Projection of tooth pulp afferents to the brainstem and to the cortex in the cat

Neuropathic orofacial pain: proposed mechanisms, diagnosis, and treatment considerations

The most common reason patients seek medical or dental care in the United States is due to pain or dysfunction. The orofacial region is plagued by a number of acute, chronic, and recurrent painful maladies. Pain involving the teeth and the periodontium is the most common presenting concern in dental practice. Non-odontogenic pain conditions also occur frequently. Recent scientific investigation has provided and explosion of knowledge regarding pain mechanisms and pathways and an enhanced understanding of the complexities of the many ramifications of the total pain experience. Therefore, it is mandatory for the dental professional to develop the necessary clinical and scientific expertise on which he/she may base diagnostic and management approaches. Optimum management can be achieved only by determining an accurate and complete diagnosis and identifying all of the factors associated with the underlying pathosis on a case-specific basis. A thorough understanding of the epidemiologic and etiologic aspects of dental. musculoskeletal, neurovascular, and neuropathic orofacial pain conditions is essential to the practice of evidence-based dentistry/medicine.

Rat tooth pulp projections to spinal trigeminal subnucleus caudalis are glutamate-like immunoreactive

It has been shown that glutamate-like immunoreactive axon terminals are present within the spinal trigeminal nucleus, including subnucleus caudalis. The morphology of many of these terminations is consistent with their identification as primary afferents. To establish whether primary afferent projections to subnucleus caudalis are glutamate-like immunoreactive, we injected an anterograde tract tracer into rat incisor tooth pulp, histochemically visualized this tracer within subnucleus caudalis, and then used an immunocytochemical technique to label glutamate-like immunoreactive profiles within these same sections. The anterograde tract tracer used, the B subunit of cholera toxin conjugated to horseradish peroxidase (B-HRP), is transported transganglionically and can be used to localize tooth pulp projection fibers in the spinal trigeminal nucleus. A majority of B-HRP projection fibers from rat lower incisors terminated ipsilaterally in axon terminals in the dorsal region of subnucleus caudalis. Labeled axon terminals were both scallop-shaped and smooth in profile. Small numbers of fibers containing B-HRP extended into laminae I-III caudally and were present in both the border zone between laminae IV and V and the most lateral region of lamina V rostrally. Approximately 75% of the B-HRP-labeled projection fibers were glutamate-like immunoreactive, providing evidence that the excitatory amino acid glutamate functions as a neurotransmitter in a subpopulation of these fibers. Terminals reactive for both B-HRP and glutamate-like immunoreactivity contained small, spherical round vesicles, formed asymmetric synapses, and participated in axoaxonic and axodendritic synaptic junctions. These results support the hypothesis that glutamate may be a transmitter of A delta and C fibers involved in relaying nociceptive information from the tooth pulp.

Neuronal responses in rostral trigeminal brain-stem nuclei of macaque monkeys after chronic trigeminal tractotomy

Unilateral trigeminal tractotomy was carried out at the level of the obex, just rostral to the subnucleus caudalis, in five young adult Macaca fascicularis monkeys. The animals had been trained previously to perform a behavioral shock avoidance task in response to electrical stimulation of dental pulp and facial skin. Tractotomy produced an elevation in the stimulus strength which elicited escape behavior when facial skin was stimulated but not when the tooth pulp was stimulated. Unit activity, evoked by electrical stimulation of the tooth pulp and facial skin as well as innocuous and noxious mechanical stimulation of orofacial regions, was recorded from neurons in the trigeminal main sensory nucleus and the subnuclei oralis and interpolaris of the spinal nucleus 8 to 12 weeks after tractotomy. Primary afferent input to these nuclei is unaffected by the tractotomy which is located more caudally. The tractotomy interrupts primary afferent input into the trigeminal nucleus caudalis and also intranuclear connections between caudalis and the more rostral nuclei. Forty-one units contralateral and 47 ipsilateral to the tractotomy were studied. Thirty-six of the units responded only to low-threshold mechanical or electrical stimulation of orofacial zones, 46 were responsive to innocuous mechanical and electrical stimulation of orofacial zones and also to electrical stimulation of the dental pulp. Six units responded only to dental pulp stimulation. No statistically significant differences between the populations of neurons ipsilateral and contralateral to the tractotomies were found relating to the size or location of the peripheral receptive fields, latencies, thresholds, mean firing densities, or responsiveness to the various forms of stimulation. The behavioral results suggest that trigeminal relay neurons rostral to the obex are able to signal dental pain sensation, and the physiological studies confirm that the firing of such neurons is unaffected by tractotomy. The physiological studies demonstrate that the firing patterns of relay neurons activated by natural and electrical cutaneous facial stimuli and which are located in trigeminal brain-stem nuclei rostral to the obex are also not affected by tractotomy. The cutaneous facial analgesia observed after tractotomy thus appears to be due to deafferentation of relay neurons in trigeminal nucleus caudalis rather than to alterations in coding patterns in rostrally located trigeminal neurons due to interruption of the intratrigeminal pathway between the caudal and rostral nuclear groups.

Tooth pulp-evoked potentials in the monkey: cortical surface and intracortical distribution

The distribution of tooth pulp-evoked potentials (TPEPs) was characterized in the primary motor (MI), primary somatosensory (SI) and secondary somatosensory (SII) cortices of the monkey. Bipolar electrical tooth pulp stimulation elicited TPEP components P23 and N44 over SI, P26 and N72 over MI, and P72, N161, P280, N420, P561 and N662 over SII. Muscular artifacts and extradental input did not affect the TPEP as demonstrated by experiments using a neuromuscular blocking agent and removal of the pulp, respectively. The short latency TPEPs recorded over SI and MI were evoked by low stimulus intensities and activation of A beta nerve fibers, whereas the long latency TPEPs recorded over SII required higher stimulus intensities and the additional recruitment of A delta nerve fibers. Intracortical recordings revealed polarity reversals of components P23 and N44 in area 3b, P26 and N72 in area 4, and P72, N161, P280, N420, P561 and N662 in the upper bank of the lateral sulcus (SII). Evidence presented in this study suggests that TPEPs recorded from SI and MI relate to non-nociceptive mechanisms while TPEPs recorded from SII relate to nociceptive mechanisms.

The central projections of tooth pulp afferent neurons in the rat as determined by the transganglionic transport of horseradish peroxidase

Transganglionic transport of horseradish peroxidase (HRP) or horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) was used to map in detail the central projections of trigeminal primary afferent neurons that innervate the dental pulp organ of the rat. In each of ten animals, 0.5-2.0 microliters of enzyme solution was injected into the pulp chamber of the first maxillary molar tooth. Postmortem examination of the decalcified teeth in all cases showed that the HRP/HRP-WGA remained confined to the pulp chamber and pulp roots, with no spread of enzyme into periapical tissues. HRP-labeled tooth pulp afferent fibers projected to all four rostrocaudal subdivisions of the ipsilateral trigeminal brainstem nuclear complex (TBNC) and to the upper cervical spinal cord. The labeled terminal fields formed a column that stretched relatively uninterrupted from just caudal to the rostromedial tip of the trigeminal principal sensory nucleus to at least the C2 segment of the spinal cord. The density of the afferent projection varied markedly from one rostrocaudal level of the TBNC to the next but was heaviest in an area encompassing the caudal one-half of the principal sensory nucleus and the rostral two-thirds of pars oralis. Fibers projected only lightly to pars caudalis, where they terminated preferentially in laminae I, IIa, and the junctional zone between laminae IV and V. HRP-labeled terminals in C1 and C2 were located almost exclusively in laminae I. In the dorsoventral axis, the terminal fields in the TBNC were located in a surprisingly dorsal part of the complex, well within what has been shown by others to be largely an area of termination for mandibular division fibers. Most fibers ended in medial parts of the TBNC, with the exception of two modestly labeled terminal fields located in the lateral aspects of rostral pars oralis and rostral pars caudalis. No labeled fibers terminated in the contralateral TBNC or contralateral cervical spinal cord.