Noradrenaline Decreases Spike Voltage Threshold and Induces Electrographic Sharp Waves in Turtle Medial Cortex in vitro

Abstract

The noradrenergic modulation of neuronal properties has been described at different levels of the mammalian brain. Although the anatomical characteristics of the noradrenergic system are well known in reptiles, functional data are scarce. In our study the noradrenergic modulation of cortical electrogenesis in the turtle medial cortex was studied in vitro using a combination of field and intracellular recordings. Turtle EEG consists of a low voltage background interspersed by spontaneous large sharp waves (LSWs). Noradrenaline (NA, 5–40 µM) induced (or enhanced) the generation of LSWs in a dose-dependent manner. Pharmacological experiments suggest the participation of α and β receptors in this effect. In medial cortex neurons NA induced a hyperpolarization of the resting potential and a decrease of input resistance. Both effects were observed also after TTX treatment. Noradrenaline increased the response of the cells to depolarizing pulses, resulting in an upward shift of the frequency/current relation. In most cells the excitability change was mediated by a decrease of the spike voltage threshold resulting in the reduction of the amount of depolarization needed to fire the cell (voltage threshold minus resting potential). As opposed to the mechanisms reported in mammalian neurons, no changes in the frequency adaptation or the post-train afterhyperpolarization were observed. The NA effects at the cellular level were not reproduced by noradrenergic agonists. Age- and species-dependent properties in the pharmacology of adrenergic receptors could be involved in this result. Cellular effects of NA in turtle cortex are similar to those described in mammals, although the increase in cellular excitability seems to be mediated by a different mechanism.

Hippocampal memory consolidation during sleep: a comparison of mammals and birds

“…During sleep, and to a lesser extent wakefulness, the reptilian medial cortex-the developmental homologue of the mammalian and avian hippocampus-generates distinct high-amplitude, sharp-waves that share several characteristics in common with mammalian hippocampal sharpwaves. The sharp-waves in both groups increase following sleep deprivation and respond similarly to pharmacological agents (Hartse & Rechtschaffen, 1982;Gaztelu García-Austt & Bullock, 1991;Hartse, 1994;Lorenzo, Macadar & Velluti, 1999;Lorenzo & Velluti, 2004;Rattenborg, 2007). Unlike mammalian sharp-waves, however, reptilian sharp-waves do not occur in conjunction with slow-oscillations (as measured by the EEG) in the dorsal cortex, an area thought to be homologous to neocortical primary visual and somatosensory areas (Medina & Reiner, 2000;).…”