Extracellular and intracellular recordings from neurones in the substantia gelatinosa Rolandi

Spinal dorsal horn neurones play an important role in processing sensory information received from primary afferent fibers. The application of the patch-clamp technique to thin slices of rat spinal cord has enabled the study of ionic channels in visually identified dorsal horn neurones. The small soma of these neurones isolated from the slice by means of a novel method of `entire soma isolation' has become a convenient model for investigating the properties and distributions of ionic channels. The present review summarizes results of recent experiments studying different types of voltage-gated Na⁺ and K⁺ channels expressed in dorsal horn neurones. Uneven distribution of the channels between the soma, axon and dendrites appears to play a major role in determining the neuronal excitability. The contribution of the soma, axon and dendrites to generation and propagation of the action potentials in central neurones is discussed.

Clonidine and glutamate were applied by iontophoresis to cells in the superficial 3 laminae of the spinal cord in the anaesthetised rat. Only cells that were excited by glutamate (up to 150 nA) were studied. Some spontaneously active cells could be excited by clonidine (up to 100 nA). However, when applied to non-spontaneous cells, clonidine had no effect at any dose level. When ejected in a cyclic pattern alternating with glutamate ejection, clonidine powerfully amplified the response of many cells to the glutamate stimulus. This effect was seen only on cells with small-amplitude spikes and low-threshold (LT) receptive fields. The amplification was often sustained and could outlast the clonidine ejection by several minutes. Clonidine had a long-lasting inhibitory effect on the responses to glutamate of cells with high-threshold (HT) or wide-dynamic-range (WDR) receptive fields. Clonidine appeared to selectively decrease the responsiveness of WDR cells to noxious stimulation. It is suggested that an amplification of the response of LT cells to other excitatory inputs could contribute to the analgesic action of clonidine.

The electrophysiological properties of 87 neurons in the deep dorsal horn (laminae III VI) of the rat spinal cord have been investigated in vitro. Two preparations have been used; the transverse spinal cord slice preparation from the third or fourth lumbar segments of 14–16 -day-old rats (71 cells) and a hemisected lumbar spinal cord preparation from 10–12-day-old rats (16 cells).

The input impedances (range 11–128 MΩ), membrane potentials ( -67 ± 8mVS.D.), action potential amplitude (77 ± 11.8mV) duration (1.4 ± 0.5ms) and afterpotentials, were effectively identical in the neurons recorded from the two preparations. Neurons in both preparations when activated with long-duration (1–2s) outward current pulses showed a single steady-state firing range with little Actaptation of firing frequency or action potential amplitude. This pattern of responses was unaffected by changing the membrane potential.

Orthodromic synaptic activity could be elicited in the neurons by stimulating either the small dorsal root remnants in the slice or the dorsal roots in the hemisected spinal cord. The responses evoked by single stimuli of increasing intensity varied in different neurons in both preparations. The commonest response (32/62) consisted of a short-latency, short-duration composite excitatory postsynaptic potential which generated one or two spikes with no further spiking activity at longer latency when the stimulus intensity was increased beyond threshold. In 20 neurons, graded stimulation produced a graded response with recruitment, at high intensities, of a discharge of action potentials lasting several hundred milliseconds. A small number of cells (4) responded to the single stimulus with a train of action potentials lasting several seconds.

Stimulating adjacent dorsal roots in the hemisected cord preparation could evoke quite different responses from the neurons.

The heterogeneity of the types of orthodromic responses obtained in both preparations, in spite of the almost uniform intrinsic membrane properties, is likely to rcflect differences in the strength, location and type of afferent and interneuronal input to different dorsal horn cells.

The importace of the spinothalamic tract in pain transmission makes it an attractive candidate for study with respect to the effects of antinociceptive compounds. We have been interested in the analgesic actions of opioids and noradrenergic agents at the spinal level and have investigated the effects of these agents on extracellularly recorded nociceptive dorsal horn neurons in the rat. Spinothalamic tract cells were identified by antidromic activation from the somatosensory thalamus. Morphine was administered by bathing the spinal cord in an artificial cerebrospinal fluid solution which contained a known concentration of drug. We observed a dose-related inhibition, naloxone-reversible in some cases, of activity produced by spinally administered morphine in identified rat spinothalamic tract cells and dorsal horn nociceptive neurons. Morphine had no effect on stimulus-evoked responses of low threshold dorsal horn neurons.