Nitric oxide signalling underlies salicylate-induced increases in neuronal firing in the inferior colliculus: a central mechanism of tinnitus?

The tinnitus-inducing agent salicylate reduces cochlear output but causes hyperactivity in higher auditory centres, including the inferior colliculus (the auditory midbrain). Using multi-electrode recording in anaesthetised guinea pigs (Cavia porcellus), we addressed the hypothesis that salicylate-induced hyperactivity in the inferior colliculus involves nitric oxide signalling secondary to increased ascending excitatory input. In the inferior colliculus, systemic salicylate (200 mg/kg i.p., 0 h) markedly increased spontaneous and sound-driven neuronal firing (3-6 h post drug) with both onset and sustained responses to pure tones being massively increased. Reverse microdialysis of increasing concentrations of salicylate directly into the inferior colliculus (100 μM-10 mM, from 0 h) failed to mimic systemic salicylate. In contrast, it caused a small, transient, increase in sound-driven firing (1 h), followed by a larger sustained decrease in both spontaneous and sound-driven firing (2-5 h). When salicylate was given systemically, reverse microdialysis of the neuronal nitric oxide synthase inhibitor L-methyl arginine into the inferior colliculus (500 mM, 2-6 h) completely blocked the salicylate-induced increase in spontaneous and sound-driven neuronal firing. Our data indicate that systemic salicylate induces neuronal hyperactivity in the auditory midbrain via a mechanism outside the inferior colliculus, presumably upstream in the auditory pathway; and that the mechanism is ultimately dependent on nitric oxide signalling within the inferior colliculus. Given that nitric oxide is known to mediate NMDA receptor signalling in the inferior colliculus, we propose that salicylate activates an ascending glutamatergic input to the inferior colliculus and that this is an important mechanism underlying salicylate-induced tinnitus.

Release of Norepinephrine in the Preoptic Area Activates Anteroventral Periventricular Nucleus Neurons and Stimulates the Surge of Luteinizing Hormone

The role of norepinephrine (NE) in regulation of LH is still controversial. We investigated the role played by NE in the positive feedback of estradiol and progesterone. Ovarian-steroid control over NE release in the preoptic area (POA) was determined using microdialysis. Compared with ovariectomized (OVX) rats, estradiol-treated OVX (OVX+E) rats displayed lower release of NE in the morning but increased release coincident with the afternoon surge of LH. OVX rats treated with estradiol and progesterone (OVX+EP) exhibited markedly greater NE release than OVX+E rats, and amplification of the LH surge. The effect of NE on LH secretion was confirmed using reverse microdialysis. The LH surge and c-Fos expression in anteroventral periventricular nucleus neurons were significantly increased in OVX+E rats dialyzed with 100 nm NE in the POA. After Fluoro-Gold injection in the POA, c-Fos expression in Fluoro-Gold/tyrosine hydroxylase-immunoreactive neurons increased during the afternoon in the A2 of both OVX+E and OVX+EP rats, in the locus coeruleus (LC) of OVX+EP rats, but was unchanged in the A1. The selective lesion of LC terminals, by intracerebroventricular N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, reduced the surge of LH in OVX+EP but not in OVX+E rats. Thus, estradiol and progesterone activate A2 and LC neurons, respectively, and this is associated with the increased release of NE in the POA and the magnitude of the LH surge. NE stimulates LH secretion, at least in part, through activation of anteroventral periventricular neurons. These findings contribute to elucidation of the role played by NE during the positive feedback of ovarian steroids.