RESUMEN
UNLABELLED: Abstract Background: Chemo-somatosensory evoked potentials (CSSEPs) elicited by chemical stimulation (CO2 gas) of the nasal mucosa have been shown to be sensitive enough to pick up even weak analgesic effects. With the present study we wanted to investigate whether CSSEPs are also a sensitive tool to capture endogenous pain inhibitory mechanisms elicited by conditioned pain modulation (CPM; where a first conditioning stimulus reduces the sensitivity for a second test stimulus) with a conditioning stimulus of rather low noxious load. METHODS: Seventeen healthy participants were tested for CPM effects (conditioning stimulus: tonic heat pain with intensities around the pain threshold induced via a thermode; test stimulus: chemonasal stimulation (73% and 78% CO2)) on CSSEPs and on self-report ratings. RESULTS: We found significant CPM effects in the CSSEPS, with reduced amplitudes and prolonged latencies at several electroencephalogram (EEG) recording positions when using the lower CO2 concentration (73% CO2). In contrast to the visible inhibitory effects on the CSSEPs, subjective ratings of the test stimulus did not reflect CPM action. DISCUSSION: The experimental pain model using CO2 stimuli to elicit CSSEPs proved to be sensitive enough to capture weak CPM effects elicited by a conditioning stimulus of rather low noxious load. The usage of such mild noxious conditioning stimuli-in contrast to stimuli of higher noxious load (e.g., cold pressor test)-has the advantage that the activation of other types of pain inhibitory mechanisms in parallel (like attentional distraction, stress-induced analgesia) can be avoided.
Asunto(s)
Condicionamiento Psicológico , Potenciales Evocados Somatosensoriales/fisiología , Umbral del Dolor/fisiología , Dolor/fisiopatología , Adulto , Dióxido de Carbono/efectos adversos , Electroencefalografía , Femenino , Humanos , Masculino , Dolor/etiología , Dimensión del Dolor , Psicofísica , Adulto JovenRESUMEN
Heterotopic noxious conditioning stimulation (HNCS) has been thought to give access to the diffuse noxious inhibitory controls (DNIC) in man, which can be activated in wide-dynamic-range neurons by noxious stimulation from remote areas of the body and form the neurophysiological basis of the phenomenon 'pain inhibits pain'. The latter phenomenon suggests that the subjective experience of pain is a prerequisite for an inhibitory action. The necessity of using painful stimuli as conditioning and as test stimuli to produce inhibitory effects was investigated in the present study, using a HNCS paradigm. Twenty young men received conditioning stimuli created by tonic heat at painful and non-painful levels, using either hot water (hand) or thermode (forearm). The test stimuli were phasic heat stimuli (thermode) at painful and non-painful levels applied to the cheek. Only painful but not non-painful heat as conditioning stimulus increased the heat pain threshold and decreased the ability to discriminate between painful heat of different intensities. These two findings are in accord with an inhibitory effect depending on a painful conditioning stimulus. However, the intensity ratings of the test stimuli indicated inhibitory effects of the conditioning stimuli also upon non-painful levels. Furthermore, non-painful heat as conditioning stimulus also appeared to be capable of decreasing the ratings of the test stimuli at painful levels. The latter two findings suggest: (i) that very strong but subjectively still non-painful stimulation can trigger pain inhibitory effects and (ii) that also subjectively non-painful stimuli are affected by inhibitory influences during HNCS.