Its release is stimulated by gastrin and acetylcholine and inhibited by somatostatin. Men can be physically stimulated without feeling aroused and thus causing an erection. Melatonin production is stimulated by darkness and inhibited by light. During ovulation, cells are constantly stimulated to divide while ovulatory cycles continue. Business activity was accordingly both stimulated and challenged. The pain receptors may be stimulated by head trauma or tumors and cause headaches. Glycogen synthesis is also stimulated by the insulin receptor via IRS-1. Fat production in adipocytes is strongly stimulated by insulin. This ensures that individual photosensitive cells are continually stimulated in different degrees. Glucagon release is stimulated by low blood glucose or insulin levels, and during exercise. In myocytes, glycogen degradation may also be stimulated by neural signals. Glycolysis is thus stimulated when energy charge falls. Over a longer period, root growth is also stimulated. Saliva production may be pharmacologically stimulated by sialagogues such as pilocarpine and cevimeline. It is also stimulated by agonists and arachidonic acid.