Circuit breaking the synaptic connections of the superficial dorsal horn of the spinal cord

The dorsal horn of the spinal cord is the site of the first synapse in sensory pathways for all of the bodily tissues, except those innervated by the trigeminal nerve. A huge variety of cutaneous, deep somatic and visceral primary afferent fibres terminate there, where subsequent integration occurs with other inputs prior to onward transmission by second order neurons to more rostral regions of the CNS. Sensory processing of information carried by thinly myelinated and unmyelinated nerve fibres, which encompasses a diverse range of sensory homeostatic modalities, such as pain, temperature, affective touch and itch, occurs predominantly in the superficial dorsal horn (laminae I and II). Lamina I is the principal output region of the superficial dorsal horn, whereas lamina II (the substantia gelatinosa) is essentially composed solely of interneurons. In recent years emphasis has been placed on the ascending projections from lamina I, on the basis that, at least in primates and carnivores, about half of the spinothalamic tract, which is critical for human pain and temperature sensation, originates in lamina I. Although these analyses have shed considerable light on the physiology of the principal output pathway from the superficial dorsal horn (Craig, 2003), integration and intrinsic processing by spinal neurons prior to the output stage has proved a complex problem. In part this is due to the bewilderingly complex cytoarchitecture of the superficial dorsal horn as well as the technical difficulty of recording from these neurons, which are amongst the smallest in the spinal cord. Prior in vitro studies of spinal cord slices have identified at least nine specific subsets of lamina II neurons that show distinct biophysical and anatomical properties (Grudt & Perl, 2002), but a holistic understanding of the inputs these neurons receive, how they interact with each other as well as their relationship with projection neurons has seemed a distant aspiration until recently. The study of Zheng and colleagues in a recent issue of The Journal of Physiology (Zheng et al. 2010) is the latest in a series of influential papers from Edward Perl's laboratory that have identified modular circuits within lamina II that are thought to be organized functionally, and therefore represent specific neuronal pathways for subsets of primary afferent fibres. In a technical tour de force, Zheng et al. used paired whole cell recordings in vitro to identify the synaptic connections of a defined subset of lamina II GABAergic neurons that expressed green fluorescent protein (GFP) under control of the mouse prion promoter (GFP-PrP). They showed that there were monosynaptic inhibitory connections between GFP-labelled neurons and islet (inhibitory) neurons and between GFP-labelled neurons and vertical (excitatory) neurons. Moreover, the GFP-labelled neurons received primary afferent inputs from nerve fibres that expressed the heat-sensitive receptor TRPV1 as well as the cooling-sensitive receptors TRPM8/TRPA1, whereas islet cells received inputs from TRPV1-expressing nerve fibres, but not from TRPM8/TRPA1-expressing fibres. Notwithstanding the technical skill and determination needed for this study, the results are significant for several reasons. Firstly, they identify functional interactions between specific classes of lamina II interneurons that enable modification of sensory processing prior to transmission to output neurons. Secondly, they confirm, albeit using a small sample size, that biophysically and anatomically distinct spinal neurons do receive inputs from specific classes of primary afferents, which are probably functionally distinct, i.e. they may contribute to different sensations. This latter conclusion has been difficult to reconcile at the whole animal level, as prior studies published in this journal (Graham et al. 2004) did not identify any systematic relationship between biophysical and receptive properties of lamina II neurons in vivo. Whether this is due to differences in the classification schema used by Grudt & Perl (2002) and that used by Graham et al. (2004) or due to the reduced preparation used in in vitro experiments is open to interpretation, but an ex vivo preparation (Koerber & Woodbury, 2002), where receptive fields are preserved, but which is sufficiently flexible to permit cell physiology studies of spinal neurons may present a more tractable solution to this issue. By virtue of the study design, the GFP-PrP neurons studied by Zheng et al. were a homogeneous population of GABAergic interneurons with known biophysical and anatomical properties (tonic-central). Whether other inhibitory interneurons display similar homogeneity is unknown, but immunocytochemical studies reveal that there may be up to eight neurochemically distinct groups on the basis of GABA/glycine expression and co-expression of other neurochemicals such as neuropeptides, receptors, enzymes or Ca2+-binding proteins (Todd & Koerber, 2005). How the GFP-PrP neurons fit in with the neurochemical classification scheme is currently unknown, but if a coherent model of neuronal circuitry in the superficial dorsal horn is to be achieved it will be a significant task to integrate these separate classification schema. Although excitatory interneurons were mentioned only briefly in the study of Zheng et al. they are, on the basis of weight of numbers, the predominant class of interneurons in the spinal cord, comprising about 70% of the neurons in laminae I and II. The major challenge for the future will be to systematically characterize these populations, and to integrate data from excitatory and inhibitory interneurons as well as projection neurons into a single model that could be tested in vivo.