Opiate-Induced Suppression of Rat Hypoglossal Motoneuron Activity and Its Reversal by Ampakine Therapy

Background

Hypoglossal (XII) motoneurons innervate tongue muscles and are vital for maintaining upper-airway patency during inspiration. Depression of XII nerve activity by opioid analgesics is a significant clinical problem, but underlying mechanisms are poorly understood. Currently there are no suitable pharmacological approaches to counter opiate-induced suppression of XII nerve activity while maintaining analgesia. Ampakines accentuate α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor responses. The AMPA family of glutamate receptors mediate excitatory transmission to XII motoneurons. Therefore the objectives were to determine whether the depressant actions of μ-opioid receptor activation on inspiratory activity includes a direct inhibitory action at the inspiratory premotoneuron to XII motoneuron synapse, and to identify underlying mechanism(s). We then examined whether ampakines counteract opioid-induced depression of XII motoneuron activity.

Methodology/Principal Findings

A medullary slice preparation from neonatal rat that produces inspiratory-related output in vitro was used. Measurements of inspiratory burst amplitude and frequency were made from XII nerve roots. Whole-cell patch recordings from XII motoneurons were used to measure membrane currents and synaptic events. Application of the μ-opioid receptor agonist, DAMGO, to the XII nucleus depressed the output of inspiratory XII motoneurons via presynaptic inhibition of excitatory glutamatergic transmission. Ampakines (CX614 and CX717) alleviated DAMGO-induced depression of XII MN activity through postsynaptic actions on XII motoneurons.

Conclusions/Significance

The inspiratory-depressant actions of opioid analgesics include presynaptic inhibition of XII motoneuron output. Ampakines counteract μ-opioid receptor-mediated depression of XII motoneuron inspiratory activity. These results suggest that ampakines may be beneficial in countering opiate-induced suppression of XII motoneuron activity and resultant impairment of airway patency.     

Neural drive to tongue protrudor and retractor muscles following pulmonary C-fiber activation.

Hypoglossal (XII) nerve recordings indicate that pulmonary C-fiber (PCF) receptor activation reduces inspiratory bursting and triggers tonic discharge. We tested three hypotheses related to this observation: 1) PCF receptor activation inhibits inspiratory activity in XII branches innervating both tongue protrudor muscles (medial branch; XIImed) and retractor muscles (lateral branch; XIIlat); 2) reduced XII neurogram amplitude reflects decreased XII motoneuron discharge rate; and 3) tonic XII activity reflects recruitment of previously silent motoneurons. Phrenic, XIImed, and XIIlat neurograms were recorded in anesthetized, paralyzed, and ventilated rats. Capsaicin delivered to the jugular vein reduced phrenic bursting at doses of 0.625 and 1.25 mug/kg but augmented bursting at 5 mug/kg. All doses reduced inspiratory amplitude in XIImed and XIIlat (P < 0.05), and these effects were eliminated following bilateral vagotomy. Single-fiber recordings indicated that capsaicin causes individual XII motoneurons to either decrease discharge rate (n = 101/153) or become silent (n = 39/153). Capsaicin also altered temporal characteristics such that both XIImed and XIIlat inspiratory burst onset occurred after the phrenic burst (P < 0.05). Increases in tonic discharge after capsaicin were greater in XIImed vs. XIIlat (P < 0.05); single-fiber recordings indicated that tonic discharge reflected recruitment of previously silent motoneurons. We conclude that PCF receptor activation reduces inspiratory XII motoneuron discharge and transiently attenuates neural drive to both tongue protrudor and retractor muscles. However, tonic discharge appears to be selectively enhanced in tongue protrudor muscles. Accordingly, reductions in upper airway stiffness associated with reduced XII burst amplitude may be offset by enhanced tonic activity in tongue protrudor muscles.     

Clonidine regularizes substantia nigra dopamine cell firing

The effects of clonidine on the activity of single substantia nigra dopamine neurons were studied in the chloral hydrate anesthetized rat. Although clonidine did not affect the firing rate of the cells, it regularized the firing pattern and decreased burst firing at 2-8 micrograms kg-1 i.v. These effects were antagonized by the alpha 2-antagonist yohimbine. Yohimbine (1.0 mg kg-1) deregularized the firing pattern and increased the firing rate as well as the burst firing. The regularization produced by clonidine is discussed in terms of synaptic efficacy. The results might explain the therapeutic effects of clonidine in certain neuropsychiatric disorders.     

Modulation of phrenic motoneuron excitability by ATP: consequences for respiratory-related output in vitro.

On the basis of the high level of P2X receptor expression found in phrenic motoneurons (MN) in rats (Kanjhan et al., J Comp Neurol 407: 11-32, 1999) and potentiation of hypoglossal MN inspiratory activity by ATP (Funk et al., J Neurosci 17: 6325-6337, 1997), we tested the hypothesis that ATP receptor activation also modulates phrenic MN activity. This question was examined in rhythmically active brain stem-spinal cord preparations from neonatal rats by monitoring effects of ATP on the activity of spinal C4 nerve roots and phrenic MNs. ATP produced a rapid-onset, dose-dependent, suramin- and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium-sensitive increase in C4 root tonic discharge and a 22 +/- 7% potentiation of inspiratory burst amplitude. This was followed by a slower, 10 +/- 5% reduction in burst amplitude. ATPgammaS, the hydrolysis-resistant analog, evoked only the excitatory response. ATP induced inward currents (57 +/- 39 pA) and increased repetitive firing of phrenic MNs. These data, combined with persistence of ATP currents in TTX and immunolabeling for P2X2 receptors in Fluoro-Gold-labeled C4 MNs, implicate postsynaptic P2 receptors in the excitation. Inspiratory synaptic currents, however, were inhibited by ATP. This inhibition differed from that seen in root recordings; it did not follow an excitation, had a faster onset, and was induced by ATPgammaS. Thus ATP inhibited activity through at least two mechanisms: 1) a rapid P2 receptor-mediated inhibition and 2) a delayed P1 receptor-mediated inhibition associated with hydrolysis of ATP to adenosine. The complex effects of ATP on phrenic MNs highlight the importance of ATP as a modulator of central motor outflows.     

Variability of motoneuron activation and the modulation of force production in a postural reflex of the hermit crab abdomen

The tri-phasic reflex in hermit crab (Pagurus pollicarus) abdomen is triggered by local mechanoreceptors and is essential for postural control. The reflex consists of three stereotypical phases: a brief, high-frequency burst, a transient cessation of firing, and a late-discharge that is much lower in frequency than the initial burst. To better understand the reflex generation of force, variability of motoneuron discharge in each of five parameters of reflex activation was assessed. An intracellular current injection routine was used to correlate each of these parameters with force production. Phase 3 motoneuron firing frequency showed the greatest correlation with force production. Phase 3 spike rate increased as a function of phase 2 duration, but the relationship between phase 2 duration and force produced by the reflex was weak. Junction potential amplitude decreased as phase 2 duration increased, and we hypothesize that this trend counteracts the increased phase 3 frequency, explaining the weak relationship of phase 2 duration and force production. Surprisingly, when phase 3 frequency was held constant and phase 2 was increased in duration, the concurrent decrease in junction potential amplitude did not reduce force production.     

Episodic hypoxia induces long-term facilitation of neural drive to tongue protrudor and retractor muscles.

Hypoxic episodes can evoke a prolonged augmentation of inspiratory motor output called long-term facilitation (LTF). Hypoglossal (XII) LTF has been assumed to represent increased tongue protrudor muscle activation and pharyngeal airway dilation. However, recent studies indicate that tongue protrudor and retractor muscles are coactivated during inspiration, a behavior that promotes upper airway patency by reducing airway compliance. These experiments tested the hypothesis that XII LTF is manifest as increased inspiratory drive to both tongue protrudor and retractor muscles. Neurograms were recorded in the medial XII nerve branch (XIIMED; contains tongue protrudor motor axons), the lateral XII nerve branch (XIILAT; contains tongue retractor motor axons), and the phrenic nerve in anesthetized, vagotomized, paralyzed, ventilated male rats. Strict isocapnia was maintained for 60 min after five 3-min hypoxic episodes (arterial Po(2) = 35 +/- 2 Torr) or sham treatment. Peak inspiratory burst amplitude showed a persistent increase in XIIMED, XIILAT, and phrenic nerves during the hour after episodic hypoxia (P < 0.05 vs. sham). This effect was present regardless of the quantification method (e.g., % baseline vs. percent maximum); however, comparisons of the relative magnitude of LTF between neurograms (e.g., XIIMED vs. XIILAT) varied with the normalization procedure. There was no persistent effect of episodic hypoxia on inspiratory burst frequency (P > 0.05 vs. sham). These data demonstrate that episodic hypoxia induces LTF of inspiratory drive to both tongue protrudor and retractor muscles and underscore the potential contribution of tongue muscle coactivation to regulation of upper airway patency.     

d-serine regulation of the timing and architecture of the inspiratory burst in neonatal mice

d-serine, released from mouse medullary astrocytes in response to increased CO₂ levels, boosts the respiratory frequency to adapt breathing to physiological demands. We analyzed in mouse neonates, the influence of d-serine upon inspiratory/expiratory durations and the architecture of the inspiratory burst, assessed by pwelch's power spectrum density (PSD) and continuous wavelet transform (CWT) analyses. Suction electrode recordings were performed in slices from the ventral respiratory column (VRC), site of generation of the respiratory rhythm, and in brainstem-spinal cord (en bloc) preparations, from the C5 ventral roots, containing phrenic fibers that in vivo innervate and drive the diaphragm, the main inspiratory muscle.In en bloc and slice preparations, d-serine (100 μM) reduced the expiratory, but not the inspiratory duration, and increased the frequency and the regularity of the respiratory rhythm. In en bloc preparations, d-serine (100 μM) also increased slightly the amplitude of the integrated inspiratory burst and the area under the curve of the integrated inspiratory burst, suggesting a change in the recruitment or the firing pattern of neurons within the burst. Time-frequency analyses revealed that d-serine changed the burst architecture of phrenic roots, widening their frequency spectrum and shifting the position of the core of firing frequencies towards the onset of the inspiratory burst. At the VRC, no clear d-serine induced changes in the frequency-time domain could be established. Our results show that d-serine not only regulates the timing of the respiratory cycle, but also the recruitment strategy of phrenic motoneurons within the inspiratory burst.     

Spontaneously hypertensive rats exhibit an enhanced mydriatic response to clonidine. Evidence for enhanced sensitivity of central alpha 2-adrenoceptors

Comparisons among spontaneously hypertensive (SHR), Kyoto Wistar (KW), and Wistar (W) rats were made of the functional states of central nervous system (CNS) alpha 2-adrenoceptors (clonidine-induced mydriasis) and nonvascular peripheral presynaptic alpha 2-adrenoceptors (clonidine-induced inhibition of the neurogenic twitch of the isolated vas deferens). While there were no differences among the strains of rats in the concentration of clonidine required to produce a 50% inhibition of the electrically evoked contractile response of the vas deferens, there was a significant reduction in the mean effective concentration (ED50) of clonidine to induce mydriasis in SHR as compared with KW and W rats. These observations indicate that CNS alpha 2-adrenoceptors may be functionally more sensitive in SHR. The data also suggest that the sensitivity of nonvascular presynaptic alpha 2-adrenoceptors, at least in the vas deferens, is not altered in hypertensive animals.     

Contrasting effects of presynaptic alpha2-adrenergic autoinhibition and pharmacologic augmentation of presynaptic inhibition on sympathetic heart rate control

Presynaptic alpha2-adrenergic receptors are known to exert feedback inhibition on norepinephrine release from the sympathetic nerve terminals. To elucidate the dynamic characteristics of the inhibition, we stimulated the right cardiac sympathetic nerve according to a binary white noise signal while measuring heart rate (HR) in anesthetized rabbits (n = 6). We estimated the transfer function from cardiac sympathetic nerve stimulation to HR and the corresponding step response of HR, with and without the blockade of presynaptic inhibition by yohimbine (1 mg/kg followed by 0.1 mg.kg(-1).h(-1) iv). We also examined the effect of the alpha2-adrenergic receptor agonist clonidine (0.3 and 1.5 mg.kg(-1).h(-1) iv) in different rabbits (n = 5). Yohimbine increased the maximum step response (from 7.2 +/- 0.8 to 12.2 +/- 1.7 beats/min, means +/- SE, P < 0.05) without significantly affecting the initial slope (0.93 +/- 0.23 vs. 0.94 +/- 0.22 beats.min(-1).s(-1)). Higher dose but not lower dose clonidine significantly decreased the maximum step response (from 6.3 +/- 0.8 to 6.8 +/- 1.0 and 2.8 +/- 0.5 beats/min, P < 0.05) and also reduced the initial slope (from 0.56 +/- 0.07 to 0.51 +/- 0.04 and 0.22 +/- 0.06 beats.min(-1).s(-1), P < 0.05). Our findings indicate that presynaptic alpha2-adrenergic autoinhibition limits the maximum response without significantly compromising the rapidity of effector response. In contrast, pharmacologic augmentation of the presynaptic inhibition not only attenuates the maximum response but also results in a sluggish effector response.     

Pre- and postsynaptic mechanisms in the clonidine- and oxymetazoline-induced inhibition of gastric motility in the rat

The inhibitory effect of clonidine (non-selective alpha2-adrenoceptor agonist) and oxymetazoline (alpha2A-adrenoceptor selective agonist) was compared on basal and stimulated gastric motor activity (gastric tone and contractions) using the balloon method in the rat. It was shown that oxymetazoline (0.2-1.7 micromol/kg, i.v.) decreased the basal motility, while clonidine (1.9-3.8 micromol/kg, i.v.) failed to affect it. When motility was stimulated centrally by insulin (5 IU/rat, i.v.), both clonidine (1.9-3.8 micromol/kg, i.v.) and oxymetazoline (0.1-3.4 micromol/kg, i.v.) inhibited the gastric motor activity. However, while the effect of clonidine was antagonized by the non-selective alpha2-adrenoceptor antagonist yohimbine (5 micromol/kg, i.v.) and the alpha2A-adrenoceptor selective antagonist BRL 44408 (3 micromol/kg, i.v.), the effect of oxymetazoline was only partially affected. Prazosin (alpha1- and alpha2B-adrenoceptor antagonist, 0.07-0.28 micromol/kg, i.v.) also failed to reverse the effect of oxymetazoline. Furthermore, when gastric motility was stimulated peripherally by activation of postsynaptic cholinergic muscarinic receptors by the combination of carbachol (0.14 micromol/kg, i.v.) and hexamethonium (37 micromol/kg, i.v.), clonidine (3.8 micromol/kg, i.v.) failed to affect the increased motor activity, however, oxymetazoline (0.8-3.4 micromol/kg, i.v.) exerted a pronounced inhibition. These results suggest that different mechanisms may be involved in the inhibitory effect of clonidine and oxymetazoline; while clonidine reduces the gastric motility by activation of presynaptic alpha2-adrenoceptors, postsynaptic component in the effect of oxymetazoline has also been raised.     

Regulation of chicken pineal arylalkylamine-N-acetyl transferase by postsynaptic alpha 2-adrenergic receptors

The present investigation sought to characterize the adrenergic inhibition of arylalkylamine-N-acetyltransferase in cultured chicken pineal glands. Arylalkylamine-N-acetyltransferase, the melatonin rhythm generating enzyme, displays daily oscillations of activity that are driven by a circadian oscillator. Norepinephrine released at sympathetic nerve endings inhibits the enzyme and appears to play a role in maintaining a circadian rhythm of melatonin release. Chicken pineal glands were isolated in organ culture and the effects of adrenergic agents on the night time peak of N-acetyltransferase activity were studied. Norepinephrine and clonidine prevented 50 to 65% of the nocturnal rise of N-acetyltransferase activity. When applied at middark, norepinephrine and clonidine caused a 50 to 65% inhibition of N-acetyltransferase activity in 2 hours. Dose-response studies indicated clonidine was 100 times more potent than norepinephrine or cirazoline at inhibiting N-acetyltransferase activity. Inhibition of N-acetyltransferase activity was also observed, at micromolar concentration with epinephrine, UK 14,304 and alpha-methylnorepinephrine but not with phenylephrine, isoproterenol or dopamine. Epinephrine and clonidine actions were antagonized by yohimbine but not by prazosin. Destruction of the presynaptic compartment by bilateral superior cervical ganglionectomy did not affect the clonidine-induced inhibition of N-acetyltransferase and its reversal by yohimbine. It is concluded that the adrenergic inhibition of N-acetyltransferase activity in chicken pineal gland probably occurs via stimulation of postsynaptic alpha 2-adrenergic receptors.     

Evidence that glycine and GABA mediate postsynaptic inhibition of bulbar respiratory neurons in the cat.

Experiments were carried out on decerebrate cats to identify transsynaptic mediators of spontaneous postsynaptic inhibition of bulbar inspiratory and postinspiratory neurons. Somatic membrane potentials were recorded through the central micropipette of a coaxial multibarreled electrode. Blockers of type A gamma-aminobutyric acid (GABA-A) and glycine receptors were iontophoresed extracellularly from peripheral micropipettes surrounding the central pipette. Effective antagonism was demonstrated by iontophoresis of agonists with antagonists; application of strychnine antagonized the action of glycine but not GABA, and application of bicuculline antagonized the action of GABA but not glycine. In both types of neurons, iontophoresis of either antagonist depolarized the somatic membrane and increased input resistance throughout the respiratory cycle. Bicuculline preferentially depolarized the somatic membrane in both types of neurons during inactive phases. Strychnine increased the firing rate of inspiratory neurons during inspiration despite maintenance of somatic membrane potential at preiontophoresis levels. Tetrodotoxin reduced the effects of iontophoresed bicuculline and strychnine, suggesting that the action of the antagonists required presynaptic axonal conduction. The present results suggest that presynaptic release of both GABA and glycine contributes to tonic postsynaptic inhibition of bulbar respiratory neurons. GABA-A receptors appear to contribute to inhibition during inactive phases in inspiratory and postinspiratory neurons, whereas glycinergic mechanisms appear to contribute to inspiratory inhibition in inspiratory neurons.     

Recurrent inhibition of human firing motoneurons (experimental and modeling study)

Recurrent inhibition between tonically activated single human motoneurons was studied experimentally and by means of a computer simulation. Motor unit activity was recorded during weak isometric constant-force muscle contractions of brachial biceps (BB) and soleus (SOL) muscles. Three techniques (cross correlogram, frequencygram, and interspike interval analysis) were used to gauge the relations between single motor unit potential trains. Pure inhibition was detected in 5.6% of 54 BB motoneuron pairs and in 5.2% of 43 SOL motoneuron pairs. In 27.8% (BB) and 23.7% (SOL) presumed inhibition symptoms were accompanied by a synchrony peak; 37% (BB) and 48.8% (SOL) exhibited synchrony alone. The demonstrated inhibition was very weak, at the edge of detectability. Computer simulations were based on the threshold-crossing model of a tonically firing motoneuron. The model included synaptic noise as well as threshold and postsynaptic potential (PSP) amplitude change within interspike interval. Inhibition efficiency of the model neurons increased with IPSP amplitude and duration, and with increasing source firing rate. The efficiency depended on target motoneuron interspike interval in a manner similar to standard deviation of ISI. The minimum detectable amplitude estimated in the simulations was about 50V, which, compared with the experimental results, suggests that amplitudes of detectable recurrent IPSPs in human motoneurons during weak muscle contractions do not exceed this magnitude. Since recurrent inhibition is known to be progressively depressed with an increase in the force of voluntary contraction, it is concluded that the recurrent inhibition hardly plays any important role in the isometric muscle contractions of constant force.     

Sensory responses of neurons in the medial septal area to modulation of the theta-activity by alpha2-adrenergic receptor agonist clonidine

It was shown by us earlier that bilateral intracerebroventricular injection of alpha2-adrenoreceptor agonist clonidine produced a dose-dependent effect on theta oscillations in the septohippocampal system of awake rabbits. A relatively low dose of clonidine (0.5 microgram) attenuated and a high dose (5 micrograms) significantly enhanced the rhythmic activity. It was suggested that the effect of the low dose of clonidine is mediated by presynaptic alpha2-adrenoreceptors were as postsynaptic alpha2-adrenoreceptors. In this article sensory neuronal responses in the medial septal area (MS) were analyzed against the background of the theta activity modulation by different clonidine doses. Different effects of the low and high doses of the agonist were revealed. The low dose of clonidine (0.5 microgram in 5 microliters into each lateral ventricle) which produced a decrease in the theta activity resulted in attenuation of excitation and enhancement of inhibition, i.e., the number of activating effects significantly decreased and inhibitory responses were more frequent and distinct. The high dose of clonidine (5 micrograms in 5 microliters) which produced a sharp increase in the theta activity led to a significant decrease in the reactions of the MS cells to sensory stimuli (from 76.8% in the control to 45% under clonidine) independently on the initial reaction character. Persisted excitatory and inhibitory responses became less distinct than the initial ones except single excitatory reactions. The results suggest that alpha2-adrenoreceptors are involved in the control of the sensory reactivity of MS neurons. A sharp decrease in neuronal reactivity during stable rhythmical oscillations developing under the influence of high dose of clonidine confirm the role of the theta rhythm in the septohippocampal system as an active filter in information selection and registration.     

Nucleus raphe obscurus modulates hypoglossal output of neonatal rat in vitro transverse brain stem slices

Nucleus raphéobscurus (NRo) modulates hypoglossal (XII) nerve motor output in the invitro transverse brain stem slice of neonatal rats (1-5 days old);chemical ablation of NRo and its focal CO₂ acidificationmodulated the bursting rhythm of XII nerves. We microinjected a 4.5 mMsolution of kainic acid into the NRo to disrupt cellular activity andobserved that XII nerve activity was temporarily abolished(n = 10). We also microinjected CO₂-acidified (pH = 6.00 ± 0.01) artificialcerebrospinal fluid (aCSF) into the NRo (n = 6), thepre-Bötzinger complex (PBC) (n = 6), as well as acontrol region in the lateral tegmental field equidistant to NRo, PBC,and the XII motor nuclei (n = 12). CO₂acidification of the control region had no effect on XII motor output.CO₂ acidification of the NRo significantly(P < 0.05) increased the burst discharge frequency ofXII nerves by 77%; integrated burst amplitude and burst durationincreased by 64% and 52%, respectively. CO₂ acidificationof the PBC significantly (P < 0.05) increased theburst discharge frequency of XII nerves by 65%, but neither integratedburst amplitude nor burst duration changed. These results demonstratethat chemical ablation of the NRo can abolish XII nerve bursting rhythmand that stimulation of the NRo with CO₂-acidified aCSF canexcite XII nerve bursting activity. From these observations, weconclude that, in transverse brain stem slices, the NRo containspH/CO₂-sensitive cells that modulate XII motor output.

     

Vestibulospinal influences on lower limb motoneurons

Galvanic vestibular stimulation (GVS) is a research tool used to activate the vestibular system in human subjects. When a low-intensity stimulus (1-4 mA) is delivered percutaneously to the vestibular nerve, a transient electromyographic response is observed a short time later in lower limb muscles. Typically, galvanically evoked responses are present when the test muscle is actively engaged in controlling standing balance. However, there is evidence to suggest that GVS may be able to modulate the activity of lower limb muscles when subjects are not in a free-standing situation. The purpose of this review is to examine 2 studies from our laboratory that examined the effects of GVS on the lower limb motoneuron pool. For instance, a monopolar monaural galvanic stimulus modified the amplitude of the ipsilateral soleus H-reflex. Furthermore, bipolar binaural GVS significantly altered the onset of activation and the initial firing frequency of gastrocnemius motor units. The following paper examines the effects of GVS on muscles that are not being used to maintain balance. We propose that GVS is modulating motor output by influencing the activity of presynaptic inhibitory mechanisms that act on the motoneuron pool.     

Developmental nicotine exposure enhances inhibitory synaptic transmission in motor neurons and interneurons critical for normal breathing

Nicotine exposure in utero negatively affects neuronal growth, differentiation, and synaptogenesis. We used rhythmic brainstems slices and immunohistochemistry to determine how developmental nicotine exposure (DNE) alters inhibitory neurotransmission in two regions essential to normal breathing, the hypoglossal motor nucleus (XIIn), and preBötzinger complex (preBötC). We microinjected glycine or muscimol (GABA_A agonist) into the XIIn or preBötC of rhythmic brainstem slices from neonatal rats while recording from XII nerve roots to obtain XII motoneuron population activity. Injection of glycine or muscimol into the XIIn reduced XII nerve burst amplitude, while injection into the preBötC altered nerve burst frequency. These responses were exaggerated in preparations from DNE animals. Quantitative immunohistochemistry revealed a significantly higher GABA_A receptor density on XII motoneurons from DNE pups. There were no differences in GABA_A receptor density in the preBötC, and there were no differences in glycine receptor expression in either region. Nicotine, in the absence of other chemicals in tobacco smoke, alters normal development of brainstem circuits that are critical for normal breathing. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 337–354, 2016     

Inspiratory neuron activity in the ventrolateral medulla of the dog

The purpose of this study was to describe the distribution and activity pattern of respiratory neurons located in the ventrolateral medulla (VLM) of the dog. Spike activity of 129 respiratory neurons was recorded in 23 ketamine-anesthetized spontaneously breathing dogs. Pontamine blue dye was used to mark the location of each neuron. Most VLM neurons displaying respiratory related spike patterns were located in a column related closely to ambigual and retroambigual nuclei. Both inspiratory and expiratory neurons were present with inspiratory units being grouped more rostrally. The predominant inspiratory neuron firing pattern was "late" inspiratory, although eight "early" types were located. All expiratory firing patterns were the late expiratory variety. Each neuron burst pattern was characterized by determining burst duration (BD), spikes per burst (S/B), peak frequency (PF), time to peak frequency (TPF), rate of rise to peak frequency (PF/TPF), and mean frequency. CO2-induced minute ventilation increases were associated with decreases in BD and TPF and increases in PF, S/B, and PF/TPF. In 11 experiments the relative influences of vagotomy and tracheal occlusion on late inspiratory units were compared. Tracheal occlusion increased late inspiratory BD and S/B but did not alter PF/TPF. Vagotomy increased BD and S/B beyond those obtained by tracheal occlusion and, in some neurons, decreased the PF/TPF. We conclude that the location of respiratory units in the VLM of the dog is similar to that in other species, the discharge pattern of VLM respiratory units is similar to those in cat VLM, and vagotomy and tracheal occlusion affect discharge patterns differently.     

Membrane potential-dependent modulation of recurrent inhibition in rat neocortex

Dynamic balance of excitation and inhibition is crucial for network stability and cortical processing, but it is unclear how this balance is achieved at different membrane potentials (V(m)) of cortical neurons, as found during persistent activity or slow V(m) oscillation. Here we report that a V(m)-dependent modulation of recurrent inhibition between pyramidal cells (PCs) contributes to the excitation-inhibition balance. Whole-cell recording from paired layer-5 PCs in rat somatosensory cortical slices revealed that both the slow and the fast disynaptic IPSPs, presumably mediated by low-threshold spiking and fast spiking interneurons, respectively, were modulated by changes in presynaptic V(m). Somatic depolarization (>5 mV) of the presynaptic PC substantially increased the amplitude and shortened the onset latency of the slow disynaptic IPSPs in neighboring PCs, leading to a narrowed time window for EPSP integration. A similar increase in the amplitude of the fast disynaptic IPSPs in response to presynaptic depolarization was also observed. Further paired recording from PCs and interneurons revealed that PC depolarization increases EPSP amplitude and thus elevates interneuronal firing and inhibition of neighboring PCs, a reflection of the analog mode of excitatory synaptic transmission between PCs and interneurons. Together, these results revealed an immediate V(m)-dependent modulation of cortical inhibition, a key strategy through which the cortex dynamically maintains the balance of excitation and inhibition at different states of cortical activity.     

Inhibitory processes of hippocampal CA1 pyramidal neurons following kindling-induced epilepsy in the rat

To determine the alterations in cellular function which may contribute to the chronic predisposition of neuronal tissue to epileptiform activity, the membrane properties and inhibitory processes of hippocampal CA1 pyramidal cells were investigated using in vitro slices prepared from commissural-kindled rats. No changes were observed in resting membrane potential, input resistance, spike amplitude, and membrane time constant of "kindled" CA1 pyramidal neurons when compared with controls. There were also no differences between control and kindled preparations in the amplitude of recurrent inhibitory postsynaptic potentials (IPSP) and in the duration of inhibition produced by either alvear (Alv) or stratum radiatum (SR) stimulation. Irrespective of group, repetitive stimulation of the Alv reduced the amplitude of the recurrent IPSP but failed to induce seizurelike activity. On the other hand, repetitive stimulation of SR frequently produced a neuronal burst discharge even though the duration and to some extent the amplitude of orthodromic inhibition was increased. On the basis of these data, it may be suggested that chronic changes in CA1 pyramidal cell membrane properties and transient reductions of inhibitory processes do not underlie the enhanced sensitivity of these neurons to seizure activity associated with kindling.