HRP Injections Research Articles

TRH regulation of tracheal tension through vagal preganglionic motoneurons

TRH-immunoreactive nerve terminals innervate the ambiguous nucleus in the rabbit. Vagal preganglionic motoneurons that innervate the trachea, were revealed by HRP histochemistry in the rostral part of the ambiguous nucleus and the dorsal motor nucleus of the vagus. HRP histochemistry plus TRH immunocytochemistry revealed that TRH-immunoreactive axon terminals synapsed on ambiguous nucleus neurons retrogradely labeled by HRP injection into tracheal smooth muscle and the superior laryngeal nerve. Microinjection of 50 ng TRH into the rostral ambiguous nucleus caused slight dilation followed by constriction, which was inhibited by atropine and vagotomy. Results show that central TRH-containing neurons regulate tracheal tension through synapses on vagal preganglionic motoneurons that innervate tracheal smooth muscle.

Preplate neurons in embryonic mouse cortex: Ultrastructural observations using photoconversion of the fluorescent lipophilic dye dil

We have begun to look directly at small numbers of afferent axons to early generated neurons that form the preplate in the developing mouse cortex. The carbocyanine dye Dil (1’1, dioctadecyl-3,3,3’3’-tetramethyl-indocarbocyanine) has proved especially useful for this goal. DiI labels axons and their terminals with greater sensitivity and without some of the disadvantages of axon filling with HRP. The increased sensitivity provided by labeling embryonic axons with DiI has given us new insights into the development of cortical afferents. For instance, we reported originally that afferents from the thalamus were present below the cortex as early as embryonic day 15 (E15) based on HRP injections into mouse embryos. By using DiI placements into the thalamus in aldehyde-fixed brains, we now know that thalamic fibers reach the cortex 24 hrs earlier.

An HRP study of the location of the motoneurons supplying the tensor veli palatini muscle of the rabbit

Location of the motoneurons supplying the tensor veli palatini muscle of the rabbit was examined with the retrograde labeling technique following intramuscular injection of HRP. Labeled motoneurons were ipsilaterally located in the ventral or ventromedial portion of the rostral two-thirds of the motor trigeminal nucleus at the level of about 6.0 to 8.5 mm rostral to the obex. The location of the labeled motoneurons was ventromedial to the region supplying the masseter, the temporalis, and the medial pterygoid muscles and ventral to the region supplying the anterior digastric and the mylohyoid muscles, the location which coincided with the lateral pterygoid region. The labeled motoneurons were scattered around or in this region.

Convergence of oral and extraoral information in the superior secondary gustatory nucless of the channel catfish

Neurons within the superior secondary gustatory nucleus (nGS) of the channel catfish were examined electrophysiologically for responses to mechanical and chemical stimulation of neural peripheral receptive fields (RFs). Of the 28 single units sampled, 18 had mechanosensory RFs on the extraoral epithelium, two had RFs within the oropharyngeal cavity, and eight had RFs that included both oral and extraoral surfaces. RF sizes varied from approximately 2 cm 2 on the ipsilateral lips and barbels to the whole body surface, bilaterally. No obvious correlation existed between RF pattern and recording location within the nGS. Eight of the mechanosensory nGS units also responded to amino acid taste stimuli with thresholds from micromolar to millimolar concentrations. The convergence of oral and extraoral information within the nGS determined electrophysiologically was corroborated anatomically by HRP labeling experiments. Restricted HRP injections into each of the primary gustatory nuclei of the medulla, the vagal (VL) and facial (FL) lobes, labeled fibers that appeared to terminate diffusely throughout the nGS, and injections into different portions of the nGS retrogradely labeled cells in both the FL and VL. The present electrophysiological and neuroanatomical data distinguish the convergent gustatory representation within the nGS of the catfish from the highly specific somatotopic and viscerotopic sensory maps previously identified in the FL and VL, respectively.

Evidence for GABAergic interneurons in the red nucleus of the painted turtle.

Immunocytochemical and electrophysiological evidence supporting the presence of GABAergic interneurons in the turtle red nucleus is presented. Injections of HRP into the spinal cord produced labeling of large neurons in the contralateral red nucleus. The peroxidase-antiperoxidase (PAP) method revealed smaller cells immunoreactive to an antibody against glutamate decarboxylase (GAD), the synthetic enzyme for the inhibitory neurotransmitter GABA, that were interspersed among larger immunonegative neurons. Similar small neurons were densely immunostained by antibodies to GABA-glutaraldehyde conjugates obtained from different sources and applied according to pre-embedding and postembedding protocols. Rubrospinal neurons retrogradely labeled with HRP measured 16 and 27 microns in mean minor and major cell body diameters, while GABA-like immunopositive neurons situated within the red nucleus measured 7 and 13 microns. There was very little overlap in soma size between the two cell populations. Therefore, we suggest that the GAD- and GABA-positive neurons may be local inhibitory interneurons. This notion is further supported by observations of pre-embedding immunostaining for GAD and postembedding immunostaining for GABA showing that the turtle red nucleus is amply innervated by immunoreactive axon terminals. These puncta are closely apposed to cell bodies and dendrites of both immunonegative large neurons and immunopositive small neurons. Moreover, immunogold staining at the electron microscopic level demonstrated that GABA-like immunoreactive axon terminals with pleomorphic synaptic vesicles formed symmetric synapses with cell bodies and dendrites of the two types of red nucleus cells. These ultrastructural features are commonly assumed to indicate inhibitory synapses. A moderately labeled bouton with round vesicles and asymmetric synapses was also observed. In addition, the two types of red nucleus neurons received asymmetric axosomatic and axodendritic synapses with GABA-negative boutons provided with round vesicles, features usually associated with excitatory functions. To obtain electrophysiological evidence for inhibition, intracellular recordings from red nucleus neurons were conducted using an in vitro brainstem-cerebellum preparation from the turtle. Small, spontaneous IPSPs were recorded from 7 out of 14 red nucleus cells studied. These morphological and physiological results provide strong support for concluding that the turtle red nucleus, like its mammalian counterpart, contains GABAergic inhibitory interneurons. While we have not identified the main source of input to these interneurons, in view of the scarce development of the reptilian cerebral cortex, this input is unlikely to come from the motor cortex as it does in mammals.(ABSTRACT TRUNCATED AT 400 WORDS)

Two types of motoneurons supplying dorsal fin muscles in lamprey and their activity during fictive locomotion.

The location and dendritic morphology of motoneurons supplying the dorsal fin muscles were studied in the lamprey spinal cord (Ichthyomyzon unicuspis). Motoneurons were retrogradely labelled after injection of HRP into the fin muscles or after its application on the cut ends of the ventral roots. HRP-labelled cells were subsequently reconstructed, in the horizontal and/or transverse planes. Fin motoneurons were also injected intracellularly with Lucifer Yellow and their detailed three-dimensional structure was analysed by confocal laser-scanning microscopy. Unlike myotomal motoneurons, which are closely spaced in the lateral cell column, fin motoneurons were distributed along the spinal cord separately or in pairs. They could be distinguished from motoneurons supplying trunk muscles by having a limited number of dendrites in the lateral part of the spinal cord. In addition, some fin motoneurons extend their dendrites into the dorsal column. The motor cells innervating fin muscles were divided into two types based on their dendritic morphology. Type I have a widespread dendritic tree in the rostrocaudal direction and, with few exceptions, completely restricted to the ipsilateral side. A proportion (25%) of these cells have dendrites extending into the dorsal column. Type II fin motoneurons extended their dendrites both ipsi- and contra-laterally. The contra-lateral dendrites pass below and above the central canal. The dendrites send off branches into the dorsal columns on both the ipsi- and the contra-lateral sides. Electron microscopic analysis showed that both type I and type II fin motoneurons receive numerous synaptic contacts from dorsal column axons. During fictive locomotion both types of motoneurons are active in antiphase in relation to myotomal motoneurons and to the main locomotor burst.

Identification of nociceptive neurons in the medial thalamus: morphological studies of nociceptive neurons with intracellular injection of horseradish peroxidase

Somatosensory neurons including nociceptive ones in the medial thalamus have been studied extracellularly, and are classified into three types: nociceptive-specific, wide dynamic range, and tap neurons. However, the morphological characteristics of these three neurons have not yet been clarified. We studied the morphological characteristics of the neurons by iontophoretic injection of HRP into single neurons in 32 cats. Nine wide dynamic range neurons and two tap neurons were electrophysiologically identified and successfully stained with HRP in and around the parafascicular and subparafascicular nuclei, and in the mediodorsal nucleus. All nine wide dynamic range neurons had fewer dendrites which formed scanter tufts, whereas the two tap neurons had many more dendrites which formed denser tufts: i.e. wide dynamic range neurons were of the isodendritic type, and the tap neurons were of the allodendritic type, according to Ramón-Moliner's classification. The axons of the two tap neurons ran antero-laterally whereas those of wide dynamic range neurons ran in many directions. These morphological differences suggest that tap neurons may have more specialized and fixed functions than wide dynamic range neurons.

A parsimonious description of motoneuron dendritic morphology using computer simulation

Most quantitative descriptions of neuronal dendrite morphology involve tabulations of measurements and correlations among them. The present work is an attempt to extract from such data a parsimonious set of parameters that are sufficient to describe the quantitative features of individual and pooled dendrites, including their statistical variability. A relatively simple stochastic (Monte Carlo) model was devised to simulate branching dendritic trees. The necessary parameters were then derived directly from measurements of 64 completely reconstructed dendrites belonging to six gastrocnemius alpha-motoneurons, labeled by intracellular injection of HRP. Comparison of actual and simulated dendrites was used to guide the process of parameter extraction. The model included only two processes, one to generate individual branches given their starting diameters and the second to select starting diameters for the daughter branches produced at dichotomous branching points. The stochastic process for branch generation was controlled by probability functions for branching (Pbr) and for terminating (Ptrm), together with a constant rate of branch taper. All model parameters were fixed by motoneuron measurements except for branch taper rate, which was allowed to vary within limits consistent with observed taper rates in order to generate the appropriate total number of branches. The simplest model (model 1), in which Pbr and Ptrm depended only on local branch diameter, produced simulated dendrites that fit many, but not all, characteristics of actual motoneuron dendrites. Two additional properties produced significant improvements in the fit: (1) a small but significant dependence of daughter diameters on the normalized starting diameter of the parent branch, and (2) a dependence of Pbr and Ptrm on distance from the soma as well as on local branch diameter. The process of developing this model revealed unsuspected relations in the original data that suggest the existence of fundamental mechanisms for morphological control. The final model succinctly describes a large amount of data and will enable quantitative comparisons between the dendritic structures of different types of neurons, regardless of their relative sizes.

Functional influence of interlaminar connections in the hamster's superior colliculus

The aim of this study was to determine the functional importance of intrinsic connections within the hamster's superior colliculus (SC) in the development of the visual responses of neurons in the deep layers of this nucleus. We used localized injections of lidocaine to block action potentials of fibers that passed through the superficial layers and injections of CoCl2 to block synaptic transmission in these layers. We also determined the morphology of a number of the deep layer cells recorded in these experiments by intracellular injection of HRP. Injection of lidocaine into the superficial layers completely abolished the visual- and/or optic chiasm-evoked responses of all 40 deep layer cells tested. Thus, fibers that either pass through or synapse in the superficial layers are necessary for the visual responses of deep layer neurons. Injections of CoCl2 restricted to the superficial layers significantly reduced the visual responsivity of 86% of 92 deep layer neurons tested and abolished the visual responses of 68% of these cells. Superficial layer injections of CoCl2 were equally effective in reducing the responses of neurons with dendrites that ascended into the superficial layers (all seven cells tested and recovered) and those of cells with dendrites restricted to the deep layers (six of seven cells tested and recovered). Injections of CoCl2 into the deep layers, in the region of the cell being recorded, significantly reduced the visual responses of 59% of 37 cells and abolished the visual responses of 40% of the neurons tested. Deep CoCl2 injections abolished the visual responses of three of four cells with dendrites restricted to the deep layers and only one of four cells with dendrites that ascended into the superficial layers. These results indicate that descending interlaminar axons from visually responsive superficial layer cells may be responsible for the visual responsivity of most neurons in the deep SC layers of hamster. Also, some deep layer neurons in this species may receive effective visual input through their dendrites that ascend into the superficial layers, where they are likely to be contacted by retinal axons or axon collaterals of superficial layer cells.

Cholinergic Innervation of the Cerebellum of the Rat by Secondary Vestibular Afferents

The cholinergic innervation of the cerebellar cortex of the rat was studied by immunohistochemical localization of choline acetyltransferase, radiochemical measurement of ChAT activity, and double labeling of ChAT-positive neurons with HRP injected into the cerebellum. ChAT immunohistochemistry revealed large mossy fiber rosettes as well as finely beaded terminals with different morphological characterization, laminar distribution within the cerebellar cortex, and regional differences within the cerebellum. Large "grapelike" ChAT-positive mossy fiber rosettes that were distributed primarily in the granule cell layer were concentrated, but not exclusively located, in three separate regions of the cerebellum: (1) the uvula-nodulus (lobules 9 and 10); (2) the flocculus, and (3) the anterior lobe vermis (lobules 1 and 2). Regional differences in ChAT-positive afferent terminations in the cerebellar cortex demonstrated by immunohistochemistry were confirmed by regional biochemical measurements of ChAT activity. Using ChAT immunohistochemistry in combination with HRP injections into the uvula-nodulus, we have studied the origin of the cholinergic projection. The caudal medial vestibular nucleus and to a lesser extent the nucleus prepositus hypglossus contain ChAT-positive neurons that were double labeled following HRP injections into the uvula-nodulus. We conclude that (1) there is a prominent cholinergic mossy fiber pathway to the vestibulocerebellum, (2) this pathway originates primarily in the caudal third of the medial vestibular nucleus, and (3) this cholinergic pathway likely mediates secondary vestibular information related to postural adjustment.

The origin of trochlear motoneurons in the larval sea lamprey, Petromyzon marinus L. An HRP study

The origin of trochlear motoneurons in larval lampreys was studied by injection of HRP into the orbit. Immature motoneurons had ventricular attachments, the position of which with respect to the ventricular sulci was used to study their regions of origin. Motoneurons originate from the region between the sulcus intermedius dorsalis and the sulcus intermedius ventralis, which in other parts of the brain were identified as the visceromotor and viscerosensory columns. Both ipsilateral and contralateral immature motoneurons were found. The significance of these findings is discussed in relation to the origins of the trochlear nerve and its nucleus in vertebrates.

Ganglion cell types of the turtle retina that project to the optic tectum: Intracellular HRP injections of retrogradely, rhodamine-marked cell bodies.

The turtle retina has been shown to have a variety of different morphological ganglion cell types as well as distinct physiological ganglion cell types. The major projection of the retina to the brain in nonmammalian vertebrates is to the optic tectum. In this study, we address the question of which retinal ganglion cell types project to the optic tectum in the turtle. Fluorescent rhodamine-labeled microspheres were used to trace the retinal ganglion cell projection to the superficial layers of the optic tectum. The fluorescent ganglion cell somata, retrogradely marked by transport from the contralateral optic tectum, were impaled with micropipettes containing rhodamine-horseradish peroxidase solution and this dye was iontophoresed into the cells under visual control. Most of the morphological ganglion cell types described in Golgi studies (Kolb, 1982; Kolb et al., 1988) were stained. Thus, the small cell types G1, G2, G3, G5, G6, and G7; the medium-sized types G10, G11, G12, G13, and G14; and the large-sized types G15, G16, G19, G20, and G21 project to the optic tectum in the turtle. We have added a new type, G2a, which proves to have some differences from the original G2 in branching pattern. We were unable to stain the small type G4, the medium-sized types G8 and G9, and the large cell types G17 and G18: this suggests that they might not project to the superficial layers of the dorsolateral optic tectum, at least, in the turtle.

Claustrum in the hedgehog (Erinaceus europaeus) brain: cytoarchitecture and connections with cortical and subcortical structures.

The cytoarchitecture of the claustrum in the hedgehog (Erinaceus europaeus) brain, the morphology of its neurons, and the efferent connections with cortical and subcortical structures were studied with the Nissl and Klüver-Barrera, the Golgi, and the horseradish peroxidase methods. It was found that the claustrum is a well developed nucleus in the hedgehog telencephalon and, as in other mammals, is divided into dorsal and ventral parts. In Golgi-stained sections, spiny multipolar cells are the predominant neurons of both the dorsal and the ventral claustrum and are projection neurons. Aspiny multipolar neurons with fewer, often beaded, dendrites constitute a minority in both divisions and are interneurons. Injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) in the prefrontal, motor, somatosensory, auditory and visual areas, and HRP or WGA-HRP injections in the thalamus showed that: (1) the claustroneocortical projections originate in the dorsal claustrum and are distributed to the entire neocortex; these projections are mainly ipsilateral but some also originate contralaterally; (2) the claustroneocortical projections show a rough topographic organization; there exists a substantial degree of overlap; and (3) the claustrothalamic projection, arising throughout the dorsal claustrum, is strictly ipsilateral. No evidence of a thalamoclaustral projection was found. The present results suggest that, although the hedgehog has been referred to as a "paleocortical mammal" owing to the great development of its rhinencephalic structures in comparison with its small neocortex, the dorsal claustrum is well developed and is connected with all neocortical areas as well as with the thalamus, establishing it as a key structure in the hedgehog forebrain.

Ultrastructural characteristics of callosal neurons in deep layers of cat primary auditory cortex (AI)

An electron microscope study of retrogradely labelled nonpyramidal neurons has been carried out in layers V-VI of the primary auditory cortex (AI) after HRP injections into the contralateral AI of cats. From 2 to 9 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 15.8 +/- 1.7% (on the average) of the somatic surface of these neurons. All of the revealed synapses on the somata of these callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. An average length of these synaptic contacts in sections was 1.6 +/- 0.1 mm. HRP-labelled axon terminals of callosal fibres in layers V-VI contained round synaptic vesicles and formed asymmetric synapses on spines and dendrites. Possible functional significance of axo-somatic synapses in formation of impulsation patterns of the callosal neurons is discussed.

Intracellular HRP study of nociceptive neurons within the ventrobasal complex of the cat thalamus

Locations and morphological characteristics of nociceptive specific (NS) and wide dynamic range (WDR) neurons of the ventrobasal (VB) thalamic complex were studied using an intracellular HRP injection technique in cats anesthetized with urethane and chloralose. Both NS and WDR neurons were found within the marginal zone of the VB complex. Their somata were confined to the VB complex, but dendrites were extended into the structures of the thalamus surrounding the VB complex. NS neurons were found in the caudal part of the VB complex, whereas WDR neurons were found more rostrally. Examinations of their locations ithin the marginal zone of the VB complex confirmed the somatotopic organization, as found previously. We also confirmed the previous reports that there are 3 different types of low-threshold mechanoreceptive (LTM) neurons within the VB complex. They are type I, type II and type III neurons. Both NS and WDR neurons were either type I or type II neurons. Obvious morphological differences were not found between LTM neurons and two classes of nociceptive VB neurons investigated.

Brainstem Reticular Nuclei that Project to the Cerebellum in Rats A Retrograde Tracer Study; pp. 39–53

The nuclear origins of projections from the brainstem reticular formation to the cerebellum were examined using four retrograde tracer substances: horseradish peroxidase, wheat germ agglutinin-horseradish peroxidase conjugate, Fluoro-Gold, and rhodamine beads. Tracer injections were made into each of the three major longitudinal zones of the cerebellar cortex (vermis, paravermal hemisphere, and lateral hemisphere) as well as into the various deep cerebellar nuclei. Counts of retrogradely labeled cells were done on a large sample of select cases. The data generated by these cell counts indicate that the strongest reticulocerebellar projections arise from the three specialized pre-cerebellar reticular nuclei: the lateral reticular nucleus, the medullary paramedian reticular nucleus, and the reticulotegmental nucleus. The presumed noradrenergic locus coeruleus (A6 cell group) was also densely packed with retrogradely labeled neurons. However, strong reticulocerebellar projections also arose from other presumed catecholamine cell groups such as those in the ventrolateral medulla (the A1/C1 complex) and the caudal pons (A5). Substantial cerebellar projections originated from most of the various presumed serotonergic brainstem raphe cell groups (particularly raphe obscurus in the medulla), as well as from the presumed cholinergic Ch5 cell group (the pedunculopontine pars compact us nucleus). Labeled cells were also seen in several nonaminergic isodendritic reticular nuclei thought to be involved in visuomotor activity (e.g. paragigantocellularis dorsalis, raphe interpositus, and the pontine dorsomedial tegmental area), as well as in the lateral reticular zone of the medulla and lower pons (reticularis dorsalis and parvocellularis). Tracer injections into the deep nuclei produced relatively greater numbers of labeled neurons in large-celled medial reticular nuclei associated with skeletomotor activity, such as gigantocellularis. magnocellularis, and pontis caudalis. Reticular nuclei conspicuous in their lack of projections to the cerebellum included reticularis ventralis in the medulla, pontis oralis, and both subdivisions of the midbrain reticular formation (cuneiformis and subcuneiformis). As a whole, the various isodendritic reticular nuclei project most strongly to midline cerebellar structures (vermal cortex or fastigial nuclei), less strongly to the paravermal cortex or interposed nuclei, and least strongly to the lateral cortex or dentate nucleus. Within individual reticular nuclei, the morphology of labeled neurons is identical to that reported previously by this laboratory subsequent to spinal or cortical HRP injections, thus strengthening this laboratory's hypothesis that the various brainstem reticular nuclei can be distinguished on the basis of neuronal morphology.

Factors influencing survival of transplanted embryonic motoneurones in the spinal cord of adult rats

The survival of transplanted embryonic motoneurones in the initially intact spinal cord of adult rats was studied and compared to that previously observed in the motoneurone-depleted cord. Embryonic (ED 11–12) spinal grafts prelabeled with 5-bromo-2′-deoxyuridine (BrDUr) were placed in the intact lumbar cord of the hosts. To provide a target for grafted embryonic motoneurones and to guide their axons to it, the contralateral extensor hallucis longus (EHL) muscle with its nerve attached was transferred paravertebrally. The nerve stump was implanted in the cord at the site of transplantation. Eight to 14 weeks later BrDUr-labeled motoneurone-like cells had migrated outside the grafts into the host's neuropil, preferentially into the anterior horn. Following injection of HRP into the implanted EHL muscle 6–17 weeks after transplantation a few retrogradely labeled motoneurones were seen in the host's anterior horn around the grafts. The lumbar cord of the rats with neuromuscular implants but without embryonic grafts had no retrogradely labeled cells. However, most animals, both with and without embryonic grafts, had retrogradely labeled motoneurones in the thoracic cord, which may contribute to the reinnervation of the implanted muscle. Thus, although some embryonic motoneurones can survive, migrate into the proper location, and probably innervate a host muscle when transplanted into the intact spinal cord, their number was significantly fewer than that in the motoneurone-depleted cord. The results show that reduction of the host's motoneurone pool increases chances of their survival.

Techniques for intra-axonal recording of electrical activity from single nerve fiber in vitro and in situ

The method usually applied for recording the electrical activity of single nerve fiber is extracellular recording on isolated single nerve fiber. It is difficult to obtain stable and satisfactory results from direct intracellular recording of single nerve fiber, except for some special biological materials, such as Loligo giant axon. We present in this paper a technique for recording intracellular resting membrane potential and action potential from single nerve fiber both in vitro and in situ, using glass microelectrode and a special mirror-base plate for fixing the preparation. Besides, we also report a method for labelling single nerve fiber by means of injection of HRP into the axon.

Organization of ascending projections from the optic tectum and mesencephalic pretectal gray inRana pipiens

The ascending projections from the dorsal mesencephalon to the thalamus and pretectum in Rana pipiens were investigated by using the anterograde and retrograde transport of HRP with regard to two major issues: (1) the degree of tectotopic organization in the projections, and (2) their cells of origin. The results indicate that the spatial organization of the tecto-thalamic tract is specifically related to the laminar organization of the contributing tectal efferent neurons. Axons of neurons in the superficial portion of tectal layer 8 exit the tectum through layer 9 and travel in the superficial portion of the dorsal and ventral tecto-thalamic tracts and innervate the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, and corpus geniculatum. The distribution of terminals within these structures varied with the tectal HRP-injection site. HRP injections in the ventral tecto-thalamic tract retrogradely labeled neurons in the superficial portion of tectal layer 8 across the lateral and caudal portion of the tectal lobe. HRP injections into the dorsal tecto-thalamic tract, at the level of the pretectum, retrogradely labeled pyriform neurons in the superficial portion of tectal layer 8 in the rostral and medial portions of the tectal lobe. With regard to the deep tectal layers, axons from pyramidal neurons in layer 6 and ganglionic neurons in layer 8 leave the tectum through layer 7, travel in both the dorsal and ventral tecto-thalamic tracts, and are located internal to the axons of the pyriform neurons of superficial tectal layer 8. The majority of the ganglionic neurons project to the posterior lateral ventral nucleus and the anterior lateral nucleus. The distribution of terminals within these nuclei did not display a tectotopic organization. A second major projection to the thalamus originates from the mesencephalic pretectal gray and innervates the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, the anterior lateral nucleus, dorsal and ventral divisions of the ventral lateral thalamus, and the nucleus of Bellonci. Other axons from the mesencephalic pretectal gray terminate in the contralateral, medial portions of the posterior lateral dorsal thalamus, the ventral lateral thalamus, and the anterior lateral nucleus. The isthmo-tectal projection was also retrogradely labeled following tectal injections of HRP. This pathway travels in the most ventral portion of the ventral tecto-thalamic tract; its axons passed over the lateral margin of the endopeduncular nucleus bilaterally, and crossed the midline in the caudal portion of the optic chiasm. Extensive, bead-like varicosities were observed on these axons both in the endopeduncular nucleus and in the posterior optic chiasm.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomical evidence for an intergeniculate leaflet in Rana pipiens

The projections of the nucleus of Bellonci and the anterior thalamic nucleus in Rana pipiens appear to be remarkably similar to those that have been described for the mammalian intergeniculate leaflet. The connections of these nuclei were examined using both the anterograde and retrograde transport of horseradish peroxidase. Afferents to the neuropil of Bellonci and its nucleus include bilateral projections from the retina, the contralateral nucleus of Bellonci, and anterior thalamic nucleus as well as bilateral projections from the pretectum and the ipsilateral suprachiasmatic nucleus. Efferent projections observed following HRP injections in the anterior thalamus consist of three components: (1) a ventral hypothalamic-suprachiasmatic and commissural projection, (2) a dorsal descending tract to the pretectum and tectum, and (3) a ventral descending tract to the somatomotor brainstem.