The somatic representations of the trigeminal (V) and hypoglossal (XII) nerves in the rat brainstem were examined by using retrograde horseradish peroxidase (HRP) transport from peripheral nerves. HRP applied to either the mandibular division of V, or the V motor root, revealed the V motor nucleus (Mot V), a largely ellipsoid collection of multipolar motoneurons extending rostrally from the level of the facial genu to the rostral border of the principal sensory V nucleus. The distributions of major and minor axes of Mot V cell bodies were unimodal, ranging from 12 to 60 pn and 7 to 30 q, respectively, while their somal diameters (D = 2Jarealn) and areas were somewhat bimodally distributed. A smaller collection of cell bodies was also labelled along the medial trajectory of the V motor root in the ventrolateral pons. These cells were, as a rule, smaller than those in Mot V. A V motoneuron subnucleus innervating the anterior digastric and mylohyoid muscles (two main jaw openers) was also identified by HRP application to the mylohyoid nerve. This subnucleus extended the entire length of Mot V; at caudal levels it occupied the ventromedial margin; midway it almost disappeared, reappearing rostrally where the labelled cells were diffuse. Most labelled mylohyoid axons coursed dorsomedially and rostrally toward the floor of the fourth ventricle before turning laterally as a V genu to exit the pons with the V motor root.To delineate the somatotopic organization of the V mesencephalic nucleus (Mes V), HRP was applied to the whole V mandibular ganglion, the V motor root, the lingual, inferior alveolar, mylohyoid, and auriculotemporal nerves, individually or collectively, or the infraorbital nerve. In whole mandibular cases, labelled Mes V cells extended from the level of caudal Mot V to the level of the rostral superior colliculus. Mes V had a widened caudal base and a tapered rostral end that occupied the lateral margin of the central gray. Mes V cells were unipolar with unimodal distributions of major and minor axes which ranged from 12 to 42 pn and 5 to 25 pn, respectively. Labelled cells were visible at all levels of Mes V, but primarily caudally, following inferior alveolar nerve applications. Infraorbital nerve applications also labelled Mes V cells, though only caudally. Applications of HRP to the mylohyoid or auriculotemporal nerves labelled few, if any, Mes V cells. None at all were observed following lingual nerve HRP applications. Retrogradely labelled cells were also observed in the supratrigeminal nucleus following either whole mandibular or inferior alveolar applications and in the dorsolateral parvicellular reticular formation in whole mandibular cases. XI1 motoneurons innervating tongue extensor muscles were labelled following HRP application to the ascending ramus of XII. These cells formed a subnucleus largely in the ventromedial central gray of the medulla. The distributions of major and minor axes of these cells were unimodal and ranged from 17 to 45 pn and 9 to 29 pn, respectively.

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.

Retrograde tracing, immunocytochemical, and histochemical methods were used to determine the manner in which different classes of trigeminal (V) ganglion cells respond to transection of their axons during infancy. Retrograde tracing with true blue (TB), histochemistry using the plant lectin Bandieraea simplicifolia-I (BS-I), and immunocytochemistry using an antiserum directed against substance P (SP) were carried out in the V ganglion and V brainstem complex of normal adult rats. In the adult V ganglion, 11.9 +/- 1.9% of the cells that sent axons into the infraorbital nerve (ION) contained SP-like immunoreactivity (SPLI) and 26.9 +/- 3.6% bound the lectin BS-I. Only 2.7 +/- 1.6% of ION cells were labelled by both the SP antiserum and BS-I. Transection of the ION on the day of birth had very different effects upon primary afferent neurons containing SPLI and those labelled by BS-I. We have previously shown that such lesions result in a significant expansion of the portion of SpC innervated by primary afferents containing SPLI and we have also provided data consistent with the proposal that ganglion cells recognized by an antiserum directed against SP are more likely than other primary afferent neurons to survive neonatal axotomy. In the present study, combination of retrograde tracing with TB and lectin binding histochemistry showed that cells recognized by BS-I were selectively lost after neonatal ION transection. Only 14.2 +/- 4.4% of the ION ganglion cells that projected into this nerve at the time of the lesion and that survived neonatal axotomy were BS-I positive when the animals reached adulthood. Neonatal ION transection also resulted in a permanent reduction in the density of BS-I binding in SpC. Bandieraea simplicifolia-I binding in the brainstem ipsilateral to the damaged nerve was almost completely gone within 1 day of the nerve transection and recovered only partially by the time the rats were 2 months of age. In alternate sections tested with the SP antiserum, there was a slight reduction in the density of SPLI in the deafferented SpC on postnatal days 4 and 5, but this change never approached that observed for BS-I binding.

A combination of [3H]thymidine labelling and retrograde tracing with either horseradish peroxidase (HRP) or true blue (TB) was used to determine whether V primary afferent neurons born on different embryonic (E) days were differentially susceptible to neonatal transection of the infraorbital nerve (ION). In one experiment, rat fetuses were exposed to [3H]thymidine on E-8.5, 9.5, 10.5, 11.5, 12.5, 13.5, 14.5, or 15.5, the left infraorbital nerve (ION) was transected on the day of birth, and both the regenerate and intact IONs were labelled with HRP when the animals reached adulthood. The percentage of HRP labelled cells that were also heavily labelled by [3H]thymidine was calculated for both the intact ganglion and that ipsilateral to the damaged nerve for each animal. A consistently higher percentage of double labelled cells on the lesioned rather than on the intact side for a given E-day was taken as an indication that cells born on the day in question had an increased probability of survival relative to the entire population of V ganglion cells that contributed axons to the ION. Cells born late in gestation on E-12.5 through 14.5 were significantly more likely than early born (E-9.5 through 11.5) cells to survive neonatal axotomy. In a second experiment, fetuses were exposed to [3H]thymidine on either E-9.5, E-10.5, or E-14.5, the vibrissa pads on both sides of the face were injected with TB within 6 hours of birth, and the ION was transected 6-8 hours later.(ABSTRACT TRUNCATED AT 250 WORDS)