MONOSYNAPTIC CONNECTIONS BETWEEN PRIMARY AFFERENTS AND GIANT-NEURONS IN THE TURTLE SPINAL DORSAL HORN

This paper reports the occurrence of monosynaptic connections between dorsal root afferents and a distinct cell type - the giant neuron - de ep in the dorsal horn of the turtle spinal cord. Light microscope stud ies combining Nissl stain and transganglionic HRP-labeling of the prim ary afferents have revealed the occurrence of axosomatic and axodendri tic contacts between labeled boutons and giant neurons. The synaptic n ature of these contacts has been confirmed by use of electron microsco pe procedures involving the partial three-dimensional reconstruction o f identified giant neurons. Intracellular recording in spinal cord sli ces provided functional evidence indicating the monosynaptic connectio ns between dorsal root afferents and giant neurons. The recorded neuro ns were morphologically identified by means of biocytin injection and with avidin conjugates. Electrical stimulation of the ipsilateral dors al roots evoked synaptic responses with short, fixed latencies (1.6-5. 6 ms), which remained unchanged at high frequencies (10 Hz). Excitator y polysynaptic potentials were also observed. By means of pharmacologi cal procedures the short-latency response was dissected in two compone nts: one insensitive to tetrodotoxin, the other abolished by the drug. The toxin-resistant component was presumed to be sustained by small-d iameter C fibers. The synaptic response was mainly mediated by the glu tamate-AMPA receptor subtype; however, a small component mediated by N MDA receptor was also present.