All monkeys studied were male African green monkeys (Chlorocebus sabaeus), weighing from 5 to 8 kg at the beginning of the experiments. Note that these animals from St. Kitts, West Indies, have had several official taxonomic name changes during the course of our studies and are designated in various reports as Cercopithecus aethiops sabaeus, Chlorocebus aethiops sabaeus, and other variations, all being exactly the same type of monkey. The animals were given free access to water and fed standard daily rations of Harland Teklad monkey chow (20% protein and without isoniazide or other drugs or contaminants), and were hand fed or tube fed if necessary to ensure minimal nutrition and fluid requirements. They were sometimes given supplemental fresh tropical fruits, such as banana, soursop, starfruit, and guava. They were held in roofed enclosures in standard primate squeeze cages exposed to natural daylight at 17° North latitude with natural breezes and ambient temperatures. Care and treatment was in accordance with the US Public Health Service Guide for the Care and Use of Animals (37). All of the relevant protocols were approved by the institutional review committees. Antiparkinsonian drugs were not administered so as to avoid confounding results, except for l-DOPA administration to determine its effects on the model. Although most of the animals went on to participate in experiments for potential anti-Parkinson’s treatments, only baseline or pre-MPTP exposure data were included in the present analyses.

MPTP was administered intramuscularly over a 5-day period in most of the monkeys, with the total dose divided into five treatments. Four doses were administered at approximately 4 p.m., the following 8 a.m. and 4 p.m., the next 8 a.m. The fourth day was skipped, and the fifth dose administered at 8 a.m. on the fifth day. When an animal showed a significant early toxic effect, the fifth dose was skipped. The monkeys included in these analyses, however, all received either a total of 2 or 2.25 mg/kg. Some animals were re-dosed with various small repeated doses over a period of months, but these animals were analyzed separately to evaluate these different injection schedules.

Behavioral data were collected by trained observers as described earlier and in detail by Taylor et al. (33). Regular inter-rater reliability testing was done, with discussion to minimize and reconcile differences and to prevent observer drift. These data were entered into a computer at the observation site from paper data recording sheets, which were then checked for accuracy. Data were analyzed using the Statistical Analysis System. After the assumptions for the use of parametric tests were confirmed (normality and lack of heteroscedasticity), the effects of observers, individual differences between monkeys, and repeated measures over time, dose response to l-DOPA were made using analysis of variance (38). Data from any observers that were significantly different from the other observers were dropped from the dataset. Post hoc tests, after ANOVA, used Student–Newman–Keuls test (at p  <  0.05) (38). Linear regressions were determined using PROC GLM from SAS (32). Approximately 33,000 observations entered into the different analyses.

Two hundred and eighteen monkeys were studied during a baseline period of 30–90 days before treatment and then the month following MPTP administration. From their normal baseline state, the monkeys became progressively parkinsonian during the first month. After some acute reactions during treatment, such as occasional vomiting, monkeys became progressively slower in their motor movements, showed decreased normal behaviors and activities, ate and drank less, and then developed more characteristic signs of parkinsonism such as bradykinesia, incoordination, delayed initiation of movement, freezing, and muscle rigidity, tremor with intention and at rest, and difficulties with ambulation and eating and swallowing.

Every scored and rated behavior changed during the first month. The Parkscore changed from a baseline of 0.97–29.47 during the first month (F  =  305, df  =  1, 6910, p  <  0.0001). There were significant differences between monkeys, however (F  =  13.41, df  =  217, p  <  0.0001). Conversely, normal behaviors, as measured by Healthy Behavior, were significantly reduced from 21 during baseline to 4.55 in month 1 (F  =  1515, df  =  1, 6886, p  <  0.0001), also with significant differences among monkeys (F  =  9.29, df  =  217, 6886, p  <  0.0001). The components which contributed to the Parkscore are shown in Fig. 1. Every item changed (p  <  0.0001, for all except vocalize, which was p  <  0.0004). Parkinsonian behaviors increased, and other normal activities decreased. Effects of MPTP administration on the individual non-Parkinson’s behaviors were detailed previously (33).

Since factor analyses are sometimes dependent upon the specific sample of data, the same type of factor analysis was run again on these newer data. The five principal factors were replicated, and the component individual behaviors are shown in Table 4. Many of the loadings, which are shown from the new analysis, are almost identical to the previous analysis (see Table II in Taylor et al. (33)), but some of the parkinsonian behaviors show higher loadings, consistent with the fact that, as a whole, the monkeys in this sample were more severe. There was more variability in the nonparkinsonian behaviors, probably because this dataset included more data from animals with significant parkinsonism. There were no monkeys in the present data that had not received MPTP (normal monkeys), but there were extensive baseline/normative data from the same monkeys prior to MPTP administration.

We repeated the same approach described by Taylor et al. (33) initially to examine the degree of variability. The animals were categorized into five severity scores (0, 1, 2, 3, and 4) by rank of Parkscore at 1 month. It was apparent from this analysis that some animals showed much greater sensitivity to MPTP, becoming more parkinsonian more rapidly in the same fashion. Approximately 44 monkeys were in each severity group. Group 0 had values of Parkscore from 0.45 to 7.3. Since there were no normal controls in this dataset, all of these monkeys were treated at least with 2.0 mg/kg of MPTP, and some with more. The next groups had Parkscore ranges as follows: Group 1, from 7.3 to 14.3; Group 2, from 14.3 to 26.4; Group 3, from 27 to 60.6; and Group 4, from 60.7 to 98.3. The animals that became more parkinsonian in the first month were also more likely to die from some complication of the disease. To determine the impact of severity on the rate of progression or possible recovery, we analyzed these animals during the first 4 months after MPTP treatment, computing slopes by each severity grouping for each month. Groups 0, 1, and 2 remained fairly flat, with some suggestion of increases after 3 months. Based on their classifications of severity after MPTP, none of the monkeys in these severity groups were different during the baseline period, although some significant slopes in opposite directions were identified (see Fig. 2). After MPTP, Groups 0, 1, 3, and 4 showed highly significant increasing slopes in the first month, and Groups 3 and 4 showed significant decreasing slopes of Healthy Behavior at that time. Of course, mean values of Parkscore during the month ranked perfectly with severity classification (since the groupings were based on these 1 month data). Thereafter, both Parkscore and Healthy Behavior remained stable (no significant increasing slopes for Parkscore or evidence of recovery from decreasing slopes). The only exception was Group 3 in the fourth month, which showed an increasing slope for Parkscore (p  <  0.003) and decreasing Healthy Behavior (p  <  0.03). Group 4 animals showed the steepest increases in the first month and remained highly parkinsonian during this time period, although, as with initial sensitivity to MPTP, there were some individuals that did better. The Healthy Behavior scores were always inverted, with the highest scores in Groups 0 and 1. The numbers of observations per period analyzed ranged from 1,015 in the baseline to 160 in one of the groups in month 4. Because the number of animals was not constant over the period, the slopes were analyzed independently for each month so that more severe monkeys that died in a previous month would not affect the slopes in the next month, although these could have influenced changes in the mean scores. From these data, the previous report, that severity ranking in the first month would predict the outcome, continued to be true for this time period, although with significantly larger numbers of animals there are clearly some exceptions. In addition, 4 months is clearly not long enough to assume that there is no recovery, as we have learned.

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