G. Böhmer

Glycine receptor-mediated fast synaptic inhibition in the brainstem respiratory system

The involvement of glycinergic neurotransmission in the central regulation of respiration was investigated by administration of specific receptor agonists and antagonists into the 4th cerebral ventricle of the rabbit. Central respiratory activity was assessed by cycle-triggered averaging of phrenic nerve activity (PNA). Glycine reduced the peak amplitude of PNA and increased the duration of the respiratory phases. Conversely, strychnine decreased the duration of both phases, being more effective in expiration. Strychnine converted inspiratory ramp activity to a decrescendo type of pattern, with the highest discharge activity present at the onset of the inspiratory phase. Strychnine induced tonic PNA lasting throughout expiration with a decrescendo type of activity. In many cases strychnine induce apneusis with tonic PNA being at a level of about 2/3 of its maximum value during control. We conclude that glycinergic neurotransmission is important for the maintenance of the respiratory rhythm as well as the generation of normal respiratory pattern. Glycine could be involved in phasic as well as tonic inhibitory neurotransmission within the central respiratory system.

Serotonergic control of phrenic motoneuronal activity at the level of the spinal cord of the rabbit

The role of serotonin (5-hydroxytryptamine, 5-HT)-mediated modulation of phrenic motoneuronal activity was evaluated by microapplication of 5-HT and methysergide into the phrenic nuclei of the rabbit. 5-HT facilitated phrenic nerve activity (PNA) considerably resulting in a long-lasting augmentation of the peak amplitude of integrated PNA. In contrast, the blockade of intrinsically active 5-HT by methysergide decreased PNA and led to a strong reduction of the peak amplitude of integrated PNA. MDL 72222 was ineffective. Blockade of 5-HT receptors by preceding administration of methysergide effectively abolished the effects of microinjected 5-HT. Respiratory timing was unaffected by both the agonist and the antagonist. These results suggest that a considerable portion of the facilitatory influence of caudal raphe nuclei on central respiratory activity takes place at the phrenic nuclei level.

Effects of excitatory amino acids and neuropeptide Y on the discharge activity of suprachiasmatic neurons in rat brain slices

Effects of L-glutamate, AMPA, NMDA and NPY on the discharge activity of neurons located in the ventral subdivision of the suprachiasmatic nucleus were examined in submerged coronal slices of the rat hypothalamus. All substances were bath applied. Application of L-glutamate (14 neurons examined) induced an excitatory response in 8 suprachiasmatic neurons (+248.9 +/- 122.24%, mean +/- S.E.M.; P < 0.001). A biphasic response, i.e. an initial transient excitation (+54.3 +/- 8.21%; P < 0.001) succeeded by an inhibition (-66.2 +/- 9.31%; P < 0.001), was observed in 6 neurons. Application of AMPA (36 neurons examined) resulted in an excitation of 31 neurons (+209.2 +/- 58.58%; P < 0.0001). Application of NMDA (57 neurons examined) induced an excitation in 34 neurons (+253.8 +/- 91.18%; P < 0.0001), but an inhibition in 8 neurons (-757 +/- 6.52; P < 0.0001). Biphasic effects of NMDA with an excitatory component (+58.7 +/- 9.94%; P < 0.0001) succeeded by an inhibitory component (-62.0 +/- 8.07%; P < 0.0001) were observed in 13 neurons. In 5 of 13 examined cases, the inhibitory component of neuronal responses to NMDA was significantly attenuated by the simultaneous application of strychnine (attenuation was 56%; P < 0.05). The application of NPY (40 neurons examined) induced significant effects on the discharge rate of 29 suprachiasmatic neurons. 18 of these neurons were inhibited (-59.3 +/- 6.39%; P < 0.0001) whereas 11 neurons were excited (+156.6 +/- 107.222%; P < 0.001) by NPY. In 8 of 11 neurons examined, the NPY-induced inhibition was significantly attenuated by 92% during simultaneous application of strychnine (P < 0.001). In 23 NPY-sensitive neurons, the discharge activity was also affected by NMDA. Neurons excited by NPY were also excited by NMDA (8 cells). In neurons inhibited by NPY, application of NMDA induced either an inhibition (3 cells) an excitation (5 cells) or a biphasic effect (7 cells). Results suggest a direct excitatory effect of AMPA, NMDA and NPY on suprachiasmatic neurons. In contrast, inhibitory actions of NMDA and NPY are considered induced by an activation of inhibitory interneurons. Antagonistic effects of strychnine suggest an involvement of glycinergic interneurons in a subpopulation of neurons inhibited by NMDA and in most neurons inhibited by NPY. The involvement of inhibitory mechanisms in photic entrainment of the circadian system is discussed. An integrative model of excitatory and inhibitory actions of EAA and NPY on suprachiasmatic neurons is proposed.

Evidence for an involvement of NMDA and non-NMDA receptors in synaptic excitation of phrenic motoneurons in the rabbit.

The action of endogenous excitatory amino acids on phrenic motoneurons was studied in anesthetized, vagotomized, paralyzed and artificially ventilated rabbits. The NMDA receptor antagonists APV and ketamine, as well as the non-NMDA receptor antagonists GAMS and DNQX were administered by microinjection into the ventral horn of the spinal segments C3-C5. Injection of each antagonist resulted in a reversible reduction of the phrenic nerve activity. Results suggest an important function of endogenous excitatory amino acids in the excitation of phrenic motoneurons. NMDA as well as non-NMDA receptors are involved. The functional role of both receptor types in bulbospinal neurotransmission is discussed.

Medullary respiratory-related neurons with axonal connections to rostral pons and their function in termination of inspiration

In urethane-anaesthetized, paralyzed and artificially ventilated rabbits, medullary respiration-related neurons (RRU) were classified according to the phase relation of their burst discharge to phrenic nerve activity. Phase-bound inspiratory (I) or expiratory (E) neurons were discriminated from phase-spanning expiratory-inspiratory (EI) or inspiratory-expiratory (IE) units. Mechanisms of termination of inspiration by electrical stimulation of rostral pontine nuclei (Nc. parabrachialis medialis; Lc. coeruleus) were examined firstly to demonstrate whether RRU receive descending excitatory and inhibitory afferents as well as ascending efferents and secondly to analyse the time course of the neuronal pathways involved. Of 120 RRU, 38 neurons were demonstrated to receive pontine afferents. About 33% of all E neurons became orthodromically excited during rostral pons stimulation whereas 18.2% of all I cells became orthodromically inhibited. Some RRU were shown to project up to the rostral pons. 50% of these were of the phase-spanning IE type. The onset of inspiratory inhibition induced by rostral pons stimulation occurred 3.4 ms after the onset of single electrical pulse stimulation. Based on these results a neuronal model for a pontine mechanism terminating inspiration is proposed.

GABAB receptor mediated effects on central respiratory system and their antagonism by phaclofen

The role of GABAB receptors in control of central respiratory system was evaluated by cycle-triggered averaging of phrenic nerve activity (PNA) of the rabbit. Blockade of GABAB receptors of the caudal brainstem by intracerebroventricular administration of phaclofen augmented PNA, decreased the duration of inspiration and to about the same extent increased the duration of expiration thus unmasking intrinsically active GABA. Analogously, stimulation of brainstem GABAB receptors by exogenous baclofen decreased PNA. Preceding administration of larger doses of phaclofen could block the effects of baclofen. It is proposed that GABAB receptors are involved in tonic and phasic modulation of central respiratory activity.

Effects of corticotropin-releasing factor on central respiratory activity.

Effects of corticotropin-releasing factor (CRF) on central regulation of respiration were studied in urethane-anaesthetized, vagotomized, paralyzed and artificially ventilated rabbits. Injections of CRF into the IVth cerebral ventrile (i.c.v.) resulted in an increase of the neuronal tidal volume (nVt), but had only minor effects on the duration of respiratory phases. Microinjection of CRF into the ventral parabrachial region of the pons (pneumotaxic center) resulted in a reduction of nVt and of the respiration rate. Respiratory effects of CRF administered i.c.v. or into the pontine parabrachial region were antagonized by alpha-helical-CRF, indicating a receptor-mediated action of CRF. Respiratory effects of CRF were not significantly affected after blocking ganglionic transmission with chlorisondamine. Neither mean arterial blood pressure nor heart rate were significantly affected by the injection of CRF into the IVth ventricle or the rostral pons. Results suggest an involvement of CRF in the central regulation of respiratory movements. CRF may be involved in the adaptation of respiration to stress.

Evidence for a respiration-modulated cholinergic action on the activity of medullary respiration-related neurons in the rabbit

Effects of the iontophoretically administered cholinergic agonists acetylcholine, bethanechol and DMPP on the activity of medullary respiration-related neurons were examined in urethane-anaesthetized rabbits. Inhibitory effects prevailed over excitatory effects. Analysis of cholinergic effects by cycle-triggered averaging revealed three major types of neuronal responses: (i) constant alterations of spike-density throughout the whole period of activity (“constant effects”), (ii) effects increasing during the progression of the burst of discharge or effects restricted to a particular fraction of the burst (“phasic effects”) and (iii) effects which were characterized by an excitation during one respiratory phase and an inhibition during the other phase (“bi-phasic effects”). The latter type of effects was observed in phase-spanning respiration-related neurons. Phasic effects were mainly observed in inspiration-related neurons which were predominantly inhibited by stimulation of muscarinic receptors. Inspiratory Rβ-neurons in no case were phasically affected by cholinergic agents. The mean muscarinic inhibition of inspiration-related neurons increased with the progression of inspiration. The mean nicotinic inhibition of expiration-related neurons decreased with the progression of expiration. Results suggest that the efficacy of (i) a central inspiration terminating mechanism and (ii) the onset of discharge of expiratory neurons is modulated by acetylcholine.

Cholinergic effects on spike-density and burst-duration of medullary respiration-related neurones in the rabbit: An iontophoretic study ☆

Cholinoceptive properties of 180 medullary respiration-related neurones (RRN) were studied in urethane-anaesthetized rabbits. Acetylcholine (ACh) and agonists, as well as antagonists of muscarinic receptors and nicotinic receptors, were administered iontophoretically. Respiration-related neurons were classified with respect to the correlation of their activity with the activity of the phrenic nerve: phase-bound inspiratory (I) and expiratory (E) neurones and phase-spanning expiratory-inspiratory and inspiratory-expiratory neurones were discriminated. Acetylcholine altered the activity of 170 respiration-related neurones. In 49 cells the discharge-rate (f) alone was affected. In 44 respiration-related neurones the burst-duration (tbd) only was altered. In 77 respiration-related neurones the discharge rate and burst duration were affected simultaneously. Inhibitory actions of ACh prevailed over excitatory actions on inspiratory neurones. In expiratory neurones the discharge rate was increased or decreased in about the same number of cases. In most phase-spanning neurones the discharge rate was increased. Cholinergic inhibition of burst duration was observed in most respiration-related neurones. In all phase-types of respiration-related neurones muscarinic as well as nicotinic actions of ACh were demonstrated. In inspiratory and expiratory-inspiratory neurones muscarinic effects on the discharge rate prevailed over nicotinic effects. More nicotinic than muscarinic effects on discharge rate were observed in expiratory and inspiratory-expiratory neurones. Cholinergic effects on burst duration in about the same number of respiration-related neurones were mediated by muscarinic or by nicotinic receptors, respectively. Various types of cholinoceptors may be involved in these effects. The results suggest that cholinergic mechanisms play an important role in the control of the central regulation of respiratory movements. The functional significance of cholinergic effects on respiration-related neurones is discussed with special emphasis of effects on burst duration.

Involvement of fast synaptic inhibition in the generation of high-frequency oscillation in central respiratory system.

During moderate hypercapnia, spectrum analysis of efferent phrenic nerve activity (PNA) of urethane anesthetized rabbits revealed high-frequency oscillations (HFO). The spectral peak (mean frequency 111 Hz) was reversibly eliminated by the glycine receptor antagonist strychnine administered into the 4th cerebral ventricle. In contrast, blockade of brainstem GABAA receptors by bicuculline did not abolish HFO but even reinforced or induced HFO. Thus, there is evidence that fast synaptic inhibition via glycine receptors is involved in the generation of HFO within the medullary respiratory center, whereas GABA may play a modulatory role.