Distribution of Glycine/GABA Neurons in the Ventromedial Medulla with Descending Spinal Projections and Evidence for an Ascending Glycine/GABA Projection

Abstract

The ventromedial medulla (VM), subdivided in a rostral (RVM) and a caudal (CVM) part, has a powerful influence on the spinal cord. In this study, we have identified the distribution of glycine and GABA containing neurons in the VM with projections to the cervical spinal cord, the lumbar dorsal horn, and the lumbar ventral horn. For this purpose, we have combined retrograde tracing using fluorescent microspheres with fluorescent in situ hybridization (FISH) for glycine transporter 2 (GlyT2) and GAD67 mRNAs to identify glycinergic and/or GABAergic (Gly/GABA) neurons. Since the results obtained with FISH for GlyT2, GAD67, or GlyT2+GAD67 mRNAs were not significantly different, we concluded that glycine and GABA coexisted in the various projection neurons. After injections in the cervical cord, we found that 29%±1 (SEM) of the retrogradely labeled neurons in the VM were Gly/GABA (RVM: 43%; CVM: 21%). After lumbar dorsal horn injections 31%±3 of the VM neurons were Gly/GABA (RVM: 45%; CVM: 12%), and after lumbar ventral horn injections 25%±2 were Gly/GABA (RVM: 35%; CVM: 17%). In addition, we have identified a novel ascending Gly/GABA pathway originating from neurons in the area around the central canal (CC) throughout the spinal cord and projecting to the RVM, emphasizing the interaction between the ventromedial medulla and the spinal cord. The present study has now firmly established that GABA and glycine are present in many VM neurons that project to the spinal cord. These neurons strongly influence spinal processing, most notably the inhibition of nociceptive transmission.

Neurotransmitter phenotypes of descending systems in the rat lumbar spinal cord

“…In contrast only approximately 59% of ReST fibres labelled by injection of CTb to MLF contained VGLUT2 and a considerable proportion expressed VGAT. Again this is consistent with previous reports of the existence of descending inhibitory systems from the rat medulla using anterograde (Holstege, 1991;Holstege and Bongers, 1991;Antal et al, 1996) or retrograde (Vetrivelan et al, 2009;Martin et al, 2011;Hossaini et al, 2012) labelling methods. Co-localisation of GABA and glycine has been reported for numerous spinal interneurons, especially those located in the dorsal horn (e.g.…”

“…Todd and Spike, 1993) but our results indicate that most descending ReST axons labelled from the MLF are purely GABAergic or purely glycinergic. This finding is somewhat at variance with a recent report which suggested that many spinallyprojecting neurons in the ventromedial medulla express a mixture of mRNAs for GAD67 and GLYT2 (Hossaini et al, 2012). The reason for this difference is unclear; however the method employed by these authors was significantly different to the approach used here as it involved retrograde tracing and use of a mixture fluorescence in situ hybridisation probes.…”

“…see Wilson and Yoshida, 1969a;Jankowska et al, 2003;Edgley et al, 2004;Hammar et al, 2011). However, some ReST axons are GABAergic (Holstege, 1991;Antal et al, 1996) or glycinergic (Holstege and Bongers, 1991) and recent evidence suggests that these transmitters may be colocalised in the same neurons (Hossaini et al, 2012).…”

Antinociceptive effect of intracerebroventricular administration of glycine transporter-2 inhibitor ALX1393 in rat models of inflammatory and neuropathic pain

“…Previous studies have demonstrated that glycinergic neurons from the rostral ventromedial medulla (RVM) project directly to the spinal cord, and contribute to an inhibition of spinal nociception (Antal et al, 1996;Millan, 2002;Kato et al, 2006;Hossaini et al, 2012). Kato et al (2006) used in vivo patch recording to show that inhibitory postsynaptic currents (IPSCs) from the substantia gelatinosa neurons of the spinal cord evoked by electrical stimulation of the RVM were blocked by strychnine.…”

Supraspinal respiratory plasticity following acute cervical spinal cord injury

“…Moreover, increased tonic activity in the ipsilateral phrenic nerve was seen in these early stages of recovery, similar to what was reported by Lee et al (2013) in chronically injured (C2Hx) rats. We hypothesize that increased tonic activity in the ipsilateral phrenic nerve following C2Hx reflects the disruption of inhibitory control from bulbospinal and spinal circuits and results in an imbalance between descending inhibitory (Blessing, 1990; Holmes et al, 1994; Hossaini et al, 2012) and excitatory (for e.g., 5-HT tonic drive from raphe nuclei) inputs to phrenic motoneurons.…”