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Neurotoxins in the Neurobiology of Pain

Stephen D. Silberstein, MD

Migraine is a common, chronic, incapacitating, neurovascular disorder that affects an estimated 12% of thepopulation. Understanding the basic mechanisms of pain is important when treating patients with chronic paindisorders.

Pain, an unpleasant sensory and emotional experience, is usually triggered by stimulation of peripheral nervesand often associated with actual or potential tissue damage. Peripheral nerve fibers transmit pain signals from theperiphery toward the spinal cord or brain stem. The different diameter pain fibers (A and C) vary in the speed ofconduction and the type of pain transmitted (eg, sharp versus dull). When stimulated, peripheral pain fibers carry-ing sensory input from the body enter at different layers of the dorsal horn, which is then propagated toward thethalamus via the spinothalamic tract within the spinal cord. Conversely, sensory input from the face does not enterthe spinal cord but enters the brain stem via the trigeminal nerve.

This review describes in detail the neurobiological mechanisms and pathways for pain sensation, with a focuson migraine pain.

Key words: migraine, pain, mechanism, trigeminal nucleus

Abbreviations: STT spinothalamic tract, VPL ventroposterior lateral

(

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Migraine is a common, chronic, incapacitating,neurovascular disorder that affects an estimated 12%of the population.

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Migraine attacks are character-ized by episodes of throbbing head pain that may besevere. Understanding the basic mechanisms of painis important when treating patients with chronic paindisorders.

Pain is an unpleasant sensory and emotional ex-perience usually triggered by stimulation of periph-eral nerves and often associated with actual or poten-

tial tissue damage. The intracranial pain-sensitivestructures include the glossopharyngeal, vagus, trigem-inal, and upper cervical spinal nerves; the vascularstructures of the venous sinuses and their tributaries;the dural arteries; the carotid, vertebral, and basilararteries; the circle of Willis; and the proximal por-

tions of the cerebral, vertebral, and basilar branches.

2

Surrounding the large cerebral vessels, pial vessels,large venous sinuses, and dura mater is a plexus oflargely unmyelinated fibers that arise from the oph-thalmic division of the trigeminal ganglion and, in theposterior fossa, the upper cervical dorsal roots. Thisreview describes the neurobiological mechanisms andpathways for pain sensation.

PAIN SENSORY FIBERS

Peripheral nerves vary in diameter, with fibersize correlating with function. Large fibers conductfaster than small fibers; myelinated fibers conductfaster than unmyelinated fibers. The compound ac-tion potential of a peripheral nerve has 3 distinct de-flections: A being the fastest; B, the intermediate;and C, the slowest. The A fibers, which are responsi-ble for the A deflection, are myelinated sensory andmotor fibers. The A deflection is subdivided furtherinto alpha, beta, gamma, and delta peaks, with A alphafibers the largest and most rapidly conducting my-elinated fibers, and A delta fibers the smallest and

From the Jefferson Headache Center, Thomas Jefferson Uni-versity Hospital, Philadelphia, Pa.

Address all correspondence to Dr. Stephen D. Silberstein, Jef-ferson Headache Center, Thomas Jefferson University Hospi-tal, 111 South 11th Street, Suite 8130, Philadelphia, PA 19107.

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slowest of the A group of fibers. The B fibers are my-elinated visceral fibers (preganglionic autonomic fibersand some visceral afferents). The C fibers are small-caliber, unmyelinated, and slow-conducting fibers.

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For the most part, nociceptors are free nerveendings found throughout almost all human tissue.There are 4 classes of nociceptors: thermal, mechani-cal, polymodal, and silent nociceptors. Some freenerve endings (silent nociceptors) respond poorly toall stimuli. Their firing threshold is dramatically re-duced by inflammation and various chemical insultsthat may contribute to the development of secondaryhyperalgesia and central sensitization. Activation ofsilent nociceptors is thought to contribute to pain thataccompanies inflammation and certain pathologicalprocesses.

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Pain evoked by different input channels repre-sents activation of high-threshold receptor ion-channeltransducers in nociceptor peripheral terminals (nocicep-tive transduction). The mechanism by which noxiousstimuli depolarize free sensory endings and generateaction potentials is not known. The membrane of thenociceptor, however, is thought to contain proteinsthat convert the thermal, mechanical, or chemical en-ergy of noxious stimuli into a depolarizing electricalpotential. One such protein is the receptor for capsa-icin, a natural product of hot peppers. The capsaicin,or vanilloid, receptor is found exclusively in primaryafferent nociceptors and mediates the pain-producingactions of capsaicin. The capsaicin receptor respondsto noxious heat stimuli, which suggests that it is atransducer of painful heat stimuli. Local inhibition oc-curs at different relay levels in the neuraxis. Descend-ing inhibition originates in the forebrain and brainstem and terminates in the brain stem and spinal cord.Factors responsible for the local inhibition include de-creased activation of neurons, down-regulation of re-ceptors and transmitters, and cell death (disinhibition).

Nociceptors are associated with either unmyeli-nated C fibers; small, thinly myelinated A delta fibers;or, under certain circumstances, A beta fibers.

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Ther-mal and mechanical nociceptors are associated withsmall-diameter, thinly myelinated A delta fibers thatconduct signals at about 5 to 30 m/s. Polymodal noci-ceptors, activated by high-intensity mechanical, chemi-cal, or thermal (both hot and cold) stimuli, are associ-

ated with small-diameter, nonmyelinated C fibers thatconduct slowly, generally at velocities of less than 1m/s, and are responsible for dull pain and tempera-ture sensitivity. Neurotransmitters within these fibersinclude glutamate and the neuropeptides, substance P,calcitonin gene-related peptide (CGRP), and neuroki-nin A. Stimulation of C fibers results in the slow buildupof an aching, throbbing, or burning pain, whereasfaster-conducting A delta fibers transmit sharper ini-tial pain sensations.

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For example, if you were to stickyourself with a pin, the initial sharp pain would bedue to activation of the A delta fibers. After a lag ofabout 1 second, the sharp pain would be followed bya dull pain. The time delay in the 2 types of pain sen-sation is explained by the fact that A delta fiberstransmit much faster than C fibers.

In contrast, A beta fibers normally do not trans-mit pain. The neurotransmitters within these nerve fi-bers consist mainly of excitatory amino acids, such asglutamate and aspartate. Even so, following activa-tion or injury, the expression of several neuropep-tides such as neuropeptide Y, galanine, cholecystoki-nin, and substance P, is augmented. As a result,sensory information that is normally transduced fromtouch and vibration may be perceived as painful.

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Inthe absence of conduction of A alpha and A beta fi-bers, the perception of pain is not normal (eg, whenpinprick, pinch, or ice cannot be distinguished fromeach other and instead each produces the sensationof burning pain).

Action potentials initiated by a painful stimulustravel both centrally, toward the spinal cord or brainstem, and peripherally, invading branches of thesame neuron outside the area of injury (axon reflex).Peripherally released neuropeptides cause dilation ofarterioles (flare), leakage of plasma from venules(edema), and inflammation. In the meninges, theyproduce neurogenic inflammation with plasma pro-tein extravasation. Nociceptors contain glutamate re-ceptors, and glutamate is thought to play a role in pe-ripheral sensitization.

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PAIN PATHWAYS

The principal ascending spinal pain pathway isthe lateral spinothalamic tract (STT). Originatingfrom both lamina I and deep layers V-V1 of the dor-

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sal horn, the STT receives synaptic input from pri-mary nociceptive afferent neurons. The main projec-tion of this pathway is to the ventroposterior lateral(VPL) nucleus of the thalamus and, from there, tothe primary and secondary somatosensory cerebralcortical areas (S

1

and S

2

) (the STT-VPL-S

1-2

path-way). The STT-VPL-S

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pathway’s neurons of originwithin the dorsal horn contain predominantly widedynamic-range neurons and some nociceptive-specific neurons that are critical for both sensory andaffective pain processing.

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The fifth cranial nerve, arising from the trigemi-nal ganglion (semilunar or gasserian ganglion), has 3divisions: ophthalmic, mandibular, and maxillary.Anterior pain-producing structures are innervated bythe ophthalmic (first) division. Posterior regions aresubserved by upper cervical nerves.

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Central trigemi-nal primary afferent processes form the sensory root

of the trigeminal nerve, enter the brain stem at thepontine level, and terminate in the trigeminal brainstem nuclear complex. The trigeminal brain stem nu-clear complex is composed of the principal trigeminalnuclei and spinal trigeminal nuclei (subdivided intothe rostral subnuclear oralis, middle subnuclear inter-polaris, and caudal subnuclear caudalis). All contrib-ute to facial and cranial nociception.

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The brain stem spinal trigeminal nucleus is analo-gous to the dorsal horn of the spinal canal, the firstsynapse in the central nervous system. Most spinotha-lamic and trigeminothalamic tract neurons that origi-nate from the dorsal horn and project to VPL andVPM nuclei have wide dynamic-range characteris-tics.

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Second-order neurons from the trigeminal spi-nal nuclei form the trigeminothalamic tract andproject to other midbrain structures, as well as to thethalamic tract. A number of secondary neuronal

Fig 1.—Development of cutaneous allodynia during a migraine attack.

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pathways for pain also project to the somatosensorycortex, insular cortex, and cingulate cortex.

A reflex connection exists between neurons in thepons in the superior salivatory nucleus, which resultsin a cranial parasympathetic outflow that is mediatedthrough the pterygopalatine, otic, and carotid gan-glia. This normal trigeminal-autonomic reflex may behyperactive in patients with trigeminal-autonomiccephalgias (cluster headache and paroxysmal hemi-crania) and perhaps migraine.

Most trigeminal brain stem nuclear complex neu-rons that project to the contralateral thalamus arefound in the principal trigeminal nuclei. The remain-ing neurons are located in the middle subnuclear in-terpolaris, with smaller contributions from the rostralsubnuclear oralis and caudal subnuclear caudalis.The contralateral fibers from the principal trigeminalnuclei ascend with the medial lemniscus and are oftenreferred to as the trigeminal lemniscus. The spinal nu-clei project to the thalamus via a crossed pathwaythat joins the contralateral STT. The trigeminotha-lamic projections terminate preferentially in the tha-lamic VPM nucleus.

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Most VPM nuclei, some with wide dynamic-range characteristics, respond to low-threshold stim-uli. The trigeminal brain stem nuclear complex neu-rons also project to a number of diencephalic andbrain stem areas involved in the regulation of auto-nomic, endocrine, affective, and motor functions. Alltrigeminal brain stem nuclear complex neuron subnu-clei project directly to the hypothalamus.

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Unlike the STT-VPL-S

1-2

pathway, other ascend-ing pathways have a preponderance of nociceptive-specific neurons. The spino-parabrachio-amygdaloidpathway and the spino-parabrachio-hypothalamic path-way consist exclusively of nociceptive-specific neu-rons. These pathways are involved in autonomic pro-cesses and behaviors related to fear and defense.Spinothalamic and trigeminothalamic tract neuronsthat terminate in the ventromedial portion of the pos-terior nuclear complex and ventrocaudal portion ofthe mediodorsal nucleus have more nociceptive-specific neurons.

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Thus, there are functionally different ascendingpain pathways: (1) the STT-VPL-S

1-2

pathway, whichcontains mostly wide dynamic-range neurons impor-

tant for encoding the capacity to recognize the sen-sory intensity and sensory qualitative features ofpain; and (2) other pathways, which contain mostlynociceptive-specific neurons that are important forimmediate affective-motivational responses, auto-nomic and somatomotor activation, and possibly painsensation.

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Pain has both sensory and affective dimensions.In addition to being physically unpleasant, pain is as-sociated with negative emotions shaped by context,anticipations, and attitudes.

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The somatosensory cor-tex is involved in pain affect. Pain unpleasantness isin series with pain sensation intensity. Pain sensationsand affect are disrupted by STT-VPL-S

1-2

lesions be-cause this pathway makes serial interconnections tocorticolimbic structures involved in pain-related af-fect.

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The anterior cingulate cortex is part of the brain’sattentional and motivational network, projecting toprefrontal (executive functions) and supplementarymotor cortex (response selection) regions. It coordi-nates inputs from parietal areas involved in the per-ception of bodily threat with frontal cortical areas in-volved in the plans and response priorities for pain-related behavior. The anterior cingulate cortex is themost consistent brain region activated in brain imag-ing studies of pain.

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Input to lower brain stem and limbic structuresmay contribute to arousal, autonomic, and somato-motor activation. Medial thalamic nuclei project toregions involved in monitoring the overall state of thebody (insular cortex), directing attention (anteriorcingulate cortex), and assigning response priorities(anterior cingulate cortex). In addition, the anteriorcingulate cortex receives input from a ventrally di-rected somatosensory-limbic pathway that contributesvarying degrees of cognitive evaluation to pain affect.

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POTENTIAL MECHANISMS OF PAIN

Pain can be assessed at different levels—the indi-vidual level (personal suffering), the system level(how pain is generated), and the cellular and molecu-lar level (change in the individual elements of the sys-tem)—and is mediated by different input channels.Usually, the pain from intensive stimulation or injury(nociceptive pain) diminishes as healing progresses.

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Even so, another type of pain can occur with periph-eral or central nervous system malfunction. Threespatiotemporal characteristics of pain can be seenduring both normal and pathophysiological pain: (1)as pain intensity increases, the area in which it is ex-perienced often enlarges (radiation); (2) the painmay outlast the evoking stimulus; and (3) repeatednociceptive stimuli may increase the perceived painintensity, even without increased input (sensitiza-tion). Pain was initially believed to be a fixed line sys-tem activated in the periphery by nociceptors in re-sponse to an adequate noxious stimulus. Althoughtrue of nociceptive pain, it is not the case for pain hy-persensitivity or spontaneous pain, where a numberof different input channels can lead to the sensationof pain. These input channels include: (1) nociceptoractivation in the periphery by noxious mechanical,thermal, or chemical stimuli (nociceptive pain); (2)activation of sensitized nociceptors in the peripheryby low-intensity stimuli (peripheral sensitization); (3)ectopic discharge in nociceptors, originating at a neu-roma, dorsal root ganglion, peripheral nerve, or dor-sal root (peripheral nerve injury); (4) low-thresholdafferent activation in the periphery by low-intensitymechanical or thermal stimuli (in combination withcentral sensitization, synaptic reorganization, or dis-inhibition); (5) ectopic discharge in low-threshold af-ferents originating at a neuroma, dorsal root gan-glion, peripheral nerve, dorsal root (peripheral nerveinjury associated with central sensitization, synapticreorganization, or disinhibition); and (6) spontaneousactivity in central neurons (in the dorsal horn, thala-mus, or cortex).

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Pain results from: (1) nociceptive transduction; (2)peripheral sensitization; (3) changes in ion-channelexpression/phosphorylation/accumulation in primaryafferents (altered sensory neuron excitability); (4)posttranslational changes in ligand- and voltage-gated ion-channel kinetics in central (spinal cord andbrain) neurons, altering their excitability and thestrength of their synaptic inputs (central sensitiza-tion); (5) alterations in the expression of receptors/transmitters/ion channels in peripheral and centralneurons (phenotype modulation); (6) modification ofsynaptic connections caused by cell death or sprout-ing (synaptic reorganization); and (7) loss of local in-

hibition at different relay levels in the neuraxis anddescending inhibition caused by decreased activationof neurons, down-regulation of receptors/transmit-ters, and cell death, originating in the forebrain andbrain stem and terminating in the brain stem and spi-nal cord (disinhibition).

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The mammalian nervous system contains net-works that modulate nociceptive transmission. Thetrigeminal brain stem nuclear complex receivesmonoaminergic, enkephalinergic, and peptidergic pro-jections from regions known to be important in themodulation of nociceptive systems. A descending in-hibitory neuronal network extends from the frontalcortex and hypothalamus through the periaqueductalgray to the rostral ventromedial medulla and themedullary and spinal dorsal horn. The rostral ventro-medial medulla includes the nucleus raphe magnumand the adjacent reticular formation and projects tothe outer laminae of the spinal and medullary dorsalhorn. Electrical stimulation or injection of opioidsinto the periaqueductal gray or rostral ventromedialmedulla reduces nociresponsive neuron activity. Theperiaqueductal gray receives projections from the in-sular cortex and the amygdala.

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Antinociception can be measured by nociceptivereflex inhibition. In the rostral ventromedial medullaand periaqueductal gray, 3 classes of neurons havebeen identified.

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“Off-cells” pause immediately be-fore the nociceptive reflex, whereas “on-cells” are ac-tivated. Neutral cells show no consistent changes inactivation.

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Opioids activate off-cells and inhibit on-cells; nociceptive reflexes are inhibited. Thus, off-cellactivity is related to suppression of nociception,whereas on-cell activity is accompanied by enhance-ment to noxious stimuli. On- and off-cell activity ismodulated by 5-HT

1

receptor agonists.

6

These cells are believed to modulate the activityof the trigeminal nucleus caudalis and dorsal hornneurons. Increased on-cell activity in the brainstem’s pain modulation system could enhance theresponse to both painful and nonpainful stimuli.Opiate withdrawal results in increased firing of on-cells, decreased firing of off-cells, and enhanced no-ciception.

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Headache may be caused, in part, by en-hanced neuronal activity in the nucleus caudalis as aresult of enhanced on-cell or decreased off-cell ac-

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tivity. Other conditioned stimuli associated withpain and stress also can turn on the pain system andmay account, in part, for the association betweenpain and stress.

Although much is known about the mechanismsthat operate in primary afferent and dorsal horn neu-rons to produce pain, much less is known about thechanges that occur in the brain and how the affective,cognitive, and perceptual aspects of pain are gener-ated. It is clear, however, that the sensory cortex canundergo considerable plasticity in concert with thechanges that occur in subcortical structures, and suchsupraspinal plasticity is likely to play a major role inshaping the global pain experience.

Pain perception can be modulated by psychologi-cal factors, such as attention, stress, and arousal. Pla-cebo and nocebo manipulations (giving inert agentswith suggestions for reducing or enhancing symp-toms), hypnotic suggestion, attention, distraction,and emotions modulate pain-inhibitory and pain-facilitation pathways.

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Our understanding of how thismodulation occurs at a neuroanatomical level hasbeen significantly enhanced using functional neuro-imaging. Brain imaging experiments show that antici-pation of pain affects cortical nociceptive regions. In-terestingly, these brain regions are the same ones thatare directly activated during pain itself.

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Tracey et alneuroanatomically defined a key area in the networkof brain regions active in response to pain that ismodulated by attention to the painful stimulus.

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High-resolution functional magnetic resonance imag-ing was used in 9 control subjects to define brain acti-vation within the periaqueductal gray region in re-sponse to painful heat stimulation applied to thehand. Subjects were asked to either focus on or dis-tract themselves from painful stimuli, which werecued using colored lights. During the distraction con-dition, subjects rated pain intensity significantlylower than when they attended to the stimulus. Acti-vation in the periaqueductal gray region was signifi-cantly increased during the distraction condition, andthe total increase in activation was predictive ofchanges in perceived intensity. These results providedirect evidence supporting the notion that the peri-aqueductal gray region is a site for higher corticalcontrol of pain modulation in humans, which is en-

tirely consistent with the known concept that themore attention is focused on pain, the worse it gets.

PAIN IN MIGRAINE

Migraine is a primary brain disorder, a form ofneurovascular headache in which neural events re-sult in dilation of blood vessels, with further painand nerve activation. Migraine most likely resultsfrom dysfunction of brain stem pathways that nor-mally modulate sensory input. Three componentsseem to be involved: (1) the cranial blood vessels,(2) the trigeminal innervation of the vessels, and(3) the reflex connections of the trigeminal systemwith the cranial parasympathetic outflow. The keypathway for the pain is trigeminovascular inputfrom the meningeal vessels. Brain imaging studiessuggest that important modulation of the trigemi-novascular nociceptive input stems from the dorsalraphe nucleus, locus coeruleus, and nucleus raphemagnus.

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During a migraine attack, there is an inflamma-tory process (neurogenic inflammation) that occursat the site of the nerve terminal. Trigeminal nerve ac-tivation is accompanied by the release of vasoactiveneuropeptides including CGRP, substance P, andneurokinin A from the nerve terminals. These media-tors produce mast cell activation, sensitization of thenerve terminals, and extravasation of fluid into theperivascular space around the dural blood vessels. In-tense neuronal stimulation causes induction of c-

fos

(an immediate early gene product) in the trigeminalnucleus caudalis of the brain stem. Substance P andCGRP further amplify the trigeminal terminal sensi-tivity by stimulating the release of bradykinin andother inflammatory mediators from nonneuronalcells.

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Cortical spreading depression (the cause ofthe aura) can activate the trigeminal system. Al-though the brain itself is largely insensate, pain canbe generated by large cranial vessels, proximal intra-cranial vessels, or dura mater. The involvement of theophthalmic division of the trigeminal nerve and itsoverlap of structures innervated by branches of C2nerve roots explain the typical distribution of mi-graine pain over the frontal and temporal regionsand the referral of pain to the parietal, occipital,and high cervical regions.

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CENTRAL SENSITIZATION IN PAINAND MIGRAINE

Sensitization of nociceptors results in increasedspontaneous neuronal discharges. Neurons increaseresponsiveness to both painful and nonpainful stim-uli. Often the receptor fields are expanded and pa-tients feel pain over a greater part of the dermatome.Clinically, this is recognized as hyperalgesia (in-creased sensitivity to pain) and cutaneous allodynia(pain perceived in response to stimuli that are notnecessarily noxious).

Most patients with migraine exhibit cutaneous al-lodynia inside and outside their pain-referred areasduring migraine attacks. Burstein et al studied the de-velopment of cutaneous allodynia during migraine bymeasuring the pain thresholds in the head and fore-arms of a patient at several points during the mi-graine attack (1, 2, and 4 hours after onset) and com-pared the pain thresholds in the absence of anattack.

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Within 20 to 30 minutes after the initial acti-vation of the patient’s peripheral nociceptors, theybecame sensitized and mediated the symptoms ofcranial hypersensitivity. The barrage of impulses thenactivated second-order neurons and initiated theirsensitization, mediating the development of cutane-ous allodynia on the ipsilateral head. The sensitizedsecond-order neurons activated and eventually sensi-tized third-order neurons, leading to allodynia on thepatient’s contralateral head and forearms at the 2-hour measurement, a full hour after the initial allo-dynia on the ipsilateral head.

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