Final answer:
The H1 receptor is associated with weight gain and sedation, particularly due to its histamine receptor properties that influence sleep-wake regulation.
Step-by-step explanation:
The receptor type associated with weight gain and sedation is the H1 receptor. Antagonists of the H1 receptor, which is a histamine receptor, are known for their sedative properties and potential to increase appetite, potentially leading to weight gain. The other receptors mentioned, such as 5-HT (serotonergic receptors) and α1 (an alpha adrenergic receptor), have different physiological roles more related to mood regulation, anxiety behavior, and insulin release. However, the H1 receptor, due to its role in histamine response, also influences sleep-wake regulation and can induce drowsiness, hence its connection to sedation.