Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus. III. Lingual muscle motor systems

Recent techniques for tracing pathways in the central nervous system of developing and adult mammals

Over the last 20 years, the choice of neural tracers has increased manyfold, and includes newly introduced anterograde tracers that allow quantitation of single-axon morphologies, and retrograde tracers that can be combined with intracellular fills for the study of dendritic arbors of neurons which have a specific projection pattern. The combination of several different tracers now permits the comparison of multiple connections in the same animal, both quantitatively and qualitatively. Moreover, the finding of new virus strains, which infect neural cells without killing them, provides a tool for studying multisynaptic connections that participate in a circuit. In this paper, the labeling characteristics, mechanism of transport and advantages/disadvantages of use are discussed for the following recently introduced neural tracers: carbocyanine dyes, fluorescent latex microspheres, fluorescent dextrans, biocytin, dextran amines, Phaseolus vulgaris leucoagglutinin, cholera toxin and viruses. We also suggest the choice of specific tracers, depending on the experimental animal, age and type of connection to be studied, and discuss quantitative methodologies.

Fos-like immunoreactivity in the brain stem following oral quinine stimulation in decerebrate rats

The present study compared the distribution of Fos-like immunoreactivity (FLI) following intraoral stimulation with quinine monohydrochloride (QHCl) in awake intact rats to the pattern obtained in chronic supracollicular decerebrate (CD) rats. Because the behavioral rejection response to QHCl is evident in the CD rat, it was hypothesized that the pattern of FLI in the lower brain stem should be similar in both groups. Overall, the distribution of FLI in the brain stem was quite similar in both intact and CD groups, and QHCl stimulation increased FLI in the rostral (gustatory) nucleus of the solitary tract, the parabrachial nucleus (PBN), and the lateral reticular formation (RF) compared with an unstimulated control group. The CD group differed from the intact group, however, with a trend toward less FLI in the RF and a shift in the pattern of label away from the external subdivision of the PBN. CD rats also had increased FLI in the caudal nucleus of the solitary tract, with or without intraoral infusions. The distribution of QHCl-induced FLI in the brain stem of intact rats thus indicates both local sensorimotor processing as well as the influence of forebrain structures.

Inhibition of styloglossus motoneurons during the palatally induced jaw-closing reflex

The inhibition of hypoglossal motoneurons innervating the styloglossus muscle during transient jaw closing, the so-called jaw-closing reflex, was studied in cats. The application of diffuse pressure stimulation to the posterior palatal surface produced the jaw-closing reflex and inhibitory postsynaptic potentials in the styloglossus motoneurons, indicating that mechanosensory inputs from the posterior palatal mucosa sent inhibitory synaptic inputs to styloglossus motoneurons. We also demonstrated that, during the palatally induced jaw-closing reflex, the tongue extended at jaw closure and was still extended forward in the initial part of the opening phase. In all of 22 styloglossus motoneurons studied, the depression of firing was elicited after the onset of jaw closure. In 14 of 22 styloglossus motoneurons, the depression of firing was elicited in the closing phase, and in the remaining cells it was elicited in the occlusal phase. By increasing the intracellular concentration of chloride ions, the inhibitory postsynaptic potential elicited in the styloglossus motoneuron converted to a depolarizing potential. It is concluded that the inhibition of styloglossus motoneurons may be involved in the maintenance of tongue protrusions during the palatally induced jaw-closing reflex, and that inhibitory postsynaptic potentials evoked in the styloglossus motoneurons are partly due to a chloride-dependent inhibitory postsynaptic potential.

Identification of rat brainstem multisynaptic connections to the oral motor nuclei in the rat using pseudorabies virus. II. Facial muscle motor systems

The present experiments continue our investigations of the higher order afferent systems controlling the orofacial musculature. Pseudorabies virus (PRV) was injected into the buccinator, platysma, posterior digastric, and zygomatic muscles in bilaterally sympathectomized rats. Injection volumes ranged from 6 to 12 microl with average titers of 7 x 10(8) pfu/ml and maximum survival times of 96 h. The labeling patterns and distributions were similar across the individual muscles and between muscle groups (perioral vs. posterior digastric), as well as in comparison to the results from previous masticatory muscle injections. Injections produced a predictable myotopic labeling pattern in the facial motor nucleus (Mo 7) and transneuronally in regions known to project directly to Mo 7 including the red nucleus, ventrolateral parabrachial region, principal trigeminal sensory nucleus, supratrigeminal area, and the parvicellular reticular formation. Maximum survival times revealed more distant connections from a variety of nuclear zones including the periaqueductal gray, laterodorsal and pedunculopontine tegmental areas, and the substantia nigra in the midbrain, ventromedial reticular regions including the gigantocellular region and pars alpha and ventralis in the pons and medulla, and the nucleus of the solitary tract, spinal trigeminal nucleus caudalis, paratrigeminal region, and paramedian field in the medulla. The similarity of the labeling patterns and distributions of the higher order afferents resulting from PRV facial and masticatory muscle injections identifies the neural circuits that may coordinate the activity of these muscle groups during oral motor behavior.