Objective. This study assessed the neuropeptide Y (NPY) expression in healthy human dental pulp following tooth bleaching with three in-office hydrogen peroxide-based systems: Opalescence Boost© (Ultradent Products, South Jordan, UT), Pola Office© (SDI, Victoria, Australia), and Zoom© (Zoom! Bleaching System; Discuss Dental, Culver City, CA).

Materials and Methods. Forty dental pulps were collected from healthy premolars scheduled for extraction for orthodontic reasons. Teeth were divided into four groups containing ten healthy premolars each: Control group (n= 10): the teeth were not exposed to dental bleaching agents (healthy pulps assessed for normal/basal NPY values). Pola Office system group (n= 10): application of Pola office (35% H2O2) for 8 minutes. Opalescent Boost system group (n= 10): application of Opalescent Boost (40% H2O2) for 20 minutes. Zoom system group (n= 10): application of Zoom! (25% H2O2 + cold blue light) for 15 minutes. The investigators rigorously followed the manufacturer's instructions for all bleaching systems. Following the extractions, the pulpal tissue was collected, placed in a 4% formaldehyde solution in Eppendorf tubes, and processed. NPY levels were measured using enzyme-linked immunosorbent assay (ELISA).

Inclusion Criteria:

• Dental pulps were collected from non-smoking donors
• Patients who need premolar extractions for orthodontic purposes.
• These premolars must meet the following requirements:
• Caries and restoration-free,
• Complete root development (visually and radiographically confirmed),
• Normal response to sensitivity testing.

Exclusion Criteria:

• Evidence of periodontal disease,
• Traumatic occlusion
• Previous orthodontic force application.

The bleaching agent was applied using specific devices, with only the laboratory staff able to identify the corresponding group codes, which consisted of letters and numbers.