Low Dietary Uptake Efficiencies and Biotransformation Prevent Biomagnification of Octamethylcyclotetrasiloxane (D4) and Decamethylcyclopentasiloxane (D5) in Rainbow Trout.

With octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) being considered for evaluation under the UN Stockholm Convention on Persistent Organic Pollutants, which specifically acknowledges risks of biomagnification of persistent organic pollutants in traditional foods, a study into the mechanism of the biomagnification process of D4 and D5 in Rainbow trout was conducted by combining the absorption-distribution-metabolism-excretion for bioaccumulation (ADME-B) approach to determine intestinal and somatic biotransformation rates and radiochemical analyses to identify metabolite formation. High rates of intestinal biotransformation of D4 and D5 (i.e., 2.1 (0.70 SE) and 0.88 (0.67 SE) day-1, respectively) and metabolite formation [i.e., 52.0 (17 SD)% of D4 and 56.5% (8.2 SD)% of D5 were metabolized] were observed that caused low dietary uptake efficiencies of D4 and D5 in fish of 15.5 (2.9 SE)% and 21.0 (6.5 SE)% and biomagnification factors of 0.44 (0.08 SE) for D4 and 0.78 (0.24 SE) kg-lipid·kg-lipid-1 for D5. Bioaccumulation profiles indicated little effect of growth dilution on the bioaccumulation of D4 and D5 in fish and were substantially different from those of PCB153. The study highlights the importance of intestinal biotransformation in negating biomagnification of substances in organisms and explains differences between laboratory tests and field observations of bioaccumulation of D4 and D5.

Bioaccumulation of Linear Siloxanes in Fish.

The bioaccumulation behavior, including the uptake, internal distribution, depuration, and biotransformation rates, of three widely used linear methyl-siloxanes was investigated in rainbow trout. Dietary uptake efficiencies of octamethyltrisiloxane (L3), decamethyltetrasiloxane (L4), and dodecamethylpentasiloxane (L5) were 15% (3.3% standard error [SE]), 8.6% (1.4% SE), and 15% (1.8% SE), respectively, and for L3 and L4 were well below those of nonmetabolizable reference chemicals with similar octanol-water partition coefficients, suggesting significant intestinal biotransformation of L3 and L4. Somatic biotransformation rate constants were 0.024 (0.003 SE) day-1 for L3 and 0.0045 (0.0053 SE) day-1 for L4 and could not be determined for L5. Lipid-normalized biomagnification factors for L3, L4, and L5 were 0.24 (0.02 SE), 0.24 (0.01 SE), and 0.62 (0.05 SE) kg-lipid kg-lipid-1 , respectively. Bioconcentration factors standardized to a 5% lipid content fish for water in Canadian oligotrophic lakes with a dissolved organic carbon content of 7.1 mg L-1 were 2787 (354 SE) for L3, 2689 (312 SE) for L4, and 1705 (418 SE) L kg-wet weight-1 , respectively, and 3085 (392 SE) for L3, 4227 (490 SE) for L4, and 3831 (938 SE) L kg-wet weight-1 in water with a dissolved organic carbon content of 2.0 mg L-1 . A comparison of 238 bioaccumulation profiles for 166 different chemicals shows that the bioaccumulation profiles for L3, L4, and L5 are vastly different from those of other very hydrophobic compounds found in the environment. Environ Toxicol Chem 2024;43:42-51. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Modulation of nigral dopamine signaling mitigates parkinsonian signs of aging: evidence from intervention with calorie restriction or inhibition of dopamine uptake.

Identifying neurobiological mechanisms of aging-related parkinsonism, and lifestyle interventions that mitigate them, remain critical knowledge gaps. No aging study, from rodent to human, has reported loss of any dopamine (DA) signaling marker near the magnitude associated with onset of parkinsonian signs in Parkinson's disease (PD). However, in substantia nigra (SN), similar loss of DA signaling markers in PD or aging coincide with parkinsonian signs. Alleviation of these parkinsonian signs may be possible by interventions such as calorie restriction (CR), which augment DA signaling markers like tyrosine hydroxylase (TH) expression in the SN, but not striatum. Here, we interrogated respective contributions of nigral and striatal DA mechanisms to aging-related parkinsonian signs in aging (18 months old) rats in two studies: by the imposition of CR for 6 months, and inhibition of DA uptake within the SN or striatum by cannula-directed infusion of nomifensine. Parkinsonian signs were mitigated within 12 weeks after CR and maintained until 24 months old, commensurate with increased D1 receptor expression in the SN alone, and increased GDNF family receptor, GFR-α1, in the striatum, suggesting increased GDNF signaling. Nomifensine infusion into the SN or striatum selectively increased extracellular DA. However, only nigral infusion increased locomotor activity. These results indicate mechanisms that increase components of DA signaling in the SN alone mitigate parkinsonian signs in aging, and are modifiable by interventions, like CR, to offset parkinsonian signs, even at advanced age. Moreover, these results give evidence that changes in nigral DA signaling may modulate some parameters of locomotor activity autonomously from striatal DA signaling.

Bioaccumulation Screening of Neutral Hydrophobic Organic Chemicals in Air-Breathing Organisms Using In Vitro Rat Liver S9 Biotransformation Assays.

To advance methods for bioaccumulation assessment of organic substances in air-breathing organisms, the present study developed an in vitro approach for screening neutral hydrophobic organic substances for their bioaccumulation potential in air-breathing organisms consisting of (1) depletion assays for chemicals in rat liver S9 subcellular fractions, (2) in vitro-in vivo extrapolation, and (3) whole-organism bioaccumulation modeling to assess the biomagnification potential of neutral organic substances in the rat. Testing of the in vitro method on 14 test chemicals of potentially biomagnifying substances showed that the bioassays could be conducted with a high level of reproducibility and that in vitro-derived elimination rate constants were in good agreement with in vivo-determined elimination rate constants in the rat. Exploring the potential of the in vitro approach for screening organic chemicals for bioaccumulation in air-breathing organisms indicated that chemical substances that exhibit a depletion rate constant in the S9 in vitro bioassay ≥0.3 h-1 are not expected to biomagnify in rats independent of their octanol-water partitioning coefficient (KOW ) or octanol-air partitioning coefficient (KOA ). The high level of reproducibility achieved in the test, combined with the good agreement between in vitro-derived and in vivo-determined depuration rates, suggests that the in vitro approach in combination with a KOA - and KOW -based screening approach has good potential for screening chemicals in commerce for their bioaccumulation potential in air-breathing organisms in a cost-effective and expedient manner, especially if the bioassay can be automated. Environ Toxicol Chem 2022;41:2565-2579. © 2022 SETAC.

Bioaccumulation of dodecamethylcyclohexasiloxane (D6) in fish.

To investigate the bioaccumulation behavior of dodecamethylcyclohexasiloxane (D6, CAS number: 540-97-6) in fish, an OECD-305 style dietary bioaccumulation study of D6 in rainbow trout was conducted in the presence of non-metabolizable reference chemicals. The dietary uptake absorption efficiency of D6 was 14 (3 SE) % and lower than that of the reference chemicals which ranged between 22 (2 SE) to 60 (8 SE) %. The concentration of D6 in the body of the fish showed a rapid 40% drop during the first day of the depuration phase, followed by a slower decline during the remainder of the depuration period. The overall depuration rate constant of D6 was 0.016 (0.0026 SE) d-1 and significantly greater than those of PCB153 and PCB209, which were not significantly different from zero. During the depuration phase, when fish body weight did not significantly change over time, depuration of D6 appears to be almost entirely due to biotransformation in the body of the fish. The biomagnification factor of D6 in rainbow trout was 0.38 (0.14 SE) kg-lipid kg-lipid-1, indicating a lack of biomagnification. The bioconcentration factor (BCF) of D6 in Rainbow trout was estimated at 1909 (483 SE) L kg-1 wet for natural waters of mostly oligotrophic lakes in Northern Canada with an average concentration of total organic carbon of 7.1 mg L-1. Comparing the bioaccumulation profile of D6 to that of 238 similar profiles for 166 unique chemicals indicates that the bioaccumulation capacity of D6 is markedly less than that of many very hydrophobic organochlorines.

GFR-α1 Expression in Substantia Nigra Increases Bilaterally Following Unilateral Striatal GDNF in Aged Rats and Attenuates Nigral Tyrosine Hydroxylase Loss Following 6-OHDA Nigrostriatal Lesion.

Glial cell line-derived neurotrophic factor (GDNF) improved motor function in Parkinson's disease (PD) patients in Phase I clinical trials, and these effects persisted months after GDNF discontinuation. Conversely, phase II clinical trials reported no significant effects on motor improvement vs placebo. The disease duration and the quantity, infusion approach, and duration of GDNF delivery may affect GDNF efficacy in PD treatment. However, identifying mechanisms activated by GDNF that affect nigrostriatal function may reveal additional avenues to partially restore nigrostriatal function. In PD and aging models, GDNF affects tyrosine hydroxylase (TH) expression or phosphorylation in substantia nigra (SN), long after a single GDNF injection in striatum. In aged rats, the GDNF family receptor, GFR-α1, increases TH expression and phosphorylation in SN. To determine if GFR-α1 could be a mechanistic link in long-term GDNF impact, we conducted two studies; first to determine if a single unilateral striatal delivery of GDNF affected GFR-α1 and TH over time (1 day, 1 week, and 4 weeks) in the striatum or SN in aged rats, and second, to determine if soluble GFR-α1 could mitigate TH loss following 6-hydroxydopamine (6-OHDA) lesion. In aged rats, GDNF bilaterally increased ser31 TH phosphorylation and GFR-α1 expression in SN at 1 day and 4 weeks after GDNF, respectively. In striatum, GFR-α1 expression decreased 1 week after GDNF, only on the GDNF-injected side. In 6-OHDA-lesioned rats, recombinant soluble GFR-α1 mitigated nigral, but not striatal, TH protein loss following 6-OHDA. Together, these results show GDNF has immediate and long-term impact on dopamine regulation in the SN, which includes a gradual increase in GFR-α1 expression that may sustain TH expression and dopamine function therein.

Glatiramer Acetate Reverses Motor Dysfunction and the Decrease in Tyrosine Hydroxylase Levels in a Mouse Model of Parkinson's Disease.

Parkinson's disease (PD) is the second most common neurodegenerative disease and there are no effective treatments that either slow or reverse the degeneration of the dopamine (DA) pathway. Using a 4-week progressive MPTP (1-methyl-1,2,3,6-tetrahydropyridine) neurotoxin model of PD, which is characterized by neuroinflammation, loss of nigrostriatal DA, and motor dysfunction, as seen in patients with PD, we tested whether post-MPTP treatment with glatiramer acetate (GA), an immunomodulatory drug, could reverse these changes. GA restored the grip dysfunction and gait abnormalities that were evident in the MPTP treated group. The reversal of the motor dysfunction was attributable to the substantial recovery in tyrosine hydroxylase (TH) protein expression in the striatum. Within the substantia nigra pars compacta, surface cell count analysis showed a slight increase in TH+ cells following GA treatment in the MPTP group, which was not statistically different from the vehicle (VEH) group. This was associated with the recovery of BDNF (brain derived neurotrophic factor) protein levels and a reduction in the microglial marker, IBA1, protein expression within the midbrain. Alpha synuclein (syn-1) levels within the midbrain and striatum were decreased following MPTP, while GA facilitated recovery to VEH levels in the striatum in the MPTP group. Although DA tissue analysis revealed no significant increase in striatal DA or 3,4-Dihydroxyphenylacetic acid levels (DOPAC) in the MPTP group treated with GA, DA turnover (DOPAC/DA) recovered back to VEH levels following GA treatment. GA treatment effectively reversed clinical (motor dysfunction) and pathology (TH, IBA1, BDNF expression) of PD in a murine model.

Tyrosine Hydroxylase Inhibition in Substantia Nigra Decreases Movement Frequency.

Reduced movement frequency or physical activity (bradykinesia) occurs with high prevalence in the elderly. However, loss of striatal tyrosine hydroxylase (TH) in aging humans, non-human primates, or rodents does not reach the ~ 80% loss threshold associated with bradykinesia onset in Parkinson's disease. Moderate striatal dopamine (DA) loss, either following TH inhibition or decreased TH expression, may not affect movement frequency. In contrast, moderate DA or TH loss in the substantia nigra (SN), as occurs in aging, is of similar magnitude (~ 40%) to nigral TH loss at bradykinesia onset in Parkinson's disease. In aged rats, increased TH expression and DA in SN alone increases movement frequency, suggesting aging-related TH and DA loss in the SN contributes to aging-related bradykinesia or decreased physical activity. To test this hypothesis, the SN was targeted with bilateral guide cannula in young (6 months old) rats, in a within-subjects design, to evaluate the impact of nigral TH inhibition on movement frequency and speed. The TH inhibitor, α-methyl-p-tyrosine (AMPT) reduced nigral DA (~ 40%) 45-150 min following infusion, without affecting DA in striatum, nucleus accumbens, or adjacent ventral tegmental area. Locomotor activity in the open-field was recorded up to 3 h following nigral saline or AMPT infusion in each test subject. During the period of nigra-specific DA reduction, movement frequency, but not movement speed, was significantly decreased. These results indicate that DA or TH loss in the SN, as observed in aging, contributes as a central mechanism of reduced movement frequency.

Aging-related limit of exercise efficacy on motor decline.

Identifying lifestyle strategies and allied neurobiological mechanisms that reduce aging-related motor impairment is imperative, given the accelerating number of retirees and increased life expectancy. A physically active lifestyle prior to old age can reduce risk of debilitating motor decline. However, if exercise is initiated after motor decline has begun in the lifespan, it is unknown if aging itself may impose a limit on exercise efficacy to decelerate further aging-related motor decline. In Brown-Norway/Fischer 344 F1 hybrid (BNF) rats, locomotor activity begins to decrease in middle age (12-18 months). One mechanism of aging-related motor decline may be decreased expression of GDNF family receptor, GFRα-1, which is decreased in substantia nigra (SN) between 12 and 30 months old. Moderate exercise, beginning at 18 months old, increases nigral GFRα-1 and tyrosine hydroxylase (TH) expression within 2 months. In aged rats, replenishing aging-related loss of GFRα-1 in SN increases TH in SN alone and locomotor activity. A moderate exercise regimen was initiated in sedentary male BNF rats in a longitudinal study to evaluate if exercise could attenuate aging-related motor decline when initiated at two different ages in the latter half of the lifespan (18 or 24 months old). Motor decline was reversed in the 18-, but not 24-month-old, cohort. However, exercise efficacy in the 18-month-old group was reduced as the rats reached 27 months old. GFRα-1 expression was not increased in either cohort. These studies suggest exercise can decelerate motor decline when begun in the latter half of the lifespan, but its efficacy may be limited by age of initiation. Decreased plasticity of GFRα-1 expression following exercise may limit its efficacy to reverse motor decline.

Dissociation of Striatal Dopamine and Tyrosine Hydroxylase Expression from Aging-Related Motor Decline: Evidence from Calorie Restriction Intervention.

The escalating increase in retirees living beyond their eighth decade brings increased prevalence of aging-related impairments, including locomotor impairment (Parkinsonism) that may affect ~50% of those reaching age 80, but has no confirmed neurobiological mechanism. Lifestyle strategies that attenuate motor decline, and its allied mechanisms, must be identified. Aging studies report little to moderate loss of striatal dopamine (DA) or tyrosine hydroxylase (TH) in nigrostriatal terminals, in contrast to ~70%-80% loss associated with bradykinesia onset in Parkinson's disease. These studies evaluated the effect of ~6 months 30% calorie restriction (CR) on nigrostriatal DA regulation and aging-related locomotor decline initiated at 12 months of age in Brown-Norway Fischer F1 hybrid rats. The aging-related decline in locomotor activity was prevented by CR. However, striatal DA or TH expression was decreased in the CR group, but increased in substantia nigra versus the ad libitum group or 12-month-old cohort. In a 4- to 6-month-old cohort, pharmacological TH inhibition reduced striatal DA ~30%, comparable with decreases reported in aged rats and the CR group, without affecting locomotor activity. The dissociation of moderate striatal DA reduction from locomotor activity seen in both studies suggests that aging-related decreases in striatal DA are dissociated from locomotor decline.

Ceftriaxone reduces L-dopa-induced dyskinesia severity in 6-hydroxydopamine parkinson's disease model.

BACKGROUND: Increased extracellular glutamate may contribute to l-dopa induced dyskinesia, a debilitating side effect faced by Parkinson's disease patients 5 to 10 years after l-dopa treatment. Therapeutic strategies targeting postsynaptic glutamate receptors to mitigate dyskinesia may have limited success because of significant side effects. Increasing glutamate uptake may be another approach to attenuate excess glutamatergic neurotransmission to mitigate dyskinesia severity or prolong the time prior to onset. Initiation of a ceftriaxone regimen at the time of nigrostriatal lesion can attenuate tyrosine hydroxylase loss in conjunction with increased glutamate uptake and glutamate transporter GLT-1 expression in a rat 6-hydroxydopamine model. In this article, we examined if a ceftriaxone regimen initiated 1 week after nigrostriatal lesion, but prior to l-dopa, could reduce l-dopa-induced dyskinesia in an established dyskinesia model. METHODS: Ceftriaxone (200 mg/kg, intraperitoneal, once daily, 7 consecutive days) was initiated 7 days post-6-hydroxydopamine lesion (days 7-13) and continued every other week (days 21-27, 35-39) until the end of the study (day 39 postlesion, 20 days of l-dopa). RESULTS: Ceftriaxone significantly reduced abnormal involuntary movements at 5 time points examined during chronic l-dopa treatment. Partial recovery of motor impairment from nigrostriatal lesion by l-dopa was unaffected by ceftriaxone. The ceftriaxone-treated l-dopa group had significantly increased striatal GLT-1 expression and glutamate uptake. Striatal tyrosine hydroxylase loss in this group was not significantly different when compared with the l-dopa alone group. CONCLUSIONS: Initiation of ceftriaxone after nigrostriatal lesion, but prior to and during l-dopa, may reduce dyskinesia severity without affecting l-dopa efficacy or the reduction of striatal tyrosine hydroxylase loss. © 2017 International Parkinson and Movement Disorder Society.