Intracellular recordings of subnucleus reticularis dorsalis neurones revealed novel electrophysiological properties and windup mechanisms

Abstract

Non-technical summary The electrophysiological properties of subnucleus reticularis dorsalis (SRD) neurones were intracellularly studied. The work describes three different classes of cells differentiated by the absence (class I) or presence (classes II and III) of a fast postspike hyperpolarisation followed by depolarising sags and activation of low threshold calcium channels (class II) or type A potassium channels (class III); properties that entitle the different SRD cells to intrinsically regulate their temporal firing. All three classes showed subthreshold oscillatory activity leading to cumulative excitation (windup) in response to low frequency C-fibre afferent input as well as to low frequency injection of depolarising currents, thus allowing SRD cells to generate windup intrinsically. The three neuronal classes sent collateral branches to the nucleus reticularis gigantocellularis (NRGc) and the spinal cord which would provide simultaneous regulation of pain-related motor responses through the NRGc, and ascending nociceptive information via the SRD-descending fibres ending at the spinal dorsal horn.Abstract Aδ-and/or C-fibre nociceptive inputs drive subnucleus reticularis dorsalis (SRD) neurones projecting to a variety of regions including the spinal cord and the nucleus reticularis gigantocellularis (NRGc), but their electrophysiological properties are largely unknown. Here we intracellularly recorded the SRD neuronal responses to injection of polarising current pulses as well as to electrical stimulation of the cervical spinal posterior quadrant (PQ) and the NRGc. Three different classes of neurones with distinct electrophysiological properties were found: type I were characterised by the absence of a fast postspike hyperpolarisation, type II by the presence of a postspike hyperpolarisation followed by a depolarisation resembling low threshold calcium spikes (LTSs), and type III (lacking LTSs) had a fast postspike hyperpolarisation deinactivating A-like potassium channels leading to enlarged interspike intervals. All three classes generated depolarising sags to hyperpolarising current pulses and showed 3-4.5 Hz subthreshold oscillatory activity leading to windup when intracellularly injecting low-frequency repetitive depolarising pulses as well as in response to 0.5-2 Hz NRGc and PQ electrical stimulation. About half of the 132 sampled neurones responded antidromically to NRGc stimulation with more than 65% of the NRGc-antidromic cells, pertaining to all three types, also responding antidromically to PQ stimulation. NRGc stimulation induced exclusively excitatory first-synaptic-responses whilst PQ stimulation induced first-response excitation in most cases, but inhibitory postsynaptic potentials in a few type II and type III neurones not projecting to the spinal cord that also displayed cumulative inhibitory effects (inverse windup). The results show that SRD cells (i) can actively regulate different temporal firing patterns due to their intrinsic electrophysiological properties, (ii) generate windup u...

Electrophysiological Study of Supraspinal Input and Spinal Output of Cat's Subnucleus Reticularis Dorsalis (SRD) Neurons

“…6 A2 and A3) stimulation induced bimodal changes with increased activity peaking at latencies of 2–4 ms and of 9–11 ms followed by a steadily decrease in the induced activity. This bimodal distribution might represent temporal integration of information through different mechanisms such as two different levels of afferent drive, distinct grades of intrinsic excitability [19], presence of inhibitory interneurons shaping the interval between the excitatory peaks, or to a combination of these.…”

“…Bipolar stimulating needle electrodes with inter-tip separation of 5 to 8 mm were also routinely thrust into all four central foot pads to stimulate Aδ and/or C receptors and fibers driving SRD neurons [18], [19], by passing rectangular current pulses of 0.5–1 ms duration and up to 6 mA current intensity.…”

“…Extracellular single-unit recordings of SRD neurons, using epoxy-insulated tungsten microelectrodes of 12 MΩ resistance (A-M Systems, Carlsborg, WA, USA), were obtained from the region located ventrally to the cuneate nucleus and extending rostrocaudally from the obex to about 3.5 mm caudal to it, laterally from 1 to 3 mm, and dorsoventrally from 2.5 to 4.5 mm below the dorsal surface [18], [19]. After the electrode passed through the middle main cuneate nucleus, where the neurons had easily identifiable receptive fields responding to light touch, to brushing the skin [18], [19], [39] or to passive muscular manipulation [40], a neuronal silence was always present before entering the SRD.…”

“…After the electrode passed through the middle main cuneate nucleus, where the neurons had easily identifiable receptive fields responding to light touch, to brushing the skin [18], [19], [39] or to passive muscular manipulation [40], a neuronal silence was always present before entering the SRD. At this stage, the tissue was covered with warm agar and, when solidified, the electrode was further advanced and the search for SRD cells began.…”

“…We have recently shown that 40 to 60% of SRD neurons send axons to the cat's cervical spinal cord [18], [19] but the proportions of axons, if any, reaching thoracic and/or lumbar segments have not been reported in felines. Accordingly, the first aim of the present work was to elucidate this issue by electrically stimulating the spinal ipsilateral dorsolateral funiculus at cervical, thoracic and lumbar levels.…”