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.

Water-soluble hormones can be either polypeptides (proteins) or tyrosine-derivatives. The binding of a water-soluble hormone to a receptor occurs on the cell membrane, and receptor binding triggers a physiological response through a signal transduction cascade. Peptide hormones include insulin, growth hormone, oxytocin, adrenocorticotropic hormone, and many more. Tyrosine-derivative hormones include epinephrine and the thyroid hormones (T3 and T4).

Water-soluble hormones, like most hormones, are secreted into the bloodstream and carried throughout the body. As such, they are capable of eliciting responses in non-local regions of the body, and would not be classified as local regulators.

Neuroendocrine signaling describes the process when specialized neural sensors, usually in the brain, detect changes in the body. These sensors stimulate the release of a hormone to help correct deviations. The hormone can travel via the bloodstream to affect distant tissues.

In this example, sensor neurons in the hypothalamus trigger the release of the hormone vasopressin to affect the kidney.

Groups of hormones from the hypothalamus, anterior pituitary, and individual endocrine glands are often organized into a hormone cascade pathway. The flow of such a cascade occurs as follows:

1.  Signals or sensory cells in the brain trigger the hypothalamus to secrete a hormone (example: thyroid-releasing hormone) that either stimulates or inhibits release of a particular hormone from the anterior pituitary.

2.  The anterior pituitary hormone (example: thyroid-stimulating hormone) acts on a target endocrine tissue (example: thyroid gland)

3.  The endocrine tissue secretes a systemic hormone (example: thyroid hormone/T4) that creates a physiological response.  

4.  The physiological response by the target cells provides negative feedback to the brain to curtail the cascade. 

Non-tropic hormones are hormones that directly stimulate target cells to induce effects. For example, aldosterone acts directly on the kidney to promote reabsorption of sodium, which causes an increase in blood pressure. This differs from tropic hormones, which act on other endocrine glands to stimulate (or inhibit) release of a second hormone. For example, thyroid-stimulating hormone is responsible for stimulating the release for thyroid hormone by acting on the thyroid gland.

Growth hormone (GH) stimulates growth through both tropic and non-tropic mechanisms. Growth hormone's major tropic effect is to stimulate the release of insulin-like growth factors (IGFs) from the liver, which causes bone growth. Growth hormone also stimulates a number of tissues through non-tropic mechanisms to affect metabolism and raise blood glucose levels.

Calcium ions are essential for normal cellular function. Parathyroid hormone is released when blood calcium levels fall below a threshold of about 10mg per 100mL. Parathyroid hormone acts to raise blood calcium through multiple mechanisms. It works directly to increase blood calcium by causing demineralization of bone and by directly stimulating ion reabsorption in the kidneys. It also acts indirectly by stimulating conversion of vitamin D, which acts on the intestines to stimulate the uptake of calcium ions from food.

This molecule is estrogen, a lipid-soluble endocrine hormone. Estrogen acts a classic endocrine hormone. It is secreted and diffuses into the bloodstream to trigger a physiological response in target cells anywhere in the body that possess estrogen receptors.

Estrogen is a steroid hormone. Its nonpolar nature and small size allow it to diffuse through the membrane of target cell, binding to receptors within the cytoplasm. The estrogen receptor is an intracellular receptor whose activation through ligand binding typically results in alteration of gene expression.

Endocrine interactions involve a molecule (hormone) being secreted into the bloodstream to trigger a response in target cells in a different location. Reduction of blood-glucose levels by insulin and stimulation of cortisol release by adrenocorticotropic hormone are both examples of endocrine function.

Though epinephrine can act as a hormone when secreted by the adrenal medulla, the answer option indicates that it is being secreted by a neuron into a synapse. In this case, epinephrine would be acting as a neurotransmitter, causing an effect on a neighboring post-synaptic neuron. This answer does not describe an endocrine interaction.

In almost all animals, the endocrine and nervous systems are integrated to respond and control physiological responses.  

In insects, the prothoracic gland releases a hormone called ecdysone at different times to stimulate molting and maturation. Calcitonin and glucagon are hormones that are secreted by the pancreas and thyroid gland, respectively. They produce these hormones in response to ongoing monitoring of blood levels for glucose and calcium. The hormones are secreted to produce physiological responses in order to bring a return to homeostasis.  

Prostaglandins are local regulators and produce rapid responses that are confined to a small area. In the example, prostaglandins in semen stimulate the smooth muscle of the uterine wall to contract to facilitate sperm reaching an egg.

A tropic hormone is one that regulates the function of other endocrine cells or glands. Thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), adrenocorticotropic hormone (ACTH), and luteinizing hormone (LH) are the four topic hormones that are produced by the anterior pituitary.  

Prolactin is also produced by the anterior pituitary, but does not trigger a response from any other endocrine tissues. The stimulation of mammary gland growth and milk production in mammals, regulation of fat metabolism and reproduction in birds, and the regulation of salt and water balance in freshwater fish are all functions of prolactin.