Spider peripheral mechanosensory neurons are directly innervated and modulated by octopaminergic efferents.
Octopamine is a chemical relative of noradrenaline providing analogous neurohumoral control of diverse invertebrate physiological processes. There is also evidence for direct octopaminergic innervation of some insect peripheral tissues. Here, we show that spider peripheral mechanoreceptors are innervated by octopamine-containing efferents. The mechanosensory neurons have octopamine receptors colocalized with synapsin labeling in the efferent fibers. In addition, octopamine enhances the electrical response of the sensory neurons to mechanical stimulation. Spider peripheral mechanosensilla receive extensive efferent innervation. Many efferent fibers in the legs of Cupiennius salei are GABAergic, providing inhibitory control of sensory neurons, but there is also evidence for other neurotransmitters. We used antibody labeling to show that some efferents contain octopamine and that octopamine receptors are concentrated on the axon hillocks and proximal soma regions of all mechanosensory neurons in the spider leg. Synaptic vesicles in efferent neurons were concentrated in similar areas. Octopamine, or its precursor tyramine, increased responses of mechanically stimulated filiform (trichobothria) leg hairs. This effect was blocked by the octopamine antagonist phentolamine. The octopamine-induced modulation was mimicked by 8-Br-cAMP, a cAMP analog, and blocked by Rp-cAMPS, a protein kinase A inhibitor, indicating that spider octopamine receptors activate adenylate cyclase and increase cAMP concentration. Frequency response analysis showed that octopamine increased the sensitivity of the trichobothria neurons over a broad frequency range. Thus, the major effect of octopamine is to increase its overall sensitivity to wind-borne signals from sources such as flying insect prey or predators.
Widmer
Alexandre
A
Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, B3H 1X5 Canada.
Höger
Ulli
U
Meisner
Shannon
S
French
Andrew S
AS
Torkkeli
Päivi H
PH
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D001369:Axons / Q000737:chemistry / Q000648:ultrastructure; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology / Q000502:physiology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D004525:Efferent Pathways / Q000502:physiology*; D005260:Female; D008297:Male; D008465:Mechanoreceptors / Q000187:drug effects / Q000502:physiology*; D040542:Mechanotransduction, Cellular / Q000187:drug effects / Q000502:physiology*; D009655:Octopamine / Q000494:pharmacology / Q000502:physiology*; D010477:Perfusion; D010646:Phentolamine / Q000494:pharmacology; D017982:Receptors, Biogenic Amine / Q000819:agonists / Q000037:antagonists & inhibitors / Q000502:physiology*; D015290:Second Messenger Systems / Q000502:physiology; D013112:Spiders / Q000502:physiology*; D013572:Synaptic Vesicles / Q000648:ultrastructure; D013873:Thionucleotides / Q000494:pharmacology
D017982:Receptors, Biogenic Amine; D013873:Thionucleotides; C027604:norsynephrine receptor; D009655:Octopamine; D015124:8-Bromo Cyclic Adenosine Monophosphate; C016957:adenosine-3',5'-cyclic phosphorothioate; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D000262:Adenylyl Cyclases; D010646:Phentolamine