Respiratory depressant effects of GABA alpha- and beta-receptor agonists in the cat

Baclofen destabilises breathing during sleep in healthy humans: A randomised, controlled, double-blind crossover trial

AIMS

Periodic breathing is frequent in patients with severe heart failure. Apart from being an indicator of severity, periodic breathing has its own deleterious consequences (sleep-related oxygen desaturations, sleep fragmentation), which justifies attempts to correct it irrespective of the underlying disease. Animal models and human data suggest that baclofen can reconfigure respiratory central pattern generators. We hypothesised that baclofen, a GABA_B agonist, may thus be able to correct periodic breathing in humans.

METHODS

Healthy volunteers were exposed to hypoxia during sleep. Participants who developed periodic breathing (n = 14 [53 screened]) were randomly assigned to double-blind oral baclofen (progressively increased to 60 mg/d) or placebo. The primary outcome was the coefficient of variation (CoVar) of respiratory cycle total time considered as an indicator of breathing irregularity. Secondary outcomes included the CoVar of tidal volume, apnoea-hypopnoea index, sleep fragmentation index and ventilatory complexity (noise limit).

RESULTS

The analysis was conducted in 9 subjects after exclusion of incomplete datasets. CoVar of respiratory cycle total time significantly increased with baclofen during non-rapid eye movement sleep (median with placebo 56.00% [37.63-78.95]; baclofen 85.42% [68.37-86.40], P = .020; significant difference during the N1-N2 phases of sleep but not during the N3 phase). CoVar of tidal volume significantly increased during N1-N2 sleep. The apnoea-hypopnoea index, sleep fragmentation index and ventilatory complexity were not significantly different between placebo and baclofen.

CONCLUSION

Baclofen did not stabilise breathing in our model. On the contrary, it increased respiratory variability. Baclofen should probably not be used in patients with or at risk of periodic breathing.

Decreased GABAB receptor function in the cerebellum and brain stem of hypoxic neonatal rats: role of glucose, oxygen and epinephrine resuscitation

Background-

Hypoxia during the first week of life can induce neuronal death in vulnerable brain regions usually associated with an impairment of cognitive function that can be detected later in life. The neurobiological changes mediated through neurotransmitters and other signaling molecules associated with neonatal hypoxia are an important aspect in establishing a proper neonatal care.

Methods-

The present study evaluated total GABA, GABA_B receptor alterations, gene expression changes in GABA_B receptor and glutamate decarboxylase in the cerebellum and brain stem of hypoxic neonatal rats and the resuscitation groups with glucose, oxygen and epinephrine. Radiolabelled GABA and baclofen were used for receptor studies of GABA and GABA_B receptors respectively and Real Time PCR analysis using specific probes for GABA_B receptor and GAD mRNA was done for gene expression studies.

Results-

The adaptive response of the body to hypoxic stress resulted in a reduction in total GABA and GABA_B receptors along with decreased GABA_B receptor and GAD gene expression in the cerebellum and brain stem. Hypoxic rats supplemented with glucose alone and with oxygen showed a reversal of the receptor alterations and changes in GAD. Resuscitation with oxygen alone and epinephrine was less effective in reversing the receptor alterations.

Conclusions-

Being a source of immediate energy, glucose can reduce the ATP-depletion-induced changes in GABA and oxygenation, which helps in encountering hypoxia. The present study suggests that reduction in the GABA_B receptors functional regulation during hypoxia plays an important role in central nervous system damage. Resuscitation with glucose alone and glucose and oxygen to hypoxic neonatal rats helps in protecting the brain from severe hypoxic damage.

Alterations in cortical GABAB receptors in neonatal rats exposed to hypoxic stress: role of glucose, oxygen, and epinephrine resuscitation

Hypoxia in neonates can cause permanent brain damage by gene and receptor level alterations mediated through changes in neurotransmitters. The present study evaluated GABA(B) receptor alterations, gene expression changes in glutamate decarboxylase and hypoxia-inducible factor 1A in the cerebral cortex of hypoxic neonatal rats and the resuscitation groups with glucose, oxygen, and epinephrine. Under hypoxic stress, a significant decrease in total GABA and GABA(B) receptors, GABA(B) and GAD gene expression was observed in the cerebral cortex, which accounts for the respiratory inhibition. Hypoxia-inducible factor 1A was upregulated under hypoxia to maintain body homeostasis. Hypoxic rats supplemented with glucose alone and with oxygen showed a reversal of the receptor alterations and changes in GAD and HIF-1A to near control. Being a source of immediate energy, glucose can reduce the ATP-depletion-induced changes in GABA and oxygenation, which helps in encountering hypoxia. Resuscitation with oxygen alone and epinephrine was less effective in reversing the receptor alterations. Thus, our study suggests that reduction in the GABA(B) receptors functional regulation during hypoxia plays an important role in cortical damage. Resuscitation with glucose alone and glucose and oxygen to hypoxic neonatal rats helps in protecting the brain from severe hypoxic damage.

Postnatal changes in ventilation during normoxia and acute hypoxia in the rat: implication for a sensitive period

Previously, we found heightened expression of inhibitory neurochemicals and depressed expression of excitatory neurochemicals with a sudden drop in metabolic activity around postnatal day (P) 12 in rat brainstem respiratory nuclei, suggesting that this period is a critical window during which respiratory control or regulation may be distinctly different. To test this hypothesis, the hypoxic ventilatory responses (HVR) to 10% oxygen were tested in rats every day from P0 to P21. Our data indicate that (1) during normoxia (N), breathing frequency (f) increased with age, peaking at P13, followed by a gradual decline, whereas both tidal volume (V(T)) and minute ventilation (.V(E) ) significantly increased in the second postnatal week, followed by a progressive increase in V(T) and a relative plateau in .V(E); (2) during 5 min of hypoxia (H), .V(E) exhibited a biphasic response from P3 onward. Significantly, the ratio of .V(E)(H) to .V(E)(N) was generally > 1 during development, except for P13-16, when it was < 1 after the first 1-2 min, with the lowest value at P13; (3) the H : N ratio for f, V(T) and .V(E) during the first 30 s and the last minute of hypoxia all showed a distinct dip at P13, after which the V(T) and .V(E) values rose again, while the f values declined through P21; and (4) the H : N ratios for f, V(T) and .V(E) averaged over 5 min of hypoxia all exhibited a sudden fall at P13. The f ratio remained low thereafter, while those for V(T) and .V(E) increased again with age until P21. Thus, hypoxic ventilatory response is influenced by both f and V(T) before P13, but predominantly by V(T) after P13. The striking changes in normoxic ventilation as well as HVR at or around P13, together with our previous neurochemical and metabolic data, strongly suggests that the end of the second postnatal week is a critical period of development for brainstem respiratory nuclei in the rat.

Effect of prevention of lung inflation on metamorphosis and respiration in the developing bullfrog tadpole, Rana catesbeiana

We tested the hypothesis that respiratory development would be retarded in tadpoles reared in aquaria in which a barrier prevented access to the air-water interface. To test this hypothesis, we examined swimming behavior and respiration in intact tadpoles and gill and lung respiratory activity and central chemosensory responses in an in vitro brainstem preparation. The "barrier" tadpoles had significantly lower resting gill frequencies and higher lung breath attempts than control tadpoles at the same metamorphic stage. Control tadpoles swam greater distances and spent more time in the upper one third of the aquaria, while barrier tadpoles spent significantly more time at the bottom of the aquaria. There was significantly greater mortality for barrier tadpoles compared to control animals in the earliest and latest metamorphic stages. Mean body weight was significantly greater, and metamorphic rate was reduced in barrier tadpoles. Neither control nor barrier tadpole brainstem preparations demonstrated a gill ventilatory response to CO(2); however, both control and barrier preparations possessed significant lung frequency responses to central CO(2) chemoreceptor stimulation. Bath application of the GABA(A) and glycine receptor antagonists, bicuculline and strychnine, had greater effects on control tadpole gill burst activity and produced a similar large-amplitude bursting pattern in both control and barrier tadpoles, that was insensitive to CO(2) chemoreceptor stimulation. We conclude that development of the respiratory pattern was perturbed by the barrier, but the major effect was on gill ventilation rather than lung ventilation as we had expected.

Maturational changes in neuromodulation of central pathways underlying hypoxic ventilatory response

The neuromodulator systems mediating the central component of the hypoxic ventilatory response (HVR) during development are complex and diverse. The early component of the HVR is mediated through N-methyl-D-aspartate (NMDA) glutamate receptors in the caudal brainstem. The intracellular downstream signal transductions of the NMDA receptors involve protein kinase C (PKC), neuronal nitric oxide synthase (nNOS) and tyrosine kinase (TK). Activation of NMDA receptors will also lead to activation of the early gene transcription factors including AP-1 (c-fos, c-jun) and NF-kappaB which may play a role in modulation of the subsequent response to hypoxia. NMDA receptors in the caudal brainstem play a critical role in the development of the HVR and increasing dependency on NMDA receptors emerges over time. Similarly, hypoxia-induced PKC, NOS and c-Fos activation in the caudal brainstem is relatively weak in the immature animals, but this activation increases with age and the strength of the response appears to increase concomitantly with the appearance of NMDA expression. Several neurotransmitters including adenosine, gamma-aminobutyric acid (GABA), serotonin and opioids are involved in the late component of the HVR. In addition, the late phase of the HVR is mediated in part through platelet-derived growth factor (PDGF)-beta receptors. PDGF-beta receptor activation is an important contributor of the hypoxic ventilatory depression at all postnatal ages, but its role is more critical in the developing animals. Maturation of these neuromodulators, especially the NMDA and PDGF-beta receptors-mediated pathways, occurs primarily during the early postnatal period. Perturbation of these developmental processes may result in short-term or sustained alterations to the HVR and may also affect neuronal survival during hypoxia.

GABA-mediated neurotransmission in the ventrolateral NTS plays a role in respiratory regulation in the rat

Our purpose was to determine whether endogenously released GABA in the ventrolateral nucleus of the solitary tract (vlNTS) of the rat influences respiration. Experiments were carried out in anesthetized, vagotomized and spontaneously breathing rats, and diaphragm electromyogram activity was measured while drugs affecting GABAergic neurotransmission were microinjected into the vlNTS and medial NTS (mNTS). Bilateral microinjection of nipecotic acid, 5 or 25 nmol, into the vlNTS (but not in the mNTS) produced dose-dependent increases in inspiratory duration (Ti) frequently culminating in apneustic breathing. Neither unilateral microinjection of bicuculline nor CGP-35348 (GABA(B) receptor antagonist) reversed this response; however, a combination of both GABA receptor antagonists effectively reversed apneustic breathing. Bilateral microinjection of either muscimol or baclofen into the vlNTS mimicked the effect of nipecotic acid. Microinjection of the bicuculline produced apnea, whereas microinjection of CGP-35348 produced a decrease in Ti and an increase in expiratory duration. Immunohistochemical analysis of the vlNTS region revealed GABA(A) receptors densely localized to processes, whereas GABA(B) immunoreactivity was localized to cell bodies. Our data indicate that GABA activity in the vlNTS is important for respiratory function.

Apnoeic response to stimulation of peripheral GABA receptors in rats

Respiratory effects of intracarotid injection of gamma-amino-butyric acid (GABA) were investigated in two groups of rats. In the first group of 12 rats the effects of GABA were checked in the intact state, following bilateral vagotomy and GABA receptor blockade. The second group consisted of five initially vagotomized rats, challenged with GABA prior to and after bilateral carotid chemodenervation (CSN-cut). All rats were urethane and chloralose anaesthetized and spontaneously breathing. Injection of 39 micromol/kg GABA prior to and after vagotomy induced an expiratory apnoea of, respectively 5.5+/-0.84 sec and 3.9+/-0.6 sec duration (mean+/-S.E.M.), P>0.05 in all 12 rats. In breaths that followed the apnoea tidal volume increased above the control level by 23.3% (P<0.01) and 25.6% (P<0.01) pre- and post-vagotomy, respectively. Blockade of GABA receptors with bicuculline and picrotoxin abolished the inhibition of breathing. In five vagotomized rats with intact carotid sinus nerves (CSNs) intracarotid GABA challenge increased tidal volume by 39% compared with baseline breathing (P<0.05). Section of the CSNs precluded the occurrence of apnoea and undergoing respiratory changes evoked by GABA. Intracarotid GABA caused significant decrease in the mean blood pressure independent of the neural state, but the fall was delayed by CSNs neurotomy. Results of this study indicate that GABA given systemically induces apnoea followed by post-apnoeic hyperventilation. Carotid bodies are required for the ventilatory response to GABA; vagal afferents are not involved in this response.

Role of GABA within the nucleus tractus solitarii in the hypoxic ventilatory decline of awake rats

The purpose of this study was to examine our hypothesis that gamma-aminobutyric acid (GABA) in the nucleus tractus solitarii (NTS) may be related to the hypoxic ventilatory decline (HVD) and that chemoreceptor stimulation was essential to activate this mechanism. We used unanesthetized, freely moving rats in this study. An in vivo microdialysis technique was used to measure the extracellular GABA concentration ([GABA]o), and an in vivo microinjection technique was used to examine the effects of the GABA agonists and antagonists on the ventilation during hypoxia. The GABA agonists injected into the NTS attenuated the ventilation during hypoxia. By hypoxic exposure, [GABA]o was increased during the HVD. However, by carotid body denervation (CBD), this GABA increase was abolished. Although GABA antagonists microinjected into the NTS during the HVD phase significantly increased the depressed ventilation, this effect on the ventilation was abolished by CBD. These results suggest that the GABA in the NTS has a pivotal role in the HVD and that this mechanism is not activated without chemoreceptor stimulation.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

GABA receptors in the phrenic nucleus of the rat

The phrenic nucleus was identified by microinjections of N-methyl-D-aspartic acid in urethan-anesthetized adult male Wistar rats. Microinjections of GABAA and GABAB receptor agonists (muscimol and baclofen, respectively) at the same site decreased the phrenic nerve burst amplitude. Microinjections of GABAA and GABAB receptor antagonists (bicuculline and 2-hydroxysaclofen, respectively) blocked as well as reversed the effects of their respective agonists. These results were confirmed by recording extracellular action potentials from single phrenic neurons. Micropressure applications of muscimol and baclofen decreased the activity of single neurons in the phrenic nucleus; this effect was blocked as well as reversed by micropressure applications of bicuculline and 2-hydroxysaclofen, respectively. These results demonstrated the presence of GABA receptors on the neurons in the phrenic nucleus and suggested that their activation results in the decrease of the phrenic nerve burst amplitude. The importance of these results in the identification of neural circuits mediating inhibition of phrenic neurons is discussed.

High dose baclofen is neuroprotective but also causes intracerebral hemorrhage: a quantal bioassay study using the intraluminal suture occlusion method

Agonists of the GABA-A receptor are neuroprotective after experimental stroke, but studies of GABA-B agonists have contradicted each other. To further investigate whether GABA-B agonists may be neuroprotective, we devised a quantal bioassay using the intraluminal occlusion method of inducing reversible cerebral ischemia. Subjects underwent middle cerebral artery occlusion for varying amounts of time, ranging from 5 to 90 min. Behavioral outcome was measured 48 h later with a quantal observational scale: score of abnormal given for any one of asymmetric forepaw flexion on tail lift, asymmetric grip, circling, reduced exploration, seizures, or death. To the grouped response data the logistic equation was used to find the ED50, the duration of occlusion that caused one-half of the subjects to be abnormal. To find the potency ratio for each drug, we divided the ED50 for treatment by that for vehicle. We administered baclofen, a GABA-B agonist, intraperitoneally 5 min after the onset ofischemia. Baclofen (20 mg/kg) was neuroprotective (potency ratio of 3.0, P < 0.05), but a lower dose (10 mg/kg) was not. However, both doses of baclofen caused significantly more intracerebral hemorrhages than control. In awake animals, both baclofen doses caused significant increases in mean arterial pressure, but no changes in other cardiorespiratory variables. The glutamate antagonist MK-801, the GABA-A agonist muscimol, and hypothermia were all protective using the bioassay (potency ratios ranging from 1.5 to 3.0). We conclude that although baclofen (20 mg/kg) may be neuroprotective, its utility is complicated by postischemic hypertension and cerebral hemorrhages.

Inhibitions mediated by glycine and GABAA receptors shape the discharge pattern of bulbar respiratory neurons

Experiments were performed to identify the glycinergic or GABAergic nature, and the timing of discharge, of the neurons which produce chloride-dependent inhibitions on other bulbar respiratory neurons (RNs) during their silent and active phases. RNs recorded extracellularly in pentobarbital-anesthetized or decerebrate cats, were subjected to iontophoretic applications of glutamate, of the glycine antagonist strychnine, and of the GABAA receptor antagonist bicuculline. Both antagonists induced discharge or increased discharge frequency in restricted parts of the respiratory cycle without affecting the discharge frequency in other parts of the cycle. Strychnine most often elicited activity in late-inspiration and early-expiration, but also in early inspiration and in late expiration. Bicuculline was most often effective throughout the entire discharge period of each neuron with no effect during the silent period, although it also acted selectively during late-inspiration in inspiratory neurons, an effect attributed to GABAA receptor blockade. The convergence of glycinergic afferent inputs during late inspiration and early expiration suggests that glycinergic neurons may play an important role in the inspiratory to expiratory phase transition.

Respiratory effects of baclofen and 3-aminopropylphosphinic acid in guinea-pigs

1. The effects of the GABAB receptor agonists, baclofen and 3-aminopropylphosphinic acid (3-APPi) given by the subcutaneous or intracerebroventricular (i.c.v.) route were examined on minute ventilation (V), tidal volume (VT) and respiratory rate (f) due to room air and carbon dioxide (CO2)-enriched gas hyperventilation in conscious guinea-pigs. 2. Baclofen (0.3-10 mg kg-1, s.c.) produced a dose-dependent inhibition of V and f due to room air and CO2 inhalation. The maximum inhibition of room air breathing V was 85% +/- 3 and f was 74% +/- 3 at 10 mg kg-1, s.c. The maximum effects on CO2-induced hyperventilation were 68% +/- 9 and 51% +/- 6, for V and f respectively. Only the highest dose of baclofen studied (10 mg kg-1) produced a significant inhibition of VT due to room air breathing (46% +/- 6) and CO2 breathing (38% +/- 11). 3. 3-APPi (0.3-100 mg kg-1, s.c.) did not affect V, VT or f due to room air breathing or CO2 inhalation at any dose tested. Also, i.c.v. administration of 3-APPi (100 micrograms) did not affect ventilatory responses due to room air breathing or CO2 inhalation. 4. Pretreatment with the GABAB antagonist, CGP 35348 3-aminopropyl-(diethoxymethyl) phosphinic acid (3-30 mg kg-1, s.c.) blocked the respiratory depressant effects of baclofen (3 mg kg-1, s.c.) in a dose-related fashion. 5. Intracerebroventricular (i.c.v.) administration of CGP 35348 (50 micrograms) blocked the respiratory depressant effects of baclofen. CGP 35348 given alone either i.c.v. or s.c. had no effects on respiration due to room air or CO2 inhalation.6. Pretreatment with either the GABAA antagonist bicuculline (30 mg kg-1, s.c.) or the opioid antagonist, naloxone (1 mg kg-1, s.c.) had no effect on the respiratory depressant action of baclofen(3 mg kg-1, s.c.).7. These results show that baclofen inhibits ventilation due to room air breathing, and attenuates the hyperventilation response to CO2 inhalation. The peripherally acting GABAB agonist, 3-APPi had no effect on ventilation. These findings demonstrate that the respiratory depressant effects of baclofen are due to activation of CNS GABAB receptors and indicates that only GABAB receptor agonists that penetrate into the CNS may cause respiratory depression.

Intrathecal baclofen therapy for adults with spinal spasticity: therapeutic efficacy and effect on hospital admissions

A prospective trial to demonstrate the efficacy of intrathecal baclofen therapy by implanted pump for adults with spasticity due to spinal cord injury or multiple sclerosis was initiated in our hospital. Of the 140 patients assessed, 7 met the following criteria for inclusion in the study: a modified Ashworth score > 3, a spasm frequency score > 2, and an inadequate response to oral anti-spasticity drugs, (i.e., baclofen, clonidine and cyproheptadine). All patients responded to intrathecal bolus injection of baclofen in the double blind, placebo-controlled screening phase (mean bolus dose = 42.8 micrograms). Programmable Medtronic pumps were implanted in 4 patients while 3 patients received non-programmable Infusaid pumps. Post-implantation, a marked decrease in spasticity occurred with a significant reduction of the Ashworth score (mean = 1.8, p < .005), a reduced spasm score (mean = 0.8, p < .005), and an improved leg swing in the pendulum test. These effects were maintained during a follow-up of 24-41 months (average infusion dose = 218.7 micrograms/day). The gross cost-savings due to reduced hospitalizations related to spasticity was calculated by comparing the cost for the two year period before pump implantation to the same period after treatment for 6 of the 7 patients. The cost of in-hospital implantation as well as the cost of the pumps were deducted from the gross savings. There was a net cost-saving of $153,120. Our findings agree with the reported efficacy and safety of intrathecal baclofen treatment, and illustrate the cost-effectiveness of this treatment.

Effects of bicuculline on inspiratory activities in piglets

Antagonism of the alpha receptor sub-type at gamma-aminobutyric acid (GABA) recognition sites in developing pigs was evaluated using the GABAA receptor antagonist bicuculline. The effect of bicuculline infusions was to produce an increase of phrenic and hypoglossal discharge amplitudes. This bicuculline-induced effect on discharge amplitude was manifested in autopower spectra as an increase in the power of peaks located in the medium-frequency (10-50 Hz) band. More importantly, coherence estimates were increased by bicuculline administration demonstrating GABA-mediated influences on a central pattern generator with output in the 10-50 Hz band.

Effects of GABAA receptor antagonism on inspiratory activities in kittens

In anesthetized kittens (< 1 to 24 days old), the effects of GABAA receptor antagonism on phrenic, hypoglossal and cervical sympathetic discharges were examined by i.v. bicuculline infusions during hyperoxia and hypoxia. Administration of bicuculline during hyperoxia produced marked increases in the amplitudes of inspiratory nerve discharges. During hypoxic stimulation (10% O2), the amplitudes of inspiratory activities decreased towards or below those observed during hyperoxia; bicuculline reversed this depression and restored inspiratory discharges. Our results indicated that GABAA receptors were functional shortly after birth, acting to mediate influences shaping inspiratory activity during hyperoxic breathing and during conditions of increased chemical drive.

Respiratory mu-opioid and benzodiazepine interactions in the unrestrained rat

Interactions of mu-opioid receptors with the benzodiazepine system were studied by examining the modulatory effects of flumazenil (a benzodiazepine antagonist) and alprazolam (a benzodiazepine agonist) on the respiratory effects of the opioid peptide dermorphin. Dermorphin, 1-30 nmol administered i.c.v., to conscious, unrestrained rats decreased ventilation rate (VR) and minute volume (MV) dose-dependently. The ventilatory depression was antagonized by naloxone and by the benzodiazepine antagonist flumazenil. The benzodiazepine alprazolam potentiated the respiratory inhibition of a small (1 nmol) dose of dermorphin but antagonized that of a higher dose (3 nmol). The results suggest that the benzodiazepine/GABA receptor complex modulates respiratory depression induced by central mu-receptor stimulation in the rat.

Effects of GABAB receptor agonists and antagonists on the bulbar respiratory network in cat

We examined the involvement of the GABAB receptor in central respiratory mechanisms. Respiratory neurons (RNs) from the ventral respiratory group in the medulla of the cat were subjected to iontophoretic applications of the GABAB receptor agonist baclofen and the antagonists saclofen and CGP 35348. In all types of RNs baclofen decreased the firing rate. This reduction was antagonized by CGP 35348. Application of either antagonist increased the spontaneous discharge in both inspiratory and expiratory RNs. CGP 35348 excited 57% of the neurons tested, on the average by 34% with ejection currents of 100 nA. Saclofen excited 6 of 9 neurons tested. Baclofen administered systemically (8-12 mg/kg i.v.) to either anesthetized, decerebrate or intact freely moving cats, induced a selective lengthening of the inspiratory phase, an effect comparable to the apneusis induced by the NMDA antagonist MK-801. Baclofen also produced either a pronounced decrease in the amplitude of phrenic nerve discharge or an apnea, both of which were reversed by increasing paCO2. The results suggest that endogenously released GABA acting on GABAB receptors may be involved in the control of respiratory neuronal discharge.

Development of a piglet model of status epilepticus: preliminary results

STUDY OBJECTIVE

Ventilation frequently is impaired during prolonged clinical seizures and their treatment. In a pilot study, respiratory, metabolic, and hemodynamic variables were studied during induced seizures in a lightly anesthetized, spontaneously breathing piglet model.

PARTICIPANTS

Weanling, mixed-breed domestic piglets.

INTERVENTIONS

Piglets were instrumented with a tracheostomy, arterial catheter, and epidural electrodes. Conditions included hyperoxia, normothermia, and ketamine maintenance infusion throughout recordings. After baseline recordings, 2 mg/kg IV bicuculline was administered. For further model validation, piglets were randomized to infusions of diazepam (three), lorazepam (two), or saline (control; five) groups after ten minutes of untreated seizures.

MEASUREMENTS AND MAIN RESULTS

Integrated tidal volume, respiratory rate, PaCO2, pH, arterial pressure, rectal temperature, heart rate, and bipolar EEG waveforms were recorded and compared at intervals for 60 minutes. Vigorous tonic-clonic seizures occurred in all piglets, confirmed by sudden synchronization and large-amplitude EEG waveforms. Increases in heart rate, arterial pressure, tidal volume, respiratory rate, PaCO2, minute ventilation, and base deficit occurred in all piglets during seizures as compared with baseline. Five minutes after bicuculline was administered, increases in minute ventilation (4.5 +/- 0.4 L/min at baseline to 13 +/- 2.1 L/min) were accounted for by increases in both tidal volume and respiratory rate. More abrupt decreases in respiratory rate were observed in anticonvulsant-treated piglets as compared with controls. The duration of continuous seizure activity (12 +/- 1.0 minutes versus 21 +/- 3.3 minutes; P < .05) was reduced in anticonvulsant-treated piglets.

CONCLUSION

Significant increases in ventilation occur during generalized seizures in tracheostomized piglets given bicuculline. Diazepam and lorazepam infusions ameliorate seizure activity and suppress increases in respiratory rate but not minute ventilation as compared with controls. Problems with this model included baseline variability, temperature instability, and that direct respiratory stimulation from the convulsant agent may have occurred.

The role of inhibitory amino acids in control of respiratory motor output in an arterially perfused rat

The respiratory effects of drugs affecting GABAergic and glycinergic transmission were examined in order to assess the role of synaptic inhibition in breathing rhythmogenesis. Experiments were performed in the arterially perfused in situ brainstem-spinal cord preparation from adult rats (Hayashi et al., 1991, J. Neurosci. Meth. 36:63-70). Administration to the perfusate of agonists of GABAA, GABAB, and glycine receptors reduced both the frequency and amplitude of the activity recorded from the phrenic and hypoglossal nerves. Similar effects were observed following the infusion of aminooxyacetic acid (a blocker of GABA-transaminase). Picrotoxin (0.1-2 microM), bicuculline (0.05-0.2 microM), strychnine (0.1-1 microM) and phaclofen (0.1-0.2 mM) usually increased the frequency and amplitude of inspiratory bursts. Perfusion with low Cl- (8 mM) solution elicited tonic discharge followed by reversible arrest of the respiratory activity. It is concluded that synaptic inhibition is involved in the respiratory rhythm generation process in the mature mammalian brain. As data from the literature indicate that interference with central inhibitory processes does not largely affect the rhythm generation process in newborn rats, a possibility is discussed that the brainstem respiratory generator undergoes a developmental change from a 'pacemaker' driven circuit at the neonatal stage to a network requiring post-synaptic inhibition in the mature brain.

Cardiovascular responses to intrathecal administration of L- and D-baclofen in the rat

D- and L-baclofen were given intrathecally at the T2 spinal level in the anaesthetized rat. D-Baclofen, in doses of 7, 35 and 70 nmol produced graded increases in arterial pressure but heart rate remained unaffected. Responses appeared within 30 s, peaked at 2 min and decayed over the next 5 min. Injection i.v. of 70 nmol of D-baclofen failed to alter arterial pressure or heart rate. In contrast, intrathecal administration of L-baclofen decreased both arterial pressure and heart rate. The amplitude and time course of the effects depended on the dose used; 700 nmol of L-baclofen had stronger and longer effects than those induced by 70 nmol, while 7 nmol had no effect. (I.v. injection of 70 nmol of L-baclofen had similar effects to intrathecal administration but with different time course and amplitude.) When given at the T9 level at doses of 70 nmol, D- and L-baclofen had effects similar to those observed at the second thoracic level. Effects of intrathecal administration of D- and L-baclofen at T2 were prevented by pretreatment with either hexamethonium (10 mg/kg i.v.) or lidocaine (25 microliters of a 1% solution, intrathecally). The results suggest that D- and L-baclofen-sensitive receptors in the spinal cord are involved in regulating sympathetic output in pathways to the vessels and/or to the heart. In addition, our results suggest that D- or L-baclofen may not act via classical GABAB receptors or that two types of GABAB receptor exist in spinal sympathetic pathways.

Neural mechanisms of swallowing: neurophysiological and neurochemical studies on brain stem neurons in the solitary tract region

Neurophysiological studies of the nuclei of the tractus solitarius (NTS) and adjacent regions have provided a partial understanding of the integrative brainstem network underlying swallowing and related functions such as respiration. The NTS is also richly endowed with an abundance of neuropeptides and other neuroactive substances, but only limited information is available on their influences on neurons involved specifically in swallowing. Since dysfunction of these neurophysiological and neurochemical regulatory mechanisms in the NTS region may be important in pathophysiological conditions such as dysphagia, increased awareness of and focus on these mechanisms are warranted. This paper outlines recent neurophysiological and neurochemical data that provide information on the afferent inputs and neurophysiological properties of neurons in NTS and adjacent caudal brainstem regions implicated in swallowing, respiration, and respiratory-related reflexes.

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.