Postnatal changes in responses of rat dorsal horn cells to afferent stimulation: a fibre-induced sensitization

Abstract

1. In vivo extracellular recordings were made of 171 dorsal horn cells in both superficial and deep laminae in urethane-anaesthetized newborn rats aged 3, 6, 10 and 21 days, and their response to single and repeated stimuli to primary afferent fibres investigated. 2. No long-latency spike responses were evoked in response to C fibre stimulation in pups at postnatal day 3 (P3) or P6, while by P10, 35 % of cells had a C fibre response. Latencies of response to A fibre skin stimulation were very long and varied widely in the youngest animals, particularly in superficial cells, but mean latencies decreased with postnatal age, from 33.1 +/- 2.78 ms at P3 to 7.3 +/- 0.3 ms at P21. The mean number of spikes evoked by a single A fibre skin stimulus was remarkably consistent between cells and not significantly different in superficial and deep laminae at each age. The mean value of 5.1 +/- 0.6 at P3 increased to 7.0 +/- 1.4 at P10. 3. Repeated stimulation of cutaneous A fibres at 0.5 Hz at twice the threshold level did not significantly alter the magnitude of the evoked response but led to shifts in latency, or 'latency jitter', which decreased with age. Deeper cells displayed more latency jitter than superficial cells. 4. Repeated stimulation of cutaneous A fibres at 0.5 Hz at twice the threshold level produced considerable sensitization in a population of dorsal horn cells in the neonate. This sensitization was unlike the classic C fibre-evoked 'wind-up' observed in adult dorsal horn. The direct A fibre-evoked activity did not increase, but the background activity increased during repetitive stimulation leading to a prolonged after-discharge beyond the stimulation period. At P6, 33 % of cells were sensitized, displaying a mean after-discharge of 70.6 +/- 18 s. At P10, only 6 % were sensitized, with a mean after-discharge of 63 s, and by P21, sensitization was no longer observed. 5. The present study demonstrates that the postsynaptic activity evoked in neonatal dorsal horn cells by cutaneous afferents differs considerably from that in adults. The results may account for the known behavioural reflex sensitization to low-intensity cutaneous stimulation observed in neonatal rats and man.

Figure 1. Postnatal latencies of A fibre-evoked response

Scatter plots showing the latency of the A fibre-evoked response plotted against depth of the cell from the cord surface, for the different postnatal ages examined. _○, deep cells; _▪, cells in the superficial laminae. The mean latencies ( ± s.e.m.) for the deep cells at P3, P6, P10 and P21, respectively, were: 28.0 ± 4.5, 17.9 ± 1.2, 13.1 ± 1.0 and 6.45 ± 0.4 ms. The latencies for the superficial cells were: 36.3 ± 3.4, 19.4 ± 1.2, 13.7 ± 0.9, and 7.8 ± 0.5 ms, respectively.

Figure 2. Postnatal variation in evoked latencies (s.d.)

The change in variation of response latency in individual cells on repeated A fibre stimulation with postnatal age. The variation is expressed as the standard deviation of the mean latency for the population of cells at each age. ▪, cells recorded in superficial laminae; ○, cells recorded in deep laminae.

Figure 3. Stability of evoked latency on repeated stimulation

The ‘latency jitter’ of dorsal horn cells at P3, P6 and P10. Latency jitter is the variation in the response latency recorded from an individual cell, when comparing the evoked response to each stimulus in the train (16 stimuli at 0.5 Hz). This is different from the variation in latency of the whole-cell population at a specific age (Fig. 2). ▪, cells in the superficial dorsal horn; ○, cells in the deep dorsal horn.

Figure 4. Magnitude of evoked response

Bar charts representing the mean number of spikes (+s.e.m.) evoked by each stimulus in a train of 16 A fibre stimuli (twice threshold; 0.5 Hz). P6 and P10 cells are divided into those that sensitized on repeated A fibre stimulation and those that did not.

Figure 5. Time histogram of a sensitized cell

A ratemeter record of the firing of a single P6 dorsal horn cell illustrating the sensitization seen following a train of 16 stimuli (0.5 Hz) at twice the A fibre threshold. Bin size, 0.2 s. Note the low background rate before the stimulus train and the long after-discharge.

Figure 6. Magnitude of the after-discharge

Bar charts representing the mean number of spikes (+s.e.m.) in the 40-2000 ms range after each stimulus, in a train of 16 stimuli (twice threshold, 0.5 Hz). This illustrates sensitization of dorsal horn cells not reflected in the directly evoked responses occurring within a 40 ms latency (Fig. 4).