Vascular tone is generally controlled by both the humoral component and the neuro-vegetative component. Regarding the second one, catecholaminergic sympathetic innervation of blood vessels is almost a rule in the body[1,2].
It is known that the iris is sympathetically inner-vated, both at the level of the iris dilator muscle[1,2] and at the level of iris blood vessels[3]. Cathecolamines are responsible for vasoconstriction and, in some vascular beds, vasodilation. [...]

Histamine and serotonin (5-hydroxytryptamine) are two amino acid derivatives with important biological functions [1]. Histamine, an endogenous monoamine, is synthesized from the histidine and is stored in most tissues and degraded in liver by histaminase. The most important roles of histamine are: mediation of type I allergic reactions, stimulation of stomach secretion of hydrochloric acid and pepsin (as autacoid), and functioning as a neurotransmitter (especially in the central nervous system)[2]. Regarding the vascular effects of histamine in non-ocular territories, there have not been many published literature reviews, for example, there are data reviewed for pulmonary artery[3], brain territory[4], but these data are not recent.

5-HT4 receptors are a G-protein-coupled family of re-ceptors coupled with Gs protein that stimulates the pro-duction of the intracellular signaling molecule cAMP. It has two isoforms (5-HT4S and 5-HT4L), differing in the length and sequence of their C-termini[1]. 5-HT4 are present both in the central nervous system and peripheral tissues
in the brain, are found mostly in basal ganglia and the hippocampus[2]. In the periphery, 5HT4 play an important role in the functioning of gastrointestinal tract, urinary bladder, heart and adrenal gland. Gastrointestinal 5-HT4 receptors potentiate peristalsis, and electrolyte secretion. In the urinary bladder, acti-vation of 5-HT4 receptors modulates cholinergic and purinergic transmission. Stimulation of atrial 5-HT4 receptors produces tachycardia and arrhythmias. In the adrenal gland, activation of 5-HT4 receptors releases cortisol, aldosterone and corticosterone[3]. [...]

Serotonin (5-hydroxytryptamine or 5-HT)is a mo-noamine neurotransmitter involved in regulating and modulating physiological and behavioral processes [1]. Serotonin also plays an important role in the functio-ning of enteric nervous system [2]. [...]

Pain is defined by IASP (International Association for the Study of Pain) as an unpleasant sensorial and emotional feeling which is produced by an actual or potential tissue injury. Chronic pain management is currently a major public health challenge throughout the world, especially for patients with oncologic disorders. Data show that up to 90 percent of cancer patients suffer from pain during the course of their illness and 50-80 percent receive an inadequate pain management. [...]