The 5-HT2A serotonin receptor in executive function: Implications for neuropsychiatric and neurodegenerative diseases

How Studies of the Serotonin System in Macaque Models of Menopause Relate to Alzheimer's Disease1

Serotonin plays a key role in mood or affect, and dysfunction of the serotonin system has been linked to depression in humans and animal models. Depression appears prior to or coincident with overt symptoms of Alzheimer's disease (AD) in about 50% of patients, and some experts consider it a risk factor for the development of AD. In addition, AD is more prevalent in women, who also show increased incidence of depression. Indeed, it has been proposed that mechanisms underlying depression overlap the mechanisms thought to hasten AD. Women undergo ovarian failure and cessation of ovarian steroid production in middle age and the postmenopausal period correlates with an increase in the onset of depression and AD. This laboratory has examined the many actions of ovarian steroids in the serotonin system of non-human primates using a rhesus macaque model of surgical menopause with short or long-term estradiol (E) or estradiol plus progesterone (E+P) replacement therapy. In this mini-review, we present a brief synopsis of the relevant literature concerning AD, depression, and serotonin. We also present some of our data on serotonin neuron viability, the involvement of the caspase-independent pathway, and apoptosis-inducing factor in serotonin-neuron viability, as well as gene expression related to neurodegeneration and neuron viability in serotonin neurons from adult and aged surgical menopausal macaques. We show that ovarian steroids, particularly E, are crucial for serotonin neuron function and health. In the absence of E, serotonin neurons are endangered and deteriorating toward apoptosis. The possibility that this scenario may proceed or accompany AD in postmenopausal women seems likely.

5-HT2A Receptor Binding in the Frontal Cortex of Parkinson's Disease Patients and Alpha-Synuclein Overexpressing Mice: A Postmortem Study

The 5-HT_2A receptor is highly involved in aspects of cognition and executive function and seen to be affected in neurodegenerative diseases like Alzheimer's disease and related to the disease pathology. Even though Parkinson's disease (PD) is primarily a motor disorder, reports of impaired executive function are also steadily being associated with this disease. Not much is known about the pathophysiology behind this. The aim of this study was thereby twofold: (1) to investigate 5-HT_2A receptor binding levels in Parkinson's brains and (2) to investigate whether PD associated pathology, alpha-synuclein (AS) overexpression, could be associated with 5-HT_2A alterations. Binding density for the 5-HT_2A-specific radioligand [³H]-MDL 100.907 was measured in membrane suspensions of frontal cortex tissue from PD patients. Protein levels of AS were further measured using western blotting. Results showed higher AS levels accompanied by increased 5-HT_2A receptor binding in PD brains. In a separate study, we looked for changes in 5-HT_2A receptors in the prefrontal cortex in 52-week-old transgenic mice overexpressing human AS. We performed region-specific 5-HT_2A receptor binding measurements followed by gene expression analysis. The transgenic mice showed lower 5-HT_2A binding in the frontal association cortex that was not accompanied by changes in gene expression levels. This study is one of the first to look at differences in serotonin receptor levels in PD and in relation to AS overexpression.