**Question:** Substance P, actions are all except

A. Stimulates the secretion of gastric acid
B. Enhances neuronal excitability
C. Decreases blood pressure
D. Reduces inflammatory response

**Core Concept:**
Substance P is a neuropeptide that plays a crucial role in pain transmission, nociception, and inflammation. It is primarily involved in the neurogenic inflammatory response, where it acts on the sensory neurons and increases their excitability, leading to pain perception. Substance P binds to neurokinin-1 receptor (NK1R) and activates various signaling pathways, ultimately resulting in inflammation and pain.

**Why the Correct Answer is Right:**
The correct answer, D, "Reduces inflammatory response," is right because Substance P, in reality, heightens the inflammatory response rather than reducing it. It stimulates the release of various inflammatory mediators and promotes the recruitment of immune cells, exacerbating inflammation.

**Why Each Wrong Option is Incorrect:**
A. Substance P is known to increase gastric acid secretion, making option A incorrect.
B. Although Substance P does contribute to pain transmission, stating that it enhances neuronal excitability is incorrect as it increases, not reduces, neuronal excitability. This makes option B wrong.
C. Substance P generally promotes inflammation and does not decrease blood pressure. Therefore, option C is incorrect.

**Clinical Pearl:**
Substance P is a crucial mediator in various pathological conditions involving pain, inflammation, and immune response. Understanding its actions is essential for recognizing and treating conditions like fibromyalgia, chronic pain syndromes, and inflammatory skin disorders where Substance P plays a significant role.