Local circuit connections between hamster laminae III and IV dorsal horn neurons

To better understand the role of intrinsic spinal cord circuits in the integration of mechanosensory information, we studied synaptic transmission between neurons in Rexed's laminae III-IV, a major termination zone for cutaneous mechanoreceptor afferents, using dual, simultaneous whole cell electrophysiological recordings in young hamsters. Synaptic connections were detected between 32 of 106 cell pairs (linkage probability of 0.3) and were predominantly unidirectional (91%). Inhibitory connections outnumbered excitatory connections by 2:1. Amplitude of single-axon postsynaptic potentials (PSPs) was independent of postsynaptic cell input resistance. Intracellular labeling suggested that recordings were obtained from local axon interneurons. In connected cell pairs, the percentage of presynaptic action potentials that failed to evoke a postsynaptic response was 44 +/- 29%. Shape indices of PSPs suggested that synaptic contacts were widely distributed along the postsynaptic membrane. Linkage probability was unrelated to intrinsic firing properties, laminar position of the cells or the distance (<160 mum) separating them. However, PSPs in target cells following action potentials in neurons with phasic firing patterns had longer duration and lower failure rates than PSPs activated by neurons with tonic firing patterns. Thus transmission reliability at synapses between lamina III/IV interneurons overall is low, and efficacy of these connections is related to firing properties of the presynaptic cells. The observations also suggest that synaptic organization in LIII-IV is fundamentally different from the superficial dorsal horn (LI-II) where neural circuits may be composed of stereotyped units made from connections between a few functional types of neurons.