Proceedings of The Physiological Society

University of Manchester (2010) Proc Physiol Soc 19, PC233

Poster Communications

Electrophysiological properties of neurones in the Intermedius nucleus of the medulla and their responses to electrical stimulation of primary afferent pathways.

I. J. Edwards1, S. A. Deuchars1, J. Deuchars1

1. IMSB, University of Leeds, Leeds, United Kingdom.

We have previously identified the Intermedius nucleus of the Medulla (InM) as a neurochemically diverse perihypoglossal nucleus (Edwards et al., 2009;Edwards et al., 2007). The InM is a source of both excitatory and inhibitory projections to neurones in the nucleus of the solitary tract (Edwards et al., 2007). The InM receives direct afferent projections from the musculature of the neck (Neuhuber and Zenker, 1989). To determine how neurones in the InM respond to such afferent inputs we electrically stimulated the primary afferent pathway through the brainstem whilst recording from InM neurones in the whole cell patch clamp configuration in brainstem slices. 13 day old rats were transcardially perfused with oxygenated sucrose artificial cerebrospinal fluid (containing in mM: sucrose (217); NaCHO3 (26); KCl (3); MgSO4 (2); NaH2PO4 (2.5); CaCl2 (2); glucose (10)) under terminal anaesthesia (urethane 2mg/kg IP). The brainstem was dissected and placed into the same solution before slicing at 300µm and collected into a holding chamber containing oxygenated artificial cerebrospinal fluid (containing in mM: NaCl (124); NaHCO3 (26); KCl (3); MgSO4 (2); NaH2PO4 (2.5); CaCl2 (2); glucose (10)). Visualised patch clamp recordings were then taken from neurones in the InM. A bipolar stimulating electrode was inserted into the fibre tract lateral to the external cuneate nucleus to stimulate passing primary afferent fibres, as primary afferents traced from the upper cervical dorsal root ganglia pass through this region (Neuhuber and Zenker, 1989). InM neurones had a resting membrane potential of -42±3.2mV, displayed tonic firing properties in response to depolarising current pulses and a small hyperpolarisation activated current (n=21). Stimulation of the afferent pathway evoked excitatory post synaptic potentials (EPSPs) in all investigated neurones (n=11). These EPSPs had a latency of 6.7±0.43ms, which displayed minimal synaptic jitter (105±27μs) and an amplitude of 6.1±0.69mV. Bath application of the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX, 10µM, n=13) completely abolished the EPSPs, whilst the NMDA receptor antagonist (2R)-amino-5-phosphonovaleric acid (AP-5, 50µM) had no significant effect upon the EPSPs (n=7). In conclusion, InM neurones receive a strong input from primary afferent fibres which is mediated through non-NMDA receptors. This input is likely to be involved in a variety of physiological responses to changes in the position of the head relative to the neck.

Where applicable, experiments conform with Society ethical requirements