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The goal of this clinical trial is to learn about the role of noradrenergic system in the non-motor symptoms of Parkinson's disease. The main objectives it aims to answer are:

1. To explore the association between orthostatic hypertension which is low blood pressure that occurs after going from lying to standing, and several neuropsychiatric and neurocognitive nonmotor features of Parkinson's disease (PD), such as feeling tired or disinterested and depression.
2. To explore the association between central noradrenergic dysfunction, orthostatic hypertension, and nonmotor symptoms of PD by measuring hormonal response to head up tilt-table testing before and after administration of yohimbine.
3. To explore the association between central noradrenergic dysfunction, orthostatic hypertension, and nonmotor symptoms of PD by measuring participants pupils before and after administration of yohimbine

Participants will be asked to come onsite for two study visits.

Visit one will consist of:

• Discussing and signing the Informed Consent Form
• Discussing Medical History and Current Medications
• Collecting Blood samples
• Measuring heart rate and blood pressure
• Mental health screening and neurocognitive questionnaires
• Pupil test
• Test to feel vibrations

Visit two will consist of :

• Mental Health questionnaire
• IV Placement
• Blood Draws
• Administration of Yohimbine hydrochloride
• Head up tilt table
• Measuring heart rate and blood pressure
• Answering questions about anxiety, mood, and fatigue using a scale
• Pupil tests

Visit three will be a follow-up call from the Nurse Coordinator to discuss any adverse events.

This is a cross-sectional pilot study comparing 10 PD patients with Orthostatic hypertension (OH) to 10 PD patients without OH. OH is defined as a drop in blood pressure within 5 minutes of standing (measured at 1, 2, and 5 minutes). The study will include two clinical visits and a follow-up phone call 24-72 hours after the 2nd visit. Visits will last approximately 4-5 hours each. During visit two the participant will be given a 5 mg tablet of Yohimbine hydrochloride. Yohimbine hydrochloride will be administered orally during Visit 2 in order to manipulate the noradrenergic system to determine the association between OH and neuropsychiatric symptoms in those with PD. Yohimbine is not administered as a treatment in this study, but as a tool to study the noradrenergic system during some of the assessments. This will be done by measuring the amounts of hormones in the participants body produces before and after yohimbine hydrochloride administration. Yohimbine hydrochloride is not approved by the FDA for this use.

On the morning of Visit 1, the participant will be asked to not take their medications that contains dopamine, such as levodopa/carbidopa, as well as other medications that would interfere with testing. The study doctor will determine which medications need to be held for this visit. The participant will be asked to bring one dose of these medications to take during the visit, instead of at the normal time in the morning. During visit one eligibility criteria will be discussed and if participants meet the inclusion exclusion criteria the Informed Consent will be signed and Visit 1 procedures will follow.

Visit 2 will occur no more than 30 days after visit 1. Subjects will be asked to not eat, and not take morning medications containing dopamine or norepinephrine (such as levodopa/carbidopa), or any other medications that interfere with testing. The neurologist will determine which medications need to be held for this visit. An IV will be placed as per Head Up Tilt procedure. Head Up Tilt testing will be conducted twice, once prior to and once after yohimbine administration. The participant will complete a scale that measures anxiety, mood, and fatigue prior to and after yohimbine administration. Pupil testing will also be evaluated prior to yohimbine administration and after yohimbine administration. Blood will be drawn twice prior to yohimbine administration and twice after yohimbine administration. One draw will be in a lying down position, and the other will be during the head up tilt.

A follow up study phone call will be made by the study nurse coordinator 24-72 hours after study visit 2. Any adverse events which may have occurred since the study visit will be captured on the participants adverse event log.

Inclusion Criteria:

1. Participant able to provide informed consent
2. Diagnosis of Parkinson's disease confirmed by a DH neurologist according to Movement Disorder Society criteria, with the exception that "Red flag" 5a will not be used (severe autonomic failure within five years of disease onset).
3. All subjects must have CMP and CBC drawn within 6 months of study visit 1, with results in the normal range or with abnormal results not considered to be clinically significant in the investigator's opinion.
4. Female patients must be post-menopausal (at least one year) or not planning to get pregnant and have negative pregnancy test.

Exclusion Criteria:

1. Diagnosis or previous history of diabetes of any kind
2. Known autonomic neuropathy unrelated to PD
3. History of or current cardiac, liver or renal disease that, in the opinion of the investigator, may put the patient at risk because of participation in the study
4. Known condition that in the investigator's opinion would be a contraindication to HUT testing or yohimbine challenge (e.g. decompensated cardiac disease, severe positional vertigo; severe anxiety, known panic disorder69)
5. Current use of catecholaminergic medications (e.g. stimulants, droxidopa, midodrine) that cannot be held for at least three half-lives
6. Inability to hold PD medications for at least 12 hours
7. History of major depressive or bipolar disorder preceding the diagnosis of PD,69 or diagnosis or previous history of psychiatric illness that in the investigator's opinion would affect the subject's ability to successfully participate in the study.
8. Any history (other than PD) that could significantly and adversely affect neurocognitive function, such as history of traumatic brain injury (head injury with loss of consciousness > 1 hour), known dementia unrelated to Parkinson's or related diseases; developmental delay, multiple sclerosis or epilepsy with cognitive impairment, intellectual deficit, diagnosed and untreated sleep apnea; untreated syphilis; HIV with HAND; or other conditions that, based on the investigators opinion, could interfere with neurocognitive evaluation.
9. Known ophthalmologic disease such as untreated cataract, glaucoma, optic neuritis, orbital trauma, or other neuroretinal disease that might impact pupillary function
10. Severe illness within 30 days prior to enrollment.
11. Use of opiate, procholinergic, or other medications influencing pupillary function that cannot be held for three half-lives
12. In the Investigator's opinion, subject would be unable to successfully participate in the study for any reason.