Influence of halothane and methoxyflurane on regional brain and spinal cord concentrations of methionine-enkephalin in the rat

Effects of Anesthetic Administration on Rat Hypothalamus and Cerebral Cortex Peptidome

Prohormone-derived neuropeptides act as cell-cell signaling molecules to mediate a wide variety of biological processes in the animal brain. Mass spectrometry-based peptidomic experiments are valuable approaches to gain insight into the dynamics of individual peptides under different physiological conditions or experimental treatments. However, the use of anesthetics during animal procedures may confound experimental peptide measurements, especially in the brain, where anesthetics act. Here, we investigated the effects of the commonly used anesthetics isoflurane and sodium pentobarbital on the peptide profile in the rodent hypothalamus and cerebral cortex, as assessed by label-free quantitative peptidomics. Our results showed that neither anesthetic dramatically alters peptide levels, although extended isoflurane exposure did cause changes in a small number of prohormone-derived peptides in the cerebral cortex. Overall, our results demonstrate that acute anesthetic administration can be utilized in peptidomic experiments of the hypothalamus and cerebral cortex without greatly affecting the measured peptide profiles.

Short-Term Administration of Common Anesthetics Does Not Dramatically Change the Endogenous Peptide Profile in the Rat Pituitary

Cell-cell signaling peptides (e.g., peptide hormones, neuropeptides) are among the largest class of cellular transmitters and regulate a variety of physiological processes. To identify and quantify the relative abundances of cell-cell signaling peptides in different physiological states, liquid chromatography-mass spectrometry-based peptidomics workflows are commonly utilized on freshly dissected tissues. In such animal experiments, the administration of general anesthetics is an important step for many research projects. However, acute anesthetic administration may rapidly change the measured abundance of transmitter molecules and metabolites, especially in the brain and endocrine system, which would confound experimental results. The aim of this study was to evaluate the effect of short-term (<5 min) anesthetic administration on the measured abundance of cell-cell signaling peptides, as evaluated by a typical peptidomics workflow. To accomplish this goal, we compared endogenous peptide abundances in the rat pituitary following administration of 5% isoflurane, 200 mg/kg sodium pentobarbital, or no anesthetic administration. Label-free peptidomics analysis demonstrated that acute use of isoflurane changed the levels of a small number of peptides, primarily degradation products of the hormone somatotropin, but did not influence the levels of most other peptide hormones. Acute use of sodium pentobarbital had negligible impact on the relative abundance of all measured peptides. Overall, our results suggest that anesthetics used in pituitary peptidomics studies do not dramatically confound observed results.

Do inhalation general anesthetic drugs induce the neuronal release of endogenous opioid peptides?

The antagonism of some effects of inhalation general anesthetic agents by naloxone suggests that there may be an opioid component to anesthetic action. There is evidence that this opioid action component is due to neuronal release of endogenous opioid peptides. The strongest evidence is provided by studies that monitor changes in the concentration of opioid peptides in the perfused brain following inhalation of the anesthetic. Indirect or circumstantial evidence also comes from studies of anesthetic effects on regional brain levels of opioid peptides, antagonism of selected anesthetic effects by antisera to opioid peptides and anesthetic-induced changes radioligand binding to opioid receptors. It is likely that some inhalation general anesthetics (e.g., nitrous oxide) can induce neuronal release of opioid peptides and that this may contribute to certain components of general anesthesia (e.g., analgesia). More definitive studies utilizing in vivo microdialysis or autoradiography in selected areas of the brain during induction and successive states of general anesthesia have yet to be conducted.

An opioid basis for early-phase isoflurane-induced hypotension in rats

This study was conducted to more clearly delineate the possible role of endogenous opioid receptors and opioid peptides in general anesthesia-associated hypotension in rats. Exposure to 2% isoflurane in oxygen produced a triphasic change in mean arterial pressure (MAP), including an early phase in which MAP fell by -28.4 +/- 2.2%. The magnitude of this early-phase hypotension was attenuated in rats pretreated with intravenous (i.v.) mu-subtype-selective doses of either naloxone or methylnaloxone but not central doses of the selective mu-opioid antagonist beta-funaltrexamine. This early hypotensive phase was also reduced following i.v. pretreatment with antiserum against methionine-enkephalin but not beta-endorphin. These findings suggest that early-phase isoflurane-induced hypotension may be due to activation of peripheral mu-opioid receptors by an endogenous opioid peptide, possibly related to methionine-enkephalin.

Effects of halothane and methoxyflurane on the hypothalamic-pituitary-adrenal axis in rat

Effects of acute exposure (2 h) to either 1.5% halothane or 0.5% methoxyflurane on chemical mediators of the hypothalamic-pituitary-adrenal (HPA) axis were evaluated in male Sprague-Dawley rats immediately after exposure, after the righting reflex (4 h), or 24 h postexposure. Effects of these anesthetics on hippocampal corticotropin releasing factor (CRF) were also evaluated. Methoxyflurane caused significant elevations in pituitary adrenocorticotropin hormone (ACTH)-like immunoreactivities in all three of the experiment's time groups, yet halothane failed to cause the same response immediately after exposure. Serum ACTH-like immunoreactivities were significantly elevated immediately after exposure to both anesthetics, but were not elevated at 4 and 24 h postexposure. Corticosterone (CORT)-like immunoreactivities were significantly elevated by halothane in all experimental groups, and in the 2- and 24-h groups following methoxyflurane exposure. Hippocampal CRF-like immunoreactivities remained unaffected by either anesthetic. Results indicate that a 2-h exposure to either halothane or methoxyflurane results in significant activation of the rat hypothalamic-pituitary-adrenal axis, and that the activation appears to be sustained over a 24-h period.

Alterations in catecholamine turnover in specific regions of the rat brain following acute exposure to nitrous oxide

The effects of nitrous oxide (N2O) on steady-state concentrations and turnover rates of catecholamines in the olfactory bulb, hypothalamus, brain stem, hippocampus, striatum, thalamus, cerebral cortex, and spinal cord were determined in rats. Animals were exposed for 2 h to either 60% N2O or air. Immediately following exposure, all animals were injected intraperitoneally with alpha-methylparatyrosine (alphaMPT), a competitive inhibitor of tyrosine hydroxylase, and sacrificed at 0, 30, or 90 min postinjection. Brain catecholamine concentrations were determined using high-performance liquid chromatography coupled with electrochemical detection (HPLC-EC). Results indicate that N2O exposure significantly elevates steady-state concentrations of norepinephrine (NE) in the hypothalamus and striatum yet decreases amine levels in the brain stem region. Steady-state levels of dopamine (DA) were not significantly altered in any region of the CNS by N2O exposure. Acute exposure to N2O also resulted in significant decreases in the turnover rate of NE in the brain stem, yet it increased turnover of this amine in the olfactory bulb, hypothalamus, and striatum. Acute exposure to N2O resulted in a decreased turnover rate of DA in the hippocampus and striatum. In contrast, N2O appears to increase DA turnover in the olfactory bulb. These results indicate that acute exposure to N2O in rats causes region-specific alterations in steady-state levels and turnover rates of DA and NE within the central nervous system.

Effects of halothane and methoxyflurane on regional brain and spinal cord substance P-like and beta-endorphin-like immunoreactivities in the rat

Effects of acute exposure (2 hr) to either 1.5% halothane or 0.5% methoxyflurane were investigated in the Sprague Dawley rat. Pituitary (PIT) and central nervous system (CNS) substance P (SP)-like and beta-endorphin (beta-end)-like immunoreactivities were evaluated immediately after anesthetic exposure (2 h), after righting reflex (4 h) or 24 hr postexposure (24 h). Only halothane significantly reduced SP-like immunoreactivity in olfactory bulbs in both the 2-h and 4-h groups. Halothane elevated SP-like immunoreactivity of hippocampus at all three time periods, and in the hypothalamus at 2 h. Both anesthetics significantly depleted thalamic concentrations of SP-like immunoreactivity. Methoxyflurane anesthesia resulted in a drastic decrease in SP-like immunoreactivity in PIT at all three time periods periods, while halothane elevated PIT concentrations of this peptide at 4 h. Both anesthetics significantly decreased beta-end-like immunoreactivity in the olfactory bulbs and thalami at 2, 4, and 24 h. However, halothane alone significantly elevated beta-end-like immunoreactivity in the spinal cord at 24 h. Halothane significantly elevated PIT beta-end-like immunoreactivity at 2 and 24 h, while methoxyflurane significantly lowered it in the 4-h group, but elevated the levels of the same in the 24-h group. Brain stem beta-end immunoreactivity were significantly reduced at 2 h by both anesthetics, and at 4 h by methoxyflurane. Results indicate that halothane and methoxyflurane may differ significantly in their actions on SP and beta-end secreting neurons in the CNS.