256 257 258 259 *Table 46.2 Neuropeptides, Slowly Acting Transmitters, or Growth Factors* 260 261 262

Hypothalamic-Releasing Hormones
Thyrotropin-releasing hormone
Luteinizing hormone-releasing hormone
Somatostatin (growth hormone inhibitory factor)
Pituitary Peptides
Adrenocorticotropic hormone
β-Endorphin
α—Melanocyte-stimulating hormone
Prolactin
Luteinizing hormone
Thyrotropin
Growth hormone
Vasopressin
Peptides That Act on Gut and Brain
Leucine enkephalin
Methionine enkephalin
Substance P
Gastrin
Cholecystokinin
Vasoactive intestinal polypeptide
Nerve growth factor
Brain-derived neurotrophic factor
Neurotensin
Insulin
Oxytocin
Peptides From Other Tissues
Angiotensin II
Bradykinin
Carnosine
Sleep peptides
Calcitonin

263 264 causes excitation, or to increase potassium or chloride conductance, which causes inhibition. 265 266 Recycling of Small-Molecule Types of Vesicles. Vesicles that store and release small-molecule transmitters are continually recycled and used over and over again. After they fuse with the synaptic membrane and open to release their transmitters, the vesicle membrane at first simply becomes part of the synaptic membrane. However, within seconds to minutes, the vesicle portion of the membrane invaginates back to the inside of the pre-synaptic terminal and pinches off to form a new vesicle. The new vesicular membrane still contains appropriate enzyme proteins or transport proteins required for synthesizing and/or concentrating new transmitter substances inside the vesicle. 267 268 Acetylcholine is a typical small-molecule transmitter that obeys the principles of synthesis and release, as stated earlier. This transmitter substance is synthesized in the presynaptic terminal from acetyl coenzyme A and choline in the presence of the enzyme choline acetyltransferase. It is then transported into its specific vesicles. When the vesicles later release acetylcholine into the synaptic cleft during synaptic neuronal signal transmission, the acetyl-choline is rapidly split again to acetate and choline by the enzyme cholinesterase, which is present in the proteo-glycan reticulum that fills the space of the synaptic cleft. Then, once again, inside the presynaptic terminal, the vesicles are recycled, and choline is actively transported back into the terminal to be used again for synthesis of new acetylcholine. 269 270 Characteristics of Some Important Small-Molecule Transmitters. Acetylcholine is secreted by neurons in many areas of the nervous system but specifically by [... Output truncated due to length. 3172 tokens/13921 characters removed ...]