Increased vulnerability of nigral dopamine neurons after expansion of their axonal arborization size through D2 dopamine receptor conditional knockout.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Rare genetic mutations in genes such as Parkin, Pink1, DJ-1, α-synuclein, LRRK2 and GBA are found to be responsible for the disease in about 15% of the cases. A key unanswered question in PD pathophysiology is why would these mutations, impacting basic cellular processes such as mitochondrial function and neurotransmission, lead to selective degeneration of SNc DA neurons? We previously showed in vitro that SNc DA neurons have an extremely high rate of mitochondrial oxidative phosphorylation and ATP production, characteristics that appear to be the result of their highly complex axonal arborization. To test the hypothesis in vivo that axon arborization size is a key determinant of vulnerability, we selectively labeled SNc or VTA DA neurons using floxed YFP viral injections in DAT-cre mice and showed that SNc DA neurons have a much more arborized axon than those of the VTA. To further enhance this difference, which may represent a limiting factor in the basal vulnerability of these neurons, we selectively deleted in mice the DA D2 receptor (D2-cKO), a key negative regulator of the axonal arbour of DA neurons. In these mice, SNc DA neurons have a 2-fold larger axonal arborization, release less DA and are more vulnerable to a 6-OHDA lesion, but not to α-synuclein overexpression when compared to control SNc DA neurons. This work adds to the accumulating evidence that the axonal arborization size of SNc DA neurons plays a key role in their vulnerability in the context of PD.

Intestinal infection triggers Parkinson's disease-like symptoms in Pink1-/- mice.

Parkinson's disease is a neurodegenerative disorder with motor symptoms linked to the loss of dopaminergic neurons in the substantia nigra compacta. Although the mechanisms that trigger the loss of dopaminergic neurons are unclear, mitochondrial dysfunction and inflammation are thought to have key roles1,2. An early-onset form of Parkinson's disease is associated with mutations in the PINK1 kinase and PRKN ubiquitin ligase genes3. PINK1 and Parkin (encoded by PRKN) are involved in the clearance of damaged mitochondria in cultured cells4, but recent evidence obtained using knockout and knockin mouse models have led to contradictory results regarding the contributions of PINK1 and Parkin to mitophagy in vivo5-8. It has previously been shown that PINK1 and Parkin have a key role in adaptive immunity by repressing presentation of mitochondrial antigens9, which suggests that autoimmune mechanisms participate in the aetiology of Parkinson's disease. Here we show that intestinal infection with Gram-negative bacteria in Pink1-/- mice engages mitochondrial antigen presentation and autoimmune mechanisms that elicit the establishment of cytotoxic mitochondria-specific CD8+ T cells in the periphery and in the brain. Notably, these mice show a sharp decrease in the density of dopaminergic axonal varicosities in the striatum and are affected by motor impairment that is reversed after treatment with L-DOPA. These data support the idea that PINK1 is a repressor of the immune system, and provide a pathophysiological model in which intestinal infection acts as a triggering event in Parkinson's disease, which highlights the relevance of the gut-brain axis in the disease10.

Axonal Segregation and Role of the Vesicular Glutamate Transporter VGLUT3 in Serotonin Neurons.

A subset of monoamine neurons releases glutamate as a cotransmitter due to presence of the vesicular glutamate transporters VGLUT2 or VGLUT3. In addition to mediating vesicular loading of glutamate, it has been proposed that VGLUT3 enhances serotonin (5-HT) vesicular loading by the vesicular monoamine transporter (VMAT2) in 5-HT neurons. In dopamine (DA) neurons, glutamate appears to be released from specialized subsets of terminals and it may play a developmental role, promoting neuronal growth and survival. The hypothesis of a similar developmental role and axonal localization of glutamate co-release in 5-HT neurons has not been directly examined. Using postnatal mouse raphe neurons in culture, we first observed that in contrast to 5-HT itself, other phenotypic markers of 5-HT axon terminals such as the 5-HT reuptake transporter (SERT) show a more restricted localization in the axonal arborization. Interestingly, only a subset of SERT- and 5-HT-positive axonal varicosities expressed VGLUT3, with SERT and VGLUT3 being mostly segregated. Using VGLUT3 knockout mice, we found that deletion of this transporter leads to reduced survival of 5-HT neurons in vitro and also decreased the density of 5-HT-immunoreactivity in terminals in the dorsal striatum and dorsal part of the hippocampus in the intact brain. Our results demonstrate that raphe 5-HT neurons express SERT and VGLUT3 mainly in segregated axon terminals and that VGLUT3 regulates the vulnerability of these neurons and the neurochemical identity of their axonal domain, offering new perspectives on the functional connectivity of a cell population involved in anxiety disorders and depression.

Multiple episodes of sodium depletion in the rat: a remodeling of the electrical properties of median preoptic nucleus neurons.

In rat brain, the detection and integration of chemosensory and neural signals are achieved, inter alia, by the median preoptic nucleus (MnPO) during a disturbance of the hydromineral balance. This is allowed through the presence of the sodium (Na(+) ) sensor neurons. Interestingly, enkephalins and mu-opioid receptors (µ-ORs) are known for their role in ingestive behaviors and have previously been shown to regulate the excitability of MnPO neurons following a single Na(+) depletion. However, little is known about the role of these µ-ORs in the response enhancement following repeated Na(+) challenge. Therefore, we used whole-cell recordings in acute brain slices to determine neuronal plasticity in the electrical properties of the MnPO Na(+) sensor-specific neuronal population following multiple Na(+) depletions. Our results show that the population of Na(+) sensor neurons was represented by 80% of MnPO neurons after a single Na(+) depletion and was reduced after three Na(+) depletions. Interestingly, the subpopulation of Na(+) sensors responding to D-Ala(2) ,N-MePhe(4) ,Gly-ol-enkephalin (DAMGO), a specific µ-OR agonist, represented 11% of MnPO neurons after a single Na(+) depletion and the population doubled after three Na(+) depletions. Moreover, Na(+) sensor neurons displayed modifications in the discharge pattern distribution and shape of calcium action potentials after three Na(+) depletions but these changes did not occur in Na(+) sensors responding to DAMGO. Thus, the reinforced µ-OR functionality in Na(+) sensors might take place to control the neuronal hyperexcitability and this plasticity in opioid-sensitive and Na(+) detection MnPO networks might sustain the enhanced salt ingestion induced by repeated exposure to Na(+) depletion.

Intrinsic properties of the sodium sensor neurons in the rat median preoptic nucleus.

The essential role of the median preoptic nucleus (MnPO) in the integration of chemosensory information associated with the hydromineral state of the rat relies on the presence of a unique population of sodium (Na+) sensor neurons. Little is known about the intrinsic properties of these neurons; therefore, we used whole cell recordings in acute brain slices to determine the electrical fingerprints of this specific neural population of rat MnPO. The data collected from a large sample of neurons (115) indicated that the Na+ sensor neurons represent a majority of the MnPO neurons in situ (83%). These neurons displayed great diversity in both firing patterns induced by transient depolarizing current steps and rectifying properties activated by hyperpolarizing current steps. This diversity of electrical properties was also present in non-Na+ sensor neurons. Subpopulations of Na+ sensor neurons could be distinguished, however, from the non-Na+ sensor neurons. The firing frequency was higher in Na+ sensor neurons, showing irregular spike discharges, and the amplitude of the time-dependent rectification was weaker in the Na+ sensor neurons than in non-Na+ sensor neurons. The diversity among the electrical properties of the MnPO neurons contrasts with the relative function homogeneity (Na+ sensing). However, this diversity might be correlated with a variety of direct synaptic connections linking the MnPO to different brain areas involved in various aspects of the restoration and conservation of the body fluid homeostasis.