Ralph Norgren

The taste reactivity test. I. Mimetic responses to gustatory stimuli in neurologically normal rats.

One or two bottle preference tests, i.e., relative fluid consumption, constitute the primary methodology for determining acceptance or rejection of tastes in animals other than humans. These tests require organisms to initiate and maintain drinking behavior, and, therefore, can not be applied to preparations which do not eat or drink spontaneously. The taste reactivity test, a new method for assessing responses to gustatory stimuli, circumvents this shortcoming. A 50 microliter taste stimulus is injected directly into the oral cavity of a freely moving rat and the immediate response videotaped for frame by frame analysis. Each of the sapid stimuli used (4 concentrations of sucrose, NaCl, HCl, and quinine HCl) generated a stereotyped response derived from a lexicon of 4 mimetic (movements of lingual, masticatory, and facial musculature) and 5 body response components. Responses to taste stimuli were highly consistent within and between rats. For example, sapid sucrose, NaCl and HCl stimuli elicited a response sequence beginning with low amplitude, rhythmic mouth movements, followed by rhythmic tongue protrusions, and then lateral tongue movements. No body movements accompanied these mimetic responses. In contrast, quinine in concentrations at and above 3 X 10(-5) M (1/2 log step above the absolute behavioral threshold for quinine) elicited a response pattern beginning with gaping and proceeding through as many as 5 body responses. These normative data for the intact rat can be directly compared to the taste reactivity of neurally ablated preparations which do not spontaneously feed or drink. Such comparisons can be utlized in determining the neural substrates necessary for the execution and regulation of ingestive behavior.

Projections from the nucleus of the solitary tract in the rat

The axonal projections of neurons in and near the nucleus of the solitary tract have been visualized using titrated amino acid autoradiography. Axons of neurons of this nucleus ramify extensively within the nucleus itself, but much less so in the nucleus commissuralis. They also enter cranial motor nuclei within the medulla. Axons originating in the anterior part of the nucleus of the solitary tract extend to the hypoglossal, facial and probably trigeminal motor nuclei, but not to the dorsal motor nucleus of the vagus or the nucleus ambiguus. The posterior part of the nucleus of the solitary tract projects to all these motor nuclei. In the spinal cord solitary nucleus axons remain in the medial gray directly caudal to the solitary nucleus itself. The distribution becomes very weak by C3 after some fibers spread laterally into the caudal trigeminal nucleus. Fibers are labeled in the contralateral ventral columns, but they could not be unequivocably attributed to solitary neurons. Axons ascending from the nucleus of the solitary tract extend no further rostrally than the pons, where they terminate in the caudal end of the parabrachial nuclei. Although often treated as entirely separate systems, the present results indicate that secondary gustatory neurons in the anterior solitary nucleus and secondary visceral afferent neurons in the posterior solitary nucleus have very similar rostral and caudal projections. The pontine parabrachial nuclei, the rostral termination of solitary nucleus neurons, have extensive direct connections to the thalamus, the hypothalamus and the limbic forebrain. Assuming similar connections occur in other mammals, these findings establish the existence of di-synaptic visceral afferent access to the highest autonomic integrative centers in the brain.

The taste reactivity test. II. mimetic responses to gustatory stimuli in chronic thalamic and chronic decerebrate rats

The taste reactivity test described in the preceding paper was used to begin determining the capacity of brain stem structures to execute and regulate ingestive behavior. Both chronic thalamic and chronic decerebrate rat preparations were examined repeatedly, and their gustatory mimetic responses compared through frame-by-frame videotape analysis with the responses of neurologically normal controls. In response to orally injected taste stimuli, chronic decerebrate rats executed the same mimetic response components, and very similar response sequences observed in intact rats. In contrast, all taste stimuli elicited a quinine-like rejection sequence from chronic thalamic rats. In thalamic rats mimetic responses associated with ingestion were completely absent. Based on the similarities in the ingestion and rejection responses of decerebrate and intact rats, it appears that discriminative responses to taste result from integrative mechanisms complete within, or caudal to, the midbrain. Since decerebrate rats have the capacity to execute both ingestion and rejection response sequences, neural mechanisms rostral to the midbrain in some way suppress ingestion and/or releaser ejection responses in the thalamic preparation.

The central projections of the trigeminal, facial, glossopharyngeal and vagus nerves: an autoradiographic study in the rat☆

The central distributions of primary afferent axons in the facial, trigeminal (mandibular branch), glossopharyngeal, and vagal nerves of the rat have been re-examined using the autoradiographic tracing technique after injections of [3H]proline or [3H]leucine into their peripheral ganglia. Within the nucleus of the solitary tract (NST), the labeled terminals from VII, V, IX and X form a continuous distribution that spans the length of this nucleus. Sensory axons in VII terminate mainly within the lateral division of the rostral NST, although some of the terminals extend further caudally within the nucleus. Immediately caudal to the rostral NST, the distribution continues with major contributions from V and IX. Both are confined mainly to the lateral division of the NST, although some of the fibers in IX terminate within the medial division. Injections into the inferior ganglion of X confirm the extensive distribution of vagal axons as they ramify significantly within the lateral division, and virtually monopolize the medial division of the NST. Thus, the major zone of convergency for these 4 cranial nerves is the lateral division of the nucleus from the level of the entering fascicles of IX caudally to the level of the area postrema. Furthermore, only X has a crossed projection as vagal axons invade the commissural nucleus and the medial division of the contralateral NST. Vagal fibers also enter the area postrema bilaterally. Finally, some afferent fibers from VII, IX and X descend in the dorsal part of the spinal trigeminal tract and terminate within the marginal subdivision of the spinal trigeminal nucleus pars caudalis, as well as the dorsal horn of the cervical spinal cord.

The pontine taste area in the rat.

The pontine taste area relays gustatory information from the rostral pole of the solitary nucleus to both the thalamus and ventral forebrain. An electrophysiological investigation of this area was carried out in 3 stages. First, multiunit responses from the dorsal pons were mapped using sapid, thermal, and tactile stimuli applied to the anterior tongue. The gustatory zone lies within and just dorsal and ventral to the brachium conjunctivum as it enters the pons from the cerebellum. Second, gustatory stimuli were applied independently to the anterior and posterior tongue to determine whether receptors in both fields are represented in the pons. Responses with characteristics similar to those obtained from the glossopharyngeal nerve were located on the dorsal edge of the pontine gustatory zone. More ventrally the responses from the posterior tongue mimicked anterior tongue responses, but were of lesser amplitude than the largest anterior responses occurring at the ventral edge of the gustatory zone. Third, 71 single units were isolated in the dorsal pons, and tested for sensitivity to gustatory stimulation of the anterior and posterior tongue separately. More than half the units responded to gustatory stimuli--some from the anterior tongue alone, some from the posterior alone, but most responded to stimuli applied to either field. In the latter instance 7 of 10 units tested continued to respond after anesthetizing the chorda tympani with Xylocaine instilled into the middle ear, thus demonstrating a true glossopharyngeal input. This proves that gustatory information from two distinct receptive fields may converge on the same central neuron.

Taste Pathways in Rat Brainstem

By means of a combination of electrophysiological and anatomical procedures, the projections of the anterior portion of the solitary nucleus were traced to the parabrachial nuclei in the pons, structures hitherto not considered to be included in the taste pathway. Responses to taste stimuli were recorded from this pontine area. Lesions in the pontine taste area resulted in degeneration of fibers reaching the lingual area in the thalamus.

Gustatory cortex in the rat. I. Physiological properties and cytoarchitecture

The precise cytoarchitectural localization of taste-elicited cortical responses in the rat was studied using a combination of anatomical and physiological techniques. Multi-unit responses to tongue tactile, thermal and gustatory stimuli were recorded along 97 electrode penetrations positioned parallel to the lateral convexity of the brain and marking lesions were placed at the sites of transitions in these functional properties. Lesions made at sites that received different sensory inputs were consistently located within different cytoarchitectural subdivisions. In this manner, taste cortex in the rat was localized to the agranular insular cytoarchitectural region, in contrast to its traditional assignation to granular insular cortex. Instead, tongue temperature was found to be represented in the cortical area previously termed gustatory, i.e., in ventral granular cortex where layer IV attenuates.

Gustatory afferents to ventral forebrain

Abstract The pontine taste area (PTA) receives afferents from the gustatory zone of the nucleus of the solitary tract, and projects bilaterally to the thalamic taste area. Lesions of PTA also result in degenerating axons entering the substantia innominata in the ventral forebrain. The technique of antidromic activation has been used to demonstrate that pontine neurons which respond to gustatory stimuli send collaterals to both the thalamic taste area and substantia innominata. This establishes that, like olfactory input, gustatory information reaches the ventral telencephalon without first synapsing in the diencephalon.

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

Oromotor behavior results from the complex interaction between jaw, facial, and lingual muscles. The experiments in this and subsequent papers identify the sources of multisynaptic input to the trigeminal, facial, and hypoglossal motor nuclei. In the current experiments, pseudorabies virus (PRV-Ba) was injected into the jaw-opening (anterior digastric and mylohyoid) and jaw-closing muscles (masseter, medial pterygoid, and temporalis) in bilaterally sympathectomized rats. Injection volumes ranged from 2 to 21 microl with average titers of 2.8 x 10(8) pfu/ml and maximum survival times of 96 h. The labeling patterns and distributions were consistent between each of the individual muscles and muscle groups. A predictable myotopic labeling pattern was produced in the trigeminal motor nucleus (Mo 5). Transneuronally labeled neurons occurred in regions known to project directly to Mo 5 motoneurons including the principal trigeminal sensory and supratrigeminal areas, Kölliker-Fuse region, nucleus subcoeruleus, and the parvicellular reticular formation. Maximum survival times revealed polysynaptic connections from the periaqueductal gray, laterodorsal and pedunculopontine tegmental areas, and the substantia nigra in the midbrain, ventromedial pontine reticular regions including the gigantocellular region and pars alpha and ventralis in the pons and medulla, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, the results define the structure of the multisynaptic brainstem neural circuits controlling mandibular movement in the rat.

Central and peripheral vagal transport of cholecystokinin binding sites occurs in afferent fibers

The effects of various vagal lesions on cholecystokinin (CCK) binding sites in the nucleus tractus solitarii (NTS) and area postrema (AP) and the peripheral transport of CCK binding sites in the cervical vagus were examined in rats by in vitro autoradiography with [125I]CCK-8. Unilateral supraganglionic, but not subdiaphragmatic vagotomy significantly reduced CCK binding in the ipsilateral NTS. Specific unilateral afferent, but not efferent, vagal rootlet transections also significantly reduced NTS CCK binding ipsilateral to the transections. None of the vagal lesions altered CCK binding in the AP. Infraganglionic but not supraganglionic vagotomy eliminated the peripheral transport of vagal CCK binding sites. Together these results demonstrate that CCK receptors in the NTS are located on vagal afferent terminals, that CCK receptors in the AP are likely postsynaptic to a vagal afferent input and that the peripheral and central transport of vagal CCK binding sites occurs in afferent fibers.