BURST-GENERATING NEURONS IN THE DORSAL HORN IN AN IN-VITRO PREPARATION OF THE TURTLE SPINAL-CORD

1. In transverse slices of the spinal cord of the turtle, intracellula r recordings were used to characterize and analyse the responses to in jected current and activation of primary afferents in dorsal horn neur ones. 2. A subpopulation of neurones, with cell bodies located central ly in the dorsal horn, was distinguished by tile ability to generate a burst response following a hyperpolarization from rest or during a de polarization from a hyperpolarized holding potential. The burst respon se was inactivated at the resting membrane potential. 3. The burst res ponse was mediated by a low threshold Ca2+ spike assumed to be mediate d by T-type Ca2+ channels since it resisted tetrodotoxin and was block ed by 3 mM Co2+ 100 - 300 mu M Ni2+ and resembled the low threshold sp ike (LTS) described elsewhere. 4. Some burst-generating cells also dis played plateau potentials mediated by L-type Ca2+ channels. In these c ells the burst following a hyperpolarizing current pulse, applied from the resting membrane potential, facilitated the activation of the pla teau potential. Wind-up of the plateau potential was produced when the hyperpolarizing pulse generating: the burst was repeated at 0.1 - 0.3 Hz or faster. 5. The burst response and the underlying low threshold Ca2+ spike were activated synaptically by primary afferent stimuli in a voltage range hyperpolarized from the resting membrane potential. 6. Cells with bursts were morphologically distinguishable from cells wit h bursts and plateau properties. 7. Our findings in this and the prece ding paper show that the intrinsic response properties of particular s ubtypes of neurones in the dorsal horn have a profound influence on th e amplitude and time course of the responses mediated by primary affer ent fibres. We predict that these postsynaptic properties are probable targets for synaptic modulation.