In patients with Parkinson’s disease, gait disorders and difficulty with balance are often caused by the degeneration of a specific type of neuron, called cholinergic neurons, in a region of the brainstem called the pedunculopontine nucleus (PPN). Damage to this same population of neurons in the PPN is also linked to reward-based behaviours and disorders, such as addiction. Because billions of neurons are packed into the human brain, neuronal circuits that are responsible for controlling behaviours are by necessity highly intermingled. This tangled web makes it complicated for researchers to determine exactly which circuits do what. Now, researchers from Caltech have mapped out the pathways of a set of neurons responsible for the kinds of motor impairments, such as difficulty walking, found in patients with Parkinson’s disease. The team state that determining which pathways are associated with which behaviours might also improve future treatments. The opensource study is published in the journal Neuron.
Previous studies show that researchers have not been able to untangle the neural circuitry originating in the PPN to understand how both addictions and Parkinson’s motor impairments are modulated within the same population of cells. Furthermore, this uncertainty has created a barrier to treating those motor symptoms. Deep brain stimulation, in which a device is inserted into the brain to deliver electrical pulses to a targeted region, can be used to correct walking and balance difficulties in these patients, however, without knowing exactly which part of the PPN to target, the procedure can lead to mixed results. The current study applies optogenetics to selectively target and bidirectionally modulate PPN neurons both at the somata and at the terminals and further characterize the projections from these neurons to midbrain areas.
The current study began by mapping the projections of the cholinergic neurons in the PPN of a rat using a technique called Passive CLARITY Technique, or PACT. In this technique, a solution of chemicals is applied to the brain; the chemicals dissolve the lipids in the tissue and render that region of the brain optically transparent or see-through, and able to take up fluorescent markers which can label different types of neurons. Results show that the path of the PPN neurons of interest could be followed, marked by a fluorescent protein, by simply looking through the rest of the brain. Data findings show using this method, the axons of the PPN neurons were traced as they extended into two regions of the midbrain: the ventral substantia nigra, a landmark area for Parkinson’s disease that had been previously associated with locomotion; and the ventral tegmental area, a region of the brain that had been previously associated with reward.
Next, the lab used an electrical recording technique to keep track of the signals sent by PPN neurons, validating that these neurons communicate with their associated downstream structures in the midbrain and allowing the group to determine how this specific population or network of neurons affects behaviour. To do this, they used optogenetics, which allows researchers to manipulate neural activities using different colors of light.
Results show that exciting the neuronal projections in the ventral substantia nigra stimulates the animal to walk around its environment; by contrast, they could stop the animal’s movement by inhibiting these same projections. Data findings show that reward-seeking behaviour is stimulated by exciting the neuronal projections in the ventral tegmental area, and could cause aversive behaviour by inhibiting these projections. The researchers note that the global medical community could target the axonal projections in the substantia nigra for movement disorders and projections in the ventral tegmental area for reward disorders; in addition these projections in the midbrain are much easier to access surgically than their source in the PPN.
The team surmise that their findings show the cholinergic neurons from the PPN have a role in controlling behaviours and although the neurons are very densely packed and intermingled, these pathways are, to some extent, dedicated to very specialized behaviours. For the future, the researchers state that these results highlight the need for brain-wide functional and anatomical maps of these long-range neuronal projections and note that they have shown that tissue clearing and optogenetics are viable technologies in the creation of these maps.