Excitatory and inhibitory intermediate zone interneurons in pathways from feline group I and II afferents: differences in axonal projections and input

Abstract

The aim of the present study was to compare properties of excitatory and inhibitory spinal intermediate zone interneurons in pathways from group I and II muscle afferents in the cat. Interneurons were labelled intracellularly and their transmitter phenotypes were defined by using immunocytochemistry. In total 14 glutamatergic, 22 glycinergic and 2 GABAergic/ glycinergic interneurons were retrieved. All interneurons were located in laminae V-VII of the L3-L7 segments. No consistent differences were found in the location, the soma sizes or the extent of the dendritic trees of excitatory and inhibitory interneurons. However, major differences were found in their axonal projections; excitatory interneurons projected either ipsilaterally, bilaterally or contralaterally, while inhibitory interneurons projected exclusively ipsilaterally. Terminal projections of glycinergic and glutamatergic cells were found within motor nuclei as well as other regions of the grey matter which include the intermediate region, laminae VII and VIII. Cells containing GABA/glycine had more restricted projections, principally within the intermediate zone where they formed appositions with glutamatergic axon terminals and unidentified cells and therefore are likely to be involved in presynaptic as well as postsynaptic inhibition. The majority of excitatory and inhibitory interneurons were found to be coexcited by group I and II afferents (monosynaptically) and by reticulospinal neurons (mono-or disynaptically) and to integrate information from several muscles. Taken together the morphological and electrophysiological data show that individual excitatory and inhibitory intermediate zone interneurons may operate in a highly differentiated way and thereby contribute to a variety of motor synergies.

Firing and Cellular Properties of V2a Interneurons in the Rodent Spinal Cord

“…Most of the Chx10 cells have short segmental axons, whereas only a minor fraction has long projecting axons. In contrast, the Chx10 neuronal projections appear to be similar to those recently seen in excitatory, ipsilateral, lamina VII neurons in cat (Bannatyne et al, 2009). We propose that the short projecting Chx10 neurons are those involved in coordination of left-right alternation during locomotion (Crone et al, 2008).…”

Bicuculline-Sensitive Primary Afferent Depolarization Remains after Greatly Restricting Synaptic Transmission in the Mammalian Spinal Cord

“…some afferents may require minimally trisynaptic pathways on some afferents but may generate more direct negative feedback onto homonymous terminals (Wall, 1958;Olave et al, 2002;Bannatyne et al, 2009). Regardless, given that most of the DRP remains after synaptic transmission reduction, it seems likely that many low-threshold afferents produce PAD by more direct circuits.…”

“…In contrast, recent anatomical evidence demonstrated that only a single interneuron is interposed in some disynaptic circuits that could produce PAD of lower threshold afferents (Olave et al, 2002;Bannatyne et al, 2009). In addition, nonspiking spinal microcircuits in turtle have been shown to be sufficient to generate afferent-evoked PAD in TTX-insensitive high-threshold afferents, further implicating more direct negative feedback mechanisms controlling afferent activity (Russo et al, 2000).…”

Genetically Defined Inhibitory Neurons in the Mouse Spinal Cord Dorsal Horn: A Possible Source of Rhythmic Inhibition of Motoneurons during Fictive Locomotion

“…That is, unless the transmitter phenotype differs between these two species, there are at least three distinct populations of inhibitory neurons located in this region of the spinal cord. Bannatyne et al (2009) have recently characterized inhibitory interneurons in the lumbar intermediate gray matter in the cat. They identified neurons that receive low-threshold muscle afferent input and project ipsilaterally, including to motor pools.…”

“…The GFPϩ neurons in this study may be homologous to any of a number of inhibitory interneurons identified in the cat (Jankowska, 1992;Bannatyne et al, 2009). For example, they may be a subpopulation of interneurons receiving group II input (Jankowska, 1992).…”