Hormones and other signaling molecules generate physiological responses by binding to specific receptor proteins in or on target cells. Only cells that have receptors for the secreted molecule (hormone, neurotransmitter, neurohormone, etc.) are target cells; other cells are nonresponsive.

There are five different classifications for molecular signaling. These are as follow:

Endocrine signaling: Secreted molecules diffuse into the bloodstream and trigger responses in target cells throughout the body.

Paracrine signaling: Signaling molecules diffuse locally and trigger a response in neighboring cells.

Autocrine signaling: Secretion of molecules that diffuse locally and trigger a response on the same cells that secreted them.

Synaptic signaling: Secretion of neurotransmitters that diffuse across synapses and trigger a response in cells or target tissues (neurons, glands, and muscles).

Neuroendocrine signaling: Secretion of hormones from neuronal cells that diffuse into the bloodstream and trigger a response on cells throughout the body.

Melatonin is a modified amino acid that is secreted by the pineal gland. The pineal gland contains light-sensitive cells and has nervous connections to the eyes that affect its secretory activity. As a result, melatonin regulates functions related to light, circadian rhythm, and seasonal alterations based on the amount of daylight.

Melatonin is secreted at night, and the amount released depends on the length of the night. In winter, for example, more melatonin is released. Melatonin is believed to target a group of neurons in the hypothalamus called the suprachiasmatic nucleus (SCN), which functions as a biological clock.

Melatonin also affects skin pigmentation in many vertebrates.

Since melatonin is a modified amino acid, its structure is very different from that of corticosteroids, which are synthesized from the lipid cholesterol. Melatonin has two rings (similar to tryptophan), a modified ether group (from the carboxylic acid of the amino acid), and an amide group (from the amine group of the amino acid). Corticosteroids have four rings and multiple hydroxyl and ketone groups.

Hormones and other signaling molecules generate physiological responses by binding to specific receptor proteins in or on target cells. Only cells that have receptors for the secreted molecule (hormone, neurotransmitter, neurohormone, etc.) are target cells; other cells are nonresponsive.

There are five different classifications for molecular signaling. These are as follow:

Endocrine signaling: Secreted molecules diffuse into the bloodstream and trigger responses in target cells throughout the body.

Paracrine signaling: Signaling molecules diffuse locally and trigger a response in neighboring cells.

Autocrine signaling: Secretion of molecules that diffuse locally and trigger a response on the same cells that secreted them.

Synaptic signaling: Secretion of neurotransmitters that diffuse across synapses and trigger a response in cells or target tissues (neurons, glands, and muscles).

Neuroendocrine signaling: Secretion of hormones from neuronal cells that diffuse into the bloodstream and trigger a response on cells throughout the body.

Melatonin is a modified amino acid that is secreted by the pineal gland. The pineal gland contains light-sensitive cells and has nervous connections to the eyes that affect its secretory activity. As a result, melatonin regulates functions related to light, circadian rhythm, and seasonal alterations based on the amount of daylight.

Melatonin is secreted at night, and the amount released depends on the length of the night. In winter, for example, more melatonin is released. Melatonin is believed to target a group of neurons in the hypothalamus called the suprachiasmatic nucleus (SCN), which functions as a biological clock.

Melatonin also affects skin pigmentation in many vertebrates.

Since melatonin is a modified amino acid, its structure is very different from that of corticosteroids, which are synthesized from the lipid cholesterol. Melatonin has two rings (similar to tryptophan), a modified ether group (from the carboxylic acid of the amino acid), and an amide group (from the amine group of the amino acid). Corticosteroids have four rings and multiple hydroxyl and ketone groups.

Growth-hormone-releasing hormone (GHRH) causes release of growth hormone (GH) from the anterior pituitary.

The posterior pituitary is responsible for antidiuretic hormone (ADH) and oxytocin release. The thyroid releases thyroid hormones (T3 and T4), which control metabolic rates. The pancreas releases glucagon and insulin to control blood sugar levels.

The hormone responsible for the reduction of calcium in the blood is calcitonin. This hormone promotes the assimilation of calcium ions into the bone matrix from the blood. This reduces the concentration of calcium ions in the blood and increases the concentration found in the bone. Meanwhile, PTH has the opposite effect by taking calcium ions from the bone matrix and releasing them into the blood.

Growth-hormone-releasing hormone (GHRH) causes release of growth hormone (GH) from the anterior pituitary.

The posterior pituitary is responsible for antidiuretic hormone (ADH) and oxytocin release. The thyroid releases thyroid hormones (T3 and T4), which control metabolic rates. The pancreas releases glucagon and insulin to control blood sugar levels.

Prolactin is responsible for the production of milk, while oxytocin is responsible for the ejection of milk. The question asks for the hormone that causes production, so the answer is prolactin. Prolactin is released from the anterior pituitary.

Oxytocin is released from the posterior pituitary and the mammary glands are responsible for releasing milk.

The hormone responsible for the reduction of calcium in the blood is calcitonin. This hormone promotes the assimilation of calcium ions into the bone matrix from the blood. This reduces the concentration of calcium ions in the blood and increases the concentration found in the bone. Meanwhile, PTH has the opposite effect by taking calcium ions from the bone matrix and releasing them into the blood.

The two hormones known for their effect on plasma calcium levels are calcitonin and parathyroid hormone (PTH). When calcium levels are high, calcitonin is released by the thyroid gland to stimulate the uptake of serum calcium into bone. This effectively decreases calcium levels in the blood. PTH has the opposite effect and is released by the parathyroid gland.

The three remaining answer choices are not known for their effect on calcium levels in the blood.