Evidence for involvement of amino acid neurotransmitters in anesthesia and naloxone induced reversal of respiratory paralysis

Differential Anaesthetic Effects following Microinjection of Thiopentone and Propofol into the Pons of Adult Rats: A Pilot Study

Identifying the central nervous system sites of action of anaesthetics is important for understanding the link between their molecular actions and clinical effects. The aim of the present pilot study was to compare the anaesthetic effect of bilateral microinjections of propofol and thiopentone (both 200 μg/μl, in Intralipid and 0.9% saline respectively) into a recently discovered anaesthetic-sensitive region in the rat brainstem, the “mesopontine tegmental anaesthetic area” (MPTA). Microinjections (1 μl per side) were made into the MPTA of fifteen male Sprague-Dawley rats. The effect of each agent on spontaneous behaviour, postural control and nociceptive responsiveness was subjectively assessed according to established criteria. The main finding was that thiopentone induced an “anaesthesia-like” state, including complete atonia and loss of righting ability, in 20% of the subjects. Overall, thiopentone significantly reduced postural control and had a moderate antinociceptive effect compared to saline microinjections (P<0.01 and 0.05, respectively, Wilcoxon test). In contrast, propofol did not induce “anaesthesia” in any animal tested, although a similar antinociceptive effect to that of thiopentone was observed (P<0.05, Wilcoxon test). In summary, propofol and thiopentone have different effects when microinjected into the MPTA. While both agents reduced reflex withdrawal to a nociceptive stimulus, only thiopentone induced an “anaesthesia-like” state.

Methodological considerations in rat brain BOLD contrast pharmacological MRI

RATIONALE AND OBJECTIVES

Blood oxygen level dependent (BOLD) contrast pharmacological magnetic resonance imaging (phMRI) is an increasingly popular technique that allows the non-invasive investigation of spatial and temporal changes in rat brain function in response to pharmacological stimulation in vivo. Rat brain BOLD contrast phMRI is, at present, established in few neuropharmacological laboratories, and various issues associated with the technique require attention. The present review is primarily aimed at psychopharmacologists with no previous experience of phMRI, who are interested in the practical aspects that phMRI studies entail.

RESULTS AND DISCUSSION

Experimental and analytical considerations, including anaesthesia, physiological monitoring, drug dose and delivery, scanning protocols, statistical approaches and the interpretation of phMRI data, are discussed.

Immunohistochemical localization of toluene-induced c-Fos protein expression in the rat brain

Although toluene is a widely abused substance, the neuronal populations and pathways mediating its effects are not well understood. Using c-Fos protein as a marker for neuronal activation, the present study investigated the pattern of c-Fos induction at 1h after various doses (0, 300, 750, and 1000 mg/kg, i.p.) of toluene injection in adult male rats. Quantitative analysis of Fos-immunoreactive neurons indicated toluene dose-related induced c-Fos immunoreactivity in the majority of structures examined. The structures included several cortex subareas (primary motor cortex, secondary motor cortex, somatosensory cortex, frontal association cortex, cigulate cortex area 1, cigulate cortex area 2, prelimbic cortex, infralimbic cortex, retrosplenial agranular cortex, ventral orbital cortex, lateral orbital cortex, and piriform cortex), ventral tegmental area, nucleus accumbens shell, thalamic nuclei (mediodorsal, lateral posterior, and laterodorsal ventrolateral) and pontine nuclei. However, the substantia nigra, caudate putamen, nucleus accumbens core, subthalamic nucleus, hippocampus and cerebellum were almost unaffected. The data demonstrate that toluene dose-related induced a unique pattern of c-Fos immunoreactivity. The widespread distribution of toluene-induced c-Fos expression seen in this study can be linked to the profound alterations in physiological function and behavior produced by this solvent.

Orexinergic neurons and barbiturate anesthesia

Orexins (OXs) regulate sleep with possible interactions with brain noradrenergic neurons. In addition, noradrenergic activity affects barbiturate anesthesia. As we have also recently reported that OXs selectively evoke norepinephrine release from rat cerebrocortical slices we hypothesized that barbiturate anesthesia may result from of an interaction with central orexinergic systems. To test this hypothesis, we performed a series of in vivo and in vitro studies in rats. In vivo, the effects of i.c.v. OX A, B and SB-334867-A (OX1 receptor antagonist) on pentobarbital, thiopental or phenobarbital-induced anesthesia times (loss of righting reflex) was assessed. In vitro effects of barbiturates and SB-334867-A on OX-evoked norepinephrine release from cerebrocortical slice was examined. In Chinese hamster ovary cells expressing human OX1/OX2 receptors OX A- and B-evoked increases in intracellular Ca2+ were measured with and without barbiturates. OX A and B significantly decreased pentobarbital, thiopental and phenobarbital anesthesia times by 15-40%. SB-334867-A increased thiopental-induced anesthesia time by approximately by 40%, and reversed the decrease produced by OX A. In vitro, all anesthetic barbiturates inhibited OX-evoked norepinephrine release with clinically relevant IC50 values. A GABAA antagonist, bicuculline, did not modify the inhibitory effects of thiopental and the GABAA agonist, muscimol, did not inhibit norepinephrine release. In addition there was no interaction of barbiturates with either OX1 or OX2 receptors. Collectively our data suggest that orexinergic neurons may be an important target for barbiturates, and GABAA, OX1 and OX2 receptors may not be involved in this interaction.

Effect of increased brain GABA concentrations on breathing in unanesthetized newborn rabbits

Evidence suggests that gamma-aminobutyric acid (GABA) is involved in control of breathing and in the hypoxia-related ventilatory depression in newborns. However, this evidence is obtained mainly from studies on anesthetized animals. Because anesthesia may interfere with the GABA system, the objectives of our study were to examine effects of GABA on ventilation (V(E)) and ventilatory response to hypoxia and to reveal effects of repeated hypoxia on GABA concentrations in unanesthetized newborns. The study was performed in rabbits in two age groups: 1-3 days old (group I) and 10-14 days old (group II). To increase brain endogenous GABA concentrations, rabbits were injected with an inhibitor of GABA transaminase, aminooxyacetic acid (AOAA; 20 mg/kg i.p.). To prevent postmortem formation of GABA, at the end of experiments the rabbits received an inhibitor of glutamic acid decarboxylase, IP-3-mercaptopropionic acid (100 mg/kg i.p.). Animals were studied in normoxia alone, or they were exposed for 15 min to 8% O(2) before and 10 and 35 min after saline or AOAA. GABA concentrations were measured in brainstem, cerebrum, and cerebellum by means of a capillary electrophoresis. In group I, AOAA had no respiratory effects. In group II, AOAA decreased V(E), tidal volume, and mean inspiratory flow in normoxia and reversed V(E) decline during hypoxia 10 min after the injection, GABA concentrations were not age dependent and the highest in the brainstem. Repeated hypoxia increased the cerebellar GABA concentrations and had no effect in group I. These results imply that in unanesthetized rabbits, GABAergic neurotransmission in the respiratory control network becomes functional by the 2nd week of life, but it does not contribute to the biphasic ventilatory response to moderate hypoxia. In contrast, GABA-evoked block of the cerebellar inhibitory input during hypoxia may be responsible for the reversal of the V(E) decline in unanesthetized newborns.

Cardiorespiratory response to bicuculline during resistive loaded breathing in anesthetized rabbits

We hypothesized that hypoventilation induced by resistive loaded breathing may result in part from an increase in gamma-aminobutyric acid (GABA) concentration in the central nervous system. Accordingly, ventilatory depression should be minimized by GABA receptor blockade. The effects of subseizure doses of the GABAA receptor antagonist bicuculline on the cardiorespiratory variables and cortical activities were evaluated in two groups of urethane anesthetized rabbits, breathing either through an inspiratory resistive load (IRL) or not. Bicuculline induced cardiorespiratory changes which consisted mainly of an augmented respiratory rate, through shortening of expiratory duration, and of bradycardia. Bicuculline effects did not significantly differ between both groups and were accompanied by high amplitude delta rhythmic cortical activities. These data show that GABAA receptors may exert a tonic depressive effect on the respiratory circuit and suggest that endogenous GABA release is not augmented by an acute increase in the work of breathing.