Recovery from methamphetamine induced long-term nigrostriatal dopaminergic deficits without substantia nigra cell loss

Abstract

After administration of methamphetamine (METH) (2×2 mg/kg, 6 h apart) to vervet monkeys, long term but reversible dopaminergic deficits were observed in both in vivo and post-mortem studies. Longitudinal studies using positron emission tomography (PET) with the dopamine transporter (DAT)-binding ligand, [¹¹C]WIN 35,428 (WIN), were used to show decreases in striatal WIN binding of 80% at 1 week and only 10% at 1.5 years. A post-mortem characterization of other METH subjects at 1 month showed extensive decreases in immunoreactivity (IR) profiles of tyrosine hydroxylase (TH), DAT and vesicular monoamine transporter-2 (VMAT) in the striatum, medial forebrain bundle and the ventral midbrain dopamine (VMD) cell region. These IR deficits were not associated with a loss of VMD cell number when assessed at 1.5 years by stereological methods. Further, at 1.5 years, IR profiles of METH subjects throughout the nigrostriatal dopamine system appeared similar to controls although some regional deficits persisted. Collectively, the magnitude and extent of the dopaminergic deficits, and the subsequent recovery were not suggestive of extensive axonal degeneration followed by regeneration. Alternatively, this apparent reversibility of the METH-induced neuroadaptations may be related primarily to long-term decreases in expression of VMD-related proteins that recover over time.

Introduction

The neurotoxic effects of methamphetamine (METH) on the striatal dopamine system are often characterized by long-term reductions in tyrosine hydroxylase (TH), the dopamine transporter (DAT), vesicular monoamine transporter-2 (VMAT) and dopamine (DA) levels [4], [19], [27], [46], [47], [55], [69]. These deficits persist from weeks in rodents to several years in humans [32] and non-human primates [71] and, accordingly, have been attributed to neurodegeneration. However, those results generally have not provided direct evidence for the loss of nerve terminals and/or their corresponding substantia nigra cell bodies. Moreover, studies that have included determination of cell loss after neurotoxic METH exposure have not unequivocally established the correlate between substantia nigra cell loss and long-term METH-induced deficits. For example, decreases in substantia nigra cell number have been shown in the mouse [26], [57] and not the rat [55] but neither study used unbiased stereological methods that are considered essential for obtaining accurate cell number estimates [41], [68]. Nonetheless, long-term striatal dopaminergic deficits have generally been attributed to degeneration of striatal axonal terminals with an apparent sparing of cell bodies [47], [53], [59]. In this instance, the implicit supposition is that degeneration of striatal terminals occurred while indeterminate lengths of their remaining axons and corresponding cell bodies remained intact.

In our previous METH studies in the vervet monkey [34], [35], [36], [37], we showed that acute METH administration protocols (2 doses of 2 mg/kg; 4 h apart) produced striatal dopamine deficits similar to those previously characterized as long-term METH neurotoxicity in both rodents and non-human primates [37], [51], [52], [58], [61], [65]. However, further characterization of those long-term METH effects with 6-[¹⁸F]fluoro-l-DOPA (FDOPA)-PET showed that an essentially complete recovery of DA synthesis capacity occurred over 8 months. Recently, an analogous recovery phenomenon for the striatal dopamine system has also been reported in rodents given neurotoxic doses of METH [6], [18]. Generally, this recovery pattern has been attributed to degeneration/regeneration of striatal nerve terminals and/or compensatory collateral sprouting from residual terminals. By inference, the reversibility of the METH-induced changes was necessarily contingent on the preservation of residual nigrostriatal axons and ventral midbrain dopamine (VMD) cell body integrity.

In the present study, we obtained further evidence for this phenomenon of reversible METH-induced dopaminergic deficits. In longitudinal PET studies on individual subjects, the magnitude of [¹¹C]WIN 35,428 (WIN) binding decreases was assessed in striatum at 1 week and the extent of subsequent recovery at 1.5 years. In other METH subjects, METH IR-profiles of TH, DAT and VMAT were characterized at 1 month post-METH in the VMD cell region, the medial forebrain bundle (MFB) projecting from the VMD, and in the striatum. Those studies were intended to establish if the striatal decreases in WIN binding extended throughout the nigrostriatal dopamine pathway and whether other indices of dopamine system integrity were also decreased. This regional analysis was again conducted at 1.5 years to determine the extent of recovery in those parameters in subjects whose PET studies at that time had shown significant striatal WIN-binding increases. Lastly, to establish whether VMD cell loss was associated with this profile of METH-induced neurotoxicity, cell numbers were determined in VMD regions.