Differentiation of two respiratory areas in the cat medulla using kainic acid

Respiratory responses evoked by blockades of ionotropic glutamate receptors within the Bötzinger complex and the pre-Bötzinger complex of the rabbit

The respiratory role of excitatory amino acid (EAA) receptors within the Bötzinger complex (BötC) and the pre-Bötzinger complex (pre-BötC) was investigated in alpha-chloralose-urethane anaesthetized, vagotomized, paralysed and artificially ventilated rabbits by using bilateral microinjections (30-50 nL) of EAA receptor antagonists. Blockade of both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors by 50 mM kynurenic acid (KYN) within the BötC induced a pattern of breathing characterized by low-amplitude, high-frequency irregular oscillations superimposed on tonic phrenic activity and successively the disappearance of respiratory rhythmicity in the presence of intense tonic inspiratory discharges (tonic apnea). KYN microinjections into the pre-BötC caused similar respiratory responses that, however, never led to tonic apnea. Blockade of NMDA receptors by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 1, 10 and 20 mM) within the BötC induced increases in respiratory frequency and decreases in peak phrenic amplitude; the highest concentrations caused tonic apnea insensitive to chemical stimuli. Blockade of non-NMDA receptors by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 10 and 20 mM) within the BötC produced only less pronounced increases in respiratory frequency. Responses to D-AP5 in the pre-BötC were similar, although less pronounced than those elicited in the BötC and never characterized by tonic apnea. In the same region, CNQX provoked increases in respiratory frequency similar to those elicited in the BötC, associated with slight reductions in peak phrenic activity. The results show that EAA receptors within the investigated medullary subregions mediate a potent control on both the intensity and frequency of inspiratory activity, with a major role played by NMDA receptors.

Respiratory changes induced by kainic acid lesions in rostral ventral respiratory group of rabbits

The role played by the Bötzinger complex (BötC), the pre-Bötzinger complex (pre-BötC), and the more rostral extent of the inspiratory portion of the ventral respiratory group (iVRG) in the genesis of the eupneic pattern of breathing was investigated in anesthetized, vagotomized, paralyzed, and artificially ventilated rabbits by means of kainic acid (KA, 4.7 mM) microinjections (20-30 nl). Unilateral KA microinjections into all of the investigated VRG subregions caused increases in respiratory frequency associated with moderate decreases in peak phrenic amplitude in the BötC and pre-BötC regions. Bilateral KA microinjections into either the BötC or pre-BötC transiently eliminated respiratory rhythmicity and caused the appearance of tonic phrenic activity ("tonic apnea"), whereas injections into the rostral iVRG completely suppressed inspiratory activity. Rhythmic activity resumed as low-amplitude, high-frequency oscillations and displayed a progressive, although incomplete, recovery. Combined bilateral KA microinjections (BötC and pre-BötC) caused persistent (>3 h) tonic apnea. Results show that all of the investigated VRG subregions exert a potent control on both the intensity and frequency of inspiratory activity, thus suggesting that these areas play a major role in the genesis of the eupneic pattern of breathing.

Effects of kainic acid in the parabrachial region for ongoing respiratory activity and reflexive respiratory suppression

We previously reported that the electrical stimulation of gastrocnemius muscle nerve afferents given at a suprathreshold intensity for C-fiber afferents induces naloxone-reversible reflexive respiratory suppression ('after suppression'). The effects of kainic acid (KA) microinjections into the parabrachial area (nucleus parabrachialis lateralis: NPBL and nucleus parabrachialis medialis: NPBM) on (1) ongoing respiratory activity and (2) the 'after suppression' were studied in chloralose-urethane anesthetized, bivagotomized, paralyzed, and artificially ventilated cats. A large dose of KA (1.91 nmol in 0.1 microliters) microinjected into the unilateral NPBL induced significant long-lasting respiratory facilitation, while a subsequent KA injection into the ipsilateral NPBM induced significant, long-lasting respiratory depression. A small dose of KA (0.48 nmol in 0.1 microliters) into the unilateral NPBL (right side) induced significant respiratory facilitation, and the 'after suppression' effect was eliminated. A small dose into the unilateral NPBM (right side) caused initial transient respiratory facilitation followed by respiratory depression before 'after suppression' was restored. Subsequent KA injections into the NPBL on the other side (left side) significantly augmented respiration. The 'after suppression' effect was again eliminated after an injection of KA into the bilateral NPBL. It was concluded that NPBL may exhibit tonic inhibitory activities on respiration and play a critical role in the 'after suppression' effect, since an injection of KA into the NPBM counteracted both of these effects in the NPBL.

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.

Respiratory failure without pulmonary edema following injection of a glutamate agonist into the ventral medullary raphe of the rat

Injection of ibotenic acid (IA), a glutamate agonist, into the ventral medullary raphe (VMR; especially the nucleus raphe magnus) of the rat produced respiratory failure and death following a predictable course of events. The response to the IA injection was characterized initially by increased respiratory frequency and was followed by pulmonary arterial hypertension, systemic arterial hypoxemia, acidosis, and hypothermia. Within 90 min apnea occurred as a terminal event in all animals. Gravimetric, bronchoalveolar lavage protein, and histological analyses revealed no evidence of pulmonary edema. Intracerebral (VMR) pretreatment with PPP, a sigma receptor agonist, or scopolamine, a muscarinic cholinergic antagonist, prevented pulmonary failure and death even though postmortem histological analysis showed VMR cell loss and gliosis consequent to the cytotoxic IA injection. Based on the results of the study, it is suggested that the VMR has a role in regulation of pulmonary blood flow. Preliminary pharmacological studies suggested that a disruption of glutamatergic and cholinergic mechanisms mediates the lethal pulmonary phenomenon.

Antagonistic effects of somatostatin and substance P on respiratory regulation in the rat ventrolateral medulla oblongata

Substance P (SP) in the dose range 0.75-1.5 nmol exerts a potent stimulatory effect on ventilation after microinjection into the rat ventrolateral medulla oblongata (VLM; n. reticularis lateralis, n. paragigantocellularis lateralis). A significant but less pronounced effect is also seen in the dorsal medulla (DM; n. tractus solitarius). Somatostatin (0.6-1.8 nmol) inhibited ventilation and induced apnoea after microinjection into the VLM but not the DM. Serial microinjections of the two peptides showed a reciprocal antagonistic action in the VLM but not in the DM. The apnoea-inducing effect of SOM was blunted by SP while SOM reduced the ventilatory stimulation induced by SP. Extracellular single unit recordings were performed following the microiontophoretic application of SP and/or SOM to respiratory-related and non-respiratory-related neurons in the VLM and DM. Although a heterogeneous population of neurons were recorded from, the majority of respiratory-related units in the VLM responded with excitation to SP and inhibitory to SOM. A direct interaction between the peptides was seen in some respiratory-related units. The neurons not responding to either of the peptides were usually non-respiratory. Dorsal to the VLM, the type of response to the two peptides was less likely to be antagonistic and a wider distribution of response types were recorded. The results indicate a direct physiological antagonism between SP and SOM regarding their effects on respiratory regulation elicited in the VLM.

Endogenous activation of NMDA and non-NMDA glutamate receptors on respiratory neurones in cat medulla

The aim of this study was to evaluate the involvement of dicarboxylic amino acid neurotransmission in the periodic discharges of respiratory neurones. Respiratory neurones of the ventral and dorsal respiratory groups in the medulla of the cat were subjected to iontophoretic applications of (1) N-methyl-D-aspartate (NMDA) and a blocker of the NMDA subtype of glutamate receptor, D-2-amino-7-phosphonoheptanoic acid (AP7) and (2) an agonist and an antagonist of the non-NMDA subtypes of receptor: quisqualate and 6,7-dinitroquinoxaline-2,3-dione (DNQX), respectively. All five main types of respiratory neurones (all-, early- and late-inspiratory, transitional "off-switch", late expiratory) were excited by NMDA and quisqualate. Both agonists increased the peak firing rate but exerted different effects on the discharge pattern of respiratory neurones, within the respiratory cycle. Quisqualate induced discharges in the "silent" period of the neurone more readily than did NMDA which, in turn had a more pronounced effect during the burst period of the neurone. The effects of quisqualate and NMDA were suppressed by prior application of their selective antagonists, AP7 and DNQX. These antagonists decreased the spontaneous neuronal discharge of all cell types, throughout the entire firing phase, by a maximum of 24-63% with AP7 and by 30-50% with DNQX. The non-selective antagonist, gamma-D-glutamyl-glycine and the selective NMDA antagonists, CPP and MK-801, were also effective. It is concluded that respiratory neurones, of all types, within the medullary respiratory network are subjected to endogenous glutamate-like excitations, which may possibly shape the respiratory train of action potentials through the sequential activation of non-NMDA and NMDA subtypes of receptor.

Kainic acid on the rat ventral medullary surface depresses hypoxic and hypercapnic ventilatory responses

Kainic acid, topically applied to the ventral surface of the medulla immediately caudal to the trapezoid body in the urethane/chloralose anaesthetised rat, led to a depression of ventilation and a sustained rise in blood pressure; ventilatory responses to hypercapnia (10% carbon dioxide) and hypoxia (11% oxygen) were slightly depressed. Widespread application of kainic acid to an area at and slightly rostral to the rootlets of the hypoglossal nerve produced a stimulation of ventilation and an unsustained rise in blood pressure. Apnea ensued 12-28 min after application. Ventilatory responses to hypercapnia and hypoxia were markedly attenuated; more discrete bilateral application revealed two regions, one immediately rostral and lateral to the hypoglossal rootlets and the other over the point of exit of the hypoglossal nerve rootlets, which specifically contributed to the diminution of the chemosensory responses. These results raise questions about the medullary circuitry which mediates the chemoreflex regulation of breathing.

Functional significance of the dorsal respiratory group in adult and newborn rats: in vivo and in vitro studies

The involvement of the dorsal part of the medulla (the so-called dorsal respiratory group: DRG) in the networks participating in respiratory function was investigated in newborn (in vitro) and adult (in vivo) rats. In the dorsal part of the medulla of the isolated brainstem of newborn rats, no respiratory neurons were found and stimulations or lesions neither modified nor suppressed the respiratory output. On the contrary, similar experiments suggest that sites in the ventral medulla have a fundamental importance for respiration. In adult rats, lesion of the DRG areas by electrocoagulation induced transient changes in respiratory timing, and resulted in a significant decrease in the amplitude of the contralateral phrenic output. These results suggest that the dorsal part of the medulla is not involved in controlling respiratory activity in the newborn rat. In adults, no definite conclusion can be reached, but the functional role of the DRG, if any, is probably restricted.

Functions of the retrofacial nucleus in chemosensitivity and ventilatory neurogenesis

The hypothesis was evaluated that neurons within the retrofacial nucleus of medulla integrate afferent stimuli from the central chemoreceptors. In decerebrate, vagotomized, paralyzed and ventilated cats, activity of the phrenic nerve was monitored. Peak integrated phrenic activity increased in hypercapnia; the frequency of phrenic bursts typically declined slightly. The retrofacial nucleus was ablated by radio-frequency lesions or neurons within this nucleus were destroyed by microinjections of kainic acid. Results were similar following lesions or injections. Following unilateral ablations, peak phrenic activity was greatly reduced at normocapnia and hypercapnia; the frequency of phrenic bursts typically rose. Both frequency and peak phrenic activity fell further after the contralateral destruction with a cessation of all phasic phrenic discharge being observed in most animals. Injections of kainic acid in regions rostral, caudal or medial to the retrofacial nucleus produced no consistent changes in phrenic activity. We conclude that neuronal activities in the region of the retrofacial nucleus are important both in the integration of stimuli from the central chemoreceptors and in defining the discharge patterns of respiratory neurons.

Localization by kainic acid lesions of neurones transmitting the carotid chemoreceptor stimulus for respiration in rat

1. An attempt has been made to test the hypothesis that in the nucleus of the tractus solitarius (NTS) in the rat, the most caudal region of synaptic terminals of the carotid sinus nerve, just caudal to the obex, represents mainly the site of synapse of chemoreceptor fibres from the carotid body. 2. Under halothane anaesthesia, the neurotoxin kainic acid was used to lesion this region and a second region, immediately rostral to obex, where terminals are thought to arise mainly from baroreceptor fibres of the carotid sinus nerve. 3. Measurements based on the distribution of fluorescent dye co-injected with the kainic acid showed that the two groups of 100 nl microinjections were centered 0.82 mm apart and that the injectate spread through mean distances of 0.57 mm (caudal microinjections) and 0.52 mm (rostral microinjections). Nissl staining was used to determine cellular degeneration. The caudal lesions mostly involved ventrolateral and commissural subnuclei of NTS and the rostral lesions involved lateral and dorsolateral subnuclei. 4. Ventilatory sensitivity to hypoxia was tested under light halothane anaesthesia, 1 day after lesioning. To enhance the responses, the contralateral carotid sinus nerve was sectioned prior to experiments. Caudal lesions reduced the ventilatory response to inspired oxygen (20.9-9.6% O2) by a mean of 67% and rostral lesions by 18% of the effect produced by carotid sinus nerve section on that side. Subsequent section of the carotid sinus nerve on the side of the NTS lesion confirmed that caudal lesions produced effects comparable to those of carotid body denervation; rostral lesions did not. 5. These results strongly support the hypothesis that chemoreceptor and baroreceptor afferent fibres in the carotid sinus nerve synapse at substantially separable sites in the nucleus of the tractus solitarius. The identification of the site in NTS caudal to the obex as the principal site of carotid chemoreceptor synapses places them close to but not upon respiratory premotor neurones of the same nucleus.

Somatostatin in the control of respiration

Somatostatin has been found to induce apnoea when applied into the brain ventricular system (Fuxe et al. 1982, Härfstrand et al. 1985). The site of action of somatostatin was suggested to be in the dorsal respiratory neurons in the medulla oblongata of the rat (Fuxe et al. 1982) where high somatostatin-like immunoreactivity has been detected (Kalia et al. 1984a). In the present study we wanted to further localize the site(s) of action of somatostatin on respiration by microinjection of somatostatin into the medulla oblongata of the cats. We could not detect any inhibitory effect of somatostatin on respiration after microinjection into the nuclear complex of the solitary tract. On the other hand microinjection of somatostatin into the region of nucleus paragigantocellularis lateralis consistently caused apnoea. This finding further supports the idea that this structure functions as an integrative area of respiratory drive inputs.

Modulation of carotid sinus afferent input to nucleus tractus solitarius by parabrachial nucleus stimulation

There is increasing evidence that the parabrachial nucleus (PBN) may be integrally involved in cardiovascular reflex regulation. In cats in which anesthesia was induced with pentobarbital and maintained with alpha-chloralose, we studied the effects of PBN stimulation on cardiovascular afferent inputs to nucleus tractus solitarius (NTS), the site of first central termination for cardiovascular afferent fibers. Electrical stimulation of PBN resulted in an initial excitation followed by prolonged inhibition of the spontaneous activity of NTS neurons activated by ipsilateral carotid sinus nerve (CSN) stimulation. In 53 units recorded extracellularly in and around NTS, the number of action potential responses to ipsilateral CSN stimulation was reduced 73 +/- 3% by a prior conditioning stimulus to PBN at an interval of 30-60 msec. CSN input to 10 units excited by selective baroreceptor stimulation was inhibited by the PBN conditioning stimulus, as were convergent inputs from contralateral CSN, vagus, and renal nerves. The inhibitory influence of the PBN stimulus lasted as long as 450 msec. We examined the mechanism for these phenomena in additional intracellular recording experiments. In 57 units, PBN stimulation evoked a long lasting (65-359 msec) membrane potential hyperpolarization. In 42 cells, the PBN evoked inhibitory postsynaptic potential (IPSP) was preceded by an excitatory postsynaptic potential (EPSP). CSN and convergent inputs were inhibited when timed to occur during the PBN induced IPSP. Conversely, CSN and convergent afferent nerves inhibited PBN input to NTS neurons with no associated change in membrane potential (n = 9 of 14). These data demonstrate for the first time a potent modulatory influence of PBN on NTS neurons processing cardiovascular afferent input.

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)

Organization of synaptic transmission in the mammalian solitary complex, studied in vitro

1. Synaptic transmission and neuronal morphology were studied in the nucleus tractus solitarius and in the dorsal vagal motor nucleus (solitary complex), in coronal brain-stem slices of rat or cat, superfused in vitro. 2. Electrical stimulation of afferent fibres of the solitary tract evoked two different types of post-synaptic response recorded intracellularly in different solitary complex neurones. Labelling with horseradish peroxidase showed that these two sorts of orthodromically evoked responses were correlated with different post-synaptic neuronal morphologies. 3. The majority of recorded neurones (n = 93) showed a prolonged reduction in excitability following the initial solitary-tract-evoked excitatory post-synaptic potential (e.p.s.p.). A smaller number of neurones (n = 53) showed a prolonged increase in excitability following solitary tract stimulation. In no case did the solitary tract stimulation induce a burst of action potentials at high frequency. 4. The time-to-peak and the half-width of the initial solitary-tract-evoked e.p.s.p. were shorter in neurones with prolonged increased excitability than in those with prolonged reduced excitability. In neurones with prolonged reduced excitability, this e.p.s.p. was followed by a hyperpolarization lasting 60-100 ms. The latency of this inhibitory post-synaptic potential (i.p.s.p.) was 3-5 ms longer than that of the initial e.p.s.p. and its reversal potential was 10 mV more negative than the reversal potential of the response measured following application of gamma-aminobutyric acid or glycine. In neurones with prolonged increased excitability, at a membrane potential of -40 to -50 mV, the initial solitary tract e.p.s.p. was followed by a prolonged depolarization lasting 100-400 ms. 5. Background synaptic activity was high in neurones with prolonged increased excitability, consisting of unitary e.p.s.p.s with an amplitude of more than 0.8 mV. This activity was increased for a period of 300-800 ms following solitary tract stimulation. Spontaneous excitatory potentials of more than 0.5 mV were not seen in neurones with prolonged reduced excitability. In these neurones, after intracellular injection of choride ions, reversed unitary i.p.s.p.s formed a background activity which was increased following stimulation of the solitary tract. 6. Neurones with prolonged reduced excitability were found in the medial, ventral and ventrolateral part of the nucleus tractus solitarius and in the dorsal vagal motor nucleus where they were identified by their antidromic response to stimulation ventral and lateral to the tractus solitarius.(ABSTRACT TRUNCATED AT 400 WORDS)

The excitation by neurotensin of nucleus tractus solitarius neurons induces apneustic breathing

The possible involvement of neurotensin in the regulation of respiratory drive has been tested on single brainstem respiratory related neurons and on the global respiratory output. The neuropeptide was locally applied either by microiontophoresis or by pressure injection in the dorsal and ventral respiratory areas of the anesthetized bivagotomized cat. Effects of neurotensin applications were studied, on the one hand on the firing discharge of respiratory related neurons and on the other hand on the phrenic nerve activity and on arterial blood pressure. An increase of the firing frequency of respiratory related neurons was induced by neurotensin applied by iontophoresis or by pressure injection (0.005-33.5 fmol/s) on single neurons. In the latter case, neurotensin was active at concentration 10(3) times lower than glutamate. A bilateral apneustic pattern was induced on the phrenic nerve activities by microinjection of neurotensin (0.23-0.54 pmol/s) in one ventrolateral nucleus tractus solitarius without alteration of arterial blood pressure. These results suggest that the release of neurotensin in the nucleus tractus solitarius regulates respiratory rhythmogenesis by increasing the inspiratory duration.

Effects of stimulation of phrenic afferent fibers on medullary respiratory neurons in cat

The effects of electrical stimulation of both cervical branches (C5 and C6) of the right phrenic nerve on medullary respiratory neuron activity were studied in anesthetized, spontaneously breathing cats. In 14 cats, the stimulation of the thin phrenic afferents had no effect on the inspiratory duration and evoked excitatory or inhibitory responses in only 3/86 inspiratory neurons tested. In 3 cats, the stimulation decreased the inspiratory duration and 26/26 inspiratory neurons showed a shortened discharge without modification of their discharge frequency. Although the effects of the stimulation were not analysed by averaging techniques, it is concluded that phrenic afferents do not exert an important control on the medullary respiratory neuron discharge.

Localization of mu- and delta-opioid receptors in cat respiratory areas: an autoradiographic study

Autoradiography after in vitro binding with selective ligands for either mu ([3H](Tyr-D-Ala-Gly-(NMePhe)-Gly-ol] or delta ([3H](Tyr-D-Thr-Gly-Phe-Leu-Thr)) opioid receptor types revealed the presence of variable amounts of radioactive labeling in the cat brainstem. Areas involved in the respiratory rhythmogenesis were among the most prominently labeled structures. The pneumotaxic center, including the nucleus parabrachialis medialis and the Kolliker-Füse nucleus, contains a very high density of delta binding sites while the dorsal respiratory nucleus which corresponds to the nucleus tractus solitarius, is more heavily labeled by the mu ligand. The neuroanatomical differences in the distribution of opioid receptors correlates well with the pharmacological responses induced by administration of specific mu- or delta ligands.