NERVUS-TERMINALIS GANGLION OF THE BONNETHEAD SHARK (SPHYMA-TIBURO) - EVIDENCE FOR CHOLINERGIC AND CATECHOLAMINERGIC INFLUENCE ON 2 CELL-TYPES DISTINGUISHED BY PEPTIDE IMMUNOCYTOCHEMISTRY

The nervus terminalis is a ganglionated vertebrate cranial nerve of un known function that connects the brain and the peripheral nasal struct ures. To investigate its function, we have studied nervus terminalis g anglion morphology and physiology in the bonnethead shark (Sphyrna tib uro), where the nerve is particularly prominent. Immunocytochemistry f or gonadotropin-releasing hormone (GnRH) and Leu-Pro-Leu-Arg-Phe-NH2 ( LPLRFamide) revealed two distinct populations of cells. Both were acet ylcholinesterase positive, but LPLRFamide-immunoreactive cells consist ently stained more darkly for acetylcholinesterase activity. Tyrosine hydroxylase immunocytochemistry revealed fibers and terminal-like punc ta in the ganglion, primarily in areas containing GnRH-immunoreactive cells. Consistent with the anatomy, in vitro electrophysiological reco rdings provided evidence for cholinergic and catecholaminergic actions . In extracellular recordings, acetylcholine had a variable effect on baseline ganglion cell activity, whereas norepinephrine consistently r educed activity. Electrical stimulation of the nerve trunks suppressed ganglion activity, as did impulses from the brain in vivo. During ele ctrical suppression, acetylcholine consistently increased activity, an d norepinephrine decreased activity. Muscarinic and, to a lesser exten t, alpha-adrenergic antagonists both increased activity during the ele ctrical suppression, suggesting involvement of both systems. Intracell ular recordings revealed two types of ganglion cells that were disting uishable pharmacologically and physiologically. Some cells were hyperp olarized by cholinergic agonists and unaffected by norepinephrine; the se cells did not depolarize with peripheral nerve trunk stimulation. A nother group of cells did depolarize with peripheral trunk stimulation ; a representative of this group was depolarized by carbachol and hype rpolarized by norepinephrine. These and other data suggest that the bo nnethead nervus terminalis ganglion contains at least two cell populat ions that respond differently to acetylcholine and norepinephrine. The bonnethead nervus terminalis ganglion appears to differ fundamentally from sensory and autonomic ganglia but does share some features with the neural circuits of forebrain GnRH systems. (C) 1995 Wiley-Liss, In c.