Selective action of noradrenaline and serotonin on neurones of the spinal superficial dorsal horn in the rat

Abstract

The superficial dorsal horn of the spinal cord (SDH; laminae I and II) receives strong input from thin primary afferent fibres and is involved in nociception, pain, temperature sensing and other experiences. The SDH also is the target of serotonergic and adrenergic projections from the brain stem. The interaction between descending pathways that utilize particular mediators and the neurone population of the SDH is poorly understood. To explore this issue, in rat spinal cord slices during whole-cell recordings from identified SDH neurones, noradrenaline (NA) or serotonin (5HT) were briefly applied in the superfusing artificial cerebrospinal fluid. The action of these agents proved specifically related to the type of SDH neurone and its dorsal-root afferent input. Vertical, radial and tonic central lamina II cells consistently expressed outward current to both NA and 5HT, but transient central and Substance P (SP)-insensitive lamina I cells were unaffected directly by either NA or 5HT. Extended islet cells responded with outward current to NA and inward current to 5HT. Lamina I SP-sensitive cells expressed an outward current regularly to NA. 5HT had inhibitory effects on Adelta and C fibre input to all types of SDH neurones. NA inhibited C fibre input to transient central neurones. The present results support the idea that descending systems may have multiple functions, including but not limited to nociceptive modulation.

Figure 1. Effects of NA and 5 HT in SDH vertical cells

A, confocal image of the biocytin-stained vertical cell from which the electrophysiological recordings (B–D) were obtained. B, action potential firing pattern of the vertical neurone (A) in response to a 1 s duration depolarizing pulse. C, chart recordings from the neurone (A) illustrating NA- and 5HT-induced outward currents in voltage clamp (holding potential: −60 mV). D, voltage-clamp, whole-cell recordings showing that the Aδ fibre EPSCs evoked by dorsal root stimulation in the neurone (A) were suppressed by 5HT but not by NA. E, schematic summarizing the effects of exogenous NA and 5HT on vertical neurones and their DR inputs. Vertical neurones are inhibited by both NA and 5HT; their DR input is suppressed solely by 5HT. DR, dorsal root; C, caudal; D, dorsal; R, rostral; V, ventral; −, inhibition; +, excitation.

Figure 2. Effects of NA and 5 HT in transient central cells

A, confocal image of the biocytin-stained transient central cell from which the electrophysiological recordings (B–D) were obtained. B, action potential firing pattern of this transient central neurone in response to a 1 s duration depolarizing pulse. C, chart recordings showing lack of direct effect on the transient central cell by either NA or 5HT (holding potential: −60 mV). D, voltage-clamp, whole-cell recordings showing C fibre monosynaptic EPSCs evoked by dorsal root stimulation were suppressed by both NA and 5HT. E, schematic summarizing the effects of NA and 5HT on transient central neurones and their DR inputs. Transient central neurones do not show direct action by NA and 5HT; their DR input is inhibited by both NA and 5HT. DR, dorsal root; C, caudal; D, dorsal; R, rostral; V, ventral; −, inhibition; +, excitation.

Figure 3. Effects of NA and 5 HT in extended islet neurones and their dorsal root inputs

A, confocal image of a biocytin-stained extended islet neurone from which the electrophysiological recordings (B–D) were obtained. B, action potential firing pattern of this extended islet neurone in response to a 1 s depolarizing pulse. C, chart recordings showing NA-induced outward current and 5HT-induced inward current. D, voltage-clamp, whole-cell recordings demonstrating suppression of the C fibre EPSCs evoked by dorsal root stimulation by 5HT but not by NA. E, schematic summarizing the effects of NA and 5HT on extended islet neurones and their DR inputs. The extended islet neurones show inhibition by NA and excitation by 5HT; their DR input is inhibited by 5HT. DR, dorsal root; C, caudal; D, dorsal; R, rostral; V, ventral; −, inhibition; +, excitation.

Figure 5. Effects of NA and 5HT in lamina I, substance P (SP)-sensitive neurones

A, confocal image of a biocytin-stained lamina I SP-sensitive, neurone from which the electrophysiological recordings (B–D) were obtained. B, action potential firing pattern of this lamina I neurone in response to a 1 s duration depolarizing pulse. C, chart recordings showing SP-induced inward current, NA-induced outward current, and absence of response to 5HT. D, voltage-clamp whole-cell recordings showing that C fibre monosynaptic EPSCs evoked by dorsal root stimulation are suppressed by 5HT but not by NA. E, schematic summarizing the effects of NA and 5HT on lamina I, SP-sensitive neurones and their DR inputs. Lamina I, SP-sensitive neurones are inhibited by NA; their DR input is inhibited by 5HT. DR, dorsal root; C, caudal; D, dorsal; R, rostral; V, ventral; −, inhibition; +, excitation.

Figure 4. 5HT-induced action potentials in extended islet cells evoke GABA-mediated inhibition of transient central cells (simultaneous recordings from two synaptically connected neurones)

A, confocal image of a connected pair of neurones. Presynaptic cell: extended islet cell. Postsynaptic cell: transient central cell. B, action potential firing patterns of the two neurones in response to a 1 s depolarizing pulse. C, current-clamp chart recording (bottom trace) showing 5HT-induced depolarization and action potentials. Middle trace expands the action potential trace. Upper trace shows postsynaptic IPSPs evoked by the 5HT-generated presynaptic action potentials. D, schematic summarizing the effects of NA and 5HT on a SDH circuit formed by an extended islet neurone and a transient central neurone with their DR inputs. Extended (dendrite) islet neurones are excited by descending 5HT-mediated connections, and in turn inhibit transient central neurones by a GABA-mediated connection. A–C are from same pair. APs, action potentials; DR, dorsal root; Pre, presynaptic; Post, postsynaptic; C, caudal; D, dorsal; R, rostral; V, ventral; −, inhibition; +, excitation.