Octopamine-Mediated Neuromodulation of Insect Senses

Octopamine functions as a neuromodulator, neurotransmitter, and neurohormone in insect nervous systems. Octopamine has a prominent role in influencing multiple physiological events: (a) as a neuromodulator, it regulates desensitization of sensory inputs, arousal, initiation, and maintenance of various rhythmic behaviors and complex behaviors such as learning and memory; (b) as a neurotransmitter, it regulates endocrine gland activity; and (c) as a neurohormone, it induces mobilization of lipids and carbohydrates. Octopamine exerts its effects by binding to specific proteins that belong to the superfamily of G protein-coupled receptors and share the structural motif of seven transmembrane domains. The activation of octopamine receptors is coupled with different second messenger pathways depending on species, tissue source, receptor type and cell line used for the expression of cloned receptor. The second messengers include adenosine 3′,5′-cyclic monophosphate (cAMP), calcium, diacylglycerol (DAG), and inositol 1,4,5-trisphosphate (IP₃). The cAMP activates protein kinase A, calcium and DAG activate protein kinase C, and IP₃ mobilizes calcium from intracellular stores. Octopamine-mediated generation of these second messengers is associated with changes in cellular response affecting insect behaviors. The main objective of this review is to discuss significance of octopamine-mediated neuromodulation in insect sensory systems.     

The PM1 neurons,movement sensitive centrifugal visual brain neurons in the locust: anatomy,physiology, and modulation by identified octopaminergic neurons

The locust’s optic lobe contains a system of wide-field, multimodal, centrifugal neurons. Two of these cells, the protocerebrum-medulla-neurons PM4a and b, are octopaminergic. This paper describes a second pair of large centrifugal neurons (the protocerebrum-medulla-neurons PM1a and PM1b) from the brain of Locusta migratoria based on intracellular cobalt fills, electrophysiology, and immunocytochemistry. They originate and arborise in the central brain and send processes into the medulla of the optic lobe. Double intracellular recording from the same cell suggests input in the central brain and output in the optic lobe. The neurons show immunoreactivity to gamma-amino-butyric acid and its synthesising enzyme, glutamate decarboxylase. The PM1 cells are movement sensitive and show habituation to repeated visual stimulation. Bath application of octopamine causes the response to dishabituate. A very similar effect is produced by electrical stimulation of one of an octopaminergic PM4 neuron. This effect can be blocked by application of the octopamine antagonists, mianserin and phentolamine. This readily accessible system of four wide-field neurons provides a system suitable for the investigation of octopaminergic effects on the visual system at the cellular level.     

Electrical stimulation of the macaque ventral tegmental area drives category-selective learning without attention

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Biogenic amines in the ponerine ant Harpegnathos saltator: serotonin and dopamine immunoreactivity in the brain

Many studies suggest a role for biogenic amines in a variety of insect behaviors including intraspecific aggression. In ants, despite a rich behavioral repertoire and prominent aggressive interactions, little is known about the potential impact of biogenic amines. This may partly be due to the general lack of information about aminergic systems in the ant brain. The present study investigates serotonergic and dopaminergic neuronal systems in the brain of the ponerine ant Harpegnathos saltator. In H. saltator, intraspecific aggression is important for the regulation of reproduction. This species, therefore, is amenable to comparative studies of aminergic neuronal effects on long-term changes in aggression. Using immunocytochemistry and confocal microscopy, we found that in the brains of sterile workers, the distributions of serotonergic and dopaminergic neuronal processes differed substantially. In addition, branching patterns of serotonergic neurons showed marked differences between males and females. Brains of workers after 3 days and 3 weeks of aggressive interactions revealed no marked differences in serotonergic and dopaminergic neurons compared to those of reproductive and non-aggressive individuals. We conclude that different levels of intraspecific aggression do not involve profound anatomical changes in serotonergic and dopaminergic neurons. Subtle changes may be masked by inter-individual variances.     

Phasic bursting activity of paraventricular neurons is modulated by temperature and angiotensin II

1. Impulse activity of phasically firing (bursting) paraventricular neurons, which are assumed to be of the vasopressinergic type, have been extracellularly recorded in brain slices of the rat.

2. Analysis of burst patterns during temperature changes, angiotensin II application and combined application of both stimuli demonstrated that certain burst parameters are effected much stronger than the mean firing rate and also for a longer period of time.

3. The most sensitive parameter was the intraburst frequency which is considered to be the most effective parameter for increased vasopressin release.

4. These data indicate that there are functionally relevant changes in the impulse patterns which are not necessarily manifested in the mean firing rate.

Neurohormone D induces ionic current changes in cockroach central neurones

The octapeptide neurohormone D (NHD), a member of the family of adipokinetic hormones (AKH-peptides), increases the frequency of spontaneous activity in dorsal unpaired median (DUM) neurones isolated from the terminal ganglion of the cockroach Periplaneta americana. The increase in spike frequency is accompanied by changes in the shape and the amplitude of the single action potentials, e.g. a more pronounced afterhyperpolarization. Effects of NHD on membrane currents were investigated in these DUM cells with whole-cell voltage-clamp measurements. A voltage-independent Ca²⁺ current flowing at the resting potential (I_Ca,R) was found. NHD, at nanomolar concentrations, enhanced this I_Ca,R in a concentration-dependent manner. 0.1 mM Cd²⁺markedly reduced I_Ca,R and in this case I_Ca,R was hardly potentiated by NHD.In the presence of NHD a fast activating Ca²⁺-dependent K⁺current sensitive to charybdotoxin and to low concentrations of tetraethylammonium was augmented. The enhanced afterhyperpolarization of action potentials can be accounted for by the increase in the Ca²⁺-dependent K⁺ current.The changes of the membrane currents induced by NHD are discussed with respect to further effects on the spike pattern and in relation to the previously described mode of action of AKH-peptides in other preparations.Abbreviations NHD neurohormone D - AKH adipokinetic hormone     

Serotonin-induced changes in the excitability of cultured antennal-lobe neurons of the sphinx moth Manduca sexta

The modulatory actions of 5-hydroxy-tryptamine (5HT or serotonin) on a morphologically identifiable class of neurons dissociated from antennal lobes of Manduca sexta at stages 9–15 of the 18 stages of metamorphic adult development were examined in vitro with whole-cell patch-clamp recording techniques. Action potentials could be elicited from approximately 20% of the cells. These cells were used to examine effects of 5HT (5 × 10^–6 to 5 × 10^–4 M) on cell excitability and action-potential waveform. 5HT increased the number of spikes elicited by a constant depolarizing current pulse and reduced the latency of responses. 5HT also led to broadening of action potentials in these neurons and increased cell input resistance. Modulation of potassium channels by 5HT is likely to contribute to these responses. 5HT causes reversible reduction of at least 3 distinct potassium currents, one of which is described for the first time in this study. Because effects of 5HT on antennal-lobe neurons in culture mimic those observed in situ in the brain of the adult moth, in vitro analysis should contribute to elucidation of the cellular mechanisms that underlie the modulatory effects of 5HT on central olfactory neurons in the moth.