Howard B. Gutstein

Oral Ketamine Preanesthetic Medication in Children

The authors sought to define a dose of oral ketamine that would facilitate induction of anesthesia without causing significant side effects. Forty-five children (ASA Physical Status 1 and 2; aged 1-7 yr) were assigned randomly in a prospective, double-blind fashion to three separate groups that received either 3 mg/kg, 6 mg/kg, or no ketamine mixed in 0.2 ml/kg cola-flavored soft drink. They also were evaluated preoperatively and postoperatively for acceptance of oral ketamine as a premedicant, reaction to separation from parents, emotional state, and emergence phenomena. The authors detected no episodes of respiratory depression, tachycardia, or arterial hemoglobin desaturation before, during, or after surgery. The 6 mg/kg dose was well accepted; provided uniform, predictable sedation within 20-25 min; and allowed calm separation from parents and good induction conditions. The 3 mg/kg dose did not always cause sedation and calm separation from parents. Neither dose of ketamine increased the incidence of laryngospasm, prolonged recovery times, or caused emergence phenomena. The authors conclude that an oral dose of 6 mg/kg ketamine is easily administered and well accepted in young children and provides predictable, satisfactory premedication without significant side effects.

Endogenous opioids: overview and current issues

It is now well established that opiate drugs, such as heroin and morphine exert their primary effects by mimicking naturally occurring substances, termed the opioid peptides or endorphins. Over the last 25 years, we have learned a great deal about the basic biology of the endogenous opioid system, both in terms of receptors and endogenous ligands. We have come to appreciate the molecular and biochemical complexity of this system, its widespread anatomy, and its diverse functions. These functions subsume a ‘housekeeping role’ that include a modulatory role of gastrointestinal, endocrine and autonomic functions; a sensory role, particularly prominent in inhibiting responses to noxious stimuli; an emotional role, evident in the powerful rewarding and addicting properties of endogenous and exogenous opioids; and a cognitive role, manifest in opioid modulation of learning and memory. Although we have learned a great deal about the pre-and post-synaptic elements, tissue-specific expression and molecular functions of this system, it is unclear if we have discovered all the relevant players or if more endogenous ligands and receptors will be identified. The issue of the number of preand postsynaptic elements and their relationship relates to a feature of this system that seems paradoxical and must be resolved if we are to understand fully its functioning and its relevance to drug abuse. Let us call this the ‘apparent paradox’ and return to it later in this chapter. We need a solid understanding of the endogenous

MK-801 inhibits the development of morphine tolerance at spinal sites

The N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 has been shown to attenuate tolerance development in rats. In this study, we show that MK-801 inhibits tolerance to the antinociceptive effects of morphine, as assessed by the tail-flick test, in spinalized rats. These results suggest that NMDA receptor antagonists inhibit opiate tolerance at spinal sites, and also provide strong evidence that the effects of MK-801 are not due to its ability to interfere with associative learning, but instead to inhibition of non-associative mechanisms of opiate tolerance.

Opioid effects on mitogen-activated protein kinase signaling cascades

Background: The molecular mechanisms underlying both beneficial and undesirable opioid actions are poorly understood. Recently, the three currently known mammalian mitogen‐activated protein kinase (MAPK) signaling cascades (extracellular signal‐related kinase [ERK], stress‐activated protein kinase, and p38 kinase) were shown to play important roles in transducing receptor‐mediated signaling processes. Methods: To determine whether any of these kinase cascades were activated by opioids, mu, delta, or kappa opioid receptors were transiently introduced into COS‐7 cells together with MAPKs tagged to allow recognition by specific antibodies, and then exposed to opioids. Mitogen‐activated protein kinase activation was determined by an in vitro MAPK activation assay. In addition, C6 glioma cells with either mu, delta, or kappa receptors stably introduced were exposed to opioids and MAPK activation determined by in vitro activation assay or antibody detection of activated forms. Results: Transient experiments in COS cells revealed potent stimulation of ERK by mu and delta receptor activation, weak stimulation of stress‐activated protein kinase by all receptor types, and no activation of p38. In stably transfected C6 glioma cells, only ERK activation was observed. Extracellular signal‐related kinase induction was rapid, peaking 5 min after stimulation, and its activation was receptor‐type specific. Mu and delta receptor stimulation activated ERK, but kappa stimulation did not. Conclusions: These results show that acute opioid signaling is not only inhibitory, but can strongly activate an important signaling cascade. Extracellular signal‐related kinase activation may contribute to desirable responses to opioids, such as analgesia and sedation, and also to undesirable adaptive responses, such as tolerance, physical dependence, and possibly addiction. Further study of this system could provide greater insight into the molecular mechanisms underlying these clinical problems.

Mu and kappa opioid receptors in periaqueductal gray and rostral ventromedial medulla

THE periaqueductal gray (PAG) and rostral ventrome-dial medulla (RVM) are important brain stem pain modulating regions. Recent evidence suggests that κ opioids antagonize the effects of μ opioids in the RVM. However, the anatomical relationship between μ and κ opioid receptors in PAG and RVM is not well characterized. This study examined relationships between μ and κ opioid receptor immunoreactivity (IR) and mRNA in PAG and RVM. Brain slices were processed for either immunocytochemistry or in situ hybridization. We found considerable anatomical overlap of μ and κ opioid IR and mRNA in the RVM and PAG. These results provide an anatomical basis for recent behavioral and electrophysiological findings in RVM, and suggest modulatory interactions between μ and κ opioids in PAG.

Does chronic nociceptive stimulation alter the development of morphine tolerance

Conflicting results exist concerning the issues of whether chronic nociceptive stimulation (a) increases or decreases the effectiveness of morphine analgesia, and (b) facilitates or inhibits the development of narcotic tolerance. We carried out a series of experiments with appropriate controls in order to examine these two issues and their possible relationship. In experiment 1, rats received complete Freunds adjuvant (CFA), a chronic nociceptor, injected into a single hind paw or anesthesia without injection, together with morphine or placebo pellets in a 2 x 2 study design. The data indicate that the presence of the chronic nociceptive stimulus significantly facilitated the development of tolerance to morphine analgesia as measured using tail-flick latency (TFL) testing. Experiment 2 was designed to compare the analgetic effectiveness of an acute injection of morphine in rats experiencing chronic nociceptive stimulation and in controls. CFA was injected in the right hindpaw, and nine days later TFLs were tested after morphine doses of 1 and 2 mg/kg s.c. The data obtained showed that chronic nociceptive stimulation significantly reduced the effectiveness of morphine at the 1 mg/kg dose. However, baseline TFLs appeared to be shorter in rats treated with CFA, suggesting that the decrease in morphine effectiveness could be due to a general increase in pain sensitivity. Therefore, a third experiment was performed, using a less intense thermal stimulus to prolong baseline TFLs and accentuate any potential differences. Sixteen rats either received CFA or served as controls. TFLs were then measured at baseline and one hour after a 0.5 mg/kg dose of morphine.(ABSTRACT TRUNCATED AT 250 WORDS)

Pre- and Posttranslational Regulation of β-Endorphin Biosynthesis in the CNS: Effects of Chronic Naltrexone Treatment

Abstract: There appear to be two anatomically distinct β‐endorphin (βE) pathways in the brain, the major one originating in the arcuate nucleus of the hypothalamus and a smaller one in the area of the nucleus tractus solitarius (NTS) of the caudal medulla. Previous studies have shown that these two proopiomelanocortin (POMC) systems may be differentially regulated by chronic morphine treatment, with arcuate cells down‐regulated and NTS cells unaffected. In the present experiments, we examined the effects of chronic opiate antagonist treatment on βE biosynthesis across different CNS regions to assess whether the arcuate POMC system would be regulated in the opposite direction to that seen after opiate agonist treatment and to determine whether different βE‐containing areas might be differentially regulated. Male adult rats were administered naltrexone (NTX) by various routes for 8 days (subcutaneous pellets, osmotic minipumps, or repeated intraperitoneal injections). Brain and spinal cord regions were assayed for total βE‐ir, different molecular weight immunoreactive β‐endorphin (βE‐ir) peptides, and POMC mRNA. Chronic NTX treatment, regardless of the route of administration, reduced total βE‐ir concentrations by 30–40% in diencephalic areas (the arcuate nucleus, the remaining hypothalamus, and the thalamus) and the midbrain, but had no effect on βE‐ir in the NTS or any region of the spinal cord. At the same time, NTX pelleting increased POMC mRNA levels in the arcuate to ∼ 140% of control values. These data suggest that arcuate POMC neurons are up‐regulated after chronic NTX treatment (whereas NTS and spinal cord systems remain unaffected) and that they appear to be under tonic inhibition by endogenous opioids. Chromatographic analyses demonstrated that, after chronic NTX pelleting, the ratio of full length βE1–31 to more processed βE‐ir peptides (i.e., βE1–27 and βE1–26) tended to increase in a dose‐dependent manner in diencephalic areas. Because βE1–31 is the only POMC product that possesses opioid agonist properties, and βE1–27 has been posited to function as an endogenous anatgonist of βE1–31, the NTX‐induced changes in the relative concentrations of βE1–31 and βE1–27/βE1–26 may represent a novel regulatory mechanism of POMC cells to alter the opioid signal in the synapse.

β-Endorphin processing and cellular origins in rat spinal cord

&NA; While enkephalin and dynorphin peptides have been well characterized in the spinal cord, the cellular localization of &bgr;‐endorphin (&bgr;E) and the processing of pro‐opiomelanocortin (POMC) to &bgr;E and other non‐opioid peptides in the cord have not been extensively investigated. Other investigators have characterized the various &bgr;E forms present in rat spinal cord regions. Previous studies have also suggested that spinal POMC content is entirely derived from supraspinal sources. However, high proportions of &bgr;E precursors present in spinal cord sieving profiles led us to suspect the presence of POMC cell bodies intrinsic to the cord. In this study, we performed thoracic spinal cord lesions on a group of animals and demonstrated the persistence of about one‐third of control levels of &bgr;E immunoreactivity (&bgr;E‐IR) below the level of the lesions. We also characterized POMC processing in various regions of the spinal cord both before and after lesioning. These data suggested that there may be intrinsic POMC/endorphinergic neuronal systems in the spinal cord.

Effects of Chronic Morphine Treatment on β‐Endorphin‐Related Peptides in the Caudal Medulla and Spinal Cord

Abstract: The effects of chronic morphine treatment on β‐endorphin (βE)‐immunoreactive (βE‐ir) peptide levels were determined in the rat caudal medulla and different areas of the spinal cord. Seven days of morphine pelleting had no effect on total βE‐ir peptides in the caudal medulla. In contrast, it significantly increased βE‐ir peptide concentrations in the cervical and thoracic regions of the spinal cord compared with placebo‐pelleted controls, whereas in the lumbosacral region this trend did not reach statistical significance. Injections of the opiate receptor antagonist naloxone 1 h before the rats were killed had no effect on the morphine‐induced increases in the cord. Chromatographic analyses revealed that enzymatic processing of βE‐related peptides in the spinal cord seemed unaffected by the morphine and/or naloxone treatments. In light of previous data showing that morphine down‐regulates βE biosynthesis in the hypothalamus, the present results suggest that the regulation of βE‐ir peptides in the spinal cord is distinct from that found in other CNS areas. These data provide support for previous results suggesting that βE‐expressing neurons may be intrinsic to the spinal cord.