A 40-year-old female with major depressive disorder and comorbid chronic pain has been on venlafaxine 75 mg daily for 8 weeks with only partial response. The PMHNP titrates the dose to 225 mg daily. At her 4-week follow-up on the higher dose, she reports improved mood and reduced pain but her blood pressure has increased from a baseline of 118/74 mmHg to 142/92 mmHg. Which of the following best explains the pharmacological basis for these clinical observations?