Colocalization of tyrosine hydroxylase and GAD65 mRNA in mesostriatal neurons

Arcuate dopaminergic/GABAergic neurons project within the hypothalamus and to the median eminence

Cotransmission, meaning the release of multiple neurotransmitters from one synapse, allows for increased diversity of signaling in the brain. Dopamine (DA) and γ-aminobutyric acid (GABA) are known to coexpress in many regions such as the olfactory bulb and the ventral tegmental area. Tuberoinfundibular dopaminergic neurons (TIDA) in the arcuate nucleus of the hypothalamus (Arc) project to the median eminence (ME) and regulate prolactin release from the pituitary, and prior work suggests dopaminergic Arc neurons also cotransmit GABA. However, the extent of cotransmission, and the projection patterns of these neurons have not been fully revealed. Here, we used a genetic intersectional reporter expression approach to selectively label cells that express both tyrosine hydroxylase (TH) and vesicular GABA transporter (VGAT). Through this approach, we identified cells capable of both DA and GABA cotransmission in the Arc, periventricular (Pe), paraventricular (Pa), ventromedial, and the dorsolateral hypothalamic nuclei, in addition to a novel population in the caudate putamen. The highest density of labeled cells was in the Arc, 6.68% of DAPI-labeled cells at Bregma -2.06 mm, and in the Pe, 2.83% of DAPI-labeled cells at Bregma -1.94 mm. Next, we evaluated the projections of these DA/GABA cells by injecting an mCherry virus that fluoresces in DA/GABA cells. We observed a cotransmitting DA/GABA population, with projections within the Arc, and to the Pa and ME. These data suggest DA/GABA Arc neurons are involved in prolactin release as a subset of TIDA neurons. Further investigation will elucidate the interactions of dopamine and GABA in the hypothalamus.NEW & NOTEWORTHY Cotransmitting dopaminergic (DA) and γ-aminobutyric acid (GABA)ergic (DA/GABA) neurons contribute to the complexity of neural circuits. Using a new genetic technique, we characterized the locations, density, and projections of hypothalamic DA/GABA neurons. DA/GABA cells are mostly in the arcuate nucleus (Arc), from which they project locally within the arcuate, to the median eminence (ME), and to the paraventricular (Pa) nucleus. There is also a small and previously unreported group of DA/GABA cells in the caudate putamen.

Dopaminergic neurons establish a distinctive axonal arbor with a majority of non-synaptic terminals

Chemical neurotransmission typically occurs through synapses. Previous ultrastructural examinations of monoamine neuron axon terminals often failed to identify a pre- and postsynaptic coupling, leading to the concept of "volume" transmission. Whether this results from intrinsic properties of these neurons remains undefined. We find that dopaminergic neurons in vitro establish a distinctive axonal arbor compared to glutamatergic or GABAergic neurons in both size and propensity of terminals to avoid direct contact with target neurons. While most dopaminergic varicosities are active and contain exocytosis proteins like synaptotagmin 1, only ~20% of these are synaptic. The active zone protein bassoon was found to be enriched in dopaminergic terminals that are in proximity to a target cell. Finally, we found that the proteins neurexin-1α^SS4- and neuroligin-1^A+B play a critical role in the formation of synapses by dopamine (DA) neurons. Our findings suggest that DA neurons are endowed with a distinctive developmental connectivity program.

Morphological changes of glutamatergic synapses in animal models of Parkinson's disease

The striatum and the subthalamic nucleus (STN) are the main entry doors for extrinsic inputs to reach the basal ganglia (BG) circuitry. The cerebral cortex, thalamus and brainstem are the key sources of glutamatergic inputs to these nuclei. There is anatomical, functional and neurochemical evidence that glutamatergic neurotransmission is altered in the striatum and STN of animal models of Parkinson’s disease (PD) and that these changes may contribute to aberrant network neuronal activity in the BG-thalamocortical circuitry. Postmortem studies of animal models and PD patients have revealed significant pathology of glutamatergic synapses, dendritic spines and microcircuits in the striatum of parkinsonians. More recent findings have also demonstrated a significant breakdown of the glutamatergic corticosubthalamic system in parkinsonian monkeys. In this review, we will discuss evidence for synaptic glutamatergic dysfunction and pathology of cortical and thalamic inputs to the striatum and STN in models of PD. The potential functional implication of these alterations on synaptic integration, processing and transmission of extrinsic information through the BG circuits will be considered. Finally, the significance of these pathological changes in the pathophysiology of motor and non-motor symptoms in PD will be examined.

Parkinson's disease as a result of aging

It is generally considered that Parkinson's disease is induced by specific agents that degenerate a clearly defined population of dopaminergic neurons. Data commented in this review suggest that this assumption is not as clear as is often thought and that aging may be critical for Parkinson's disease. Neurons degenerating in Parkinson's disease also degenerate in normal aging, and the different agents involved in the etiology of this illness are also involved in aging. Senescence is a wider phenomenon affecting cells all over the body, whereas Parkinson's disease seems to be restricted to certain brain centers and cell populations. However, reviewed data suggest that Parkinson's disease may be a local expression of aging on cell populations which, by their characteristics (high number of synaptic terminals and mitochondria, unmyelinated axons, etc.), are highly vulnerable to the agents promoting aging. The development of new knowledge about Parkinson's disease could be accelerated if the research on aging and Parkinson's disease were planned together, and the perspective provided by gerontology gains relevance in this field.