Anatomical consequences of neonatal infraorbital nerve transection upon the trigeminal ganglion and vibrissa follicle nerves in the adult rat

Central terminal sensitization of TRPV1 by descending serotonergic facilitation modulates chronic pain

The peripheral terminals of primary nociceptive neurons play an essential role in pain detection mediated by membrane receptors like TRPV1, a molecular sensor of heat and capsaicin. However, the contribution of central terminal TRPV1 in the dorsal horn to chronic pain has not been investigated directly. Combining primary sensory neuron-specific GCaMP3 imaging with a trigeminal neuropathic pain model, we detected robust neuronal hyperactivity in injured and uninjured nerves in the skin, soma in trigeminal ganglion, and central terminals in the spinal trigeminal nucleus. Extensive TRPV1 hyperactivity was observed in central terminals innervating all dorsal horn laminae. The central terminal TRPV1 sensitization was maintained by descending serotonergic (5-HT) input from the brainstem. Central blockade of TRPV1 or 5-HT/5-HT3A receptors attenuated central terminal sensitization, excitatory primary afferent inputs, and mechanical hyperalgesia in the territories of injured and uninjured nerves. Our results reveal central mechanisms facilitating central terminal sensitization underlying chronic pain.

Trigeminal nerve injury ErbB3/ErbB2 promotes mechanical hypersensitivity

BACKGROUND

Chronic constriction injury of the trigeminal infraorbital nerve results in transient analgesia followed by whisker pad mechanical allodynia in rats. Neuregulin 1 expressed on axonal membranes binds receptor tyrosine kinase ErbB, promoting Schwann cell development and remyelination. This study investigated whether orofacial mechanical allodynia is signaled by ErbB3-ErbB2 heterodimers in injured nerves.

METHODS

Whisker pad mechanical allodynia (von Frey stimuli) was quantified in wild type rats and in transgenic rats with Sleeping Beauty transposon mutation for neuregulin 1 transgene. Pain-related behavior was retested after intraperitoneal injection of the ErbB2 inhibitor Lapatinib, an agent shown by others to reduce breast cancer pain. Infraorbital nerve injury was evaluated histologically with myelin and neuronal biomarkers. ErbB3 changes over time were measured with western blots.

RESULTS

Whisker pad mechanical hypersensitivity began in week 2 in wild type rats (3.11 ± 5.93 g vs. 18.72 ± 0.00 g after sham surgery, n = 9, P < 0.001), indicating trigeminal neuropathic pain, but was not evident in transgenic rats (odds ratio: 1.12, 95% confidence interval: 0.38-3.35). Initiation of statistically significant mechanohypersensitivity was delayed until week 6 after surgery in transgenic rats (3.44 ± 4.60 g vs. 18.72 ± 0.00 g, n = 4, P < 0.001). Mechanical allodynia, which persisted 8 weeks in wild type rats was alleviated by Lapatinib (15 ± 3.89 g vs. 2.45 ± 1.13 g, n = 6, P < 0.001). Infraorbital nerve damage was verified histologically. Statistically significant ErbB3 increases (weeks 5 and 10) in wild type and transgenic rats (week 10) coincided with time points when mechanical hypersensitivity was present.

CONCLUSION

The Neuregulin 1-ErbB3-ErbB2 complex is a causal mechanism in nerve injury-induced trigeminal neuropathic pain. Understanding peripheral glial mechanisms after nerve injury will improve neuropathic pain treatment.

Comparison of microsuture, interpositional nerve graft, and laser solder weld repair of the rat inferior alveolar nerve

PURPOSE

Intraosseous repair of nerves involves difficulty of access and there is concern that bone healing may interfere with repair outcomes. The present report describes the effect of 3 separate repair techniques on recovery from section of the rat intraosseous inferior alveolar nerve, with reference to the mental nerve distal and the trigeminal ganglion proximal to the nerve section.

MATERIALS AND METHODS

Unilateral exposure of the inferior alveolar nerves of 28 rats was achieved through bone windows. Nerves were sectioned and rats were assigned to 1 of 4 groups (n = 7): untreated controls, microsuture repair, interpositional nerve grafts from the femoral nerve, or laser solder weld repair. Animals were sacrificed 1 year after surgery for histologic evaluation of the mental nerve, inferior alveolar nerve, and trigeminal ganglion compared with unoperated contralateral nerves.

RESULTS

Compared with the unoperated contralateral nerves, nerve section substantially decreased mental nerve fiber number, mental nerve myelination, mental nerve fiber diameter, inferior alveolar nerve vascularity, trigeminal neuron number, and trigeminal neuron horseradish peroxidase tracer uptake and increased trigeminal ganglion degenerate neurons (P < .001). All 3 forms of repair substantially decreased these effects (P < .05). Interpositional nerve graft was least effective (P < .05). Nonetheless, mental nerve fiber diameter was significantly decreased compared with unsectioned nerves after microsuture and laser solder weld repair (P < .05).

CONCLUSIONS

Intraosseous repair of the inferior alveolar nerve decreases peripheral and central signs of degeneration. Clinical hyperesthesia after repair may reflect a predominance of small fibers after recovery.

Prenatal exposure to ethanol affects postnatal neurogenesis in thalamus

The number of neurons in the ventrobasal thalamus (VB) in the adolescent rat is unaffected by prenatal exposure to ethanol. This is in sharp contrast to other parts of the trigeminal-somatosensory system, which exhibit 30-35% fewer neurons after prenatal ethanol exposure. The present study tested the hypothesis that prenatal ethanol exposure affects dynamic changes in the numbers of VB neurons; such changes reflect the sum of cell proliferation and death. Neuronal number in the VB was determined during the first postnatal month in the offspring of pregnant Long-Evans rats fed an ethanol-containing diet or pair-fed an isocaloric non-alcoholic liquid diet. Offspring were examined between postnatal day (P) 1 and P30. The size of the VB and neuronal number were determined stereologically. Prenatal exposure to ethanol did not significantly alter neuronal number on any individual day, nor was the prenatal generation of VB neurons affected. Interestingly, prenatal ethanol exposure did affect the pattern of the change in neuronal number over time; total neuronal number was stable in the ethanol-treated pups after P12, but it continued to rise in the controls until P21. In addition, the rate of cell proliferation during the postnatal period was greater in ethanol-treated animals. Thus, the rate of neuronal acquisition is altered by ethanol, and by deduction, there appears to be less ethanol-induced neuronal loss in the VB. A contributor to these changes is a latent effect of ethanol on postnatal neurogenesis in the VB and the apparent survival of new neurons.

Intracisternal administration of mitogen-activated protein kinase inhibitors reduced mechanical allodynia following chronic constriction injury of infraorbital nerve in rats

The present study investigated the role of mitogen-activated protein kinase (MAPK) in orofacial neuropathic pain following chronic constriction injury of the infraorbital nerve (ION-CCI). Experiments were carried out on male Sprague-Dawley rats weighing between 200 and 230 g. The ION was separated from adhering tissue, and two ligatures (5-0 chromic gut) were tied loosely around it. We examined the air-puff thresholds (mechanical allodynia), scores of pinprick (mechanical hyperalgesia), and face grooming frequency for acetone application (hypersensitivity for cold stimulation) - 3, 3, 6, 9, 12, 15, 20, 25, 30, and 40 days after surgery. ION-CCI produced mechanical allodynia, hyperalgesia, and cold hypersensitivity. We investigated whether administration of MAPKs inhibitors blocks ION-CCI-induced mechanical allodynia. Intracisternal administration with PD98059 or SB203580, a MEK inhibitor or a p38 MAPK inhibitor, respectively, significantly inhibited ION-CCI-induced mechanical allodynia in the orofacial area. These results indicate that the ION-CCI produced behavioral alterations in the orofacial area and those central MAPKs pathways contribute to orofacial neuropathic pain. Our findings suggest that MAPKs inhibitors have a potential role in treatment for orofacial neuropathic pain.

Relationship between physiological response type (RA and SA) and vibrissal receptive field of neurons within the rat trigeminal ganglion

Cells within the trigeminal ganglion (Vg) encode all the information necessary for the rat to differentiate tactile stimuli, yet it is the least-studied component in the rodent trigeminal somatosensory system. For example, extensive anatomical and electrophysiological investigations have shown clear somatotopic organization in the higher levels of this system, including VPM thalamus and SI cortex, yet whether this conserved schemata exists in the Vg is unknown. Moreover although there is recent interest in recording from vibrissae-responsive cells in the Vg, it is surprising to note that the locations of these cells have not even been clearly demarcated. To address this, we recorded extracellularly from 350 sensory-responsive Vg neurons in 35 Long-Evans rats. First, we determined three-dimensional locations of these cells and found a finer detail of somatotopy than previously reported. Cells innervating dorsal facial features, even within the whisker region, were more dorsal than midline and ventral features. We also show more cells with caudal than rostral whisker receptive fields (RF), similar to that found in VPM and SI. Next, for each vibrissal cell we determined its response type classified as either rapidly (RA) or slowly (SA) adapting. We examined the relationship between vibrissal RF and response type and demonstrate similar proportions of RA and SA cells responding to any whisker. These results suggest that if RA and SA cells encode distinct features of stimuli, as previously suggested, then at the basic physiological level each whisker has similar abilities to encode for such features.

Reinnervation of a single vibrissa after nerve excision in the adult rat

After a survival time of 180 days following the excision of a 2 mm segment of the vibrissal nerve to the gamma straddler vibrissa in the adult rat, a retrograde fluorescent single-labelling experiment revealed that 46% of the injured vibrissal sensory neurones had regenerated their peripheral processes. Peripheral collateral sprouting was not involved in the reinnervation of the denervated gamma vibrissa, as proved by a retrograde fluorescent double-labelling experiment. The regenerating nerve fibres did not invade the intact neighbouring vibrissae of the gamma vibrissa, and the sensory nerve fibres of the intact vibrissae were not translocated to the denervated gamma vibrissa. Thus, the sensory function of the denervated gamma vibrissa was restored exclusively by the regeneration of the damaged vibrissal nerve.

Functional Connections Formed by Saphenous Nerve Terminal Sprouts in the Dorsal Horn Following Neonatal Sciatic Nerve Section

The rostrocaudal distribution of saphenous nerve inputs into the lumbar dorsal horn from L2 to L6 has been investigated in urethane anaesthetized rats whose left sciatic nerve was cut and ligated at birth. In normal cord, electrical stimulation of the saphenous nerve evoked dorsal horn spikes in L2 to caudal L4. Few or no spikes were evoked in L5. After neonatal sciatic nerve section, saphenous nerve stimulation evoked spikes throughout segments L2 to L6. Dorsal horn cell receptive fields were also altered following neonatal sciatic nerve section. A somatotopic map of the lumbar enlargement in normal rats was constructed from the receptive fields (RFs) of adjacent dorsal horn cells. Cells with RFs in the saphenous skin region were concentrated in L3 and rostral L4 and very few were found in L5. After neonatal sciatic nerve section, however, a substantial number of cells with low threshold saphenous skin RFs were also found in caudal L4 and throughout L5. These results show that the central saphenous nerve terminal sprouts that grow into the sciatic terminal region following neonatal sciatic nerve section (Fitzgerald, 1985, J. Comp. Neurol., 240, 414-422; Fitzgerald et al., 1990, J. Comp. Neurol., 300, 370-385) form functional connections. This results in dorsal horn cells that are not normally influenced by saphenous nerve inputs developing substantial low threshold RFs in saphenous nerve skin regions.

Neuropeptide Y in trigeminal ganglion following chronic constriction injury of the rat infraorbital nerve: is there correlation to somatosensory parameters?

The aim of this study was to investigate neuropeptide Y (NPY) levels in trigeminal ganglia following infraorbital nerve injury. Two experimental procedures were performed in three groups of rats: a unilateral chronic constriction injury (CCI) to the infraorbital nerve (n=13), nerve manipulation without CCI (n=13) and unoperated controls (n=8). All rats underwent baseline and regular assessment of mechanical withdrawal threshold (Von Frey) and reaction to pin prick as well as free behavior evaluations. CCI to the infraorbital nerve induced significant hyperalgesia and allodynia within 9-12 days. At 6 days seven rats were euthanized and trigeminal ganglia harvested for immunocytochemical (ICC) studies. The study was ended at 14 days when all rats were euthanized and their ganglia harvested for ICC and radioimmunoassay (RIA) studies. An increase in NPY levels was seen in the ipsilateral ganglia of manipulated and CCI rats at 6 days, when rats displayed no pain-related behavior. At 14 days, ICC and RIA both detected significant increases in NPY levels in the ipsilateral ganglia of CCI and manipulated rats but not in unoperated controls. The possible roles of NPY in pain modulation and nerve injury are discussed in light of these findings.

Reduction of pentylenetetrazole-induced seizure activity in awake rats by seizure-triggered trigeminal nerve stimulation

Stimulation of the vagus nerve has become an effective method for desynchronizing the highly coherent neural activity typically associated with epileptic seizures. This technique has been used in several animal models of seizures as well as in humans suffering from epilepsy. However, application of this technique has been limited to unilateral stimulation of the vagus nerve, typically delivered according to a fixed duty cycle, independently of whether ongoing seizure activity is present. Here, we report that stimulation of another cranial nerve, the trigeminal nerve, can also cause cortical and thalamic desynchronization, resulting in a reduction of seizure activity in awake rats. Furthermore, we demonstrate that providing this stimulation only when seizure activity begins results in more effective and safer seizure reduction per second of stimulation than with previous methods. Seizure activity induced by intraperitoneal injection of pentylenetetrazole was recorded from microwire electrodes in the thalamus and cortex of awake rats while the infraorbital branch of the trigeminal nerve was stimulated via a chronically implanted nerve cuff electrode. Continuous unilateral stimulation of the trigeminal nerve reduced electrographic seizure activity by up to 78%, and bilateral trigeminal stimulation was even more effective. Using a device that automatically detects seizure activity in real time on the basis of multichannel field potential signals, we demonstrated that seizure-triggered stimulation was more effective than the stimulation protocol involving a fixed duty cycle, in terms of the percent seizure reduction per second of stimulation. In contrast to vagus nerve stimulation studies, no substantial cardiovascular side effects were observed by unilateral or bilateral stimulation of the trigeminal nerve. These findings suggest that trigeminal nerve stimulation is safe in awake rats and should be evaluated as a therapy for human seizures. Furthermore, the results demonstrate that seizure-triggered trigeminal nerve stimulation is technically feasible and could be further developed, in conjunction with real-time seizure-predicting paradigms, to prevent seizures and reduce exposure to nerve stimulation.

Structure and innervation of the vibrissal follicle-sinus complex in the Australian water rat, Hydromys chrysogaster

Light and electron microscopic techniques were used to examine the structure and innervation of the mystacial vibrissal follicle-sinus complex (F-SC) in the Australian water rat. The F-SCs of this semiaquatic rodent show the same morphologic elements described in terrestrial rats but differ in size, structure, and innervation. Most striking is the size of the water rat's caudal F-SCs, measuring 6.3 mm in length and 2.4 mm in diameter. The sinus system is divisible into a ring sinus and a cavernous sinus and shows a distinct asymmetry. At the highest level of the cavernous sinus, the outer root sheath forms a ridge in the direction of the trabeculae, which bind the ridge to the capsule. A ringwulst is present only in small and medium-sized F-SCs. The mean number of myelinated axons counted in the deep vibrissal nerve (DVN) of most caudal F-SCs was 537, indicating an innervation density of the water rat's vibrissal system at least 2.5 times as high as that of terrestrial rats. The total number of nerve fibers of the small superficial nerves was less than 10% of that of the DVN. These fibers innervate almost exclusively the area of the inner conical body. Structural specializations of the water rat F-SC are discussed as an analogous development in mammals adapted to the aquatic environment, primarily in terms of thermoregulation, whereas its high degree of innervation is assessed to lend support to the hypothesis that the vibrissal system is of special significance in aquatic mammals.

Long-term observation of the effect of peripheral nerve injury in neonatal and young rats

The purpose of this study was to observe functional recovery and motoneuron death after nerve transection-and-repair in neonatal versus young animals. One hundred nine Lewis rats underwent posterior tibial nerve transection-and-repair at 6 or 22 days of age. Fifty-two and fifty-seven nerves at the 6- and 22-day times were used for endpoint analysis at 1, 3, 10, and 14 months. These assessments included serial functional walking track analysis, electrophysiologic studies, muscle mass evaluation, motoneuron counts with retrograde horseradish peroxidase tracing, and histologic and morphometric nerve analysis. Walking track analysis and nerve conduction velocity indicated significantly poorer functional regeneration in the 6-day-old group than in the 22-day-old group. Muscle mass in the 6-day-old group did not recover as well as in the 22-day-old group. Motoneuron numbers stained with horseradish peroxidase were less in the 6-day-old group than in the 22-day-old group. In contrast, morphometric analysis did not reach significance. This study suggests that the same nerve injury sustained in a neonatal rat is less likely to demonstrate functional recovery than one sustained in a young rat.

Effect of neonatal capsaicin and infraorbital nerve section on whisker-related patterns in the rat trigeminal nucleus

In the present study, we investigated the effect of neonatally administered capsaicin on whisker-related pattern formation in the rat trigeminal complex. Both normal whisker-related patterns of barrelettes and the modified patterns seen after neonatal section of the infraorbital nerve were assessed. Capsaicin caused no change in the pattern or size of cytochrome oxidase (CO) barrelettes in the principal trigeminal nucleus (Vp) or trigeminal nucleus interpolaris (Vi) or caudalis (Vc). Injections of horseradish peroxidase (HRP) or wheatgerm agglutinin conjugated to HRP (WGA-HRP) into the posteroorbital (PO) whisker follicle in vehicle-treated animals showed that WGA labelled a larger number of trigeminal ganglion cells than HRP (203 +/- 23; cf. 158 +/- 19), with an increased labelling of small-diameter neurons (HRP: 25.9 +/- 7.7 microm; WGA: 23.2 +/- 7.2 pm). Capsaicin caused a loss of smaller diameter cells but had no effect on the location, cross-sectional area, or rostrocaudal extent of the transganglionically labelled HRP terminations in Vp, Vi, Vc, and cervical dorsal horn. WGA-HRP labelling revealed similar, but less dense, central terminal areas as HRP and an additional area of superficial terminals in the caudal medulla; these were also unaffected by capsaicin treatment. After infraorbital nerve section, CO patches and transganglionically labelled afferent terminations, corresponding to innervated nonmystacial whiskers, were approximately doubled in size. Capsaicin had no effect on the increased size of these spared whisker patches or their afferent terminal areas. These results suggest that barrelette formation is not dependent on unmyelinated afferents and that the changes in response properties seen after capsaicin, such as increased receptive fields, reflect functional changes rather than anatomical expansion of afferent terminal areas.

Peripheral and central predictors of whisker afferent morphology in the rat brainstem

Prior studies suggest that whisker afferents have but one central projection pattern, despite their association with differing peripheral receptors that predict central morphology in other systems. Target factors in barrelettes are thought to dictate afferent projection patterns; yet, barrelettes differ in their size, shape and development. We tested the hypothesis that whisker afferents have differing morphologies that are predicted by peripheral and central factors. Branching patterns and collaterals of 78 Neurobiotin-stained afferents were compared in rats. Fibers from one whisker had precisely somatotopic projections but highly varied morphologies. For the entire sample, analysis of variance revealed significant intrafiber variance in collateral number and arbor shape that was attributed to the target subnucleus. Significant interfiber variance did not reflect response adaptation rate, direction sensitivity, whisker row origin or parent fiber bifurcation in the trigeminal root. Instead, we found the following. 1) Mandibular fibers had more elongated arbors than maxillary axons. In subnuclei interpolaris and principalis, mandibular fibers had larger arbors with more boutons/collateral than maxillary axons; in oralis and interpolaris, mandibular fibers had fewer collaterals than those of the maxillary division. 2) Upper lip whisker axons had more boutons than those from the B-D row in all subnuclei. 3) Rostral whisker are afferents had larger arbors and more boutons than those from middle or caudal arcs due to significant arc effects in interpolaris and oralis. Thus, whisker afferents are not structurally uniform, and some morphological features are predictable. Intrafiber variance is attributed to the central target; interfiber variance reflects maxillary versus mandibular origin, upper lip origin and whisker rostrocaudal arc.

Plasticity of metabolic whisker maps in somatosensory brainstem and thalamus of mice with neonatal lesions of whisker follicles

We employed the autoradiographic deoxyglucose method to study metabolic whisker maps of the adult mouse somatosensory brainstem and thalamus after the neonatal removal of left whisker follicles C1, C2 and C3. Left whiskers B1-3 and D1-3 were deflected to metabolically activate the somatosensory pathway. Unoperated mice that were stimulated in the same fashion served as controls. Whisker stimulation resulted in an ipsilateral increase in metabolic activity in the three trigeminal brainstem structures in which the whiskers are represented topologically by segments of high cytochrome oxidase activity, i.e. subnucleus caudalis, subnucleus interpolaris and nucleus principalis. In the two subnuclei of mice with lesions and of controls, there was an increase in metabolic activity of the representations of the deflected whiskers, whereas the metabolic activity of representations A1-3 and E1-3 was low. Apart from these similarities, the metabolic activation of the representations originally representing whiskers C1-3 was remarkably greater in mice with lesions than in controls. This increase reached statistical significance in subnucleus caudalis and approached statistical significance in subnucleus interpolaris. In nucleus principalis the deprived territory was only partially activated and the degree of metabolic activation was less than in the subnuclei. In the thalamic ventrobasal complex of mice with lesions metabolic activity was unpatterned whereas two areas of metabolic activation were distinct in controls. Hence, the removal of whisker follicles in newborn mice resulted in the suppression of localized metabolic responses to whisker stimulation in the thalamus, whereas in the brainstem stimulus-related activity was prominent and the deprived territory became responsive to the stimulation of whisker follicles adjacent to the lesion. Apparently, the modification of the whisker representation at the first synapse of the pathway induces a diminution of localized responsivity in the thalamus.

Effect of vibrissae deprivation on follicle innervation, neuropeptide synthesis in the trigeminal ganglion, and S1 barrel cortex plasticity

Deprivation of vibrissae from an early age causes plasticity in S1 barrel cortex. This method of deprivation is most likely to induce plasticity by altering the balance of primary afferent activity from the deprived and spared vibrissae. To study whether or not induction or expression of this type of plasticity might be affected by follicle nerve injury caused by the deprivation technique, three different methods of detecting nerve injury were used: counting axon numbers in the distal follicle nerve, quantifying morphological changes in axons, and measuring neuropeptide expression in the trigeminal ganglion cells. First, nerves innervating follicles chronically deprived of vibrissae from birth had the same number of myelinated and unmyelinated axons as nerves from normally reared animals. Second, axons innervating deprived follicles showed no morphological changes in myelination or mitochondria characteristic of damaged nerves. Third, the corresponding nerve cell bodies in the trigeminal ganglion did not show upregulation of galanin or neuropeptide Y expression. In contrast, animals receiving mild injury of the follicle nerve endings (by cauterization of the follicle) showed profound changes in axonal myelination and mitochondria and increases in neuropeptide expression. These results imply that vibrissae deprivation does not act by inducing injury of the follicular nerve, suggesting that changes in the balance of follicle nerve activity are the cause of cortical plasticity. Consistent with this notion, a fourth experiment demonstrated that trimming the vibrissae induces cortical plasticity comparable to that induced by complete vibrissae removal.

Central plasticity in rat trigeminal primary sensory neurons innervating vibrissae after neonatal peripheral nerve injury

This study was undertaken to investigate whether a neonatal vibrissa lesion in the rat may lead to a central plasticity in undamaged vibrissae innervating primary sensory neurons. One vibrissa follicle was removed on the day of birth. After 3-4 months, choleragenoid-horseradish-peroxidase was injected in an adjacent vibrissa follicle either within the row of the missing follicle or across the rows and in the corresponding contralateral follicle. The trigeminal ganglion and nucleus caudalis was studied following retrograde and transganglionic transport of the tracer. The number of labeled cells was not significantly different on the two sides, whereas the central terminal field of labeling was considerably increased on the side of the lesion, but only when the injected follicle was located in the same row as the missing one. Cortical plasticity after neonatal vibrissae follicle lesions may thus be associated with a corresponding plasticity in primary sensory neurons innervating the vibrissae.

Neonatal infraorbital nerve damage and the development of eating behavior in the rat

It has been previously shown that bilateral infraorbital nerve (ION) transection in adult rats has little effect upon body weight regulation or eating behavior. However, in neonatal mouse, unilateral ION cut produces a profound decrease in body weight, beginning around the time of weaning. To help clarify the role of the ION in the development and sensorimotor control of eating solid food in rodents, the present experiment examined the effects of unilateral, neonatal ION transection in rats, upon body weight regulation and post-weaning eating behaviors. Comparison of normal and lesioned groups of rats, up to postnatal day (PND) 61, revealed no significant difference in mean adjusted (for sex) body weight. In addition, no significant differences were detected between the groups on post-weaning (PND 26 to PND 61) measures of mean adjusted (for weight) food intake, responsiveness to food, biting ability or inefficiency of mandibulation. At the end of the experiment, the effectiveness of the lesion was histologically evaluated. A significant 48.5% mean reduction in the cross-sectional area of the ophthalmic-maxillary portion of the trigeminal ganglion was observed on the lesioned side, relative to the intact side. There appears to be a differential influence of unilateral, neonatal ION cut upon eating in rat and mouse.

Rapid increase in mitochondrial volume in nucleus magnocellularis neurons following cochlea removal

Second-order auditory neurons in nucleus magnocellularis (NM) of the chick brainstem undergo a series of rapid metabolic changes following unilateral cochlea removal, culminating in the death of 25% of NM neurons. Within hours of cochlea removal, ipsilateral NM neurons show marked increases in histochemical staining for the mitochondrial enzymes succinate dehydrogenase and cytochrome oxidase. We investigated corresponding ultrastructural changes in NM neurons by preparing animals undergoing unilateral cochlea removal for transmission electron microscopy. We quantified changes in NM mitochondrial volume by stereological methods and qualitatively compared mitochondrial morphology between NM neurons destined to survive and those destined to die after cochlea removal. Within hours of cochlea removal, ipsilateral NM neurons show striking increases in mitochondrial volume (84% at 6 hours and 236% at 12 hours after cochlea removal compared to unoperated, control animals). At 2 week survival times, ipsilateral NM neurons contain fewer mitochondria than contralateral neurons. Surprisingly, anesthesia alone causes short-term increases in NM mitochondrial volume. Animals anesthetized with pentobarbital and ketamine and sacrificed 6 or 12 hours later showed a 45% increase in mitochondrial volume compared to previously unanesthetized animals. NM neurons destined to die within days of cochlea removal can be identified within several hours after deafferentation by the appearance of their ribosomes. We observed qualitative differences in mitochondrial morphology in dying neurons. Mitochondria in neurons destined to die consistently showed mitochondrial swelling and vacuolization indicative of metabolic dysfunction. Similar mitochondrial changes have been reported when mitochondria take up excess calcium. Ultrastructural changes in NM after cochlea removal display features of both programmed and pathological cell death, in which increased intracellular calcium is thought to play a role.

Organization of the proximal, orbital segment of the infraorbital nerve at multiple intervals after axotomy at birth: a quantitative electron microscopic study in rat

Although much is known of the central consequences of infraorbital nerve (ION) transection at birth, little is known about the effects of this lesion on the organization of the ION itself. To advance our understanding of how deafferentation alters the developing trigeminal neuraxis, 19 newborn rats were subjected to left ION section and perfused 1, 2, 4, 7, 17, or 90 days later. Left IONs were removed in the orbit proximal to the nerve injury site, and axon numbers, types, and fasciculation patterns were assessed with light and electron microscopic methods. Complete axon counts demonstrated that the axotomized ION contained an average (+/- SD) of 13,945 +/- 10,335, 14,112 +/- 3,501, 16,531 +/- 1,904, 9,045 +/- 1,465, 7,018 +/- 4,212, and 8,672 +/- 1,030 axons at the above-listed ages, respectively. These values are well below the 33,059 axons in the normal adult ION (Jacquin et al. [1984] Brain Res. 290:131-135) and the 42,219 axons in the newborn ION (Renehan and Rhoades [1984] Brain Res. 322:369-373). The axotomized ION also contained lower than normal percentages of myelinated axons (26.7% +/- 6.3% on postnatal day 90 vs. 59.7% +/- 6.2% in normal adults). Unmyelinated fibers constituted the vast majority of the remaining fiber types; degenerating fibers never accounted for > 1.6% of all the axons. The number of fascicles making up the axotomized ION overlapped significantly with those found in the normal newborn and adult ION. We conclude that 1) extensive, though variable, axon elimination occurs proximally within one day of the lesion; 2) the 74% reduction in fiber number seen at 90 days is not reliably achieved until postnatal day 7; 3) the higher than normal proportion of unmyelinated axons in the injured ION may underly many of the known effects of neonatal ION injury on the developing whisker-barrel neuraxis; 4) gross changes in ION fasciculation patterns are not prerequisite to injury-induced pattern alterations in the developing trigeminal system.

Recovery of original nerve supply after hypoglossal-facial anastomosis causes permanent motor hyperinnervation of the whisker-pad muscles in the rat

Hypoglossal-facial anastomosis (HFA), used in humans for the treatment of facial palsy, was experimentally performed in adult female Wistar rats. The time course of facial reinnervation and the extent of the new motor nerve supply of the vibrissal muscles that develops after HFA were estimated by counting all motoneurons in the brainstem labeled by injection of horseradish peroxidase (HRP) into the whisker pad; muscle innervation by motor endplates was not studied. In untreated animals, HRP injection labels 1,254 +/- 54 (mean +/- S.D.; n = 6) motoneurons, localized exclusively in the lateral subdivision of the facial nucleus. Immediately following HFA, this number drops to zero. The first HRP-labeled motoneurons appear in the hypoglossal nucleus at 28 days postoperation (dpo) and at 56 dpo their number reaches 1,096 +/- 48. Unexpectedly, the facial nerve, whose proximal stump has been left as blind end during surgery, additionally sends axons to the facial periphery. This resprouting is first detected at 42 dpo with HRP-marked neurons throughout the facial nucleus lacking somatotopic organization. The number of these labeled neurons also rises with time, and at 56 dpo, a total of 1,797 +/- 142 facial and hypoglossal motoneurons, that is, 43% more motoneurons than in normal animals, supplies the whisker pad. This hyperinnervation, that is, the projection of more motoneurons into the target muscle than under normal conditions--further increases to 1,978 +/- 92 motoneurons at 224 dpo and may provide a new animal model for studying the competitive relationships between motoneurons in their search for peripheral targets.

The innervation of the mystacial pad of the rat as revealed by PGP 9.5 immunofluorescence

The innervation of the mystacial pad in the rat was investigated with the aid of antihuman protein gene product (PGP) 9.5 immunofluorescence. PGP 9.5 is ubiquitin carboxyl-terminal hydrolase, which is distributed throughout neuronal cytoplasm. This technique revealed all previously known innervation as well as a wide variety of small-caliber axons and some endings of large-caliber afferents that had not been observed before. Newly revealed innervation affiliated with vibrissal-follicle sinus complexes included 1) fine-caliber, radially oriented processes in the epidermal rete ridge collar; 2) a loose network of fine-caliber, circumferentially arrayed processes in the centrifugal part of the mesenchymal sheath at the level of the ring sinus; 3) a loose haphazard network of fine-caliber and medium-caliber processes in the mesenchymal sheath and among the trabeculae of the cavernous sinus; 4) a loose network of circumferentially arrayed processes within the mesenchymal sheath of the cavernous sinus and in close proximity to the basement membrane; 5) a dense network of reticular-like endings provided by large-caliber afferents to the mesenchymal sheath in the upper part of the cavernous sinus; and 6) fine-caliber innervation to the dermal papilla at the base of all vibrissal shafts. In the intervibrissal skin, a dense distribution of fine-caliber individual and clustered profiles was detected in the epidermis. In addition to previously known innervation, Merkel endings were consistently observed in the epidermis at the mouths of guard hairs, loose networks of fine-caliber axons were found around the necks of occasional guard hairs, and fine-caliber profiles were frequently affiliated with vellus hairs. Vascular profiles were heavily innervated throughout the dermis. Axons and motor end plates of the facial nerve innervation to papillary muscles also were labeled. Transection of the infraorbital nerve eliminated all but the facial nerve innervation. Unilateral removal of the superior cervical ganglion eliminated the innervation to the dermal papillae but caused no other noticeable reduction. PGP 9.5-like immunofluorescence was also moderately expressed in apparent Schwann cells, in Merkel cells only in the external root sheath of vibrissal follicles, and in apparent dendritic and/or Langerhans cells usually located in the epidermis and occasionally in the follicles. PGP 9.5-like immunofluorescence persisted in highly vacuolated profiles along the usual courses of medium to large-caliber axons 2 weeks after nerve transection. The possible functional role of the newly discovered innervation is considered along with that of previously identified afferents.

Unmyelinated innervation of sinus hair follicles in rats

Unmyelinated nerve fibres comprise approximately one third of the innervation of rodent sinus hair follicles but their function is unknown. They may play a role as high-threshold sensory fibres, or may be autonomic efferents controlling the vascular sinus. In the present experiments capsaicin and surgical sympathectomy were used to establish whether these unmyelinated fibres are afferent fibres or autonomic efferents. The deep vibrissal nerves of mystacial follicles (C1 and C4) and a non-mystacial follicle (the postero-orbital, PO) were assessed in normal adult animals (n = 6) and compared with those treated with neonatal capsaicin (n = 6) or bilateral superior cervical ganglionectomy (n = 7). In capsaicin-treated animals, counts of fibres in the deep vibrissal nerves from all follicles showed normal numbers of myelinated axons, but approximately 80% reduction in unmyelinated fibres (normal mean +/- SD: C1 94 +/- 10, C4 89 +/- 9, PO 85 +/- 6; after neonatal capsaicin: C1 17 +/- 8, C4 16 +/- 6, PO 18 +/- 6; n = 6, P < 0.001 for all follicles). After sympathectomy there was no significant reduction in myelinated or unmyelinated fibre numbers. Labelling of PO follicles with WGA-HRP showed minimal numbers of labelled cells (0-10) within the superior cervical ganglion, also suggesting minimal sympathetic innervation. This sparse sympathetic supply to the follicle was further demonstrated by a lack of tyrosine hydroxylase reactivity within the follicle complex; tissues outside the dermal capsule showed reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

NGF augmentation rescues trigeminal ganglion and principalis neurons, but not brainstem or cortical whisker patterns, after infraorbital nerve injury at birth

Prior studies indicate that neonatal nerve injury kills many trigeminal (V) first- and second-order cells, and interrupts pattern formation in the brainstem and cerebral cortex. Yet it is not known whether effects upon cell survival and pattern formation are causally related. To determine whether axotomized V ganglion cells can be rescued by an exogenous trophic agent, rats received 5 mg/kg of nerve growth factor (NGF) prior to, and every day after, infraorbital nerve section on the day of birth until sacrifice on postnatal day (PND) 1, 3, 5, 7, or 14. Other animals received identical lesions without NGF. Ganglion cell numbers were significantly reduced by PND1 in pups not given NGF, while NGF-treated rats displayed no significant cell loss through PND7. However, NGF did not permanently rescue V neurons because ganglion cell numbers were reliably reduced by PND14. Cell numbers in V nucleus principalis were reduced by PND1 in pups not given NGF, while NGF-treated animals displayed no cell loss through PND14. NGF's rescue of second-order cells is probably an indirect effect of NGF action upon V ganglion cells because, in other newborns, NGF failed to maintain principalis cells after direct lesion of the left V ganglion. To determine whether preventing cell death permits whisker-related pattern formation, other rats also received NGF prior to and after infraorbital nerve section at birth. After 3-14 days, patterns were assessed in the brainstem and cortex with cytochrome oxidase histochemistry and serotonin immunocytochemistry. Whisker-related patterns failed to develop as in cases not given NGF. These data indicate that communication with the periphery is necessary for the maintenance of central whisker-related patterns. They also suggest that V ganglion cells can be rescued, albeit temporarily, from rapid injury-induced death by NGF, thereby delaying injury-induced cell death in nucleus principalis. However, the mechanism(s) responsible for injury-induced pattern alterations in the developing V system remains to be elucidated.

Sequential differentiation of sensory innervation in the mystacial pad of the ferret

The mystacial pad of the ferret has an elaborate sensory innervation provided by three types of terminal nerves that arise from the infraorbital branch of the trigeminal nerve. Deep and superficial vibrissal nerves innervate nearly exclusive targets in the large follicle-sinus complexes (F-SCs) at the base of each tactile vibrissa. Dermal plexus nerves innervate the fur between the vibrissae. Each type of nerve provides a similar variety of sensory endings, albeit to different targets. In this study, Winkelmann and Sevier-Munger reduced silver techniques revealed that most of the endings differentiate postnatally in an overlapping sequence like that observed previously in the rat. Afferents from the deep vibrissal nerves begin to differentiate first, followed successively by those from superficial vibrissal nerves and the dermal plexus. Within each type of nerve, Merkel endings begin to differentiate first, followed successively by lanceolate endings and circumferential endings. In the ferret, the differentiation of the intervibrissal fur and its innervation is slightly delayed but substantially overlaps the development of the vibrissal innervation, whereas in the rat it occurs almost entirely later. There was no evidence of a transient exuberant or misplaced innervation or other secondary remodeling. Differentiating afferents and endings are located only in the sites normally seen in the adult, suggesting a high degree of afferent-target specificity. In the ferret, innervation is virtually lacking in one target--the inner conical body of the F-SCs, which is densely innervated in the rat. This lack was due to a failure of innervation to develop rather than to a secondary elimination of a transient innervation.

Cholecystokinin concentrations and peptide immunoreactivity in the intact and deafferented medullary dorsal horn of the rat

To further address the hypothesis that cholecystokinin (CCK) in the medullary dorsal horn (MDH) arises from intrinsic or higher-order neurons, CCK-8-specific radioimmunoassay (RIA) and immunohistochemical (IHC) experiments were carried out in adult rats after trigeminal tractotomy. RIA of punches from deafferented superficial layers of the MDH revealed no significant change in CCK levels vs. the control right side. In this same area, IHC revealed modest reductions in CCK, gastrin, and substance P staining. Calcitonin gene-related peptide (CGRP) staining was reduced substantially. Gastrin immunoreactive cell bodies, present normally in inner lamina II, were reduced in number. RIA and IHC methods were also used to assess MDH CCK concentrations in adult rats subjected to left infraorbital nerve section at birth. The left medulla contained significantly higher levels of CCK than the control right medulla (1.27 +/- 0.19 vs. 0.97 +/- 0.11 ng/mg protein). IHC revealed a dense band of CCK-like staining in laminae I and II ipsi- and contralateral to the lesion. Thus, neonatal deafferentation elevates medullary CCK. To determine if the neonatal lesion-induced increase in medullary CCK is due to primary afferent or higher-order reorganization, RIA and IHC experiments were run after infraorbital nerve section at birth and trigeminal tractotomy in adulthood. RIA revealed no significant change in CCK levels caudal to the tractotomy, although they were higher than control levels in 9 of 12 cases. IHC revealed modest reductions in CCK, substance P, and gastrin staining that resembled the reductions observed in tractotomy-alone cases. These data suggest that 1) most MDH CCK is of non-primary afferent origin, 2) gastrin immunoreactivity in layer II probably originates in CCK-containing cells intrinsic to layer II, the expression of which is dependent upon trigeminal primary afferent input, 3) neonatal V deafferentation induces increased CCK in the superficial MDH, reflecting reorganized intrinsic or higher-order inputs, and 4) higher-order substance P in the MDH is robust.

Dual innervation of the rat vibrissa: responses of trigeminal ganglion cells projecting through deep or superficial nerves

The rat vibrissal follicle-sinus complex is innervated by a deep vibrissal nerve (DVN) and several smaller fascicles traveling in the dermis [conus or superficial vibrissal nerves, (SVNs)]. The function of the SVNs is unknown, although it has been suggested in a comparative study that they form part of a diffuse, multivibrissal system. Anatomical and electrophysiological methods were used to test this hypothesis and to determine if DVN and SVN fibers have differing response profiles. No ganglion cells were double-labeled after retrograde tracer injections in the DVN and SVNs of single follicles. Electron microscopy showed that selective transection of the DVN caused no SVN degeneration or vice versa. Thus, the dual innervation of the vibrissa arises from separate ganglion cells that project through separate nerves. Ganglion cells with A-row vibrissa receptive fields were studied before and after cutting the DVN and/or SVNs to the responsive vibrissa in order to identify their peripheral trajectories. In this sample, 83% projected through a DVN and 17% via a SVN. SVN or DVN cells were not spontaneously active. All cells responded to single vibrissae only; none were responsive to intervibrissal hairs or skin. Latencies to electrical stimulation were similar for DVN and SVN cells. Adaptation rates and threshold measurements were also similar in the two groups: 60% of the DVN cells and 80% of the SVN cells gave slowly adapting responses to sustained vibrissal displacement; threshold displacements ranged from less than 1 degrees to greater than 15 degrees for both SVN and DVN cells. Direction sensitivity was found in all DVN and SVN slowly adapting cells, with most cells responding to movements in one or two quadrants. For SVN cells, sequential circumferential nerve sections indicated that the fiber's directional sensitivity matched the direction of the fiber's entry into the follicle. The two groups differed in their responses to pushing in or pulling on the hair shaft. All the DVN cells were responsive to both of these stimuli, while for SVN cells pushing activated only 40% and none were responsive to pulling the hair. Another difference in the two groups was that no injury discharges occurred after cutting SVNs, but were present in 44% of DVN cells. These data suggest that DVN and SVNs are similar in the majority of response properties. There is also no evidence to support the hypothesis that SVNs provide diffuse, multivibrissal inputs.

The rat's postero-orbital sinus hair: II. Normal morphology and the increase in peripheral innervation with adjacent nerve section

The morphology and innervation of the postero-orbital (PO) sinus hair has been studied in normal rats and in adult animals in which an adjacent nerve, the infraorbital nerve, was sectioned on postnatal day 0 or day 7. The normal morphology of the follicle was similar to that of mystacial sinus hairs. However, the normal innervation differed from mystacial follicles in three respects: (1) instead of a separate innervation, the deep vibrissal nerve (DVN) and dermal plexus were supplied by a common follicle and skin nerve, named here the postero-orbital cutaneous nerve, a branch of the zygomaticofacial nerve; (2) the entry of the DVN through the capsule was highly variable; in some cases fascicles entered in close proximity, but in others they were widely distributed around the capsule; and (3) two or three small nerves, called here anastomosing nerves, were found to leave the PO follicle. These arose from the DVN after it had passed through the capsule to the cavernous sinus. The anastomosing nerves passed back through the capsule and ascended on the outer surface of the follicle to join the dermal plexus. Each nerve contained 1-4 myelinated fibres and 11-35 unmyelinated fibres. Infraorbital (IO) nerve section on day 0 caused a 19% (P less than 0.001, n = 8) increase in numbers of fibers to the DVN on the lesioned side. Most of the increase was due to unmyelinated fibres with no significant change in myelinated axons. No change in axon numbers in the DVN occurred after day 7 lesions. Labelling of the mystacial pad and the PO follicle did not result in any double labelling of cells in the trigeminal ganglion, in either normal or lesioned animals, making it improbable that the increased numbers of unmyelinated axons arose from rerouting of infraorbital fibres. It is suggested that the increased innervation of the PO follicle may arise by the rescue of ganglion cells from developmentally programmed cell death.

Orofacial pain sensitivity in adult rats following neonatal infraorbital nerve transection

A modification of the formalin test was used to assess orofacial pain sensitivity in adult rats that received infraorbital nerve transection at birth. Normal and neonatally lesioned adult animals received an injection of either 5% formalin or saline vehicle into the whiskerpad and the duration of whiskerpad rubbing was observed for 45 min. Normal rats given formalin exhibited the previously reported biphasic pattern of rubbing. Neonatally lesioned rats given formalin did not exhibit this pattern, and were indistinguishable from either of the saline control groups.

Cell death in the developing trigeminal nuclear complex of the rat

The time course and distribution of cell death in the trigeminal nuclear complex of the rat has been examined with the aid of sections stained for Nissl substance and succinic dehydrogenase activity. Pyknotic figure counts in the principal trigeminal nucleus and in each of the three spinal trigeminal subnuclei revealed that cell death commences at E19, in the region of the junction between the principal nucleus and the subnucleus oralis, close to the site of entry of trigeminal afferents into the brainstem. Cell death subsequently spreads rostrally and caudally into the rest of the principal and spinal trigeminal nuclei. Cell death ceases simultaneously, at about P10, in all parts of the trigeminal nuclear complex examined. Neurons could not be reliably distinguished from glial cells in prenatal animals, but data for neuronal numbers postnatally indicate that much of this cell death is indeed due to loss of neurons. The data suggest that, in the trigeminal nuclear complex, only half the number of neurons produced survive to maturity. These findings are of significance for those investigators using this system in studies of plasticity.

The rat's postero-orbital sinus hair: I. Brainstem projections and the effect of infraorbital nerve section at different ages

The central terminations, in the trigeminal nucleus, of afferents from the rat's postero-orbital (PO) sinus hair have been investigated with transganglionic transport of horseradish peroxidase (HRP) and succinic dehydrogenase (SDH) histochemistry. The normal pattern of terminations has been compared with that found after section of an adjacent nerve, the infraorbital (IO) nerve, at three ages: neonatal, 1 week old, and adult. The PO afferent fibres have three separate representations in the brainstem--in trigeminal sensory nucleus principalis (Vp) and rostral subnucleus oralis (Vo), in trigeminal subnucleus interpolaris (Vi), and in caudal trigeminal subnucleus caudalis (Vc) and C1 dorsal horn. In coronal sections the areas of terminations were seen as oval patches lying ventrolaterally in Vp, Vo, and Vi and ventromedially in Vc and C1. Following neonatal IO nerve section the terminal areas were approximately doubled in Vp, Vo, and Vi but were unchanged in Vc and C1. IO nerve section at day 7 also caused a significant, though smaller (1.4x compared with 2.0x), increase in the terminal areas in the rostral three nuclei, without changing Vc and C1. However, no significant change in area occurred after adult IO nerve section. SDH histochemistry at 3 to 4 weeks of age showed patches of terminals on both normal and lesioned sides consistent with those seen after HRP. Previous studies have reported increased functional representation of surrounding intact skin regions, including the PO sinus hairs, after neonatal but not adult, IO nerve section. The present results show that there are concomitant anatomical changes. Like the functional results, the extent of the anatomical changes are dependent on the maturity of the rat when lesioned.

Expression of ALZ-50 immunoreactivity in the developing principal sensory nucleus of the trigeminal nerve: effect of transecting the infraorbital nerve

Many neurons in the CNS die as a consequence of normal development. As these neurons die, they may be programmed to produce 'death proteins'. We explored the possibility that an antigen recognized by ALZ-50, a protein expressed in Alzheimer's-type neurofibrillary tangles, is generated during the process of neuronal death. The effects of transecting the infraorbital nerve on the expression of ALZ-50 immunoreactivity and neuronal death in the principal sensory nucleus of the trigeminal nerve (PSN) was examined. In normal rats, a small number of PSN neurons was ALZ-50-positive on postnatal day (P) 3. Transections on the day of birth (i.e. during the period of naturally occurring neuronal death) led to a 5-fold increase in the number of immunoreactive neurons expressing a 56-kDa protein on P3. In contrast, lesions on P25 (i.e. after the period of naturally occurring neuronal death) did not induce any neurons to exhibit ALZ-50 immunoreactivity. Thus, the 56-kDa protein recognized by ALZ-50 appears to be a death protein which is transiently expressed during the period of naturally occurring neuronal death. It is appealing to speculate that the pathological degeneration described in Alzheimer's brains results from the up-regulation of a quiescent developmental program.

Central projections of primary sensory neurons innervating different parts of the vibrissae follicles and intervibrissal skin on the mystacial pad of the rat

The cell bodies and central projections of neurons innervating the vibrissae follicles and adjacent skin in the rat were investigated by retrograde and transganglionic transport of HRP. The cell bodies of neurons innervating the vibrissa follicle via the deep vibrissa nerve (DVN) were the largest, followed by those innervating the follicle via the superficial vibrissa nerve (SVN). The smallest cell bodies were those innervating the intervibrissal skin. The DVN neurons terminated centrally as an almost uninterrupted column through the trigeminal sensory nuclear complex. The DVN projections to nucleus caudalis and C1 dorsal horn were entirely restricted to laminae III, IV, and V. Besides the projections to lamina V, the DVN projections were strictly localized somatotopically at all levels replicating the peripheral organization of the vibrissae. The SVNs projected sparsely to midlevels of the main sensory nucleus but not to nuclei oralis and interpolaris. The main SVN projections appeared in laminae I-III of nucleus caudalis. In addition, a small projection to lamina V was observed. The projections to laminae II and III were organized mediolaterally in a similar way as the DVN projections; those to laminae I and V were less restricted. The intervibrissal skin neurons projected sparsely to the caudal main sensory nucleus and to the border between nuclei oralis and interpolaris. The projections to nucleus caudalis were restricted to laminae I-III and V and were organized in a similar way as the SVN projections.

Birthdates of trigeminal ganglion cells contributing axons to the infraorbital nerve and specific vibrissal follicles in the rat

Prenatal labelling with [3H]-thymidine was combined with retrograde tracing techniques in adult rats to determine the birthdates of the trigeminal (V) ganglion cells that contributed axons to the infraorbital nerve (ION) and the generation of the subsets of ION cells that innervated specific vibrissae follicles (C-1 and C-5). The V ganglion cells contributing axons to the ION are born between embryonic (E-, E-0 = the day of conception) days 9.5 and 14.5. The percentages (normalized so that they total 100%) of the total V ganglion population born on E-9.5 through E-14.5 were 5.8, 25.7, 19.8, 23.4, 21.0, and 4.4%, respectively. The distribution of birthdates for the V ganglion cells that were retrogradely labelled from the ION closely matched that for the ganglion as a whole. All of these neurons were also born on E-9.5 through E-14.5, and the percentages born on each day were 6.3, 23.6, 18.1, 24.0, 23.6, and 4.4%. Finally, a similar distribution of birthdates was obtained for the V ganglion cells that were retrogradely labelled after injection of retrograde tracers into either the C-1 or C-5 vibrissae follicles. We were unable to detect any distinctive spatial distributions for either all V ganglion or ION cells born on a specific embryonic day. Furthermore, neurons with a given birthdate and that innervated a given follicle were distributed throughout the entire region containing all of the ganglion cells supplying the follicle in question. Therefore, it appears that the V ganglion cells contributing axons to the ION are born over the entire period of ganglion neurogenesis and further that the organization of the ION's innervation of the periphery is not a function of cell birthdate.

Chronic functional consequences of adult infraorbital nerve transection for rat trigeminal subnucleus interpolaris

In adult rats, transection of the infraorbital nerve and subsequent regeneration have been shown to result in altered somatotopic organization and changes in response properties of primary afferents within the trigeminal ganglion. The present study examined how these changes affect the postsynaptic targets of these neurons within subnucleus interpolaris of the trigeminal brainstem. Extracellular recordings were made from 330 cells in normal rats and 424 cells in rats surviving 57-290 days after transection of the infraorbital nerve in adulthood. Adult infraorbital nerve transection resulted in significant functional reorganization within subnucleus interpolaris. Relative to normal rats, the major changes can be summarized as follows: (1) a decrease in the dorsoventral extent of infraorbital representation; (2) a disruption of inter- and intradivisional somatotopic organization; (3) an increase in the proportion of cells with no discernible receptive field; (4) an increase in receptive field size for cells with infraorbital receptive field components; (5) the appearance of a significant proportion of cells with discontinuous receptive fields; (6) an increase in the proportion of cells exhibiting interdivisional convergence; (7) significant changes in the types of receptor surfaces activating local-circuit neurons with infraorbital receptive field components; (8) the appearance of a significant proportion of cells exhibiting convergence of different receptor surfaces; (9) significant changes in the dynamic response characteristics of cells with infraorbital receptive field components; and (10) an increase in the proportion of spontaneously active infraorbital-responsive cells. The changes observed were quite similar to those reported in adult subnucleus interpolaris following neonatal infraorbital nerve transection. The majority of changes observed in both studies can be most parsimoniously explained by alterations of primary afferents. However, central mechanisms may be more likely substrates for others. Regardless of the mechanism, the mature rodent trigeminal system appears capable of considerable functional reorganization following peripheral nerve damage.

Numbers of axons innervating mystacial vibrissa follicles in newborn and adult rats

Electron-microscopic techniques were used to determine the numbers of axons in the deep vibrissal nerves innervating the C1 and C4 follicles in newborn and adult rats. All counts were made from thin sections taken after the nerve had entered the follicle capsule (FC). In newborn animals, the nerves supplying the C1 (n = 10) and C4 (n = 10) follicles contained an average (means +/- standard deviation) of 355.0 +/- 40.0 and 233.9 +/- 19.2 axons, respectively. In the adult animals (n = 10 for C1 and n = 9 for C4), the respective values were 314.4 +/- 26.2 and 233.3 +/- 34.4 axons. There were no significant differences between the values for the counts from the neonates and adults for either follicle (p greater than 0.01, independent t tests). In the vibrissal nerves of neonates, both degenerating axons and occasional growth cones were visible. Such profiles were not observed in the nerves taken from adults.

Trigeminal projections to contralateral dorsal horn originate in midline hairy skin

The present study tested the hypothesis that the trigeminal (V) primary afferent projection to the contralateral dorsal horn originates in midline hairy skin. A prior study (Jacquin et al., 1990) showed that this crossed projection is heaviest to ophthalmic regions of medullary and cervical dorsal horns, and that it does not arise from V ganglion cells that innervate cornea, nasal mucosa, or cerebral dura mater. Here, retrograde double-labeling methods were used to show that many ophthalmic ganglion cells that innervate midline hairy skin via the supraorbital nerve project to the contralateral medullary and upper cervical dorsal horns. Diamidino yellow injections into the right dorsal horn labeled an average of 104 cells in the left V ganglion. Of these contralaterally projecting ganglion cells, an average of 45% were also labeled by horseradish peroxidase (HRP) injections into the left supraorbital nerve, and 25% were also labeled by HRP injections into the midline opthalmic hairy skin. However, only 2% were labeled by HRP injections restricted to left supraorbital vibrissae follicle nerves. Almost all of the double-labeled cells were located in the dorsal one-half of the V ganglion, and they did not differ in size from single-labeled cells. On the basis of these and prior data, we conclude that a high percentage of contralaterally projecting V ganglion cells originate in midline hairy skin. It is also likely that the contralaterally projecting V ganglion cells serve a low-threshold mechanoreceptive function, given the relatively large ganglion cells and axons giving rise to this pathway and their central terminations in dorsal horn laminae III-V.

Topography of the facial musculature within the facial (VII) motor nucleus of the neonatal rat

WGA-HRP, HRP and fluorescent tracers were used to determine the representation of the facial muscles in the facial motor nuclear complex (FMNC) of the newborn rat. Tracer injections of the superficial cervical and anterior mandibular portions of platysma, the orbicularis oculi muscle, the nasolabial musculature and the posterior auricular musculature revealed an adult-like topographic organization across FMNC subnuclei. Tracer delivery to individual vibrissa follicle loci of the whiskerpad also demonstrated an adult-like musculotopic organization within the lateral subnucleus.

Regenerative organization of the trigeminal ganglion following mental nerve section and repair in the adult rat

Sequential double-fluorescence labeling techniques were employed to determine the regenerative somatotopic organization of first-order mandibular neurons following mental nerve transection and surgical repair in the adult rat. Twenty-four ganglia from 12 adult rats were examined microscopically in the following double-labeling paradigm: i) Fast Blue was injected directly into the mental nerves bilaterally; ii) 7 days later the nerves were transected and immediately rejoined by microscopic suture techniques; iii) Diamidino Yellow was then injected directly into the regenerated nerve, distal to the point of repair, 30, 60, and 90 days postrepair; and iv) the animals were sacrificed 3 days later and the ganglia removed for fluorescent microscopic examination. Results were compared with 12 ganglia each of unrepaired/resected controls and sham surgery controls made in parallel. The organization of fluorescence-labeled mandibular cells followed an orderly somatotopic distribution along the lateral dorsoventral axis of the trigeminal ganglion in all groups. The difference in mean total number of fluorescence-labeled cells within and between groups was insignificant or minimal. There was no evidence of heteronymous (nonmandibular) or homonymous (mandibular) sprouting following neuronal regeneration. Regeneration, as determined by the presence of double-labeled cells, was negligible if the transection injury was not repaired but significant 30 days following repair. Additionally, mandibular regeneration gradually improved, as shown by the significant increase of double-labeling at 60 and 90 days postrepair. However, 90 days later, the percentage of regenerated cells had not reached sham control conditions. The results of these studies suggest that following nerve transection and immediate repair in the adult rat: i) mental sensory neuronal perikarya regenerate from and maintain an organized somatotopic area within the mandibular division of the trigeminal ganglion; ii) reorganization by collateral sprouts from nonmental sensory mandibular and/or nonmandibular trigeminal ganglion cells is not evident or is negligible in the adult rat; and iii) regeneration of resected trigeminal sensory neurons is a gradual process which is enhanced by immediate surgical intervention.

Topographic organization of the peripheral projections of the trigeminal ganglion in the fetal rat

Retrograde tracing with true blue (TB) and diamidino yellow (DY) was used to determine the topography of the peripheral projections of the trigeminal (V) ganglion in rats on embryonic day 16 (E-16; E-0 was the day of conception). On E-16, the earliest age at which we were able to accomplish retrograde tracing successfully, the topographic organization of the V ganglionic projection to the periphery was quite adult-like. Cells projecting to the vibrissa pad were restricted to the ophthalmic-maxillary portion of the ganglion, with those innervating dorsal row follicles located medially and those supplying ventral row follicles located laterally. Injections of tracer into ophthalmic skin and/or the cornea labeled cells that were tightly clustered in the most dorsal and anteromedial portion of the ophthalmic-maxillary region. Injections of tracer into the lower jaw or the skin just rostral to the ear labeled cells that were restricted to the lateral, mandibular part of the ganglion. None of the combinations of injections we carried out resulted in large numbers of double-labeled V ganglion cells. Injection of TB into the vibrissa pad and DY into the upper lip produced a small number of double-labeled ganglion cells. This was also the case for paired injections of TB and DY into the lower jaw and lip, respectively. No more than 15 such cells were observed in a ganglion. These findings suggest that the substantial cell death that has been reported to occur in prenatal V ganglion development (Davies and Lumsden, 1984) is probably not involved in the correction of major peripheral targeting errors by the axons of V ganglion cells.

Structure-function relationships in rat brainstem subnucleus interpolaris: VII. Primary afferent central terminal arbors in adults subjected to infraorbital nerve section at birth

Prior studies in this series have clarified the normal organization of subnucleus interpolaris and the response of higher-order neurons to neonatal deafferentation. The present report describes the response of individual rat trigeminal primary afferents to transection of the infraorbital (IO) nerve on the day of birth. Physiologically characterized afferents in adult animals were labeled by intraaxonal injection of horseradish peroxidase (HRP). Qualitative and quantitative examination of the interpolaris collaterals of 62 recovered neurons revealed: 1) an increase in the transverse area of vibrissa afferent terminal arbors, 2) a decrease in the number of boutons per collateral of vibrissa afferents, 3) a decrease in the bouton density of both vibrissa and guard hair primary afferents, 4) a decrease in the circularity of guard hair afferent arbors, 5) an increase in the number of collaterals given off by nociceptive fibers, and 6) abnormal primary afferent topography. The data support the hypothesis that vibrissa afferents respond to neonatal axotomy by central arbor expansion, but not by sprouting. Arbor expansion provides a morphological substrate for the abnormal histochemical staining patterns seen in animals subjected to IO damage in the early postnatal period.

Structure-function relationships in rat brainstem subnucleus interpolaris: V. Functional consequences of neonatal infraorbital nerve section

In a prior study (Jacquin et al., '86c), the response properties and projections of neonatally axotomized trigeminal (V) primary afferents were studied in the adult rat. Here, single-unit recording, electrical stimulation, and receptive field (RF) mapping techniques were also used to assess the functional consequences of neonatal infraorbital nerve section upon postsynaptic cells in V brainstem subnucleus interpolaris (SpVi). Of 904 cells studied, 385 were from normal adults and 519 were from neonatally deafferented adults. Infraorbital nerve section at birth resulted in: (1) a substantial reduction in those areas of SpVi containing cells with infraorbital RFs, and only a slight increase in areas solely responsive to noninfraorbital surfaces, (2) an absence of orderly topography within cells expressing regenerate primary afferent inputs, (3) a slight increase in mean discharge latency to V ganglion or thalamic shocks, (4) an increased relative percentage of cells orthodromically activated by diencephalic or cerebellar shocks, (5) a decreased relative percentage of mystacial vibrissa-sensitive local circuit neurons, with a corresponding increase in local circuit nociceptors and unresponsive cells, (6) an increased relative percentage of mystacial nociceptors, virbrissae, guard hair, and/or skin sensitive cells projecting to thalamus and/or cerebellum, (7) an increased percentage of local circuit neurons with RFs including more than one vibrissa, whereas projection neurons did not differ from normal in the number of vibrissae composing their RFs, and (8) an increased relative percentage of cells expressing interdivisional and intermodality convergence, split RFs, spontaneous activity, directional high velocity, and neuroma sensitivity. Thus neonatal nerve section produces changes in topography, inputs, projection status, and responses of surviving postsynaptic neurons. Although many of these centrally observed alterations can be attributed to altered peripheral projections in axotomized V primary afferents, others must reflect central reorganization. The central mechanisms responsible for the synthesis of deafferentation-induced RFs remain to be elucidated.

Target dependence of hypoglossal motor neurons during development in maturity

We have investigated the target dependence of hypoglossal motor neurons in postnatal rats by transecting the hypoglossal nerve and preventing reinnervation of the tongue. After transection in early postnatal life, approximately 60% of hypoglossal motor neurons die and surviving neurons are markedly atrophic compared to contralateral controls. In maturity, there is also substantial neuronal atrophy and about 30% of motor neurons appear to die after the procedure. However, most hypoglossal neurons in adults survive transection for periods up to 1 year. The adult response is present by 3 weeks of age. The time course of neuronal atrophy and death after permanent target deprivation was investigated in adult animals. One month after the hypoglossal nerve was deflected, there was marked axonal atrophy, although somatic atrophy was minimal. By 3 months after the procedure substantial neuronal atrophy and apparent cell loss (about 30%) had occurred. There was little change between 3 and 6 months. We conclude that hypoglossal motor neurons are influenced by connections with their targets in postnatal life. Even in maturity, neurons require target connections for maintenance of axonal and somatic morphology. However, the majority of motor neurons in adult animals can survive target deprivation for prolonged periods.