Convergence of Lemniscal and Local Excitatory Inputs on Large GABAergic Tectothalamic Neurons.

Large GABAergic (LG) neurons form a distinct cell type in the inferior colliculus (IC), identified by the presence of dense VGLUT2-containing axosomatic terminals. Although some of the axosomatic terminals originate from local and commissural IC neurons, it has been unclear whether LG neurons also receive axosomatic inputs from the lower auditory brainstem nuclei, i.e., cochlear nuclei (CN), superior olivary complex (SOC), and nuclei of the lateral lemniscus (NLL). In this study we injected recombinant viral tracers that force infected cells to express GFP in a Golgi-like manner into the lower auditory brainstem nuclei to determine whether these nuclei directly innervate LG cell somata. Labeled axons from CN, SOC, and NLL terminated as excitatory axosomatic endings, identified by colabeling of GFP and VGLUT2, on single LG neurons in the IC. Each excitatory axon made only a few axosomatic contacts on each LG neuron. Inputs to a single LG cell are unlikely to be from a single brainstem nucleus, since lesions of individual nuclei failed to eliminate most VGLUT2-positive terminals on the LG neurons. The estimated number of inputs on a single LG cell body was almost proportional to the surface area of the cell body. Double injections of different viruses into IC and a brainstem nucleus showed that LG neurons received inputs from both. These results demonstrated that both ascending and intrinsic sources converge on the LG somata to control inhibitory tectothalamic projections.

The TLC: a novel auditory nucleus of the mammalian brain.

We have identified a novel nucleus of the mammalian brain and termed it the tectal longitudinal column (TLC). Basic histologic stains, tract-tracing techniques and three-dimensional reconstructions reveal that the rat TLC is a narrow, elongated structure spanning the midbrain tectum longitudinally. This paired nucleus is located close to the midline, immediately dorsal to the periaqueductal gray matter. It occupies what has traditionally been considered the most medial region of the deep superior colliculus and the most medial region of the inferior colliculus. The TLC differs from the neighboring nuclei of the superior and inferior colliculi and the periaqueductal gray by its distinct connections and cytoarchitecture. Extracellular electrophysiological recordings show that TLC neurons respond to auditory stimuli with physiologic properties that differ from those of neurons in the inferior or superior colliculi. We have identified the TLC in rodents, lagomorphs, carnivores, nonhuman primates, and humans, which indicates that the nucleus is conserved across mammals. The discovery of the TLC reveals an unexpected level of longitudinal organization in the mammalian tectum and raises questions as to the participation of this mesencephalic region in essential, yet completely unexplored, aspects of multisensory and/or sensorimotor integration.

Stimulation of melatonin receptors decreases calcium levels in xenopus tectal cells by activating GABA(C) receptors.

To investigate the physiological effects of melatonin receptors in the Xenopus tectum, we have used the fluorescent indicator Fluo-4 AM to monitor calcium dynamics of cells in tectal slices. Bath application of KCl elicited fluorescence increases that were reduced by melatonin. This effect was stronger at the end of the light period than at the end of the dark period. Melatonin increased gamma-aminobutyric acid-C (GABA(C))-receptor activity, as demonstrated by the ability of the GABA(C)-receptor antagonists, picrotoxin and TPMPA, to abolish the effects of melatonin. In contrast, neither the GABA(A)-receptor antagonist bicuculline nor the GABA(B)-receptor antagonist CGP 35348 diminished the effects of melatonin. RT-PCR analyses revealed expression of the 3 known melatonin receptors, MT1 (Mel1(a)), MT2 (Mel1(b)), and Mel1(c). Because the effect of melatonin on tectal calcium increases was antagonized by an MT2-selective antagonist, 4-P-PDOT, we performed Western blot analyses with an antibody to the MT2 receptor; the data indicate that the MT2 receptor is expressed primarily as a dimeric complex and is glycosylated. The receptor is present in higher amounts at the end of the light period than at the end of the dark period, in a pattern complementary to the changes in melatonin levels, which are higher during the night than during the day. These results imply that melatonin, acting by MT2 receptors, modulates GABA(C) receptor activity in the optic tectum and that this effect is influenced by the light-dark cycle.

Effects of tectal grafts on sound detection deficits induced by inferior colliculus lesions in hooded rats.

The midbrain inferior colliculus (IC), a major integrating center for auditory processing, provides a model for structural, functional, and behavioral recovery. The present study examined the role of IC in spatial sound detection, and the effects of neural transplantation in sparing of behavioral performance. Hooded rats were presented noise bursts at ambient noise levels and 15 dB above this level randomly at one of eight locations in the horizontal plane, and rats were required to suppress licking upon detecting signal presentations. Following training, rats received bilateral IC lesions, bilateral lesions followed in 1 wk by bilateral tectal grafts (embryonic Day 18), or were sham operated. During repeated testing 15 to 30 days and 40 to 45 days following surgical procedures, rats with lesions showed impaired detection task performance when compared to grafted or sham animals. Detection ratios were statistically higher for grafted rats than for rats with lesions at repeated testing times. Performance of grafted animals was not statistically separable from the sham control group; however, grafted rats never achieved preoperative detection rates nor rates as high as sham rats. Postmortem implantation of diI crystals unilaterally into grafts demonstrated fiber labeling in the ipsilateral IC and lateral lemniscus, and retrograde labeling of neurons in the remaining host IC and dorsal nucleus of lateral lemniscus. Combined with results of previous studies, the results of this study suggest that sparing of detection performance for sounds occurring at random spatial locations may be attributable to partial integration of host-graft pathways.

The spinomesencephalic tract in the cat: its cells of origin and termination pattern as demonstrated by the intraaxonal transport method.

The cells of origin and terminal areas of the feline spinomesencephalic tract were investigated by the intraaxonal transport method. Following injection of wheat germ agglutinin-horseradish peroxidase conjugate into the cervical and lumbar enlargements, anterograde labelling was observed in several regions of the dorsal midbrain. The main terminal areas were the periaqueductal gray matter, the intercollicular nucleus, the posterior pretectal nucleus and the nucleus of Darkschewitsch. In addition, there was a sparse projection to the cuneiform nucleus and the anterior pretectal nucleus. The superior colliculus was virtually devoid of labelling except for a weak termination in the caudal part of the deep layers. Although there was a considerable overlap, the projection from the cervical spinal cord to the periaqueductal gray matter terminated more rostrally than that from the lumbar segments, indicating the presence of a somatotopic organization. The retrograde labelling seen after tracer injection into the midbrain terminal areas revealed that the cells of origin were located mainly in the upper cervical segments and in the cervical and lumbar enlargements; in the latter parts of the cord an overwhelming majority were situated in lamina I, with smaller fractions in laminae IV and V, whereas in the upper cervical segments and in the less densely labelled thoracic and sacral segments a much larger proportion of the peroxidase-positive neurons were found in the deep laminae. About 75% of the labelled cells were located contralateral to the injection site. The functional implication of the present results are discussed in relation to somatosensory activity in the mesencephalon. It is suggested that several regions of the dorsal midbrain have an important somesthetic function including that of pain.

GABA-containing neurons in the thalamus and pretectum of the rodent. An immunocytochemical study.

Antisera produced by immunizing rabbits with GABA conjugated to bovine serum albumin reacted, after purification, strongly with GABA fixed with glutaraldehyde to rat brain macromolecules, but insignificantly with other fixed amino acids (Storm-Mathisen et al. 1983). Sections through the diencephalon of perfusion-fixed mouse and rat brains showed a highly selective labeling pattern after incubation with these antisera. All cells of the reticular nucleus appeared to be stained. Smaller proportions of stained perikarya occurred in the dorsal and ventral subdivisions of the lateral geniculate body, in the medial geniculate body, in the lateroposterior nucleus, and in all nuclei of the pretectum. Labeled cell bodies were only rarely encountered in the ventrobasal complex, and were not found in the anterior and medial groups of thalamic nuclei. Stained axons were particularly concentrated in the ventrobasal complex, and in the stria medullaris, stria terminalis and inferior thalamic peduncle. The arrangement and density of labeled bouton-like dots varied markedly among nuclei, the highest densities occurring in the paraventricular and parataenial nuclei, and in the ventral subdivision of the lateral geniculate body. The mean staining intensity of the thalamic neuropil was lower than that of nearby structures, such as the hypothalamus and zona incerta. The present results on direct immunocytochemical detection of GABA are consistent with, and extend, data from immunocytochemical studies of the GABA-synthetizing enzyme, glutamic acid decarboxylase.

Projections to the midbrain tectum in Salamandra salamandra L.

Following unilateral iontophoretic application of HRP into the optic tectum of Salamandra salamandra, retrogradely HRP-filled cells were found bilaterally in the pretectum, tegmentum isthmi, the reticular formation, pars medialis, and in the nucleus vestibularis magnocellularis. The area octavo-lateralis projects only to the caudal part of the tectum. Ipsilateral projections were noted from the dorsal gray columns of the cervical spinal cord, the dorsal tegmentum, the thalamus dorsalis pars medialis, thalamus dorsalis, pars anterior (to the rostral one-third of the tectum), the thalamus ventralis (in its entire rostro-caudal extent), and the preoptico-hypothalamic complex. Retrogradely filled cells were identified in deeper layers of the contralateral tectum. There are two telencephalic nuclei projecting ipsilaterally to the tectum via the lateral forebrain: the ventral part of the lateral pallium, and the posterior strioamygdalar complex.