What are the neural bases (causes) of mental diseases such as psychotic disorders (e.g. mania, schizophrenia), Parkinson's disease, and Alzheimer's disease?

They all have neurochemical bases (neurotransmitters).Yours is a very broad question, so I'll just address one I know best, Alzheimer's.

Alzheimer's is generally caused by problems with acetylcholine (ACh), a major neurotransmitter of the peripheral nervous system, parasympathetic nervous system, and gads of other systems in the body.

With Alzheimer's, cortical neurons and cholinergic neurons (ACh-releasing neurons) degenerate in major cholinergic pathways, particularly in the basal nucleus of Meynert, which projects to the entire neocortex, and the striatum (caudate nucleus and putamen), which contains ACh in its circuit neurons.

The disease is associated with a 60-90% loss of choline acetyltransferase (ChAT) in the cerebral cortex, which is the enzyme responsible for the production of ACh, due to the increasing absence of the aforementioned major cholinergic pathways.

Studies have also shown that on a histological-basis, Alz. is characterized by the presence of:

Neurofibrillary tangles (NFTs) in neurons, which are essentially degenerated neurofilaments that have preternatural clumps of the tauprotein,

Senile (or neuritic) plaques, which are degenerated nerve cell processes made of abnormal clusters of beta-amyloid protein chunks (which contribute to the relatively large aggregate of Hirano bodies in the hippocampus), and

The shrinkage of connections between neurons in the brain, which means there are few nerve cells and synapses.

As you can see, acetylcholine plays a huge role in Alzheimer's. However, it also has a huge role in myasthenia gravis and basic brain function.

Reference: Hannah Cho