Leonard L. Firestone

Pain processing during three levels of noxious stimulation produces differential patterns of central activity

&NA; Previous functional imaging studies have demonstrated a number of discrete brain structures that increase activity with noxious stimulation. Of the commonly identified central structures, only the anterior cingulate cortex shows a consistent response during the experience of pain. The insula and thalamus demonstrate reasonable consistency while all other regions, including the lentiform nucleus, somatosensory cortex and prefrontal cortex, are active in no more than half the current studies. The reason for such discrepancy is likely to be due in part to methodological variability and in part to individual variability. One aspect of the methodology which is likely to contribute is the stimulus intensity. Studies vary considerably regarding the intensity of the noxious and non‐noxious stimuli delivered. This is likely to produce varying activation of central structures coding for the intensity, affective and cognitive components of pain. Using twelve healthy volunteers and positron emission tomography (PET), the regional cerebral blood flow (rCBF) responses to four intensities of stimulation were recorded. The stimulation was delivered by a CO2 laser and was described subjectively as either warm (not painful), pain threshold (just painful), mildly painful or moderately painful. The following group subtractions were made to examine the changing cerebral responses as the stimulus intensity increased: (1) just painful − warm; (2) mild pain − warm; and (3) moderate pain − warm. In addition, rCBF changes were correlated with the subjective stimulus ratings. The results for comparison ‘1’ indicated activity in the contralateral prefrontal (area 10/46/44), bilateral inferior parietal (area 40) and ipsilateral premotor cortices (area 6), possibly reflecting initial orientation and plans for movement. The latter comparisons and correlation analysis indicated a wide range of active regions including bilateral prefrontal, inferior parietal and premotor cortices and thalamic responses, contralateral hippocampus, insula and primary somatosensory cortex and ipsilateral perigenual cingulate cortex (area 24) and medial frontal cortex (area 32). Decreased rCBF was observed in the amygdala region. These responses were interpreted with respect to their contribution to the multidimensional aspects of pain including fear avoidance, affect, sensation and motivation or motor initiation. It is suggested that future studies examine the precise roles of each particular region during the central processing of pain.

Regional brain activity changes associated with fentanyl analgesia elucidated by positron emission tomography

Recent positron emission tomography (PET) studies have demonstrated areas of pain processing in the human brain.Given the inhibitory effects of opioids on neuronal activity, we predicted that fentanyls analgesic effects would be associated with suppression of pain-evoked responses in these distinct brain areas. To test this, PET was used to measure cerebral blood flow responses, as reflections of regional neuronal activity, to painful and nonpainful thermal stimuli both in the absence and presence of fentanyl in humans. During each PET scan in nine healthy volunteers a tonic heat source was placed against the subjects left forearm, delivering a preset temperature of either 40 degrees C (nonpainful) or 47-48 degrees C (painful). Subjects underwent eight blood flow studies, each consisting of 50 mCi [(15) O]water injection and a PET scan. The first four studies were performed during placebo administration in the stimulus sequence: nonpainful, painful, painful, nonpainful. This sequence was then repeated during intravenous (IV) administration of fentanyl 1.5 mg/kg. Significant differences in regional cerebral blood flow (rCBF) between the placebo and the fentanyl conditions during nonpainful and painful stimuli were identified using statistical parametric mapping. It was found that pain increased rCBF in the anterior cingulate, ipsilateral thalamus, prefrontal cortex, and contralateral supplementary motor area. Fentanyl increased rCBF in the anterior cingulate and contralateral motor cortices, and decreased rCBF in the thalamus (bilaterally) and posterior cingulate during both stimuli. During combined pain stimulation and fentanyl administration, fentanyl significantly augmented pain-related rCBF increases in the supplementary motor area and prefrontal cortex. This activation pattern was associated with decreased pain perception, as measured on a visual analog scale. In contrast to our hypothesis, these data indicate that fentanyl analgesia involves augmentation of painevoked cerebral responses in certain areas, as well as both activation and inhibition in other brain regions unresponsive to pain stimulation alone. (Anesth Analg 1997;84:120-6)

Anesthesia Sensitivity in Mice that Lack the β3 Subunit of the γ-Aminobutyric Acid Type A Receptor

BackgroundThe mammalian gamma-aminobutyric acid type A (GABAA) receptor, a likely target of anesthetic action, exhibits remarkable subunit heterogeneity. In vitro expression studies suggest that there is subunit specificity to anesthetic responses at the GABAA receptor. The authors tested whether ge

Anaesthetic potencies of primary alkanols: implications for the molecular dimensions of the anaesthetic site.

1 We have redetermined the anaesthetic potencies (EC50s) for a series of primary alkanols, to resolve uncertainties about the molecular dimensions of the anaesthetic site resulting from the use of data from different laboratories. 2 For each alkanol, concentration‐response relationships for loss of righting reflex (LRR) were plotted for over one hundred tadpoles, and the median effective concentrations determined. Aqueous concentrations present during potency assays were determined independently, and for alkanols with chain length greater than nonanol, correction was made for depletion from the aqueous phase. 3 The EC50 s were found to decrease logarithmically with increasing number of carbon atoms in the hydrocarbon chain of the alkanol (CN), such that, on average, each additional methylene group was associated with an approximately four fold increase in potency. 4 The relationship between log EC50 and CN was best described by the quadratic equation, log EC50 = 0.022 (±0.0038) CN2 + 0.76 (±0.051) CN + 3.7 (±0.14) (r2 = 0.9951). 5 A previously described correlation between the apparent changes in the free energy of binding of an additional methylene group both to luciferase and to the sites for LRR in tadpoles was not confirmed. 6 A cut‐off in potency beyond dodecanol was established in experiments where tadpoles were maintained in supersaturated solutions of tridecanol for 20 h without demonstrable LRR. 7 These findings indicate that the soluble enzyme firefly luciferase does not adequately model the anaesthetic site. Specifically, there are discrepancies in the position of cut‐off, and the apparent changes in the free energy of binding, per methylene group, of an alkanol to luciferase do not parallel that for tadpoles.

Pharmacologic and behavioral responses of inbred C57BL/6J and strain 129/SvJ mouse lines.

Gene-targeting technology is creating an explosion in the number of animals available with single gene mutations that affect the function of the central nervous system. Most gene-targeted mice are produced on a mixed genetic background of C57BL/6J and substrains of Strain 129. Understanding the behavioral characteristics and responses to various drugs of these parental strains is vital to interpreting data from gene-targeted mice. We directly compared C57BL/6J and Strain 129/SvJ mouse lines on several behavioral paradigms and in response to several hypnotic and anesthetic drugs. Compared to Strain 129/SvJ mice, C57BL/6J animals are more sensitive to the hypnotic effects of midazolam, zolpidem, and propofol, less sensitive to etomidate and ethanol, and do not differ in sensitivity to Ro15-4513 or pentobarbital. These strains do not differ in their sensitivity to the motor ataxic effects of the volatile anesthetics enflurane or halothane. However, Strain 129/SvJs are more sensitive to the immobilizing effects of halothane but not enflurane. Motor coordination differs initially, but with repeated testing strain differences are no longer apparent. Strain 129/SvJ mice are more anxious on the elevated plus maze and open-field activity assays. Thus, these mouse strains harbor polymorphisms that influence some, but not all, traits of interest to behavioral neuroscientists.

Safety and efficacy of intranasal ketamine for the treatment of breakthrough pain in patients with chronic pain: A randomized, double-blind, placebo-controlled, crossover study

&NA; Few placebo‐controlled trials have investigated the treatment of breakthrough pain (BTP) in patients with chronic pain. We evaluated the efficacy and safety of intranasal ketamine for BTP in a randomized, double‐blind, placebo‐controlled, crossover trial. Twenty patients with chronic pain and at least two spontaneous BTP episodes daily self‐administered up to five doses of intranasal ketamine or placebo at the onset of a spontaneous BTP episode (pain intensity ≥5 on a 0–10 scale). Two BTP episodes at least 48 h apart were treated with either ketamine or placebo. Patients reported significantly lower BTP intensity following intranasal ketamine than after placebo (P<0.0001), with pain relief within 10 min of dosing and lasting for up to 60 min. No patient in the ketamine group required his/her usual rescue medication to treat the BTP episode, while seven out of 20 (35%) patients in placebo group did (P=0.0135). Intranasal ketamine was well tolerated with no serious adverse events. After ketamine administration, four patients reported a transient change in taste, one patient reported rhinorrhea, one patient reported nasal passage irritation, and two patients experienced transient elevation in blood pressure. A side effect questionnaire administered 60 min and 24 h after drug or placebo administration elicited no reports of auditory or visual hallucinations. These data suggest that intranasal administration of ketamine provides rapid, safe and effective relief for BTP.

Human brain activity response to fentanyl imaged by positron emission tomography.

Positron emission tomography (PET) is a noninvasive imaging technique that can be used to observe drug actions on human brain in vivo.We used15 O-water PET scanning in six volunteers to examine the effects on regional cerebral activity as reflected by regional cerebral blood flow (rCBF) of a small intravenous bolus of fentanyl. rCBF was compared between scans obtained after fentanyl or a placebo using three separate statistical criteria including a pixel-by-pixel t statistic; significance was stringently defined at P values < 0.01. Anatomic locations of regional cerebral activity changes were verified by aligning rCBF PET scans with cranial magnetic resonance images using mathematical coregistration. Fentanyl administration was associated with significant increases in rCBF consistent with regional neuronal activation in both cingulate and orbitofrontal and medial prefrontal cortices, as well as caudate nuclei. These areas are responsive to nociceptive stimuli and are involved in avoidance learning, reward and addiction, visceromotor control, maintenance of attention, and pain-related affective behavior. Significant decreases were noted in both frontal and temporal areas and the cerebellum, a distribution far less extensive than that of opiate receptors in general. These data indicate that fentanyls effects are highly localized and specifically affect cerebral regions associated with a range of pain-related behaviors. (Anesth Analg 1996;82:1247-51)

Normal electrophysiological and behavioral responses to ethanol in mice lacking the long splice variant of the γ2 subunit of the γ-aminobutyrate type A receptor

The γ subunit of the γ-aminobutyric acid type A receptor (GABAA-R) is essential for bestowing both normal single channel conductance and sensitivity to benzodiazepines on native GABAA-Rs. The long splice variant of the γ2 subunit (γ2L) has been postulated to be essential in mediating the modulatory actions of ethanol at the GABAA-R. In order to evaluate this hypothesis, gene targeting was used to delete the 24bp exon which distinguishes γ2L from the short splice variant (γ2S). Mice homozygous for this exon deletion (γ2L−/−) are viable and indistinguishable from wild-type (γ2L+/+) mice. No γ2L mRNA was detected in these mice, nor could γ2L-containing GABAA-R protein be detected by specific antibodies. Radioligand binding studies showed the total amount of γ2 subunit protein to be not significantly changed, suggesting that γ2S replaces γ2L in the brains of the knockout animals. Electrophysiological recordings from dorsal root ganglion neurons revealed a normal complement of functional receptors. There was no difference in the potentiation of GABA currents by ethanol (20–200 mM) observed in neurons from γ2L+/+or γ2L−/− mice. Several behavioral effects of ethanol, such as sleep time, anxiolysis, acute functional tolerance, chronic withdrawal hyperexcitability and hyperlocomotor activity were also unaffected by genotype. It is concluded that γ2L is not required for ethanol’s modulatory action at the GABAA-R or whole animal behavioral effects.

In vivo imaging of human limbic responses to nitrous oxide inhalation.

Human behavioral studies have shown that nitrous oxide, in subanesthetic concentrations, impairs psychomotor function, cognitive performance, and learning and memory processes.However, the cerebral mechanisms of such effects remain unknown. Positron emission tomography (PET) was used to map the brain areas associated with nitrous oxide effects. Regional cerebral blood flow (rCBF) was measured in eight volunteers, during room air (control) or 20% nitrous oxide (nitrous oxide) inhalation using15 O-water, to reflect regional neuronal activity. To control for the possibility that 20% nitrous oxide uncoupled cerebral blood flow and metabolism, in four of the subjects, regional cerebral metabolic rate (rCMR) was also measured using18 F-deoxyglucose during the two experimental conditions. Results of rCBF and rCMR scans were compared between conditions using the statistical parametric mapping method, and areas of nitrous oxide-related activation or deactivation were identified at a significance level of 0.005. Percent changes in rCBF scan pixels from these activated or deactivated areas were then compared with those of stereotactically corresponding rCMR scan pixels with t statistics (P < 0.05 was defined as a significant difference). It was found that cerebral blood flow and metabolism were not uncoupled by 20% nitrous oxide, since percent changes in rCBF and rCMR, detected during nitrous oxide inhalation, did not differ significantly from each other (P < 0.05). Nitrous oxide inhalation was associated with significant activation in the anterior cingulate cortex, a limbic area known to mediate psychomotor and cognitive processes. Deactivation was found in the posterior cingulate, hippocampus, parahippocampal gyrus, and visual association cortices in both hemispheres; the former two regions are known to mediate learning and memory. These areas identified by PET in vivo may provide the neuroanatomical basis for the behavioral responses associated with subanesthetic nitrous oxide inhalation. (Anesth Analg 1996;83:291-8)

In vivo imaging of nitrous oxide-induced changes in cerebral activation during noxious heat stimuli.

Background Although previous studies have provided some insight into the pharmacologic aspects of nitrous oxide analgesia, the neural circuits mediating its antinociceptive effect remain relatively unexplored. Positron emission tomography was used in nine volunteers to identify the loci of nitrous oxide‐modulated cerebral responses to a peripheral noxious stimulus. Methods Nitrous oxide‐pain interactions were studied by comparing regional cerebral blood flow responses to a 48 degrees Celsius tonic heat stimulus, applied to each volunteers left forearm, during room air inhalation with those obtained while 20% nitrous oxide was administered. Two cerebral blood flow scans were obtained with the15 O‐water technique during each condition. Locations of specific regional activation related to pain, and nitrous oxide, were identified using the statistical parametric mapping method, with a significance level of P < 0.01. Pain was rated by visual analog scale and the values were compared using Wilcoxon rank sum analysis. Results Pain produced cerebral activation in the contralateral thalamus, anterior cingulate, and supplementary motor area. Adding nitrous oxide during pain stimulation abolished activation in these areas but was associated with activation in the contralateral infralimbic and orbitofrontal cortices. In parallel, mean visual analog scale scores decreased significantly from 67 +/‐ 4 (SEM) to 54 +/‐ 5 (P < 0.05). Conclusions Nitrous oxide, at 20% concentration, appears to modulate pain processing in the brains medial pain system, and also activates the infralimbic and orbitofrontal cortices. The potential contribution of the affected brain areas to nitrous oxide analgesia is discussed.