Publisher Summary This chapter takes a view of biogenic amines in the insect nervous system and to assess critically the evidence for their roles as neurotransmitters, neuromodulators and neurohormones. It attempts to answer several questions such as where biogenic amines are located in the insect nervous system; how are they synthesized and inactivated; and what physiological roles do they perform and how do they bring about their effects. Particular emphasis is placed on the correlation of biochemical, physiological, pharmacological and anatomical information from systems containing identified aminergic neurones. The chapter points out instances where data from one or more of the approaches is either missing or conflicts with that from another and suggests experiments to resolve these points. The function of biogenic amines in the insect nervous system has been compared with that in the vertebrate nervous system. In this context the chapter mentions the role of biogenic amines as chemical messengers in the vertebrate nervous system.

Biogenic Amines in the Insect Nervous System

Less than two decades ago Sutherland and Rall (1958; Rall et al., 1957) discovered a heat stable factor now known as adenosine 3′,5′-monophosphate, which accumulated in particulate fractions of liver homogenates exposed to epinephrine. This factor activated the breakdown of glycogen by soluble cytoplasmic enzymes. Although epinephrine activates glycogenolysis in intact hepatocytes, the hormone has no such effect on the soluble enzymes. Sutherland and Rall (1960; Rall and Sutherland, 1958, 1961, 1962) proposed that epinephrine might trigger the glycogenolytic response of hepatocytes by activating the synthesis of this nucleotide (cyclic AMP) whose structure (Fig. 1) by then had been shown to contain a unique cyclic phosphate bond (Lipkin et al., 1959). The polypeptide hormone glucagon also triggers the glycogenolytic response of liver, and activates the synthesis of cyclic AMP. From these and related observations on the actions of other hormones, the concept arose that cyclic AMP functioned as an intracellular “second messenger”, synthesized in response to certain hormones and which, by activating the appropriate sequence of enzymes, produced the specific biologic response of the target cell to the hormone (Sutherland et al., 1965).

The role of cyclic nucleotides in central synaptic function.

Publisher Summary This chapter reviews octopamine and some related noncatecholic amines in invertebrate nervous systems. In mammalian nervous tissue, it is now reasonably well established that catecholamines (dopamine and noradrenaline) and the indole alkylamine 5-hydroxytryptamine (5-HT. serotonin) serves as neurotransmitters. They are present in and synthesized by neurons, are released on stimulation, act on the postsynaptic membrane, and are inactivated by reuptake into the presynaptic terminal. These amines have long had a central position in theories of the etiology of psychiatric disorders. In addition to major monoamines, or classical neurotransmitters, whose role and importance is relatively understood, there exists in the nervous tissue of all species examined to date a group of monoamines described variously as exotic amines, trace amines, or microamines. Microamines have long been known to be pharmacologically active. The presence of various microamines in nervous tissues is now well established. It remains to be determined whether they interact with receptors and, if so, how.

Octopamine and some related noncatecholic amines in invertebrate nervous systems.

Sensitization of the gill and siphon withdrawal reflex in the marine mollusc, Aplysia california, is a simple form of learning Underlying this behavioral changes is a cascade of biochemical events. The first step in this cascade is postulated to be an increase in cAMP within the sensory neurons of the abdominal ganglion. We have developed a labeling protocol with 32Pi which permits us to measure the synthesis of cAMP within a single sensory neurons. Application of serotonin for 5 min was found to triple the content of [32P]cAMP in sensory neurons. The response is specific to serotonin: dopamine, a transmitter that does not produce sensitization, did not increase cAMP. Physiological stimulation of facilitator neurons also resulted in a 3.5-fold increase of cAMP in sensory neurons but not in other cells of the ganglion. We studied the time course of the increase of cAMP in sensory cells stimulated with serotonin and found that it parallels closely the time course of the short term form of presynaptic facilitation. We also have determined the effects of transmitters on the synthesis of cAMP in other identified neurons of the ganglion. The bag cells responded specifically to serotonin. R15, which has been shown to be hyperpolarized both the serotonin and by dopamine, responded to both transmitters by increased synthesis synthesis of cAMP. Thus, the dopamine- and serotonin-sensitive cyclase can be localized to both the same and different cells. Other cells did not respond to serotonin or to dopamine, indicating that a transmitter-sensitive adenylate cyclase is a specific property and is not present in all neurons.

https://www.jneurosci.org/content/jneuro/2/12/1682.full.pdf

Facilitatory transmitter causes a selective and prolonged increase in adenosine 3':5'-monophosphate in sensory neurons mediating the gill and siphon withdrawal reflex in Aplysia

Octopamine in invertebrates and vertebrates. A review.

Incubation of abdominal ganglia, obtained from Aplysia californica weighing 120--260 g, with octopamine or serotonin led to marked increases in cAMP (cyclic adenosine 3′,5′-monophosphate) levels after 10 min of incubation; these increases were blocked by phentolamine and methysergide, respectively. Incubation for 21 hr with octopamine also produced an elevation in cAMP levels but prolonged incubation with serotonin did not. Both these amines increased incorporation of 32P or 33P into a specific protein with apparent molecular weight of 118,000, upon incubation with the ganglion for 21 hr. The selective effect on this phosphoprotein was previously shown to be blocked by phentolamine (for octopamine effect) or methysergide (for serotonin effect) and could be mimicked by addition of dibutyryl cAMP to the medium.



Incubation for more than 15 hr was required to generate a stable phosphoprotein pattern in which 5 phosphoprotein peaks could be reliably identified on polyacrylamide gels. With shorter incubations there was marked variability between ganglia with respect to the proteins which incorporated radioactive phosphate. The dibutyryl cAMP effect on phosphorylation of specific protein could not be demonstrated with incubations shorter than 15 hr.



The specific effect on the phosphoprotein with molecular weight 118,000 was not observed in a number of large identified cells removed from the ganglion after 21 hr of incubation with radioactive phosphate and dibutyryl cAMP. The effect was also not observed in connective nerves and the bag cell clusters, but was consistently present in that portion of the ganglion remaining after these dissections. Upon subcellular fractionation the phenomenon was not observed in a crude nuclear fraction but was consistently present in a crude mitochondrial fraction. All these results are consistent with the possibility that the change in phosphoprotein metabolism produced by dibutryl cAMP may be localized in the neuropil, which contains all the synaptic contacts in this ganglion.

Cyclic AMP and amine effects on phosphorylation of specific protein in abdominal ganglion of Aplysia californica; localization and kinetic analysis

Phosphorylation of a protein (or proteins) of molecular weight 120,000 in the Aplysia abdominal gan- glion, as measured by incorporation of (32P) or (33P)sodium phosphate in vitro followed by separation of the phospho- proteins on sodium dodecyl sulfate-polyacrylamide gels, was specifically stimulated by incubation in the presence of the putative neurotransmitters octopamine or seroto-

Irwin B. Levitan

Papers

Cyclic AMP and amine effects on phosphorylation of specific protein in abdominal ganglion of Aplysia californica; localization and kinetic analysis

Octopamine- and Serotonin-Stimulated P1 in the Abdominal Ganglion of Aplysia ca (neurotransmitters/dibutyryl cyclic AMP)