Rat somatosensory (SmI) cortex: I. Characteristics of neuronal responses to noxious stimulation and comparison with responses to non-noxious stimulation

The role of the basal ganglia in nociception and pain

The involvement of the basal ganglia in motor functions has been well studied. Recent neurophysiological, clinical and behavioral experiments indicate that the basal ganglia also process non-noxious and noxious somatosensory information. However, the functional significance of somatosensory information processing within the basal ganglia is not well understood. This review explores the role of the striatum, globus pallidus and substantia nigra in nociceptive sensorimotor integration and suggests several roles of these basal ganglia structures in nociception and pain. Electrophysiological experiments have detailed the non-nociceptive and nociceptive response properties of basal ganglia neurons. Most studies agree that some neurons within the basal ganglia encode stimulus intensity. However, these neurons do not appear to encode stimulus location since the receptive fields of these cells are large. Many basal ganglia neurons responsive to somatosensory stimulation are activated exclusively or differentially by noxious stimulation. Indirect techniques used to measure neuronal activity (i.e., positron emission tomography and 2-deoxyglucose methods) also indicate that the basal ganglia are activated differentially by noxious stimulation. Neuroanatomical experiments suggest several pathways by which nociceptive information may reach the basal ganglia. Neuroanatomical studies have also indicated that the basal ganglia are rich in many different neuroactive chemicals that may be involved in the modulation of nociceptive information. Microinjection of opiates, dopamine and gamma-aminobutyric acid (GABA) into the basal ganglia have varied effects on pain behavior. Administration of these neurochemicals into the basal ganglia affects supraspinal pain behaviors more consistently than spinal reflexive behaviors. The reduction of pain behavior following electrical stimulation of the substantia nigra and caudate nucleus provides additional evidence for a role of the basal ganglia in pain modulation. Some patients with basal ganglia disease (e.g., Parkinson's disease, Huntington's disease) have alterations in pain sensation in addition to motor abnormalities. Frequently, these patients have intermittent pain that is difficult to localize. Collectively, these data suggest that the basal ganglia may be involved in the (1) sensory-discriminative dimension of pain, (2) affective dimension of pain, (3) cognitive dimension of pain, (4) modulation of nociceptive information and (5) sensory gating of nociceptive information to higher motor areas. Further experiments that correlate neuronal discharge activity with stimulus intensity and escape behavior in operantly conditioned animals are necessary to fully understand how the basal ganglia are involved in nociceptive sensorimotor integration.

Characterization of responses of primary somatosensory cerebral cortex neurons to noxious visceral stimulation in the rat

In pentobarbital-anesthetized rats, responses of single neurons in primary somatosensory cortex (SI) to graded noxious visceral (colorectal distention, CRD) and cutaneous stimulation were recorded. One-hundred fifteen SI neurons were identified on the basis of spontaneous activity, 66 of which responded to CRD. CRD resulted in facilitation of neuronal activity in 33% and inhibition of activity in 52% of these cells. Fifteen percent had mixed facilitated/inhibited responses to varying CRD pressures. Cutaneous receptive fields were identified in 71% of CRD-responsive neurons, with low-threshold or wide dynamic range responses in most cases. Nearly all cutaneous receptive fields were small contralateral sites. Responses to CRD were independent of neuronal depth within the cortex. These data support a role of primary somatosensory cerebral cortical neurons in visceral nociception.

Nociceptive C fibre input to the primary somatosensory cortex (SI). A field potential study in the rat

In the present study, noxious thermal stimulation of the skin with short pulses of CO2-laser radiation was used to identify a cutaneous nociceptive C fibre input to SI and investigate the organization of this input in halothane-nitrous oxide anaesthetized rats. Noxious CO2-laser stimulation of the glabrous skin of the hindpaw consistently evoked late surface positive field potentials in SI (average onset latency 226 ms, peak latency 296 ms). It was demonstrated that these late potentials were evoked by an input from nociceptive C fibres, using a combination of latency measurements, anodal block of A fibre conduction and graded intensities of stimulation. Compared to the tactile evoked potentials in SI, the nociceptive C fibre evoked potentials were more widespread and exhibited a crude somatotopical organization. Intracortically, both tactile and nociceptive C fibre evoked potentials reversed polarity and exhibited a peak negativity in laminae III-IV. The nociceptive C fibre evoked potentials exhibited an additional peak negativity in laminae V-VI. The latter potential had a different time course as compared to the nociceptive C fibre potential evoked in laminae III-IV. In conclusion, an input from cutaneous nociceptive C fibres to SI was demonstrated for the first time in animal experiments. The input to SI from tactile receptors and cutaneous C nociceptors were differently organized.

Response properties of primary somatosensory cortical neurons responsive to cold stimulation of the facial skin and oral mucous membrane

The distribution and response characteristics of the primary somatosensory cortical (SI) neurons activated by cold stimulation of the facial skin and oral mucous membrane were studied in cats. The discharge activities of 53 cold-sensitive SI neurons that responded to a decrease in temperature of the facial skin and/or oral mucous membrane were recorded. Each of these neurons was classified according to its responsiveness to mechanical stimulation as follows: LTM (low-threshold mechanoreceptive, 14/53), WDR (wide dynamic range, 39/53) and NS (nociceptive-specific, none identified). Encoding and non-encoding SI cold-sensitive neurons were identified, according to their responsiveness to decremental thermal stimulation. Only 14 cold-sensitive SI neurons demonstrated increased firing frequencies when subjected to incremental stimulus intensity increases and were classified as the encoding-type, whereas 39 non-encoding-type neurons did not.

Drastic changes of ventromedial medulla neuronal properties induced by barbiturate anesthesia. II. Modifications of the single-unit activity produced by Brevital, a short-acting barbiturate in the awake, freely moving rat

In the preceding study, we have found that pentobarbital, a powerful barbiturate substance, strongly modified the ventromedial medulla (VMM) physiology in relation to nociception: indeed, in the same rats, during time-separated similar VMM penetrations, we have recorded, under pentobarbital, 'new' neuronal groups as compared to the awake state, such as the units exclusively driven (excited or inhibited) by cutaneous innocuous or noxious stimulations and the multimodal multireceptive neurons inhibited by non-noxious and noxious stimuli. Still under pentobarbital, we have also recorded the same units found as the rats were awake, i.e., the multimodal multireceptive neurons exclusively excited by various innocuous and noxious stimuli. However, the spontaneous and nociceptive activities of these units were strongly modified as compared to awake animals. Using Brevital (a short-acting barbiturate substance) administration, we have, in the present study, tried to understand the mechanisms underlying these drastic modifications. In particular, one of the questions was whether or not the 'new' neuronal classes recorded under anesthesia resulted from a modification of the physiological properties of the unique VMM neuronal group potentially involved in nociception in awake animals: the multimodal multireceptive units. By following the VMM neuronal activities either before and after or after Brevital administration until recovery from anesthesia, we have determined that the units exclusively driven by innocuous stimulation might result from a modification of the multimodal multireceptive neurons. Alternatively, the multireceptive units inhibited by peripheral stimulations are possibly totally different neurons, silent when the animals are awake.

Comparison of human pain sensation and flexion withdrawal evoked by noxious radiant heat

The purpose of this study was to determine the reliability of flexion withdrawal magnitude as an indicator of pain sensation. In 10 healthy human volunteers, we compared the magnitude and latency of integrated biceps EMG with the subjects' rating of pain, using a visual analog scale, elicited by noxious radiant heat stimuli applied to the dorsal forearm. The magnitude and inverse latency of withdrawal, although variable, increased exponentially as a functions of stimulus temperature. The stimulus response functions for mean withdrawal magnitude and mean pain intensity were similar for lower stimulus temperatures, but at higher temperatures the withdrawal continued to increase exponentially whereas pain intensity reached a plateau. The pain intensity and withdrawal magnitude for each stimulus were poorly correlated. Under the conditions of this experiment, mean pain intensity and mean withdrawal magnitude were both well correlated with stimulus temperature, but the magnitude of withdrawal did not reliably reflect the intensity of pain sensation.

Differential effects of ascending neurons containing dopamine and noradrenaline in the control of spontaneous activity and of evoked responses in the rat prefrontal cortex

The medial prefrontal cortex receives converging projections from the mediodorsal thalamic nucleus, dopaminergic cells from the ventral tegmental area dn noradrenergic cells from the locus coeruleus. Stimulation of the ventral tegmental area inhibits the spontaneous activity of prefrontal cortical neurons and blocks the excitatory response evoked by stimulation of the mediodorsal thalamic nucleus (10 Hz). The aim of the present study was to compare the influence of dopaminergic and noradrenergic afferents on the spontaneous and evoked activity of medial prefrontal cortical neurons. In ketamine-anaesthetized rats, repetitive stimulation (20 Hz, 10 s) of the locus coeruleus produced a long-lasting post-stimulus inhibition (mean duration: 45 s) of the spontaneous activity of 56% of the tested cells. This effect was decreased markedly following selective destruction of the ascending noradrenergic pathways (local 6-hydroxy-dopamine injection) or depletion of cortical catecholamines by alpha-methyl-para-tyrosine pretreatment, suggesting that these inhibitory responses are mediated by noradrenergic neurons. The excitatory response to mediodorsal thalamus nucleus stimulation (10 Hz) could still be evoked during the post-stimulus inhibitory period induced by locus coeruleus stimulation (20 Hz, 10 s) resulting in the enhancement of signal-to-noise ratio. On the other hand, a population of prefrontal cortex neurons (26%) was found to be reproducibly activated by noxious tail pinch. This evoked response was still present during the post-stimulus inhibitory period induced by locus coeruleus stimulation but was completely suppressed during stimulation of the ventral tegmental area (10 Hz). In conclusion, these results indicate that the dopaminergic and noradrenergic systems exert a completely distinct control of information transfer in the medial prefrontal cortex.

Field potentials evoked in rat primary somatosensory cortex (SI) by impulses in cutaneous A beta- and C-fibres

A projection of cutaneous C-afferent fibres to the contralateral primary somatosensory cortex (SI) was examined in the halothane-anaesthetized rat. Field potentials evoked by electrical stimulation of the right sural nerve were recorded at the surface of an intracortically within the left SI cortex. A late surface positive potential (latency 110-190 ms, mean 136 ms) was evoked by sural stimulation at a strength that activated A- and C-fibres. A selective anodal block of impulse conduction in A-fibres proximal to the stimulating electrodes showed that impulses in C-fibres generate the late potential also in the absence of a preceding A-fibre input. Stimulation of the sural nerve at two sites caused a shift in latency of the late potential corresponding to a conduction velocity of the primary afferent fibres of less than 1.0 m/s. The distribution of the C-fibre-evoked potential in SI was similar to that of the A beta-fibre evoked 'primary' potential suggesting that the investigated projection of cutaneous C-fibres to SI has a somatotopic organization.

Cortical nociceptive responses and behavioral correlates in the monkey

Experiments were performed to characterize cerebral cortical activity and pain behavior elicited by electrical stimulation of the tooth pulp in unanesthetized monkeys. Four monkeys were trained on two different operant paradigms: two on a simple escape task and two on an appetitive tolerance-escape task. All monkeys were implanted with bipolar stimulating electrodes in the right maxillary canine tooth and subdural recording electrodes over the left primary (SI) and/or secondary (SII) somatosensory cortices. Subdural tooth pulp-evoked potentials (TPEPs) recorded over the SII consisted of components P1 (27.5 ms), N1 (40.3 ms), P2 (84.0 ms), N2 (163.5 ms), P3 (295.3 ms), and N3 (468.0 ms). The long latency component (P3-N3) was found exclusively over the SII and was elicited by high intensity stimulation. The appearance of component P3-N3 required the recruitment of A delta nerve fibers into the maxillary nerve compound action potential and was correlated with high frequencies of escape. Administration of morphine sulfate (4 mg/kg, i.m.) caused a contemporaneous reduction in escape frequency and in the amplitude of P3-N3 recorded over the SII. The relationships between TPEP amplitude, escape behavior and A delta nerve fiber activity strongly suggest that the SII is involved with nociception and pain behavior.

Electrophysiological evidence for a role of the anterolateral quadrant of the spinal cord in the transmission of noxious messages to the thalamic ventrobasal complex in the rat

Responses to noxious mechanical and thermal stimulation applied to the hindpaws were recorded extracellularly from the same neurons of the ventrobasal complex of the rat thalamus (VB) before and after lesions of various areas of the cervical cord in order to determine the pathways carrying the afferent messages. It was demonstrated that lesions of the dorsal and dorsolateral portions of the cord failed to eliminate the VB neuronal responses to noxious stimulation. By contrast, lesion of one anterolateral quadrant eliminated the responses to a noxious stimulation applied to the hindpaw contralateral to the lesion. This occurred whether the lesion was ipsilateral or contralateral to the recording site. From the present study and the data in the literature, it is concluded that the fibers of the spino-thalamic tract which are completely crossed in the spinal cord, travel in the anterolateral quadrant and project directly onto the VB, are involved in the transmission of noxious messages from the cord to the VB neurons. This conclusion indicates that the VB neuronal responses to noxious stimulation of the hindpaw ipsilateral to the recording site depend on the spinothalamic projection to the opposite ventrobasal complex. This therefore suggests that some noxious messages which reach a particular VB neuron are conveyed via the opposite VB and the existence of a thalamo-cortico-thalamic loop is discussed.

Activation of identified septo-hippocampal neurons by noxious peripheral stimulation

Septo-hippocampal neurons (SHNs) were recorded from the medial septum-diagonal band area of rats anaesthetized with either urethane or fluothane. They were identified by their antidromic response to the electrical stimulation of the fimbria. Their responses to peripheral somatic noxious and non-noxious stimulation were studied. Non-noxious natural stimulations were relatively ineffective. In contrast, 68% of the SHNs were driven by noxious stimulation. The SHNs could be driven either by mechanical or thermal stimulation. Intraperitoneal injection of bradykinin excited about half of the SHNs. Some neurons were able to encode stimulus intensity (strength of the mechanical stimulation and/or temperature of the thermal stimulation). The receptive fields of the SHNs were large, usually involving half of the body or the whole body surface. These results suggest that SHNs, which are at the origin of the cholinergic septo-hippocampal pathway, might be involved in cerebral mechanisms related to nociception.

Thalamic nociceptive systems

A role for thalamic structures in the processing of signals of nociception and pain has been suggested on the basis of clinical data since the turn of the century. Searches for a 'pain centre' by lesion or stimulation were often disappointing and the electrophysiological data were rare and usually contradictory. However, recent electrophysiological anatomical and neuropharmacological studies, made in various species (mainly rat and monkey) appear now progressively to give some clues in the understanding of pain process at the thalamic level. These studies have been mainly concerned with the areas receiving projections from ascending spinal pathways conveying noxious inputs, either directly by the spinothalamic tract or indirectly by the spinoreticulothalamic pathway. The eventual respective roles of these thalamic structures are considered. Electrophysiological recordings from thalamic structures in a model of experimental pain, arthritic rats, are also presented.

Cortical nociceptive systems

There is evidence that the cerebral cortex is involved in the perception of pain but no specific area appears to be the 'pain centre'. Limited knowledge exists on the cortical processing of the noxious input. The nociceptors are most likely to activate at least two different systems with different characteristics. One has a bilateral cortical projection, no apparent topographical pattern, low synaptic security and excites cells in large areas. This system may give rise to the widespread increase in blood flow and the widely distributed surface potentials recorded in man following a painful stimulus. Noxious stimuli also excite a system with contralateral topographical projection, high synaptic security and termination in lamina IV. This system produces EPSP-IPSP sequences in cells in a restricted cortical area. Pronounced inhibition of cells in lamina IV and more superficial layers is induced by activity in low threshold afferents. Thus, similarly as at the segmental spinal level, the nociceptive input to cortical cells is processed and integrated with the activity in other afferent systems.

Altered properties and laminar distribution of neuronal responses to peripheral stimulation in the SmI cortex of the arthritic rat

The properties of the neuronal responses to different types of mechanical peripheral stimulation were studied during electrode penetrations in the first somatosensory cortex of anaesthetized rats with polyarthritis. Very few neurons were driven by light cutaneous stimulation (such as brushing) or by intense mechanical stimulation. Most of the neurons were driven by joint movement and/or moderate pressure on the skin. These neurons could be found in all cortical layers, the majority being located in layer V. These results contrast sharply with the properties and laminar distribution of the different functional categories of cortical neurons, as observed in normal animals.