activity that peaked several seconds poststimulation. Intracellular recording revealed that individual phrenic motoneurons exhibited one of three different responses to stimulation: 1) depolarization that peaked several seconds poststimulation, 2) depolarization during stimulation and then exponential repolarization after stimulation, and 3) bistable behavior in which motoneurons depolarized to a new, relatively stable level that was maintained after stimulus termination. During the STP, excitatory postsynaptic potentials elicited by single-stimulus pulses were larger and longer. In conclusion, repetitive activation of the descending inputs to phrenic motoneurons causes a short-lasting depolarization of phrenic motoneurons, and augmentation of excitatory postsynaptic potentials, consistent with a contribution to STP.

Medical and veterinary students' structural knowledge of pulmonary physiology concepts.

PURPOSE: The goal of this study was to assess quantitatively medical and veterinary students' knowledge structures of 12 pulmonary physiology concepts before and after receiving a focused instructional block. The "goodness of fit" and internal consistency reliability of the students' knowledge structures were evaluated. Indexes of the students' structural knowledge were correlated with customary measures of student learning of the same concepts. METHOD: Knowledge structures were assessed using a questionnaire that requested similarity judgments about all possible pairs of the concepts: n(n - 1)/2 = 66 pairs. The similarity judgment data were analyzed using the individual differences (INDSCAL) model of multidimensional scaling (MDS). Dimension weights for individual students were then correlated with their final examination scores. RESULTS: A four-dimensional MDS solution provided the best structural fit to the pairwise concept-similarity data. Dimension 1 ranges from control of breathing to lung gas exchange. Dimension 2 ranges from control of breathing to respiratory mechanics. Dimension 3 separates perfusion from diffusion. Dimension 4 addresses ventilatory control. Hierarchical concept clusters are located within this framework. However, indexes of structural learning did not correlate with other measures of knowledge about the same concepts. CONCLUSION: The study outcomes, in contrast to research in other fields, suggest that structural knowledge in this domain differs from knowledge assessed by standard examinations. Further research involving other basic science or clinical concept sets is needed to verify or refute this finding.

Respiratory rhythm generation: preBötzinger neuron discharge patterns and persistent sodium current.

Considerable evidence from several laboratories (c.f., Rekling and Feldman, Ramirez et al.) is consistent with the concept that the pBc contains the kernel of the central rhythm generating network for breathing. The work summarized in this manuscript is also generally consistent with this notion. Of particular note is the observation that pre-I neurons and E-Dec neurons maintain a consistent phase relationship with phrenic nerve activity and maintain a similar peak discharge rate despite marked changes in the phrenic nerve rhythm and pattern. Other categories of respiratory neurons failed to maintain this relationship. Hence, the findings are consistent with pBc pre-I and E-Dec neurons having a key role in rhythm generation. A persistent sodium current has been postulated to underlie the rhythm generating mechanism of pacemaker neurons within the pBc. In the present study, a substantial persistent sodium current was documented in many neurons from the pBc and adjacent respiratory regions. This finding is not inconsistent with the postulated role in rhythm generation. However, it does suggest that other neuronal properties must act in concert with the persistent current to define a unique population of pacemaker neurons.

Caudal nuclei of the rat nucleus of the solitary tract differentially innervate respiratory compartments within the ventrolateral medulla.

A substantial array of respiratory, cardiovascular, visceral and somatic afferents are relayed via the nucleus of the solitary tract (NTS) to the brainstem (and forebrain). Despite some degree of overlap within the NTS, specificity is maintained in central respiratory reflexes driven by second order afferent relay neurons in the NTS. While the topographic arrangement of respiratory-related afferents targeting the NTS has been extensively investigated, their higher order brainstem targets beyond the NTS has only rarely been defined with any precision. Nonetheless, the various brainstem circuits serving blood gas homeostasis and airway protective reflexes must clearly receive a differential innervation from the NTS in order to evoke stimulus appropriate behavioral responses. Accordingly, we have examined the question of which specific NTS nuclei project to particular compartments within the ventral respiratory column (VRC) of the ventrolateral medulla. Our analyses of NTS labeling after retrograde tracer injections in the VRC and the nearby neuronal groups controlling autonomic function indicate a significant distinction between projections to the Bötzinger complex and preBötzinger complex compared to the remainder of the VRC. Specifically, the caudomedial NTS, including caudal portions of the medial solitary nucleus and the commissural division of NTS project relatively densely to the region of the retrotrapezoid nucleus and rostral ventrolateral medullary nucleus as well as to the rostral ventral respiratory group while avoiding the intervening Bötzinger and preBötzinger complexes. Area postrema appears to demonstrate a pattern of projections similar to that of caudal medial and commissural NTS nuclei. Other, less pronounced differential projections of lateral NTS nuclei to the various VRC compartments are additionally noted.

Voltage-dependent calcium signaling in rat cerebellar unipolar brush cells.

Unipolar brush cells (UBCs) are a class of excitatory interneuron found in the granule cell layer of the vestibulocerebellum. Mossy fibers form excitatory inputs on to the paint brush shaped dendrioles in the form of giant, glutamatergic synapses, activation of which results in prolonged bursts of action potentials in the postsynaptic UBC. The axons of UBCs themselves form mossy fiber contacts with other UBCs and granule cells, forming an excitatory, intrinsic cerebellar network that has the capacity to synchronize and amplify mossy fiber inputs to potentially large populations of granule cells. In this paper, we demonstrate that UBCs in rat cerebellar slices express low voltage activated (LVA) fast-inactivating and high voltage activated (HVA) slowly inactivating calcium channels. LVA calcium currents are mediated by T-type calcium channels and they are associated with calcium increases in the dendrites and to a lesser extent the cell soma. HVA currents, mediated by L-type calcium channels, are slowly inactivating and they produce larger overall increases in intracellular calcium but with a similar distribution pattern. We review these observations alongside several recent papers that examine how intrinsic membrane properties influence UBCs firing patterns and we discuss how UBC signaling may affect downstream cerebellar processing.

Respiratory motor responses to cranial nerve afferent stimulation in rats.

It was hypothesized that, because rats appear to lack a prominent disynaptic projection from the dorsal respiratory group to phrenic motoneurons (Phr), they would lack the short-latency excitation of Phr output seen in cats in response to stimulation of some cranial nerve afferents. Single-pulse superior laryngeal nerve (SLN) stimulation elicited a short-latency bilateral excitation of glossopharyngeal (IX) and hypoglossal (XII) nerves and an ipsilateral excitation of pharyngeal branch of vagus (PhX) in 67% of rats, but no excitation of Phr. Vagus (X) stimulation elicited a bilateral excitation of Phr and a predominantly ipsilateral excitation of IX and PhX. Single-pulse stimulation of SLN or X also elicited longer-latency, bilateral decreases in activity of all recorded nerves. Repetitive stimulation (50 Hz) of SLN or X suppressed inspiratory activity and prolonged expiration. Lung inflation (7.5 cmH2O) inhibited Phr and PhX activity; X stimulation inhibited Phr but prolonged PhX activity. In conclusion, rats predictably lack the SLN-induced short latency Phr excitation but exhibit other short latency reflexes for which the underlying circuitry is not clear.

Inhibition of respiratory neural discharges by clonidine and 5-hydroxytryptophan.

Activation of receptors for norepinephrine or serotonin in the central nervous system by i.v. injection of clonidine (10-50 micrograms/kg) or 5-hydroxytryptophan (20-40 mg/kg) inhibits phrenic neural discharges in anesthetized, artificially ventilated cats. Clonidine induces a rapid and complete inhibition of phrenic nerve activity which lasts for 1 to 3.2 hr. The inhibition is prevented by prior administration of phenoxybenzamine (10 mg/kg) or tolazoline (3 mg/kg). 5-Hydroxytryptophan, injected after inhibition of peripheral amino acid decarboxylase (carbidopa, 30-50 mg/kg), elicits a gradual but complete inhibition of phrenic nerve discharges which persists for 1 to 10 hr and is unaltered by alpha or beta adrenoceptor blocking agents. The inhibitions produced by clonidine and 5-hydroxytryptophan are overcome transiently during hypercapnia. Stimulation of carotid body chemoreceptors by i.a. injections of lobeline, doxapram or 0.015 N HCl in saline also briefly reinstates phrenic nerve discharges after inhibition by clonidine. Inhibition is also overcome during electrical stimulation of the carotid sinus nerve.

Neurones in a discrete region of the nucleus tractus solitarius are required for the Breuer-Hering reflex in rat.

1. The Breuer-Hering reflex consists of a shortening of inspiration and lengthening of expiration in response to afferent input from slowly adapting pulmonary stretch receptors (SAR). We hypothesized that neurones in a discrete region of the nucleus tractus solitarius (NTS) are required for producing the reflex. Accordingly, the present studies were undertaken to: (1) identify sites in the NTS in which chemical excitation of neurones inhibited phrenic nerve discharge in a manner consistent with SAR activation, (2) determine whether localized interruption of synaptic transmission prevented the Breuer-Hering reflex, and (3) determine whether these regions contained pump cells and SAR terminal afferents. Studies were carried out in urethane-anaesthetized rats. 2. Injection of picomoles of an excitatory amino acid, DL-homocysteic acid (DLH), in the NTS, at the rostrocaudal level of the area postrema and immediately medial to the tractus solitarius, silenced phrenic nerve activity similarly to that expected from SAR activation. These apnoeas lasted from 3 to 43 s and were produced with little or no change in arterial pressure or heart rate. 3. The Breuer-Hering reflex, physiologically activated by maintaining lung inflation, was transiently impaired by interruption of synaptic transmission following injections of cobalt chloride in the DLH-responsive region. 4. Pump cell (SAR interneurone) and SAR afferent activity were recorded at the site in which DLH produced apnoea. 5. Taken together, the results of chemical excitation, interruption of synaptic transmission and extracellular recording, suggest that cells within a discrete region of the NTS, probably pump cells, are necessary for the production of the Breuer-Hering reflex.

Pulmonary stretch receptor afferents activate excitatory amino acid receptors in the nucleus tractus solitarii in rats.

1. The goal of the present study was to identify potential neurotransmitter candidates in the Breuer-Hering (BH) reflex pathway, specifically at synapses between the primary afferents and probable second-order neurones (pump cells) within the nucleus tractus solitarii (NTS). We hypothesized that if activation of specific receptors in the NTS is required for production of the BH reflex, then (1) injection of the receptor agonist(s) would mimic the reflex response (apnoea), (2) injection of appropriate antagonists would impair the apnoea produced by either lung inflation or agonist injection, and (3) second-order neurones in the pathway would be excited by either lung inflation or agonists while antagonists would prevent the response to either. 2. Studies were carried out either in spontaneously breathing or in paralysed, thoracotomized and ventilated rats in which either diaphragm EMG or phrenic nerve activity, expired CO2 concentration and arterial pressure were continuously monitored. The BH reflex was physiologically activated by inflating the lungs. 3. Pressure injections (0.03-15 pmol) of selective excitatory amino acid (EAA) receptor agonists, quisqualic acid (Quis) and N-methyl-D-aspartic acid (NMDA) into an area of the NTS shown previously to contain neurones required for production of the BH reflex produced dose-dependent apnoeas that mimicked the response to lung inflation. Injection of substance P (0.03-4 pmol) did not alter baseline respiratory pattern. 4. Injections of the EAA antagonists, kynurenic acid (Kyn; 0.6-240 pmol), 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) or 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the BH region of the NTS reversibly impaired the apnoea produced by lung inflation. All three antagonists reduced or abolished the apnoeas resulting from injection of Quis or NMDA, and slowed baseline respiratory frequency. In contrast, injections of the highly selective NMDA receptor antagonist, D-2-amino-5-phosphonovaleric acids (AP5), in doses sufficient to block the apnoeic response to NMDA, neither altered the reflex apnoea evoked by lung inflation nor the baseline respiratory pattern. 5. Pump cells located within the BH region were excited by pressure injections of the broad spectrum EAA agonist, DL-homocysteic acid (DLH). Kyn reversibly blocked the excitation of pump cells in response to either lung inflation or DLH injection. 6. These findings suggest that EAAs mediate primary afferent excitation of second-order neurones in the Breuer-Hering reflex pathway, primarily through the activation of non-NMDA EAA receptor subtypes.

Defining ventral medullary respiratory compartments with a glutamate receptor agonist in the rat.

The regional organization of the ventral respiratory group (VRG) was examined with respect to generation of respiratory rhythm (breathing frequency) versus control of the respiratory motor pattern on individual nerves. In urethane-anaesthetized, neuromuscularly blocked and vagotomized Sprague-Dawley rats, arterial blood pressure (ABP) and respiratory motor outputs (phrenic, pharyngeal branch of the vagus, or superior laryngeal nerves) were recorded. The VRG organization was mapped systematically using injections of the excitatory amino acid DL-homocysteic acid (DLH; 5-20 mM, 2-6 nl) from single- or double-barrel pipettes at 100-200 microm intervals between the facial nucleus and the calamus scriptorius. Recording of respiratory neurons through the injection pipette ensured that the pipette was located within the VRG. At the end of each experiment, the injection pipette was used to make an electrical lesion, thereby marking the electrode position for subsequent histological reconstruction of injection sites. Four rostrocaudal regions were identified: (1) a rostral bradypnoea area, at the level of the Bötzinger complex, in which respiratory rhythm slowed and ABP increased, (2) a tachypnoea/dysrhythmia area, at the level of the preBötzinger complex, in which breathing rate either increased or became irregular, with little or no change in ABP, (3) a caudal bradypnoea area at the level of the anterior part of the rostral VRG in which ABP decreased and (4) a caudal 'no effect' region in the posterior part of the rostral VRG. The peak amplitude of phrenic nerve activity decreased with injections into all three rostral regions. Changes in respiratory rhythm were associated with opposite changes in inspiratory (TI) and expiratory (TE) durations after injections into either the Bötzinger complex or anterior rostral VRG, while both TI and TE decreased after injections into the preBötzinger complex. Effects on selected cranial nerves were similar to those on the phrenic nerve except that tonic activity was elicited on the superior larygneal nerve ipsilateral to injections in the Bötzinger complex and on the pharyngeal branch of the vagus ipsilateral to injections in the preBötzinger complex. These data reinforce the subdivision of the VRG into functionally distinct compartments and suggest that a further subdivision of the rostral VRG is warranted. They also suggest that region-specific influences, especially on the pattern of cranial motor discharge, can be used to assist the identification of recording sites within the VRG. However, the postulated clear functional separation of rhythm- versus pattern-generating regions was not supported.

Respiratory motoneuronal activity is altered by injections of picomoles of glutamate into cat brain stem.

The local neural circuitry underlying the control of breathing was studied by injecting nanoliter volumes of excitatory amino acids into discrete regions of cat brain stem. Experiments were performed on chloralose-urethane anesthetized, vagotomized, paralyzed, and artificially ventilated cats. Phrenic, intercostal, and recurrent laryngeal nerve discharges were recorded. Multibarrel pipettes were used for recording and pressure ejection of drugs or a dye for marking recording and ejection sites. Ejected volumes were directly monitored for every injection. Injections, proximal to neurons discharging with a respiratory periodicity, of as little of 200 fmol of L-glutamate in 200 pl of saline elicited marked, site-specific increases or decreases in respiratory motoneuronal discharge. N-Methyl-D-aspartic acid and homocysteic acid elicited similar site-specific alterations in respiratory motor output, although some details of the response could differ qualitatively. Responses to all the excitatory agents used were attenuated by concurrent injection of kynurenic acid, DL-2-amino-4-phosphonobutyric acid, or glutamic acid diethyl ester. There was no change in spontaneous phrenic nerve discharge in response to injections of equivalent or larger volumes of saline or lidocaine. These results indicate a heterogeneity in the spatial organization of the brain-stem neural circuitry underlying respiratory control, which has not been described previously. This injection technique may provide a mechanism for probing the neural circuitry underlying other behaviors.

Respiratory neurons mediating the Breuer-Hering reflex prolongation of expiration in rat.

Afferent input from pulmonary stretch receptors is important in the control of the timing of inspiratory and expiratory phases of the respiratory cycle. The current study was undertaken to identify neurons within a column of respiratory neurons in the ventrolateral medulla (termed the ventral respiratory group, VRG) that, when activated by lung inflation, produce the Breuer-Hering (BH) reflex in which lung inflation causes inspiratory termination and expiratory prolongation. Intracellular recordings of VRG neurons revealed three groups of inspiratory (I) and two groups of expiratory (E) neurons similar to previous descriptions: I-augmenting (I-Aug), I-decrementing (I-Dec), I-plateau (I-All), E-augmenting (E-Aug), and E-decrementing (E-Dec) neurons. Low-intensity, low-frequency stimulation of a vagus nerve elicited paucisynaptic EPSPs in E-Dec, I-Aug, and I-All neurons that could be divided into two groups on the basis of latency (2.8 +/- 0.1 msec, n = 10; 4.0 +/- 0.1 msec, n = 17). IPSPs were elicited in I-Aug and I-All neurons (4.8 +/- 0.1 msec, n = 12). However, only E-Dec neurons were depolarized when the BH reflex was activated by lung inflation (7.5 cm H2O) or mimicked by vagus nerve stimulation (50 Hz). All other neurons were hyperpolarized and ceased firing during BH reflex-mediated expiratory prolongation. A subset of E-Dec neurons (termed E-Decearly) discharged before inspiratory termination and could contribute to inspiratory termination. The findings are consistent with the hypothesis that a group of E-Dec neurons receives a paucisynaptic (probably disynaptic) input from pulmonary afferents and, in turn, inhibits inspiratory neurons, thereby lengthening expiration.

Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves.

The role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 micron thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4-100 microA, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of excitatory amino acids in neurotransmission of inspiratory drive to spinal respiratory motoneurons.

The role of excitatory amino acids in the transmission of bulbospinal respiratory drive to spinal motoneurons was investigated in the in vitro and in vivo spinal cord of the rat. In vitro studies were performed with a preparation of neonatal rat brain stem and spinal cord that spontaneously generates rhythmic respiratory drive to spinal respiratory motoneurons. This in vitro system allowed examination of the effects of pharmacological agents on spinal motoneuron activity, without perturbing the activity of bulbospinal neurons transmitting the respiratory drive. The amplitude of spontaneous motor discharge in spinal ventral roots containing phrenic and intercostal motor axons was reduced in a dose-dependent manner by antagonists to excitatory amino acids acting at NMDA receptors [D,L-2-amino-5-phosphonovaleric acid (D,L-AP5)] and non-NMDA receptors [kynurenic acid, gamma-D-glutamylglycine, gamma-D-glutamyltaurine, and L- and D,L-2-amino-4-phosphonobutyric acid (L-AP4,D,L-AP4)]. The order of potency of the antagonists for complete block of the motor output was L-AP4 greater than D,L-AP4 greater than kynurenic acid greater than gamma-D-glutamylglycine greater than D,L-AP5 greater than or equal to gamma-D-glutamyltaurine. Amino acid uptake inhibitors augmented the spontaneous motoneuron activity, further confirming the involvement of endogenous excitatory amino acids in transmission of respiratory drive. The results obtained in vitro with AP4, kynurenic acid, and amino acid uptake inhibitors were confirmed in vivo by bathing segments of the rat spinal cord in situ with solutions containing antagonists and uptake inhibitors. The present results suggest that an important component of the neurotransmission of bulbospinal respiratory drive involves endogenous excitatory amino acids acting at AP4-sensitive sites and other non-NMDA (quisqualate/kainate) receptors. The bulbospinal-spinal respiratory motoneuron synapse may provide a convenient model synapse in the spinal cord for detailed analysis of mechanisms underlying excitatory amino acid-mediated synaptic transmission of motor drive.

Effect of synchronous activation of medullary inspiratory bulbo-spinal neurones on phrenic nerve discharge in cat.

The effects on phrenic nerve discharge elicited by intraspinal stimulation which produced synchronous activation of bulbo-spinal inspiratory neurones were investigated in chloralose-urethane anaesthetized, paralysed, vagotomized and artificially ventilated cats. Descending respiratory axons were activated in the ventrolateral spinal cord at the second cervical level using either monopolar or bipolar stimulation (25-200 microA, 100 microseconds, 1-300 Hz). Activation of bulbo-spinal axons was confirmed by recording both orthodromic phrenic nerve excitation and antidromic spike invasion of single, inspiratory modulated units in either the dorsal respiratory group (d.r.g.) or ventral respiratory group (v.r.g.). Antidromic activation of inspiratory bulbo-spinal neurones was confirmed by the criteria of high frequency following and collision tests. Spinal cord stimulation at intensities of 100 microA antidromically activated approximately half of the inspiratory bulbo-spinal neurones in the d.r.g. and v.r.g. Stimulation pulses delivered to the spinal cord elicited an orthodromic excitation of the ipsilateral phrenic nerve lasting 2-12 ms during inspiration. The onset latency of excitation was 2-4 ms, decreasing as inspiration progressed. Following the initial excitation there was a 4-30 ms period of reduced phrenic nerve discharge. Continuous trains of stimuli (less than 100 microA, 100 microseconds, 1-300 Hz) or phrenic gated trains delivered during every fourth inspiratory or expiratory cycle had little or no effect on the duration of inspiration or expiration. Brief trains (400 ms, 50 Hz, 100 microA) of bilateral spinal cord stimulation delivered at various delays from the onset of inspiration had only a transient effect on the pattern of phrenic nerve discharge, with no noticeable effect 60 ms after termination of stimulation. Based on the assumption that synchronous activation of a portion of the central pattern generator for respiration would phase shift or reset the rhythm, we conclude that the bulbo-spinal inspiratory neurones are not responsible for generation of respiratory timing signals and play, at most, a limited role in the generation of the augmenting central inspiratory activity.

Monoamine neurotransmitter metabolism during acclimatization to hypoxia in rats.

The levels and turnovers of NE, DA and 5HT were determined in whole brain, brain stem, cervical and thoracic spinal cord and carotid bodies (CB) of rats exposed to from 1 h to 7 days of hypobaric hypoxia (PB = 450 torr). Monoamine levels decreased only transiently upon acute exposure to hypoxia. Monoamine turnover in the CNS was estimated from the average of (a) monoamine buildup following inhibition of catabolism, and (b) monoamine breakdown following inhibition of synthesis. Hypoxic effects on CNS monoamine turnover showed that: (a) NE was not affected; (b) DA was not affected in acute hypoxia, but was reduced to about 40% of normoxia control after 1-7 days hypoxia; (c) 5HT fell 50-60% during acute hypoxia but returned to and was maintained at control over 1-7 days of hypoxia; (d) acute restoration of normoxia following acute hypoxia restored 5HT and DA to control or above and in the acclimatized animal acute normoxia increased DA and 5HT turnover to about 1.4 and 1.8 X control. In the CB, DA levels gradually increased to 4 X control after 7 days of hypoxia and further increased to 6 X control upon acute restoration of normoxia. Changes in the metabolism of both central 5HT and CB DA may be related to the mechanisms mediating ventilatory acclimatization to chronic hypoxia.