Somatosensory processing by rat medial pontomedullary reticular formation neurones : responses to innocuous and noxious thermal and mechanical stimuli

Farham, Craig Jeffrey

Somatosensory processing by rat medial pontomedullary reticular formation neurones : responses to innocuous and noxious thermal and mechanical stimuli

Doctoral Thesis

1991

Publisher

University of Cape Town

Department

Division of Physiological Sciences

Faculty

Faculty of Health Sciences

Abstract

This work examines somatosensory processing in "giant" neurones of the medial pontomedullary reticular formation (PMRF) in the rat, with particular emphasis on the response to cutaneous thermal stimuli. Thermal test stimuli were employed as these were deemed to be more precisely quantifiable than other forms of cutaneous stimulation. Activity was recorded from 235 PMRF neurones in 94 female Long Evans rats (270 to 320 g) anaesthetised with urethane (1,25g/kg, i.p.). Rectal temperature was closely controlled at 38 ± 0,5°C. Standard stereotactic and extracellular recording techniques were employed. PMRF giant neurones were identified by their stereotactic location, large, stable spike amplitudes of long duration, responses to cutaneous mechanical stimuli and receptive field properties, and spontaneous discharge characteristics. Ramp, step and sine wave cutaneous thermal stimuli (35-48 °C) were applied to the glabrous skin on the hindpaw by means of a computer-controlled Peltier device. The location of the units was confirmed by subsequent histology. One hundred and eleven neurones were located in nucleus reticularis pontis caudalis (NPC), and 124 in nucleus reticularis gigantocellularis (NGC). Mechanical stimulation excited 188 of 235 (80%) PMRF neurones (ON-m cells), and inhibited 40 (17%, OFF-m cells). Seven cells (3%) had mosaic receptive fields of excitation and inhibition (complex responses, CX-m). Twenty-eight percent of neurones were responsive to both weak and intense stimuli (mixed neurones). The remainder (72%) responded only to intense mechanical stimulation of the skin (high threshold neurones). The (excitatory or inhibitory) response of the mixed neurones to intense stimuli was generally greater than to mild stimuli, Receptive fields ranged in size from restricted (hindlimbs only) to very extensive (covering the entire body surface). Neurones with small receptive fields were almost exclusively of the high threshold type, and tended to be located in NGC, while mixed neurones tended to have larger receptive fields, and were located predominantly in NPC. Some portion of the hind limbs were represented in the receptive fields of all but one of the neurones studied, while the tail and/ or trunk were represented in 77%, and the forelimbs and face in 28% of receptive fields. Most of the cells responding to cutaneous mechanical stimulation had bilateral (usually symmetric) receptive fields. Spontaneous (background) activity occurred in the absence of any deliberate sensory stimulation in 72% of PMRF neurones. The frequency of spontaneous discharge rates ranged from O to 47 spikes/ s. The coefficient of variation of the spontaneous discharge rate of a given neurone was generally less than 20% (range O to 85%). Of the 235 identified mechanosensitive PMRF neurones, 203 (86%) also responded to cutaneous thermal stimulation (43-48 °C) of the ipsilateral hind paw. Eighty percent of these responded with increased discharge rates (ON-t cells), and 20% were inhibited (OFF-t cells). The polarities of response of individual PMRF neurones to mechanical and thermal stimuli, and to repeated ipsilateral and contralateral thermal stimuli, did not differ significantly. Following transient thermal stimulation, spontaneous discharge rates largely returned to pre-stimulus levels. The thresholds of response to slow ramp (0,15°C/s) and stepped (2°C/s) thermal stimuli occurred both in the innocuous and noxious temperature ranges (below and above 42°C, respectively). The threshold temperatures showed large variability to repeated identical thermal stimuli. Despite the poor reproducibility of the threshold responses, the distribution of thresholds to thermal ramp stimuli was consistently bimodal, with peaks occurring at 39 and 43°C. The bimodality persisted even when the ipsilateral and contralateral data were pooled. The modes of these threshold distributions conform to the maximum discharge ranges for warm and noxious cutaneous receptors. Thus, it is likely that thermal input to individual PMRF neurones is derived from both types of receptors. The responses of PMRF neurones to repeated thermal stimuli were stable and reproducible with respect to magnitude and time course. The average (static) and maximum (dynamic) responses to thermal stimuli were generally small: for example, the mean of the average responses to ramp stimuli was 5,9 spikes/s ± 11,0 SD, (range -28 to 40 spikes/s), and the mean of the maximum responses was 9,3 spikes/s ± 16,1 SD, (range -46 to 65 spikes/s). The absolute change in firing rate of individual PMRF neurones, and of the population, increased monotonically as a function of the intensity of stepped cutaneous thermal stimuli in the range 40 to 48 °C. However, their resolution, based on their average and maximum responses, was poor. Incorporating the post-stimulus responses into the comparisons between different stimulus intensities marginally increased the resolution of these neurones. Thus, while the majority of PMRF neurones are able to distinguish innocuous from noxious stimuli, few are capable of encoding stimulus intensity within the noxious range (above 43 °C). The majority (70%) of PMRF neurones responded to sustained thermal stimuli with a slow increase or decrease to a new static discharge rate which was maintained with little or no adaptation. Latency to onset of response to stepped thermal stimuli varied from 1 to 50 seconds, and the time to maximal response between 5-60 seconds. Many PMRF neurones also showed marked after-discharge for periods of up to 5 minutes after removal of the stimulus. The thermal receptive fields of over 90% of PMRF neurones were large, incorporating at least both hindlimbs. The extensive receptive field sizes of individual PMRF neurones provides evidence against them having a role in stimulus location. The large number of PMRF neurones showing multimodal convergence, their small magnitude responses, their slow response times, and their large receptive fields strongly suggest that these neurones are not participating in classical sensory discrimination. Rather, they may function as stimulus detectors or alternatively play a role in associative processes.