Enriched environment confers resistance to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and cocaine: involvement of dopamine transporter and trophic factors

Abstract

We investigated, in mice, the influence of life experience on the vulnerability to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a major neurotoxin that induces a Parkinson's disease-like syndrome in humans, and to cocaine, a potent psychostimulant that promotes drug addiction. Our findings show that adult C57BL/6 mice raised in an enriched environment (EE) for only 2 months are significantly more resistant to both drugs compared with mice raised in a standard environment (SE). Indeed, EE mice showed decreased locomotor activity in response to cocaine (10 and 20 mg/kg) as well as a different pattern of c-fos expression in the striatum compared with SE mice. After MPTP treatment, SE mice showed a 75% loss of dopamine neurons, whereas EE mice showed only a 40% loss. The dopamine transporter plays a key role in mediating the effects of both drugs. We thus investigated the regulation of its expression. EE mice showed less dopamine transporter binding in the striatum and less dopamine transporter mRNA per dopamine neuron at the cellular level as demonstrated by in situ hybridization. In addition, enriched environment promoted an increase in the expression of brain-derived neurotrophic factor in the striatum. These data provide a direct demonstration of the beneficial consequences that a positive environment has in preventing neurodegeneration and in decreasing responsiveness to cocaine. Furthermore, they suggest that the probability of developing neurological disorders such as Parkinson's disease or vulnerability to psychostimulants may be related to life experience.

Figure 1.

Mice raised in an enriched environment show decreased locomotor activity in response to novelty. Motor activity was measured in activity cages made of Plexiglas and aluminum wire mesh. Locomotor activity was measured every 10 min. a, On day 1, mice were put in measurement cages for 2 hr, and their locomotor response to novelty was measured during this first exposure. EE mice showed decreased locomotor activity compared with SE mice. b, Mice were acclimated to the activity cages for 2 consecutive days (days 2 and 3). The next day, they were again put in activity cages for 50 min before receiving cocaine. Note that the locomotor activity of SE and EE mice is not different before cocaine administration. Error bars are SEM.

Figure 2.

Mice raised in an enriched environment show decreased locomotor activity in response to cocaine (coc). Locomotor activity in SE or EE mice was measured after administration of 10 mg/kg cocaine (a, b), 20 mg/kg cocaine (c, d), or saline. Measurements were done every 10 min (a, c) and were also expressed with all time points pooled together (c, d). At both doses, cocaine induced a significant increase in the locomotor activity in both SE and EE mice. Note, however, that EE mice were less responsive to cocaine than SE mice (see Results for details). ^* indicates significantly different from saline-treated animals; + indicates significantly different from cocaine-treated mice raised in an SE.

Figure 3.

Cocaine-induced c-fos expression was altered in enriched mice. Saline treatment did not induce c-fos mRNA in our experimental protocol in either SE or EE mice after in situ hybridization. However, cocaine (20 mg/kg) significantly induced c-fos expression in both groups of mice, although in different striatal areas. Indeed, EE mice showed an expression restricted to dorsal areas that receive premotor and sensorimotor cortical projections, whereas SE mice showed expression in ventromedial areas, as found by most investigators working with mice housed in an SE and investigating c-fos mRNA (rather than protein). a, b, Saline-treated mice; c, d, cocaine-treated mice.

Figure 4.

Living in an enriched environment provides protection against the pro-parkinsonian neurotoxin MPTP. a, Stereological counts of the number of TH-IR neurons in the SNc of saline- and MPTP-treated mice raised in SE or EE. Standard environment-NaCl, n = 6; SE-MPTP, n = 7; EE-NaCl, n = 8; EE-MPTP, n = 6. The enriched environment modified the number of TH-positive neurons (F_(1,26) = 12.3; p < 0.01) and interfered with the MPTP treatment (F_(1,26) = 52.1; p < 0.0001). Under control conditions, animals raised in an enriched environment had a lower number of TH-IR neurons than animals raised under standard conditions (^*p<0.05). After MPTP treatment, although both groups showed significant decreases in the number of TH-IR neurons compared with their respective controls (⁺p < 0.05), the number of surviving neurons was twice as high in EE compared with SE animals (^**p < 0.05). b, Thus, the MPTP-induced TH neuron loss was 73.4% in SE mice and 41% in EE mice. Errors bars are SEM.

Figure 5.

DAT binding is significantly decreased in the dorsal part of the striatum of EE mice (^*p < 0.05; SE, n = 8; EE, n = 8). a, The 100% level in SE mice corresponds to a PE2I binding of 97.8 fmol/mg equivalent tissue. b, This decrease is most likely attributable to lower number of DAT per neuron, because in situ hybridization for DAT mRNA showed a 75% silver grain decrease in EE compared with SE mice on microautoradiography. c, Photomicrographs of neurons labeled by in situ hybridization using oligonucleotide probes complementary to DAT mRNA and viewed under direct (top) and polarized (bottom) light illumination. Scale bar, 15 μm. Error bars are SEM.

Figure 6.

Enriched mice show increased expression of BDNF mRNA in the striatum as evidenced by in situ hybridization. Although the expression of both the full-length and the truncated form of the TrkB neurotrophin receptor [i.e., TrkB^TK+ (a, b) and TrkB^TK- (c, d), respectively] is comparable in the SE and EE mice, BDNF mRNA expression (e, f) is significantly increased throughout the brain and especially in the striatum of the mice living in the enriched environment. The mRNA expression of several dopamine-sensitive markers [i.e., D1R (g, h), D2R (i, j), PPE-A (k, l), PPE-B (m, n), and PPT (o, p)] is not affected by the enriched environment.