Age and body temperature alter inhalational anesthetic requirement; however, no human genotype is associated with inhalational anesthetic requirement. There is an anecdotal impression that anesthetic requirement is increased in redheads. Furthermore, red hair results from distinct mutations of the melanocortin-1 receptor. Therefore, the authors tested the hypothesis that the requirement for the volatile anesthetic desflurane is greater in natural redheaded than in dark-haired women.
The authors studied healthy women with bright red (n = 10) or dark (n = 10) hair. Blood was sampled for subsequent analyses of melanocortin-1 receptor alleles. Anesthesia was induced with sevoflurane and maintained with desflurane randomly set at an end-tidal concentration between 5.5 and 7.5%. After an equilibration period, a noxious electrical stimulation (100 Hz, 70 mA) was transmitted through bilateral intradermal needles. If the volunteer moved in response to stimulation, desflurane was increased by 0.5%; otherwise, it was decreased by 0.5%. This was continued until volunteers "crossed over" from movement to nonmovement (or vice versa) four times. Individual logistic regression curves were used to determine desflurane requirement (P50). Desflurane requirements in the two groups were compared using Mann-Whitney nonparametric two-sample test; P < 0.05 was considered statistically significant.
The desflurane requirement in redheads (6.2 vol% [95% CI, 5.9-6.5]) was significantly greater than in dark-haired women (5.2 vol% [4.9-5.5]; P = 0.0004). Nine of 10 redheads were either homozygous or compound heterozygotes for mutations on the melanocortin-1 receptor gene.
Red hair seems to be a distinct phenotype linked to anesthetic requirement in humans that can also be traced to a specific genotype.
INHALATIONAL anesthetic requirements are remarkably uniform in humans, mainly being affected by age and body temperature.1,2However, some anesthesiologists share an anecdotal impression that patients with natural red hair require more anesthesia than patients with other hair colors. The phenotype of nearly all red-haired individuals can be traced to distinct mutations of the melanocortin-1 receptor gene (MC1R ).3–5
The human MC1R is expressed on the surface of melanocytes and is a key regulator of intracellular signaling to the melanin biosynthetic pathway governing pigment formation. The red hair phenotype results from excess pheomelanin production. Production of this yellow-red pigment results from well-described mutations of the MC1R .3–6In contrast, when a normal (consensus) MC1R is expressed, the predominant pigment produced by melanocytes is eumelanin (dark brown) and the typical eumelanin-to-pheomelanin ratio is high.
An easily identifiable human phenotype that can be traced to a distinct genotype presents an opportunity to identify a genetic influence on anesthetic sensitivity in humans. Distinct genetic factors have been shown to contribute to anesthetic requirements in various animal species, including mice,7nematodes (Caenorhabditis elegans ),8and fruit flies (Drosophila melanogaster ).9However, a similar association has yet to be established in humans. Therefore, we tested the hypothesis that women with natural red hair have a greater desflurane requirement than women with dark hair.
Materials and Methods
With approval of the University of Louisville Human Studies Committee and written informed consent, we recruited 20 white women aged between 18 and 40 yr, with natural bright red or dark (black or dark brown) hair. The study subjects were regarded as white if they were mainly of northern European descent as indicated by self-report. The subjects were drawn from Greater Louisville, Kentucky, an urban area with a population exceeding 1,000,000. The number of subjects was based on an a priori estimate that 10 subjects in each group would provide 90% power for detecting a 0.8% difference in desflurane requirement (e.g. , 5.8% to 5.0%) between the two groups using a two-tailed, unpaired t test with an α of 0.05 and an estimate of the SD of 0.55.
Because it remains unclear whether sex could cause significant differences in anesthetic requirement,10,11only women were included. Exclusion criteria included chemical hair treatment, any history of medical or psychiatric problems, any history of chronic pain problems, possible pregnancy, body mass index greater than 30 kg/m2, recreational drug usage, and medication usage other than oral contraceptives.
Studies were started in the morning each day because circadian rhythms slightly influence anesthetic requirement.12The effect of the menstrual cycle on anesthetic requirement is unclear; however, pain threshold varies as a function of the menstrual cycle.13Therefore, studies were restricted to the first 10 days of the participants’ menstrual cycles unless they took oral contraceptives.
The subjects fasted and refrained from smoking for at least 8 h before arriving at the laboratory. No premedication was given or allowed. Routine anesthetic safety monitors were applied in an operating room setting. Core body temperature was measured from the tympanic membrane using Mon-a-Therm® thermocouples (Tyco-Mallinckrodt, Inc., St. Louis, MO). General anesthesia was induced with sevoflurane in 100% oxygen; 5 min later, a laryngeal mask airway (Laryngeal Airway Mask Co., Henley-on-Thames, United Kingdom) was inserted, and sevoflurane was discontinued. Anesthesia was then subsequently maintained solely with desflurane and ventilation was assisted until spontaneous breathing was reestablished. A forced-air warming cover positioned over the trunk (Bair Hugger; Augustine Medical, Inc., Eden Prairie, MN) was used to maintain normothermia.
There was a 45-min equilibration period after induction of anesthesia. Based on pharmacokinetic data,14this allowed sufficient time for sevoflurane concentrations to decrease to insignificant levels. It also gave time for the desflurane concentrations to equilibrate, such that the ratio of inspired (FI) to end-tidal concentrations (FA) closely approached 1.0.
After the equilibration period, a noxious electrical stimulation (100 Hz, 60–70 mA) was applied bilaterally for 10 s through needles inserted intradermally into the anterior thighs. A tetanic stimulus even 20% of this intensity is unbearable to unanesthetized subjects but is not consciously sensed during anesthesia.
An independent investigator (one out of a group of four investigators was selected for each study session) was brought into the study room just before each stimulation period to evaluate movement in response to the noxious electrical stimulation and left within a few minutes after each stimulus. A positive response was defined as gross purposeful movement of the legs or arms within the first minute after stimulation. Grimacing and head movement were not considered purposeful responses.
The independent investigator was not blinded to the hair color and skin complexion of each volunteer because of the practical and technical difficulties involved with the latter, but the independent investigator was blinded to the desflurane concentration on the vaporizer dial and those from the gas analyzer readings. In addition, the initial concentration of desflurane was randomly set between 4.5 and 7.5 end-tidal vol% to prevent the blinded investigator from guessing the actual desflurane concentration based on a uniform starting concentration.
The desflurane concentration was increased by 0.5 end-tidal vol% (e.g. , from 5.5 to 6%) when the volunteer moved in response to electrical stimulation or decreased by 0.5% when the volunteer did not move. The new end-tidal concentration was maintained for at least 15 min to allow full equilibration between alveolar and brain concentrations before repeating electrical stimulation. To prevent desensitization at the needle insertion sites, the stimulating needles were moved cranially by 1 cm after each stimulation period. The up-and-down sequence was continued until participants “crossed over” from movement to nonmovement (or vice versa ) four times. This procedure, known as the Dixon up-and-down method, is a standard technique for evaluating anesthetic potency.15
Anesthetic concentration was continuously determined from end-expired gas with a monitor accurate to within 0.1% (Datex-Engstrom, Helsinki, Finland). At equilibrium, end-expired desflurane values are essentially equal to brain concentration.16
For verification of hair color, hair samples were obtained from the nape of the volunteers’ necks, and the ratio of eumelanin to total melanin in the hair was determined spectrophotometrically as described by Ozeki et al. 17The total amount of eumelanin and pheomelanin was determined by the absorbance at 500 nm (A500), and the amount of eumelanin alone was determined by the absorbance at 650 nm (A650). Red hair has an A650:A500ratio near 0.13, whereas black hair has an A650:A500ratio near 0.30; the ratios for blond and brown hair are in between. Blood was sampled for subsequent analyses of MC1R alleles (see Appendix: Details of Genetic Analysis).
Demographic, morphometric, and spectrophotometric hair analysis data for the volunteers with red or dark hair were compared using unpaired, two-tailed t tests. In each individual, anesthetic requirement was determined by correlation of responses to noxious electrical stimulation (movement or no movement) with end-expired desflurane concentration using logistic regression. The desflurane requirement for each group was defined as the average of the individual concentrations. Desflurane requirements in the two groups were compared using the Mann–Whitney nonparametric two-sample test. Results are presented as means (95% confidence intervals); P < 0.05 was considered statistically significant.
Demographic and morphometric data were similar in the groups (table 1). Average core temperatures were similar for both groups (table 1). Hair analysis confirmed that volunteers had typical ratios of eumelanin to total melanin for red or dark hair color (table 1).
The volunteers with red hair required significantly more desflurane (mean, 6.2 [95% CI, 5.9–6.5]) than those with dark hair (mean, 5.2 [95% CI, 4.9–5.5]; P = 0.0004) (fig. 1). This represents an increase of 19% in the desflurane partial pressure.
The single nucleotide polymorphism analysis revealed that 9 of the 10 redheads had the following variant MC1R alleles (compared to consensus protein): R151C/Y152X, R151C/D294H, R151C/R160W (2 subjects), D294H/D294H, Y152×/R160W, R151C/29insA, R151C/V60L, R160W/D84E. One remaining redhead volunteer carried the R151C mutation on a single allele. In the dark-haired volunteers, 5 of 10 carried a single mutant allele (D294H, R160W, R151C [3 subjects]), and the remaining 5 showed consensus MC1R alleles at the tested locations.
Anesthetic requirement in redheads was increased 19%, a difference that was highly statistically significant (P = 0.0004). The results confirm anecdotal clinical impressions that anesthetic requirement is greater in redheads.
Inhalational anesthetic requirement is typically quantified in terms of the minimum alveolar requirement (MAC), the anesthetic concentration that prevents movement in response to skin incision in half of the population.18The reported desflurane MAC in the literature for this age group is 7.25%.19We used an analog of MAC in this study by applying repeated noxious electrical stimulation.20The mean values for desflurane requirement in both our study groups (6.2% for red hair and 5.2% for dark hair) are lower than the reported MAC but are expected with this model because anesthetic requirement depends on the type and intensity of the applied stimulation.21Skin incision is a supramaximal stimulus that fully activates pain receptors and pathways. Electrical stimulation is not, and it therefore provides graded activation of pain pathways. Movement in response to electrical stimulation can thus be blocked by partial pressures lower than those required to prevent movement in response to skin incision.20
Chemical hair analysis also indicated that the volunteers had in fact been correctly assigned to each hair color group. Results of the DNA analysis in our volunteers were consistent with previously reports.3–6Three particular mutations of the MC1R alleles (R151C, R160W, and D294H) are present in the majority of redheads, with at least one of these three alleles found in 93% of those with red hair.22These variant MC1R alleles behave as recessive mutations.6Although many other discovered allele variants do not affect function, it has been shown that MC1R variants V60L, R142H, R151C, R160W, and D294H lead to melanocortin-1 receptors that are unable to stimulate intracellular cyclic adenosine monophosphate production as efficiently as the wild-type receptor when activated.23Presumably, early single nucleotide insertions (e.g. , ins29) also result in loss of function because the frame shift leads to many other different amino acid substitutions. Our DNA analysis showed that all 10 red-haired volunteers in this study carried at least one dysfunctional or diminished function MC1R allele, and 8 carried two such alleles. The functional significance of mutation D84E, carried by one of the remaining two red haired subjects, remains unknown, but this mutation is also strongly associated with red hair, having an odds ratio of 63 for red hair relative to the consensus MC1R allele.22In the dark-haired group, there was no clear evidence for the effect of heterozygosity on anesthetic requirement.
In addition to its expression on melanocytes, the presence of MC1R also has been identified in human pituitary tissue and glial cells and in cells of the human periaqueductal gray matter.24,25The essential qualities produced by inhaled anesthetics, namely amnesia and immobility, are mediated through actions on the central nervous system. However, the central nervous system is not a major site of MC1R expression.26Furthermore, studies suggest that immobility may be mediated through the spinal cord rather than higher centers,27,28and severing of higher centers from the cord does not change MAC.28,29
A recent study by Mogil et al. 30suggests a possible role for the MC1R gene in female specific pain modulation. Women with two variant MC1R alleles displayed significantly greater analgesia in response to the κ opioid pentazocine compared to those with one or zero variant MC1R variant alleles. Whether this finding would translate into a greater underlying sensitivity to the dynorphin, the endogenous κ receptor ligand, remains unclear. Interestingly, dynorphin peptides can also bind to melanocortin receptors (including MC1R ) and act as antagonists.31However, to the extent that the results of Mogil et al. suggest an increased underlying sensitivity to certain endogenous opioids in subjects with red hair, we might expect reduced anesthetic requirement in red heads if such a system was tonically active; this would be the opposite of what we observed in this study.
Modulation of analgesic mechanism could possibly also occur from interaction between the various melanocortin receptors. MC1R is part of a family of melanocortin receptors (MC1R , MC2R , MC3R , MC4R , and MC5R )32,33that are all stimulated by the same ligands (melanocortins α-melanocyte-stimulating hormone [MSH], β-MSH, γ-MSH, and adrenocorticotropic hormone). The receptor subtypes have different physiologic functions and tissue distributions.26In fact, MC3R and MC4R are much more abundant in the central nervous system than MC1R but have similar affinities for α-MSH and adrenocorticotropic hormone.26A functional antagonism between the opioid and melanocortin systems has been suggested because the receptors are colocalized throughout the central nervous system, including the locus ceruleus, where their regulatory activities oppose each other.34Furthermore, acute intrathecal administration of the MC4R antagonist SHU9119 reduces cold and mechanical allodynia in a rat neuropathic pain model35and can be antagonized by low doses of naloxone.36
The mechanisms controlling production of α-MSH remain unclear, but most pituitary functions are controlled by negative feedback systems that increase hormone release with end-organ failure. The observation that α-MSH injection into the paraventricular hypothalamic nucleus decreases POMC gene expression is consistent with this hypothesis.37It is thus not unreasonable to postulate that MCR1 dysfunction could similarly activate a feedback system that increases central α-MSH concentration.
Sex is an additional phenotype that may be associated with anesthetic requirement. For example, Goto et al. 10report that xenon MAC for elderly Japanese women was 26% less than xenon MAC for elderly Japanese men. In contrast, the results from another study suggested that women require more desflurane using a similar model of electrical stimulation as this study.38Most recently, however, prospective11and retrospective39studies failed to identify any sex difference in MAC. Therefore, sex does not seem to have a consistent or clinically important effect on anesthetic requirement.
In summary, our results confirm anecdotal clinical impressions that anesthetic requirement is greater in redheads. The observed 19% difference between the two groups makes red hair a distinct phenotype that correlates with inhalational anesthetic requirement in humans and can be traced to a specific genotype.
The authors thank Jonathan L. Sessler, Ph.D. (Professor, University of Texas, Austin, Texas), Ozan Akça, M.D. (Assistant Professor, University of Louisville, Louisville, Kentucky), and Gilbert Haugh, M.S. (Statistician), Jane Williams (Research Technologist), and Nancy Alsip, Ph.D. (Medical Editor; all from the University of Louisville), for their assistance.
Appendix: Details of Genetic Analysis
DNA was extracted from either clotted or unclotted blood using PureGene DNA isolation kits (Gentra Systems, Inc., Minneapolis, MN). A 1,238-bp fragment encompassing the coding region of MCR1 was amplified by polymerase chain reaction (PCR) using primers described by Miller et al. 40The PCR mix contained the following in 20 μl: 50–100 ng DNA, 0.3 μm of each primer (1F: 5′-AGATGGAAGGAGGCAGGCAT-3′ and 1R: 5′-CCGCGCTTCAACACTTCAGAGATCA-3′), 0.2 μm of each dNTP, 1.5 μm MgSO4, 2.5% DMSO, 1× KOD PCR buffer, and 0.4 U KOD Hot Start Taq polymerase (Novagen Inc., Madison, WI). The DNA was amplified for 36 cycles (0.5 min at 94°C, 0.5 min at 67°C, 1.5 min at 72°C) after activating the polymerase by incubation for 2 min at 94°C. Amplified PCR products were analyzed by agarose gel electrophoresis. For single nucleotide polymorphism (SNP) analyses, the full-length PCR products were used as templates after a 1,000-fold dilution. Fragments containing the region of variants of interest (R151C, D294H, R142H, R160W, and Y152×) were amplified using the primer pairs and conditions described above, except the concentration of dNTPs was decreased 10-fold. SNPs were detected using the CEQ8000 SNP detection kit (Beckman-Coulter Inc., Fullerton, CA). Amplification and SNP interrogation primers were D294H (amplification: 5′-TCATCGTCCTCTGCCCCGAG-3′, 5′-ACACTTAAAGCGCGTGCACCGC-3′; SNP detection: 5′-TTTCTCGCCCTCATCATCTGCAATGCCATCATC-3′), R142H, R151C, R160W, and Y152× (amplification: 5′-GCAGCAGCTGGACAATGTCATT-3′, 5′-TGGTCGTAGTAGGCGATGAA-3′; SNP interrogation: R142H, 5′-TCTGCTTCCTGGGCGCCAT-CGCCGTGGACC–3′; R151C, 5′-TCGCCGTGGACCGCTACATCTCCATCTTCTACGCACTG-3′; R160W, 5′-AGATGGCCGCAACGGCTCGCC-GCGCCC-3′; Y152X, 5′-CTACATCTCCATCTTCTACGCACTGCGCTA-3′).