SCIENTIFIC ARTICLE Effects of nitrous oxide exposureon behavioral changesin mice Yiu K. Fung,PhD Michael R. Brown,DDS, MS Robert E. Sullivan, DDS,MSD Abstract This study was designedto examinethe effects of continuousexposure to low levels of nitrous oxide on several behavioral paradigmsand the occipital cortex cells of mice. Different groupsof mice wereexposed to air or two different levels of nitrous oxide (1000 ppmor 2000 ppm) 8 hr/day for eight consecutive days. The exposure to nitrous oxide was achieved by placing animalsin a specially designed,enclosed chamber. At the end of the exposureperiod, all mice weretested for motorcoordination, locomotoractivity, stereotypic behavior and anxiety level. Cellular examinationof the occipital cortex was conductedby counting both the larger neural cells and the smaller neuroglial cells in a specific region. Ourresults indicated that animals showedno deficit in motor coordination or anxiety level. Histological examinationindicated no significant difference in the numberof neural cells, neuroglial cells, or total cells countedin the control tissue, as comparedto the neuraltissue from mice exposedto nitrous oxide. Nitrous oxide-exposedmice showedreduced locomotor activity comparedto control animals; however, with the exception of one time period, this decreasewas not statistically significant. Animalsexposed to nitrous oxide showed a dose-dependentreduction in stereotypic behavior. Ourresults suggest that short-term exposureto trace levels of nitrous oxide might alter central dopaminergicneuronal activities in striatal and mesolimbicregions. Furtherresearch in this area is needed to provide moreinformation regardingthe potential effects of repeated exposureto low levels of nitrous oxide. (Pediatr Dent 15:93-98, 1993) Introduction Nitrous oxide is used in dentistry for its analgesic and dental operatory to be as high as 90,000 ppm27Other anxiolytic properties. 1-3 During use, small quantities of investigators4 have found levels as high as 7000 ppm, nitrous oxide leak into the dentist’s breathing zone.4, 5, 6 though lower nitrous oxide levels, in the hundreds of This waste nitrous oxide is measuredin parts per million ppm, also have been encountered. Maximumnitrous ox- (ppm). Levels of nitrous oxide in the dentist’s breathing ide levels ranging from 132-815 ppm were measured in zone have been found to be quite variable. Studies have the breathing zone of dentists. 7,12,17,18 Onestudy used an reported levels of waste nitrous oxide from several hun- infrared spectrophotometer with a sensitivity of 1-250 dred to several thousand ppm.7-1° Ambientlevels of ni- ppm. This study found that the analyzer frequently mea- trous oxide in the dentist’s breathing zone have been greater sured levels at its 250 ppmlimit, especially during care of than 1,000 ppm, even in the presence of scavenging equip- uncooperative children whose activity increased nitrous ment.11,12 The United States National Institute of Safety oxide leakage into the environment. Precise levels were, and Health (NIOSH) recommends a maximum atmo- however, undetermineddue to the limits of the sensor.9 In spheric13 exposureto nitrous oxide concentration of 25 ppm, another study, nine dental operatories showed a mean 8 hr/day, 40 hr/week. nitrous oxide concentration of 4304.4 ppm at the head of Chronic exposureto nitrous oxide is a concern to dental the dental chair during conscious sedation.5 Anotherstudy personnel, in part, because the gas is administered so found high levels of nitrous oxide present in all dental frequently. A recent study indicated that 28.7%of pediat- offices (N = 23), regardless of size or the presence of scav- ric dentists use nitrous oxide-oxygenfor more than 75%of enging equipment. Nitrous oxide peaks were often very their patients; 58.9%use nitrous oxide-oxygenon selected high and beyond the calibration range of t.he infrared patients; and only 12.4% never use nitrous oxide-oxygen spectrophotometer (greater than 1000 ppm). for patients. Therefore, a total of 87.6%of pediatric den- Since nitrous oxide is used so frequently, manyden- tists employnitrous oxide-oxygenin their practices. This tists are exposed to the gas several hours a week and is a dramatic increase from only 35%using nitrous oxide possibly have been for several years. The systemic ef- oxygen in 1971 and 65%in 1980.14 fects of this repeated exposure to nitrous oxide remain The pediatric dentist is especially vulnerable to ambi- unknown. However, the systemic effects of abusing ent nitrous oxide because of the frequency with which the nitrous oxide are well documented, including ataxia, gas is used, and because an increased amount of ambient paresthesias, headaches, memorydeficit, altered mood, nitrous oxide accumulates as patient behavior deterio- impotence, and an increase in miscarriages among fe- rates.9,15,16 Dental personnel are exposedto concentrations male6,19- dental27 personnel and wives of male dentists. of nitrous oxide two or three times greater than operating An earlier study showed that exposure of rats to 40% roompersonnel, y,l° Previous studies reported levels in the nitrous oxide-oxygen induced cell damage to the oc- Pediatric Dentistry: March/April, 1993 -Volume15, Number2 93 cipital cortex. 28 The potential systemic and behavioral mixture to a fume hood. Ports were pvc pipes with holes effects of chronic exposure to low levels of nitrous oxide extending deep into the chamber to create a homogeneous (1000 ppm or 2000 ppm) remain unclear. mixture of gas and to prevent stratification of the chamber Locomotoractivity and the total numberof stereotypies, air during the 8-hr exposure period. The gas coming from which include sniffing, grooming, licking, and biting are the cylinder entered a regulator, was down-regulated to a natural animal behavior. It has been suggested that do- pressure of approximately 50 psi, and entered a flowmeter paminergic activity in the nucleus accumbensis associ- to allow for the accurate delivery of the required liters per ated with the mediation of locomotor activity, while that minute. in the striatum is involved with the initiation of stereo- The animals to be exposed to nitrous oxide were placed typed behavior. 2~-31 Abnormality in these brain regions in the chamber, which was then flushed with the nitrous have been implicated in the pathophysiology of psychomo- oxide mixture to allow the gas within the chamber to tor disorders such as Parkinson’s disease, Huntington’s rapidly approximate the experimental concentration of Chorea, and schizophrenia.32’ ~4 Other behavioral changes 1000 ppmfor the initial group and 2000 ppmfor the final such as motor coordination and anxiety changes are more group. The nitrous oxide / air from the premixedcylinders complex and may involve other neuronal systems. flowed into the chamberat a rate of 16 L/min for 30 min. A humanstudy to evaluate the behavioral effects of For the next 8 hr, the nitrous oxide / air was set at a steady repeated exposure to low levels of nitrous oxide would be flow of 1.5 L/min, for adequate ventilation to meet the difficult. This animal research project creates an animal animals’ respiratory demands. This routine of flushing model feasible to evaluate several behavioral changes due followed by the flow at a steady rate, was done for both of to short-term exposureto trace levels of nitrous oxide,35, 36 the experimental exposure periods. Control animals were including changes in motor coordination, locomotor ac- placed in a separate chamber and exposed to room air. tivity, stereotyped behavior, and anxiety level. Changesin the cellular counts of the occipital cortex of the mousealso Analysisof nitrous oxideconcentration will be measured. This study will determine if exposure of The level of nitrous oxide within the chamber was mice to low levels of nitrous oxide (levels comparable to analyzed at five different times:18 time zero; 30 min after those breathed by dentists) causes a detectable change in the chamber had been flushed with 480 L of the nitrous their behavior and cortical cell count as comparedto con- oxide/air gas (1000 ppm for the initial experiment and trol animals. 2000 ppm for the final experiment); 2 hr; 4 hr; and 6 hr respectively. The samplesof nitrous oxide within the cham- Methods and materials ber were gathered in 13 ml Venaject® vacutainer tubes. Animal treatment The chamber air samples were taken by needle puncture Male adult Swiss mice (Dominion Laboratories, Omaha, of a latex port on the side of the chamber. Nitrous oxide NE) weighing 19-21 g were housed in plastic cages (four was drawn into the vacutainer tube for I min. The cham- mice per cage) in a light- and temperature- (23 + 1°C) ber air samples were subsequently analyzed by gas chro- controlled environment with a 12-hr light-dark cycle and matography. free access to water and food (Purina Lab Chow,St. Louis, Analysisof carbondioxide concentration MO). Animals were weighed and assigned randomly to control and experimental groups. They were allowed a The carbon dioxide within the chamber was tested for minimumof three days to adapt to the housing environ- the ppm concentration at 2-hr intervals during days one ment prior to experimentation. A total of 22 animals were and two of both the initial experiment (1000 ppm)and the used; eight controls, seven in N20- (1000 ppm) exposed final experiment (2000 ppm). The air was again tested for group and seven in NzO- (2000 ppm) exposed group. carbon dioxide content at 4-hr intervals on day three of Animals in control group (N = 8/group) were exposed both the initial and final experiments. The analysis was air, while those in experimental groups (N = 7/group) made by using a Precision Gas Detector, model #400. were exposed to nitrous oxide (1000 ppmor 2000 ppm)for Kitagowa Precision Gas Detector Tubes of a 0.05-1.0% 8 hr/day (9 AMto 5 PM) for eight consecutive days. All (500-10,000 ppm) sensitivity were used for the detection neurobehavioral testing was conducted on day nine be- of the carbon dioxide.