Modulation of activity of gustatory neurons in the hamster parabrachial nuclei by electrical stimulation of the ventroposteromedial nucleus of the thalamus

The parvicellular part of the ventroposteromedial nucleus of the thalamus (VPMpc) is positioned at the key site between the gustatory parabrachial nuclei (PbN) and the gustatory cortex for relaying and processing gustatory information via the thalamocortical pathway. Although neuroanatomical and electrophysiological studies have provided information regarding the gustatory projection from PbN to VPMpc, the exact relationship between PbN and VPMpc, especially the efferent projection involving VPMpc to PbN, is obscure. Here we investigated the reciprocal connection between these two gustatory relays in urethane-anesthetized hamsters. We recorded from 114 taste-responsive neurons in the PbN and examined their responsiveness to electrical stimulation of the VPMpc bilaterally. Stimulation of either or both of the ipsilateral or contralateral VPMpc antidromically activated 109 gustatory PbN neurons. Seventy-two PbN neurons were antidromically activated after stimulation of both sides of the VPMpc, indicating that taste neurons in the PbN project heavily to the bilateral VPMpc. Stimulation of VPMpc also orthodromically activated 110 of PbN neurons, including 106 VPMpc projection neurons. Seventy-eight neurons were orthodromically activated bilaterally. Among orthodromic activations of the PbN cells, the inhibitory response was the dominant response; 106 cells were inhibited, including 10 neurons that were also excited contralaterally, indicating that taste neurons in the PbN are subject to strong inhibitory control from VPMpc. Moreover, stimulation of VPMpc altered taste responses of the neurons in the PbN, indicating that VPMpc modulates taste responses of PbN neurons. These results may provide functional insight of neural circuitry for taste processing and modulation involving these two nuclei.

Deep Brain Stimulation for Neuropathic Cephalalgia

The aim of this study was to determine the efficacy of deep brain stimulation (DBS) in the treatment of various types of intractable head and facial pains. Seven patients underwent the insertion of DBS electrodes into the periventricular/periaqueductal grey region and/or the ventroposteromedial nucleus of the thalamus. We have shown statistically significant improvement in pain scores (visual analogue and McGill's) as well as health-related quality of life (SF-36v2) following surgery. There is wide variability in patient outcomes but, overall, DBS can be an effective treatment. Our results are compared with the published literature and electrode position for effective analgesia is discussed.

Effect of Amiloride on Gustatory Responses in the Ventroposteromedial Nucleus of the Thalamus in Rats

The existence of gustatory neuron types has been demonstrated in the chorda tympani nerve and the nucleus of the solitary tract (NTS) of rats and hamsters through the oral application of amiloride, a sodium channel blocker. At these lower-order levels, amiloride was shown to reduce the response to sodium and lithium salts in sodium- and sugar-oriented cells, while leaving those of acid- and quinine-oriented neurons unmodified. We extended this investigation to higher-order levels by determining whether amiloride suppressed the responses of cells at the 4th-order gustatory relay in the thalamus, which neurons were affected, the degree of suppression, and whether the subsequent neural code for sodium was altered. We stimulated the whole oral cavity of anesthetized rats with a variety of tastants while recording the responses of 42 single thalamic neurons before and after the application of amiloride. The results revealed a similar pattern to that reported in the NTS. Amiloride inhibited only sodium- and sugar-oriented neurons, and specifically their responses to sodium- or lithium-containing stimuli. Moreover, there was a significant relationship between the degree of sodium specificity of a neuron and its sensitivity to inhibition by amiloride. These results demonstrate a relationship between a cell's response profile and its susceptibility to amiloride, and so offer evidence that gustatory neuron types exist through the level of the thalamus in rats. Thus membership in a neuronal group retains functional significance based on a receptor event 4 synapses away.

Responses to Taste Stimulation in the Ventroposteromedial Nucleus of the Thalamus in Rats

Extracellular action potentials were recorded from 73 neurons in the parvicellular division of the ventroposteromedial (VPMpc) nucleus of the thalamus of anesthetized Wistar rats during gustatory, thermal, and tactile stimulation of the whole oral cavity. The stimulus array consisted of 16 room-temperature (23°C) sapid stimuli, distilled water at three temperatures (0, 23, and 37°C), and 0.1 M NaCl at three temperatures (0, 23, and 37°C). Among all 151 neurons isolated in VPMpc, 9% responded exclusively to taste, 33% to taste and temperature, none to taste and touch, but 6% to all three modalities. Discharge rates evoked by the basic tastants were 13.8 ± 1.6 (SD) spikes/s for 0.1 M NaCl, 9.3 ± 1.4 spikes/s for 0.01 M HCl, 5.1 ± 0.9 spikes/s for 0.5 M sucrose, and 4.3 ± 0.6 spikes/s for 0.01 M quinine HCl. Water evoked mean responses at 0, 23, and 37°C of 9.9 ± 1.5, 0.6 ± 0.4, and 1.3 ± 0.9 spikes/s, respectively. The mean firing rate evoked by 37 and 0°C NaCl was 15.0 ± 2.4 and 17.0 ± 2.8 spikes/s, respectively. The exponent of the NaCl concentration-response power function was 0.39. Thalamic taste cells were broadly tuned. The mean breadth-of-tuning coefficient for these 73 gustatory cells was 0.79 ± 0.02. Two cells responded predominantly with inhibition, which accounted for the majority of inhibitory responses. The taste neurons were statistically divisible into three groups: sodium-oriented ( n = 40), acid-oriented ( n = 12), and sugar-oriented ( n = 17). Four additional bitter-oriented neurons were not closely enough related to be defined as a group and were considered outliers. The sodium-oriented group could be divided into three statistically distinct subgroups, differing in the specificity of their responses to NaCl. With respect to polymodal sensitivity, spontaneous rate, evoked response rates, signal-to-noise ratio, proportions of cells responding best to basic tastants, taste neuron groups, taste spaces, and temporal responses, VPMpc neurons have characteristics that are intermediate between those of parabrachial and cortical gustatory neurons.

The Sense of Taste: Neurobiology, Aging, and Medication Effects

The sense of taste is an oral chemical sense in mammals that is involved in the choice of foods. Initial transduction of taste stimuli occurs in taste buds, which are distributed in four discrete fields in the oral cavity. Medications can affect the taste buds and ion channels in taste-bud cell membranes involved in stimulus transduction. The sense of taste gradually declines with aging, with bitter taste most affected. Neural circuits that mediate taste in primates include cranial nerves VII, IX, and X, the solitary nucleus in the brain stem, the ventroposteromedial nucleus of the thalamus, and the insular-opercular cortex. The central taste pathways process taste information about sweet, salty, sour, and bitter stimuli serially and in parallel. Medications associated with "metallic" dysgeusia and taste losses affect the taste system via unknown mechanisms.