Biphasic effects of morphine on bulbar respiratory neuronal activities in decerebrate cats

Changes in pontine and preBötzinger/Bötzinger complex neuronal activity during remifentanil-induced respiratory depression in decerebrate dogs

Introduction: In vivo studies using selective, localized opioid antagonist injections or localized opioid receptor deletion have identified that systemic opioids dose-dependently depress respiratory output through effects in multiple respiratory-related brainstem areas. Methods: With approval of the subcommittee on animal studies of the Zablocki VA Medical Center, experiments were performed in 53 decerebrate, vagotomized, mechanically ventilated dogs of either sex during isocapnic hyperoxia. We performed single neuron recordings in the Pontine Respiratory Group (PRG, n = 432) and preBötzinger/Bötzinger complex region (preBötC/BötC, n = 213) before and during intravenous remifentanil infusion (0.1-1 mcg/kg/min) and then until complete recovery of phrenic nerve activity. A generalized linear mixed model was used to determine changes in Fn with remifentanil and the statistical association between remifentanil-induced changes in Fn and changes in inspiratory and expiratory duration and peak phrenic activity. Analysis was controlled via random effects for animal, run, and neuron type. Results: Remifentanil decreased Fn in most neuron subtypes in the preBötC/BötC as well as in inspiratory (I), inspiratory-expiratory, expiratory (E) decrementing and non-respiratory modulated neurons in the PRG. The decrease in PRG inspiratory and non-respiratory modulated neuronal activity was associated with an increase in inspiratory duration. In the preBötC, the decrease in I-decrementing neuron activity was associated with an increase in expiratory and of E-decrementing activity with an increase in inspiratory duration. In contrast, decreased activity of I-augmenting neurons was associated with a decrease in inspiratory duration. Discussion: While statistical associations do not necessarily imply a causal relationship, our data suggest mechanisms for the opioid-induced increase in expiratory duration in the PRG and preBötC/BötC and how inspiratory failure at high opioid doses may result from a decrease in activity and decrease in slope of the pre-inspiratory ramp-like activity in preBötC/BötC pre-inspiratory neurons combined with a depression of preBötC/BötC I-augmenting neurons. Additional studies must clarify whether the observed changes in neuronal activity are due to direct neuronal inhibition or decreased excitatory inputs.

µ-Opioid Receptor Activation Reduces Glutamate Release in the PreBötzinger Complex in Organotypic Slice Cultures

The inspiratory rhythm generator, located in the brainstem preBötzinger complex (preBötC), is dependent on glutamatergic signaling and is affected profoundly by opioids. Here, we used organotypic slice cultures of the newborn mouse brainstem of either sex in combination with genetically encoded sensors for Ca2+, glutamate, and GABA to visualize Ca2+, glutamatergic and GABAergic signaling during spontaneous rhythm and in the presence of DAMGO. During spontaneous rhythm, the glutamate sensor SF-iGluSnFR.A184S revealed punctate synapse-like fluorescent signals along dendrites and somas in the preBötC with decay times that were prolonged by the glutamate uptake blocker (TFB-TBOA). The GABA sensor iGABASnFR showed a more diffuse fluorescent signal during spontaneous rhythm. Rhythmic Ca2+- and glutamate transients had an inverse relationship between the spontaneous burst frequency and the burst amplitude of the Ca2+ and glutamate signals. A similar inverse relationship was observed when bath applied DAMGO reduced spontaneous burst frequency and increased the burst amplitude of Ca2+, glutamate, and GABA transient signals. However, a hypoxic challenge reduced both burst frequency and Ca2+ transient amplitude. Using a cocktail that blocked glutamatergic, GABAergic, and glycinergic transmission to indirectly measure the release of glutamate/GABA in response to an electrical stimulus, we found that DAMGO reduces the release of glutamate in the preBötC but has no effect on GABA release. This suggest that the opioid mediated slowing of respiratory rhythm involves presynaptic reduction of glutamate release, which would impact the ability of the network to engage in recurrent excitation, and may result in the opioid-induced slowing of inspiratory rhythm.SIGNIFICANCE STATEMENT Opioids slow down breathing rhythm by affecting neurons in the preBötzinger complex (preBötC) and other brainstem regions. Here, we used cultured slices of the preBötC to better understand this effect by optically recording Ca2+, glutamate, and GABA transients during preBötC activity. Spontaneous rhythm showed an inverse relationship between burst frequency and burst amplitude in the Ca2+ and glutamate signals. Application of the opioid DAMGO slowed the rhythm, with a concomitant increase in Ca2+, glutamate, and GABA signals. When rhythm was blocked pharmacologically, DAMGO reduced the presynaptic release of glutamate, but not GABA. These data suggest the mechanism of action of opioids involves presynaptic reduction of glutamate release, which may play an important role in the opioid-induced slowing of inspiratory rhythm.

L-cysteine methyl ester overcomes the deleterious effects of morphine on ventilatory parameters and arterial blood-gas chemistry in unanesthetized rats

We are developing a series of thiolesters that produce an immediate and sustained reversal of the deleterious effects of opioids, such as morphine and fentanyl, on ventilation without diminishing the antinociceptive effects of these opioids. We report here the effects of systemic injections of L-cysteine methyl ester (L-CYSme) on morphine-induced changes in ventilatory parameters, arterial-blood gas (ABG) chemistry (pH, pCO₂, pO₂, sO₂), Alveolar-arterial (A-a) gradient (i.e., the index of alveolar gas-exchange within the lungs), and antinociception in unanesthetized Sprague Dawley rats. The administration of morphine (10 mg/kg, IV) produced a series of deleterious effects on ventilatory parameters, including sustained decreases in tidal volume, minute ventilation, inspiratory drive and peak inspiratory flow that were accompanied by a sustained increase in end inspiratory pause. A single injection of L-CYSme (500 μmol/kg, IV) produced a rapid and long-lasting reversal of the deleterious effects of morphine on ventilatory parameters, and a second injection of L-CYSme (500 μmol/kg, IV) elicited pronounced increases in ventilatory parameters, such as minute ventilation, to values well above pre-morphine levels. L-CYSme (250 or 500 μmol/kg, IV) also produced an immediate and sustained reversal of the deleterious effects of morphine (10 mg/kg, IV) on arterial blood pH, pCO₂, pO₂, sO₂ and A-a gradient, whereas L-cysteine (500 μmol/kg, IV) itself was inactive. L-CYSme (500 μmol/kg, IV) did not appear to modulate the sedative effects of morphine as measured by righting reflex times, but did diminish the duration, however, not the magnitude of the antinociceptive actions of morphine (5 or 10 mg/kg, IV) as determined in tail-flick latency and hindpaw-withdrawal latency assays. These findings provide evidence that L-CYSme can powerfully overcome the deleterious effects of morphine on breathing and gas-exchange in Sprague Dawley rats while not affecting the sedative or early stage antinociceptive effects of the opioid. The mechanisms by which L-CYSme interferes with the OR-induced signaling pathways that mediate the deleterious effects of morphine on ventilatory performance, and by which L-CYSme diminishes the late stage antinociceptive action of morphine remain to be determined.

Multi-Level Regulation of Opioid-Induced Respiratory Depression

Opioids depress minute ventilation primarily by reducing respiratory rate. This results from direct effects on the preBötzinger Complex as well as from depression of the Parabrachial/Kölliker-Fuse Complex, which provides excitatory drive to preBötzinger Complex neurons mediating respiratory phase-switch. Opioids also depress awake drive from the forebrain and chemodrive.

Activating α4β2 Nicotinic Acetylcholine Receptors Alleviates Fentanyl-induced Respiratory Depression in Rats

BACKGROUND

Opioid analgesics are widely used for treatment of acute, postoperative, and chronic pain. However, activation of opioid receptors can result in severe respiratory depression. There is an unmet clinical need to develop a pharmacologic therapy to counter opioid-induced respiratory depression without interfering with analgesia. Further, additional advances to confront accidental lethal overdose with the use of fentanyl and other opioids are needed. Here, the authors test the hypothesis that activation of nicotinic receptors expressed within respiratory rhythm-generating networks would counter opioid-induced respiratory depression without compromising analgesia.

METHODS

Respiratory neural discharge was measured using in vitro brainstem-spinal cord and medullary slice rat preparations. In vivo, plethysmographic recording, nociception testing, and righting reflexes were used to examine respiratory ventilation, analgesia, and sedation, respectively.

RESULTS

The administration of nicotine, selective α4β2 nicotinic receptor agonist A85380, but not α7 nicotinic receptor agonist PNU282987, reversed opioid-induced respiratory depression in neonatal pups in vitro and in vivo. In adult rats in vivo, administration of A85380 (0.03 mg/kg), but not PNU282987, provides a rapid and robust reversal of fentanyl-induced decrease in respiratory rate (93.4 ± 33.7% of control 3 min after A85380 vs. 31 ± 20.5% of control after vehicle, n = 8 each, P < 0.001), without marked side effects. The coadministration of A85380 (0.06 mg/kg) with fentanyl or remifentanil markedly reduced respiratory depression and apneas, and enhanced the fentanyl-induced analgesia, as evidenced by increased paw withdrawal latency in Hargreaves plantar test (14.4 ± 2.8 s vs. vehicle: 11.3 ± 2.4 s, n = 8 each, P = 0.013) and decreased formalin-induced nocifensive duration (2.5 ± 2.4 min vs. vehicle: 5.4 ± 2.7 min, n = 8 each, P = 0.029).

CONCLUSIONS

The novel strategy of targeting α4β2 nicotinic acetylcholine receptors has the potential for advancing pain control and reducing opioid-induced respiratory depression and overdose.

Effects on breathing of agonists to μ-opioid or GABAA receptors dialyzed into the ventral respiratory column of awake and sleeping goats

Pulmonary ventilation (V̇I) in awake and sleeping goats does not change when antagonists to several excitatory G protein-coupled receptors are dialyzed unilaterally into the ventral respiratory column (VRC). Concomitant changes in excitatory neuromodulators in the effluent mock cerebral spinal fluid (mCSF) suggest neuromodulatory compensation. Herein, we studied neuromodulatory compensation during dialysis of agonists to inhibitory G protein-coupled or ionotropic receptors into the VRC. Microtubules were implanted into the VRC of goats for dialysis of mCSF mixed with agonists to either μ-opioid (DAMGO) or GABAA (muscimol) receptors. We found: (1) V̇I decreased during unilateral but increased during bilateral dialysis of DAMGO, (2) dialyses of DAMGO destabilized breathing, (3) unilateral dialysis of muscimol increased V̇I, and (4) dialysis of DAMGO decreased GABA in the effluent mCSF. We conclude: (1) neuromodulatory compensation can occur during altered inhibitory neuromodulator receptor activity, and (2) the mechanism of compensation differs between G protein-coupled excitatory and inhibitory receptors and between G protein-coupled and inotropic inhibitory receptors.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

A μ-opioid receptor agonist DAMGO induces rapid breathing in the arterially perfused in situ preparation of rat

Ventilatory responses to opioids are complex and not yet fully understood. We evaluated concentration-dependent effects of a selective μ-opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) on respiratory output in the arterially perfused in situ rat preparation, which preserves the integrity of the ponto-medullary respiratory network. DAMGO (300-3400 nM) was added accumulatively to the perfusate. DAMGO increased inspiratory time and diminished central vagal post-inspiratory activity. At 300-500 nM DAMGO caused rapid breathing with shortening of expiratory time. The change of breathing pattern occurred within a single breath. Bilateral vagotomy did not affect the change in respiratory pattern, suggesting that it was of central origin. Additional DAMGO up to 1800 nM did not affect the rapid breathing pattern, and further elevated concentrations (up to 3400 nM) caused inconsistent results. Since the rapid breathing pattern was associated with the obliteration of vagal post-inspiratory activity, we conclude that DAMGO reconfigures the respiratory output to an inspiratory phase-dominant, rapidly alternating inspiration-expiration pattern.

PreBotzinger complex neurokinin-1 receptor-expressing neurons mediate opioid-induced respiratory depression

The analgesic properties of the opium poppy Papever somniferum were first mentioned by Hippocrates around 400 BC, and opioid analgesics remain the mainstay of pain management today. These drugs can cause the serious side-effect of respiratory depression that can be lethal with overdose, however the critical brain sites and neurochemical identity of the neurons mediating this depression are unknown. By locally manipulating neurotransmission in the adult rat, we identify the critical site of the medulla, the preBötzinger complex, that mediates opioid-induced respiratory depression in vivo. Here we show that opioids at the preBötzinger complex cause respiratory depression or fatal apnea, with anesthesia and deep-sleep being particularly vulnerable states for opioid-induced respiratory depression. Importantly, we establish that the preBötzinger complex is fully responsible for respiratory rate suppression following systemic administration of opioid analgesics. The site in the medulla most sensitive to opioids corresponds to a region expressing neurokinin-1 receptors, and we show in rhythmically active brainstem section in vitro that neurokinin-1 receptor-expressing preBötzinger complex neurons are selectively inhibited by opioids. In summary, neurokinin-1 receptor-expressing preBötzinger complex neurons constitute the critical site mediating opioid-induced respiratory rate depression, and the key therapeutic target for its prevention or reversal.

Indirect opioid actions on inspiratory pre-Bötzinger complex neurons in newborn rat brainstem slices

Findings in newborn mouse brainstem slices led to the hypothesis that depression of breathing by opioids is caused by postsynaptic K(+) channel-mediated hyperpolarization of rhythmogenic inspiratory neurons of the pre-Bötzinger complex (preBötC). Subsequent observations in newborn en bloc medullas and juvenile rats in vivo indicated that excitatory drive from retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) neurons partly counters opioid-evoked inspiratory inhibition. Our recent study in newborn rat en bloc medullas supports the latter hypothesis, whereas we found in that report that inspiratory preBötC neurons constituting the interface with the RTN/pFRG are not hyperpolarized by opioids. Here, we show that opioids also do not hyperpolarize preBötC neurons in "calibrated" newborn rat slices. This supports the previous hypothesis by us and others that opioids rather target inspiratory networks indirectly, likely primarily via presynaptic mechanisms.

Clinically relevant infusion rates of mu-opioid agonist remifentanil cause bradypnea in decerebrate dogs but not via direct effects in the pre-Bötzinger complex region

Systemic administration of mu-opioids at clinical doses for analgesia typically slows respiratory rate. Mu-opioid receptors (MORs) on pre-Bötzinger Complex (pre-BötC) respiratory neurons, the putative kernel of respiratory rhythmogenesis, are potential targets. The purpose of this study was to determine the contribution of pre-BötC MORs to the bradypnea produced in vivo by intravenous administration of clinically relevant infusion rates of remifentanil (remi), a short-acting, potent mu-opioid analgesic. In decerebrate dogs, multibarrel micropipettes were used to record pre-BötC neuronal activity and to eject the opioid antagonist naloxone (NAL, 0.5 mM), the glutamate agonist D-homocysteic acid (DLH, 20 mM), or the MOR agonist [D-Ala(2), N-Me-Phe(4), gly-ol(5)]-enkephalin (DAMGO, 100 microM). Inspiratory and expiratory durations (T(I) and T(E)) and peak phrenic nerve activity (PPA) were measured from the phrenic neurogram. The pre-BötC was functionally identified by its rate altering response (typically tachypnea) to DLH microinjection. During intravenous remi-induced bradypnea (approximately 60% decrease in central breathing frequency, f(B)), bilateral injections of NAL in the pre-BötC did not change T(I), T(E), f(B), and PPA. Also, NAL picoejected onto single pre-BötC neurons depressed by intravenous remi had no effect on their discharge. In contrast, approximately 60 microg/kg of intravenous NAL rapidly reversed all remi-induced effects. In a separate group of dogs, microinjections of DAMGO in the pre-BötC increased f(B) by 44%, while subsequent intravenous remi infusion more than offset this DAMGO induced tachypnea. These results indicate that mu-opioids at plasma concentrations that cause profound analgesia produce their bradypneic effect via MORs located outside the pre-BötC region.

Opioidergic and dopaminergic modulation of respiration

Opioids, dopamine and their receptors are present in many regions of the bulbar respiratory network. The physiological importance of endogenous opioids to respiratory control has not been explicitly demonstrated. Nonetheless, studies of opioidergic respiratory mechanisms are important because synthetic opiate drugs have respiratory side effects that in some situations pose health risks and limit their therapeutic usefulness. They can depress breathing depth and rate, blunt respiratory responsiveness to CO2 and hypoxia, increase upper airway resistance and reduce pulmonary compliance. The opiate respiratory disturbances are mainly due to agonist activation of mu- and delta-subtypes of receptor and involve specific types of respiratory-related neurons in the ventrolateral medulla and the dorsolateral pons. Endogenous dopaminergic modulation in the CNS and carotid bodies enhances CO2-dependent respiratory drive and depresses hypoxic drive. In the CNS, synthetic agonists with selectivity for D1-and D4-types of receptor slow respiratory rhythm, whereas D2-selective agonists modulate acute and chronic responses to hypoxia. D1-receptor agonists also act centrally to increase respiratory responsiveness to CO2, and counteract opiate blunting of CO2-dependent respiratory drive and depression of breathing. Cellular targets and intracellular mechanisms responsible for opioidergic and dopaminergic respiratory effects for the most part remain to be determined.

Opioid receptors on bulbospinal respiratory neurons are not activated during neuronal depression by clinically relevant opioid concentrations

Opioids depress the activity of brain stem respiratory-related neurons, but it is not resolved whether the mechanism at clinical concentrations consists of direct neuronal effects or network effects. We performed extracellular recordings of discharge activity of single respiratory neurons in the caudal ventral respiratory group of decerebrate dogs, which were premotor neurons with a likelihood of 90%. We used multibarrel glass microelectrodes, which allowed concomitant highly localized picoejection of opioid receptor agonists or antagonists onto the neuron. Picoejection of the mu receptor agonist [d-Ala(2), N-Me-phe(4), gly-ol(5)]-enkephalin (DAMGO, 1 mM) decreased the peak discharge frequency (mean +/- SD) of expiratory neurons to 68 +/- 22% (n = 12), the delta(1) agonist d-Pen(2,5)-enkephalin (DPDPE, 1 mM) to 95 +/- 11% (n = 15), and delta(2) receptor agonist [d-Ala(2)] deltorphin-II to 86 +/- 17% (1 mM, n = 15). The corresponding values for inspiratory neurons were: 64 +/- 12% (n = 11), 48 +/- 30% (n = 12), and 75 +/- 15% (n = 11), respectively. Naloxone fully reversed these effects. Picoejection of morphine (0.01-1 mM) depressed most neurons in a concentration dependent fashion to maximally 63% (n = 27). Picoejection of remifentanil (240-480 nM) did not cause any significant depression of inspiratory (n = 11) or expiratory neurons (n = 9). 4. Intravenous remifentanil (0.2-0.6 microg.kg(-1).min(-1)) decreased neuronal peak discharge frequency to 60 +/- 12% (inspiratory, n = 7) and 58 +/- 11% (expiratory, n = 11). However, local picoejection of naloxone did not reverse the neuronal depression. Our data suggest that mu, delta(1), and delta(2) receptors are present on canine respiratory premotor neurons. Clinical concentrations of morphine and remifentanil caused no local depression. This lack of effect and the inability of local naloxone to reverse the neuronal depression by intravenous remifentanil suggest that clinical concentrations of opioids produce their depressive effects on mechanisms upstream from respiratory bulbospinal premotor neurons.

5-Hydroxytryptamine 1A/7 and 4alpha receptors differentially prevent opioid-induced inhibition of brain stem cardiorespiratory function

Opioids evoke respiratory depression, bradycardia, and reduced respiratory sinus arrhythmia, whereas serotonin (5-HT) agonists stimulate respiration and cardiorespiratory interactions. This study tested whether serotonin agonists can prevent the inhibitory effects of opioids on cardiorespiratory function. Spontaneous and rhythmic inspiratory-related activity and gamma-aminobutyric acid (GABA) neurotransmission to premotor parasympathetic cardioinhibitory neurons in the nucleus ambiguus were recorded simultaneously in an in vitro thick slice preparation. The mu-opioid agonist fentanyl inhibited respiratory frequency. The 5-hydroxytryptamine 1A/7 receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin increased respiratory frequency by itself and also prevented the fentanyl-induced respiratory depression. The 5-hydroxytryptamine 4alpha agonist BIMU-8 did not by itself change inspiratory activity but prevented the mu-opioid-mediated respiratory depression. Both spontaneous and inspiratory-evoked GABAergic neurotransmission to cardiac vagal neurons were inhibited by fentanyl. 8-Hydroxy-2-(di-n-propylamino)tetralin inhibited spontaneous but not inspiratory-evoked GABAergic activity to parasympathetic cardiac neurons. However, 8-hydroxy-2-(di-n-propylamino)tetralin differentially altered the opioid-mediated depression of inspiratory-evoked GABAergic activity but did not change the opioid-induced reduction in spontaneous GABAergic neurotransmission. In contrast, BIMU-8 did not alter GABAergic neurotransmission to cardiac vagal neurons by itself but prevented the fentanyl depression of both spontaneous and inspiratory-elicited GABAergic neurotransmission to cardiac vagal neurons. In the presence of tetrodotoxin, the inhibition of GABAergic inhibitory postsynaptic currents with fentanyl is prevented by coapplication of BIMU-8, indicating that BIMU-8 acts at presynaptic GABAergic terminals to prevent fentanyl-induced depression. These results suggest that activation of 5-hydroxytryptamine receptors, particularly 5-hydroxytryptamine 4alpha agonists, may be a useful therapeutic approach in preventing opioid-evoked cardiorespiratory depression.

Acute effects of fentanyl on breathing pattern in anaesthetized subjects

BACKGROUND

The predominant effect of opioids on respiratory pattern during anaesthesia is an increase in the duration of expiration (an effect on 'timing'), but there may also be changes in tidal volume (an effect on 'drive'). Timing and drive are controlled by separate neuronal systems, but are infrequently considered individually. The effects of opioids on breathing are not well characterized clinically because changes in carbon dioxide and anaesthetic levels usually occur at the same time, and can obscure the effects of the opioid.

METHODS

To study these effects in isolation, we established stable mild hypercapnia in female patients breathing spontaneously during sevoflurane anaesthesia, and then gave fentanyl 0.5 microg kg(-1) i.v. End-tidal carbon dioxide and sevoflurane concentrations were maintained constant, and the changes in timing of inspiration, expiration and tidal volume were measured.

RESULTS

The duration of inspiration increased by 30%, and the duration of expiration increased by 95%. Tidal volume increased in proportion to inspiratory duration, and the pattern of flow during the breath was recognizably changed, with a reduction in the rate of increase of flow at the onset of inspiration.

CONCLUSIONS

Small doses of opioid given when anaesthesia and carbon dioxide are stable affect respiratory timing predominantly, but in addition changes in the pattern of motor output can be detected.

Distinct rhythm generators for inspiration and expiration in the juvenile rat

Inspiration and active expiration are commonly viewed as antagonistic phases of a unitary oscillator that generates respiratory rhythm. This view conflicts with observations we report here in juvenile rats, where by administration of fentanyl, a selective mu-opiate agonist, and induction of lung reflexes, we separately manipulated the frequency of inspirations and expirations. Moreover, completely transecting the brainstem at the caudal end of the facial nucleus abolished active expirations, while rhythmic inspirations continued. We hypothesize that inspiration and expiration are generated by coupled, anatomically separate rhythm generators, one generating active expiration located close to the facial nucleus in the region of the retrotrapezoid nucleus/parafacial respiratory group, the other generating inspiration located more caudally in the preBötzinger Complex.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Developmental gene control of brainstem function: views from the embryo

The respiratory rhythm is generated within the hindbrain reticular formation, rostrally in the vicinity of the facial nucleus and caudally within the vagal/glossopharyngeal domain. This is probably one of the best models to understand how genes have been selected and conserved to control adaptive behaviour in vertebrates. The para-facial region is well understood with respect to the transcription factors that underlie antero-posterior specification of neural progenitors in the embryo. Hox paralogs and Hox-regulating genes kreisler and Krox-20 govern transient formation of developmental compartments, the rhombomeres, in which rhythmic neuronal networks develop. Hox are master genes selecting and coordinating the developmental fate of reticular and motor neurons thereby specifying patterns of motor activities operating throughout life. Neuronal function and development are also tightly linked in the vagal/glossopharyngeal domain. At this level, bdnf acts as a neurotrophin of peripheral chemoafferent neural populations and as a neuromodulator of the central rhythmogenic respiratory circuits. A general view is now emerging on the role of developmental transcription and trophic factors allowing the coordinated integration of different neuronal types to produce, and eventually refine, respiratory rhythmic pattern in a use-dependent manner.

Distribution of mu receptors in the ventral respiratory group neurons; immunohistochemical and pharmacological studies in decerebrate cats

Immunoreactivity for mu receptors was investigated in 21 bulbar respiratory neurons, individually identified by intracellular recording and labeling with neurobiotin. In 14 of these neurons, effects of iontophoresed morphine were examined. Morphine hyperpolarized the membrane and decreased spike discharges in 4/6 augmenting inspiratory (aug-I), 4/5 postinspiratory (post-I) and 3/3 augmenting expiratory (aug-E) neurons. It had no effect on two aug-I and one post-I neurons. Strong immunoreactivity for mu receptor was detected in the soma and dendrites of 5/8 aug-I, 5/7 post-I and 6/6 aug-E neurons. In the remaining three aug-I and two post-I neurons that included cells unresponsive to morphine, weak immunoreactivity was detected only in the dendrites. These results demonstrated wide, but uneven, distribution of mu receptors in bulbar respiratory neurons and suggest their contribution to respiratory depression by opioids.