Spinal interneurons in sensorimotor integration

Abstract

Even though spinal cord research has expanded enormously during the past decades, we still lack a precise understanding of how spinal interneuron networks perfectly integrate sensory feedback with motor control, and how these neuron circuits give rise to specific functions. The present study thus has three basic aims: (1) to investigate propriospinal interneurons connecting rostral and caudal lumbar spinal cord in the rat; (2) to investigate input properties of identified spinal interneurons interposed in different pathways; (3) to investigate cholinergic terminals in the ventral horn of adult rat and cat. To realize the first aim, the B-subunit of cholera toxin (CTb) was injected into the motor nuclei at the L1 or L3 segmental level to retrogradely label propriospinal interneurons in the L5 segment of rat spinal cord. These cells had a clear distribution pattern which showed that they were located mainly in ipsilateral dorsal horn and contralateral lamina VIII. A series triple-labelling experiments revealed that about 1/4 of the CTb-positive cells were immunoreactive for calbindin and/or calretinin. It was also found that a small population of CTb labelled cells were cholinergic and were observed mainly in three locations: lamina X, the medial part of intermediate zone and lamina VIII. In addition, injection of CTb also anterogradely labelled axon terminals, which arose from the commissural interneurons (CINs) within the site of injection, crossed the midline and aroborized in the contralateral lateral motor nuclei of the L5 segment. The neurotransmitter systems in labelled axon terminals of CINs were investigated by using antibodies raised against specific transmitter-related proteins. The results showed that approximately 3/4 terminals were excitatory and among those excitatory terminals about 3/4 forming contacts with motoneurons. To achieve the second aim, 21 interneurons located in the intermediate zone and lamina VIII from 7 adult cats were characterised electrophysiologically and labelled intracellularly with Neurobiotin. Seventeen of these cells were activated monosynaptically from primary muscle afferents but the remaining four cells received monosynaptic inputs from the medial longitudinal fasciculus (MLF). Quantitative analysis revealed that cells in the first group received many contacts from excitatory terminals that were immunoreactive for the vesicular glutamate transporter 1 (VGLUT1) but those cells from the second group received few contacts of this type and were predominantly contacted by terminals immunoreactive for vesicular glutamate transporter 2 (VGLUT2). This result was as predicted because VGLUT1 is found principally in the terminals of myelinated primary afferent axons whereas VGLUT2 is located in the terminals of interneurons in the spinal cord. Interneurons in the first group were then characterised as excitatory and inhibitory on the basis of the transmitter content contained within their axon terminals. Although there was a greater density of VGLUT1 contacts on excitatory rather than inhibitory cells, the difference was not statistically significant. GABAergic terminals formed close appositions with VGLUT1 contacts on both excitatory and inhibitory cells. These appositions were likely to be axoaxonic synapses which mediate presynaptic inhibition. In addition, the densities of VGLUT1 and VGLUT2 contacts on 30 dorsal horn CINs and 60 lamina VIII CINs that were retrogradely labelled with CTb from 3 adult rats were compared. The results showed that VGLUT2 terminals formed the majority of excitatory inputs to both dorsal horn and lamina VIII CINs but dorsal horn CINs received a significantly greater density of VGLUT1/2 inputs than lamina VIII CINs. In order to achieve the third aim, i.e. whether glutamate is a cotransmitter at motoneuron axon collateral terminals in the ventral horn, a series of anatomical experiments were performed on axon collaterals obtained from motoneurons from an adult cat and retrogradely labelled by CTb in adult rats. There was no evidence to support the presence of vesicular glutamate transporters in motoneuron axon terminals of either species. In addition, there was no obvious relationship between motoneuron terminals and R2 subunit of the AMPA receptor (GluR2). However, a population of cholinergic terminals in lamina VII, which did not originate from motoneurons, was found to be immunoreactive for VGLUT2 and formed appositions with GluR2 subunits. These terminals were smaller than motoneuron terminals and, unlike them, formed no relationship with Renshaw cells. The evidence suggests that glutamate does not act as a cotransmitter with acetylcholine at central synapses of motoneurons in the adult cat and rat. However, glutamate is present in a population of cholinergic terminals which probably originate from interneurons where its action is via an AMPA receptor. In conclusion, the present studies add to the understanding of the organization of neuronal networks involved in sensorimotor integration. Propriospinal interneurons located within the lumbar segments have extensive intra-segmental projections to motor nuclei. First order interneurons interposed in reflex pathways and descending pathways receive a significantly different pattern of inputs. A similar proportion of monosynaptic excitatory input from primary afferents has been found in both excitatory and inhibitory interneurons and these two types of cells are subject to presynaptic inhibitory control of this input