Evidence for peripheral, but not central modulation of trigeminal cold receptive cells in the rat

Peptide regulators of peripheral taste function

The peripheral sensory organ of the gustatory system, the taste bud, contains a heterogeneous collection of sensory cells. These taste cells can differ in the stimuli to which they respond and the receptors and other signaling molecules they employ to transduce and encode those stimuli. This molecular diversity extends to the expression of a varied repertoire of bioactive peptides that appear to play important functional roles in signaling taste information between the taste cells and afferent sensory nerves and/or in processing sensory signals within the taste bud itself. Here, we review studies that examine the expression of bioactive peptides in the taste bud and the impact of those peptides on taste functions. Many of these peptides produced in taste buds are known to affect appetite, satiety or metabolism through their actions in the brain, pancreas and other organs, suggesting a functional link between the gustatory system and the neural and endocrine systems that regulate feeding and nutrient utilization.

Quantitative analysis of orofacial thermoreceptive neurons in the superficial medullary dorsal horn of the rat

Surprisingly little is known concerning the central processing of innocuous thermal somatosensory information. The aim of the present study was to obtain quantitative data on the characteristics of neurons in the rat superficial medullary dorsal horn (sMDH) that responded to innocuous thermal stimulation of the rat's face and tongue. Single-unit extracellular recordings were obtained in chloralose-urethane anesthetized rats. A total of 153 thermoreceptive neurons was studied. Of these, 146 were excited by cooling and inhibited by warming and were classified as COLD cells. The remaining seven cells were excited by innocuous warming of the skin or tongue. Of 123 COLD cells tested, 33% were excited by touch and 22% by pinch stimuli delivered to the thermoreceptive field. Of the 50 COLD cells tested, 46% were excited also by noxious heating (> or = 50 degrees C for 5 s). Most (82/121) of the receptive fields were located on the upper lip, 25 on the tongue, and most of the remaining on the lower lip. Receptive fields were generally small (1-5 mm2). In some experiments, electrical stimulation in the thalamus was performed, and nine COLD cells could be activated antidromically. The responses of 38 COLD cells to incremental 5 degrees C cooling steps were examined quantitatively. Thermal stimuli were applied to facial or lingual receptive fields of sMDH neurons with a computer-controlled Peltier thermode starting from 33 degrees C, decreasing to 8 or 3 degrees C, and returning to 33 degrees C. Most COLD cells (26/38) had both static and dynamic responses; 7 had mainly dynamic and 5 mainly static responses to step decreases in temperature. Rat sMDH COLD cells could be classified into three groups depending on their stimulus-response functions. The first group (Type 1, n = 19) had a bell-shaped static stimulus response function. The second group (Type 2) had a high maintained or increasing static firing rate as the temperature decreased < 18 degrees C (n = 10). Type 3 COLD cells had mainly dynamic properties (n = 7). Many of the cells in all groups were excited by noxious mechanical stimulation. Type 2 cells differed from the other two groups in that most did not respond to noxious thermal stimuli (hot) and many responded to innocuous tactile stimuli. Neurons from each of the three groups of COLD cells could be activated antidromically from contralateral thalamus. These data suggest that there is little central processing of thermal information at the first central synapse for Type 1 neurons, however, the responses of the other two types may be due to central processing and convergence. The demonstration of rat sMDH COLD cells with distinctive stimulus-response functions to thermal shifts suggests separate functional roles of these neurons in the ascending thermal sensory pathway.

Morphine alters the firing of cold-receptive neurons in the superficial dorsal horn of the medulla in the rat

Effects of morphine (1-3 mg/kg, i.v.) were tested on the innocuous cold-receptive input in the superficial dorsal horn of the medulla. The static activity of most cold-receptive (cold-specific) neurons (12/16) was reduced, whereas an enhancement (4/16) was observed in the remaining neurons. Naloxone (200 micrograms/kg, i.v.) reversed, partially or completely, the effects of morphine in 9/12 cold-receptive neurons, and enhanced the static activity of some cold-receptive neurons. Static activity, at different adapting temperatures, during a warming (10 degrees C-->40 degrees C) and a cooling (40 degrees C-->10 degrees C) sequence at steps of 5 degrees C was reduced by morphine. The effects of morphine were also tested on the static as well as the dynamic responses of 9 cold-receptive neurons. The effects of morphine on the dynamic responses were not dependent on the static firing frequency. Morphine produced similar effects, excitatory or inhibitory, on the static as well as the dynamic responses of 7/9 neurons whether the static firing frequency was high (17-33 Hz) or low (< 12 Hz). However, morphine effects on static and dynamic responses were different in the remaining 2 neurons (high static firing frequency). We suggest that the predominantly inhibitory effect of morphine on the innocuous cold receptive input in the medullary dorsal horn may explain the inhibitory effect on the perception of cooling stimuli by systemic morphine in behavioral studies.

Differential effect of ethosuximide and of electrical stimulation on inhibitory and excitatory mechanisms

Much longer trains of conditioning stimuli are required to elicit inhibition descending from the reticular formation than to elicit segmental inhibition in the trigeminal nucleus. In contrast, a single conditioning stimulus is the most effective in eliciting descending facilitation, while the test stimulus alone is most effective in eliciting segmental excitation. Ethosuximide (ESM) selectively depresses descending inhibition and to a lesser extent segmental inhibition. Thus, ESM only depresses pathways requiring repetitive stimulation, such as inhibitory pathways in the reticular formation. This action would account for ESM's specificity for absence seizures, which are probably due to paroxysmal activity in inhibitory pathways.

Transmission of low temperature information in the rat trigeminal system

Recordings have been made from cold-receptive neurons in trigeminal nucleus caudalis to define the lower receptive field temperatures at which the neurons become silent. Most cells were found to become silent at 0 degrees C, but a few were still active at skin temperatures of -5 degrees C.