Metabolic Maturation of Auditory Neurones in the Superior Olivary Complex

<div><p>Neuronal activity is energetically costly, but despite its importance, energy production and consumption have been studied in only a few neurone types. Neuroenergetics is of special importance in auditory brainstem nuclei, where neurones exhibit various biophysical adaptations for extraordinary temporal precision and show particularly high firing rates. We have studied the development of energy metabolism in three principal nuclei of the superior olivary complex (SOC) involved in precise binaural processing in the Mongolian gerbil (<i>Meriones unguiculatus</i>). We used immunohistochemistry to quantify metabolic markers for energy consumption (Na⁺/K⁺-ATPase) and production (mitochondria, cytochrome c oxidase activity and glucose transporter 3 (GLUT3)). In addition, we calculated neuronal ATP consumption for different postnatal ages (P0–90) based upon published electrophysiological and morphological data. Our calculations relate neuronal processes to the regeneration of Na⁺ gradients perturbed by neuronal firing, and thus to ATP consumption by Na⁺/K⁺-ATPase. The developmental changes of calculated energy consumption closely resemble those of metabolic markers. Both increase before and after hearing onset occurring at P12–13 and reach a plateau thereafter. The increase in Na⁺/K⁺-ATPase and mitochondria precedes the rise in GLUT3 levels and is already substantial before hearing onset, whilst GLUT3 levels are scarcely detectable at this age. Based on these findings we assume that auditory inputs crucially contribute to metabolic maturation. In one nucleus, the medial nucleus of the trapezoid body (MNTB), the initial rise in marker levels and calculated ATP consumption occurs distinctly earlier than in the other nuclei investigated, and is almost completed by hearing onset. Our study shows that the mathematical model used is applicable to brainstem neurones. Energy consumption varies markedly between SOC nuclei with their different neuronal properties. Especially for the medial superior olive (MSO), we propose that temporally precise input integration is energetically more costly than the high firing frequencies typical for all SOC nuclei.</p></div

Do current environmental conditions explain physiological and metabolic responses of subterranean crustaceans to cold ?

International audienceSubterranean environments are characterized by the quasi absence of thermal variations (±1°C within a year), and organisms living in these biotopes for several millions of years, such as hypogean crustaceans, can be expected to have adapted to this very stable habitat. As hypogean organisms experience minimal thermal variation in their native biotopes, they should not be able to develop any particular cold adaptations to cope with thermal fluctuations. Indeed, physiological responses of organisms to an environmental stress are proportional to the amplitude of the stress they endure in their habitats. Surprisingly, previous studies have shown that a population of an aquatic hypogean crustacean, Niphargus rhenorhodanensis, exhibited a high level of cold hardiness. Subterranean environments thus appeared not to be following the classical above-mentioned theory. To confirm this counter-example, we studied seven karstic populations of N. rhenorhodanensis living in aquifers at approximately 10°C all year round and we analysed their behavioural, metabolic and biochemical responses during cold exposure (3°C). These seven populations showed reduced activities, and some cryoprotective molecules were accumulated. More surprisingly, the amplitude of the response varied greatly among the seven populations, despite their exposure to similar thermal conditions. Thus, the overall relationship that can be established between the amplitude of thermal variations and cold-hardiness abilities of ectotherm species may be more complex in subterranean crustaceans than in other arthropods

Responses to cold and phylogenetic structure in the subterranean crustacean Niphargus rhenorhodanensis.

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COX activity during development in the auditory brainstem.

<p>Representative images of COX activity stainings at different developmental stages of Mongolian gerbils. Images A–D show an overview of the auditory brainstem, E–H show the lateral superior olive (LSO), I–L show the medial nucleus of the trapezoid body (MNTB) and M–P show the medial superior olive (MSO). Images were taken at P7 (A, E, I & M), P10 (B, F, J & N), P14 (C, G, K & O), and P30 (D, H, L & P). Orientation in the brainstem is given in A: D., dorsal; V., ventral; M., medial; L., lateral. Scale bar = 100 µm.</p

Metabolic maturation in the MNTB.

<p>MNTB neurones of Mongolian gerbils at P7 (A–D), P10 (E–H), P14 (I–L) or P30 (M–P) were immunohistochemically stained for Na⁺/K⁺-ATPase (A, E, I, M), synapsin (B, F, J, N), mitochondria (C, G, K, O) or GLUT3 (D, H, L, P). The sample stainings depicted for Na⁺/K⁺-ATPase and synapsin were taken from a double-staining of both markers in the same sections. Scale bar = 20 µm.</p

Control experiments for COX activity detection.

<p>(A) The majority of the colour reaction is caused by the specific COX activity. The figure depicts one example (LSO, P7) of a control experiment to test for a reaction of unspecific oxidative enzymes. To this, we incubated brain sections in staining solution for COX reaction containing either cytochrome c (left) or no cytochrome c (right). Scale bar = 100 µm. In B the mean average intensity of the unspecific reaction of 3′3-diaminobenzidine was subtracted from the intensity of the specific COX staining. White pixels represent a value above threshold. (C) COX activity in cerebellar Purkinje cells of gerbils aged P7-P30. Scale bars = 100 µm.</p

Comparison of time course of total energy consumption (E_total) and development of metabolic markers.

<p>The grey area depicts E_total per cell between low (lower margin of area) and high mean firing rates (upper margin of area) for MSO (A), LSO (B), and MNTB (C). For the low rate we used 10 Hz before and after hearing onset (both inputs and postsynaptic AP generation). As an upper estimation we used 100 Hz for all components before hearing onset and after hearing onset 200 Hz (postsynaptic AP generation) and 400 Hz (inputs), respectively. The black line represents the values for 100 Hz before and after hearing onset (both inputs and postsynaptic AP generation) depicted in Fig. 7. HO, hearing onset.</p

Energy consumption in SOC nuclei (at 100 Hz) increases during maturation.

<p>The calculated values for total ATP consumption (E_total) per cell as well as the individual components E_Vr (energy for maintenance of resting membrane potential), E_AP (energy for generation of somatic and dendritic APs, and E_post (energy for postsynaptic excitatory currents) rise during development for MSO (A), LSO (B), and MNTB (C). The firing frequency for both inputs and postsynaptic AP generation was 100 Hz. Note the difference in total energy for the three nuclei. HO, hearing onset.</p