Abstract
Superficial laminae of the spinal cord possess a considerable number of neurons with spontaneous activity as reported in vivo and in vitro preparations of several species. Such neurons may play a role in the development of the nociceptive system and/or in the spinal coding of somatosensory signals. We have used electrophysiological techniques in a horizontal spinal cord slice preparation from adult mice to investigate how this activity is generated and what are the main patterns of activity that can be found. The results show the existence of neurons that fire regularly and irregularly. Within each of these main types, it was possible to distinguish patterns of spontaneous activity formed by single action potentials and different types of bursts according to intra-burst firing frequency. Activity in neurons with irregular patterns was blocked by a mixture of antagonists of the main neurotransmitter receptors present in the cord. Approximately 82% of neurons with a regular firing pattern were insensitive to synaptic antagonists but their activity was inhibited by specific ion channel blockers. It is suggested that these neurons generate endogenous activity due to the functional expression of hyperpolarisation-activated and persistent sodium currents driving the activity of irregular neurons.
Highlights
Spontaneous activity of superficial dorsal horn neurons has been reported in acute preparations using monkeys[1], cats[2] and rats[3] as well as in several in vitro preparations[4,5]
The general picture emerging from these studies suggests that spontaneous activity is not random and that it may play an important role in the coding of sensory information
Using multielectrodes and a flexible semi-automated application for the analysis of spike trains we report the presence of a variety of spontaneous firing patterns among superficial dorsal horn neurons
Summary
Spontaneous activity of superficial dorsal horn neurons has been reported in acute preparations using monkeys[1], cats[2] and rats[3] as well as in several in vitro preparations[4,5]. Using an acute cat preparation, Cervero et al.[7] reported the existence of spinal LII neurons with persistent but irregular spontaneous activity in the 5–10 Hz frequency range. These neurons showed ‘inverted’ responses to innocuous and/or nociceptive mechanical stimulation of the skin consisting on the inhibition of ongoing activity. Li & Baccei[4] distinguished irregular, tonic and bursting neurons in LI of rat spinal cord slice preparations. No Neurons % total Mean frequency in 5 min (Hz) Mean inter-burst instant freq (Hz) Mean intra-burst instant freq (Hz) CV for classification CV intraburst
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