Experiments were done to investigate whether hypothalamic hypocretin-1 (hcrt-1; orexin-A) neurons that sent axonal projections to cardiovascular responsive sites in the nucleus of the solitary tract (NTS) co-expressed leucine-enkephalin (L-Enk), and to determine the effects of co-administration of hcrt-1 and D-Ala2,D-Leu5-Enkephalin (DADL) into NTS on mean arterial pressure (MAP) and heart rate. In the first series, in the Wistar rat the retrograde tract-tracer fluorogold (FG) was microinjected (50nl) into caudal NTS sites at which L-glutamate (0.25 M; 10 nl) elicited decreases in MAP and where fibers hcrt-1 immunoreactive fibers were observed that also contained L-Enk immunoreactivity. Of the number of hypothalamic hcrt-1 immunoreactive neurons identified ipsilateral to the NTS injection site (1207 ± 78), 32.3 ± 2.3% co-expressed L-Enk immunoreactivity and of these, 2.6 ± 1.1% were retrogradely labeled with FG. Hcrt-1/L-Enk neurons projecting to NTS were found mainly within the perifornical region. In the second series, the region of caudal NTS found to contain axons that co-expressed hcrt-1 and L-Enk immunoreactivity was microinjected with a combination of hcrt-1 and DADL in α-chloralose anesthetized Wistar rats. Microinjection of DADL into NTS elicited depressor and bradycardia responses similar to those elicited by microinjection of hcrt-1. An hcrt-1 injection immediately after the DADL injection elicited an almost twofold increase in the magnitude of the depressor and bradycardia responses compared to those elicited by hcrt-1 alone. Prior injections of the non-specific opioid receptor antagonist naloxone or the specific opioid δ-receptor antagonist ICI 154,129 significantly attenuated the cardiovascular responses to the combined hcrt-1-DADL injections. Taken together, these data suggest that activation of hypothalamic-opioidergic neuronal systems contribute to the NTS hcrt-1 induced cardiovascular responses, and that this descending hypothalamo-medullary pathway may represent the anatomical substrate by which hcrt-1/L-Enk neurons function in the coordination of autonomic-cardiovascular responses during different behavioral states.
Keywords: 3rd ventricle; 4V; 4th ventricle; AP; Ap; Com; D-Ala2,D-Leu5-Enkephalin; DADL; DMH; DMV; DOR; FG; Fluorogold; HR; ICI 154,129; L-Enk; LHA; MAP; MOR; NTS; PBS; PVp; PeF; STN; Sc; Sg; Slt; Sm; Svl; V3; VMH; XII; ZI; area postrema; arterial pressure; blood pressure regulation; cc; central canal; central opioid systems; central subnucleus of nucleus of the solitary tract; commissural subnucleus of nucleus of the solitary tract; dorsal motor nucleus of the vagus; dorsomedial hypothalamic nucleus; fasiculus gracilis; fornix; fx; gelantinus subnucleus of nucleus of the solitary tract; gr; hcrt; hcrt-1; hcrt-2; heart rate; hypocretin (orexin); hypocretin-1 (orexin-A); hypocretin-2 (orexin-B); hypoglossal nucleus; ic; internal capsule; lateral hypothalamic area; lateral subnucleus of nucleus of the solitary tract; leucine-enkephalin; mammilothalamic tract; mean arterial pressure; medial subnucleus of nucleus of the solitary tract; mt; nal; naloxone; nucleus of the solitary tract; orexin; perifornical hypothalamic area; periventricular hypothalamic area; phosphate-buffered saline; subthalamic nucleus; ventrolateral subnucleus of nucleus of the solitary tract; ventromedial hypothalamic nucleus; zona incerta; δ- and μ-opioid receptors; δ-opioid receptor; δ-opioid receptor antagonist N-diallyl-Tyr-GIy-Gly-(CH2S)Phe-Leu; μ-opioid receptor.
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