Neural substrates of behavioral aversion in lateral hypothalamus of rabbits.

Unit electrophysiology of lateral hypothalamus (LH) in rabbits has revealed two functionally contrasting, topographically distinguishable groups of neurons that relate to hedonic properties of taste stimuli. To assess the neurobehavioral role of aversion-type cells (maximally excited by aversive stimuli and inhibited by rewarding stimuli) found in the rostral part of mid-lateral LH at the level of ventromedial nucleus (vmh), intracranial self stimulation (ICSS) and stimulation-escape were studied with moveable-type electrodes in this and an adjoining caudal region of LH dominated by a contrasting type of neuron. Hedonic properties of the brain stimulation conformed to the distribution of these cellular elements. Stimulation of the rostral area supported weaker ICSS and stronger escape behavior. Aversive reactions predominated in ventral parts of the rostral area. Aversion-type cells identified electrophysiologically in this region would appear to mediate behaviorally aversive functions. These cells may play a role in the activation of feeding (possibly also drinking) and in drive-reduction reward.

Electrophysiology of taste, feeding and reward in lateral hypothalamus of rabbit.

Single-unit responses of LH cells and oral reaction were studied in food-deprived animals to intraoral injections of rewarding and aversive tastants differentially cued by tonal stimuli. Correlative tests were carried out with microstimulation. Classification and statistical analysis of task-related neurons in different AP regions of LH revealed two functionally disparate, anatomically segregated groups of cells that may play a role in hedonic reactions to tastants. A rostral group showed strong "excitatory" response to aversive test stimuli and divergent "inhibitory" or minimal response to rewarding test stimuli. More caudal cells displayed maximal excitatory response to rewarding test stimuli, which, in some cases, related to oral behavior. Microstimulation of caudal sites also evoked rhythmical oral movement. Phasic and/or tonic reactivity to taste stimuli were common among LH neurons, but most cells responded differentially to both conditioned stimuli and tastants. Only a few cells reacted selectively to the fluid stimuli.

Operant licking in rabbits for intraoral injection of basic types of tastants.

In order to compare and contrast hedonic properties of 0.75 M NaCl and 0.5 M sucrose used in behavioral electrophysiology of taste, tests were carried out of evoked patterns of orolingual response and operant licking on a FR-32 schedule to discrete intraoral injections of these stimuli and other basic types of tastants. In tests of taste reactivity, NaCl and sucrose evoked quantitatively similar numbers of orolingual response in excess of those evoked by water. NaCl was also similar to sucrose in amount of operant licking generated at the outset of the test session. Both of these stimuli were more effective than either 0.02 M HCl, 0.01 M QHCl, or water. The NaCl also did not have the suppressant effect of HCl when alternated with sucrose as the reinforcement for licking. NaCl differed from sucrose in sustaining operant licking. While NaCl would appear to share the same basic hedonic value of sucrose, long-term associative processes pertaining to postingestional consequences of fluid input and short-term sensory processes may act to limit behavioral responsivity for concentrated NaCl. Additional information was obtained on operant licking for sodium saccharin.

Electrophysiology of taste-mediated functions in parabrachial nuclei of behaving rabbit.

A single-unit analysis of parabrachial (PBN) neuronal activity associated with orolingual response to auditory-cued intraoral injections of basic type tastants and water revealed 7 major functional types of neurons. Several types dealt with mechanoreceptive orosensory input from superficial and/or deep receptors and from proprioceptors (possibly joint receptors). Some were multimodal in sensitivity. In one type, thermal or gustatory input acted to inhibit movement-related excitation in activity. Three types of taste-sensitive elements were distinguished. "Movement-taste sensitive" elements showed relations to orolingual movement and hedonic properties of fluid input. These cells may contribute to motivational control of licking activity. A tentative distinction was drawn between "taste-hedonic" and "taste" elements. One type responded to water and showed a bias toward hedonic properties of tastants. The other type exhibited no response to water and smaller responses to tastants that had no hedonic relationship. Both, however, had common properties, including inhibitory modulation of activity in apparent anticipation of fluid delivery. In addition to serving as a gustatory relay, PBN act to integrate orolingual response to taste and other orosensory input, and may operate as an output mechanism in the ventral forebrain control of ingestive behavior.

Ascending efferent projections of the gustatory parabrachial nuclei in the rabbit.

Neuroanatomical and electrophysiological techniques were used to study efferent projections of the gustatory region of the parabrachial nuclei (PBN) in rabbits. Autoradiographic analysis of [3H]leucine microinjected into electrophysiologically defined gustatory areas of the PBN revealed strong projections to the ventromedial taste component of the ventrobasal complex of the thalamus and also evidence of projections to the lateral hypothalamus, substantia innominata and nucleus centralis amygdala (ACE). Electrical stimulation in the region of ACE was shown to activate antidromically neurons in the PBN responsive to rapid stimuli, confirming gustatory projections to the ventral forebrain. An electrophysiological mapping of units in the amygdala and substantia innominata responsive to electrical stimulation of the gustatory part of the PBN agreed with the autoradiographic data. A proportion of these units were also responsive to gustatory stimuli. However, latencies of evoked response to stimulation of the PBN were unusually long. Outputs from the gustatory PBN to the ventral forebrain may include other modes of polysynaptically integrated information that are largely controlled by interneuronal networks within the PBN.

Coding of gustatory information in the pontine parabrachial nuclei of the rabbit: magnitude of neural response.

An electrophysiological analysis was carried out on 82 neurons in the parabrachial nuclei to study coding of gustatory stimuli. The magnitude of neural discharge evoked by two concentrations of each of 4 basic taste stimuli were subjected to two types of analyses to evaluate neuronal specificity and across-element spatial patterning in the coding of taste quality. Results were compared and contrasted to those in rodents. Profiles of taste response of individual neurons were analyzed mathematically by Q-technique principal components analysis and cluster analysis to identify basic neuronal patterns of response. Four distinct patterns emerged which, based on the average profiles of response, were characterized as: NaCl-dominant, NaCl-HCl, sucrose-dominant, and non-specific. Analysis of the NaCl- and sucrose-dominant types of neurons indicated that neurons with the most narrowly tuned patterns of response to the taste stimuli could not effectively code specific taste qualities. Comparison of interstimulus correlations in magnitude of response across neurons between qualitatively different tastants and intrastimulus correlations between intensities of given tastants also revealed difficulties with the spatial pattern model of coding. The discrepant findings raise questions about an assumption utilized in the analysis of the model, viz., that all elements contribute homogeneously to the patterning code.

Coding of gustatory information in the pontine parabrachial nuclei of the rabbit: temporal patterns of neural response.

As an extension of the analysis of magnitude of response to gustatory stimuli in the parabrachial nuclei5, this study sought to determine to what extent time course of neural response coded information about taste quality. A principal components analysis and stepwise discriminant analysis were used to evaluate stimulus-related differences in time course of response. Temporal patterns of response over an 8.192 s period to the 4 basic types of taste stimuli were discriminated from one another at about twice chance level. The discrimination of different qualities of taste by time course exceeded discrimination of different intensities of the stimuli. Analysis of time course restricted to the first 2,048 s of response again revealed significant overall differentiation of the stimuli, but only NaCl and sucrose were individually discriminated. Temporally coded information about gustatory stimuli, particularly in terms of onset characteristics of response, appeared inadequate for perceptual differentiation of taste quality. Such coding might, however, complement an across-neuron spatial pattern code for taste quality in simplifying the decoding of taste input for control of reflexive responses that entail a more inclusive classification of taste stimuli.

Gustatory functions of the nucleus tractus solitarius in the rabbit.

Multiunit analysis revealed a rostral region of NTS containing cells responsive to taste stimulation of rostral tongue. Using representative stimuli for the 4 basic types of taste, maximal incidence and magnitude of response was found to NaCl, followed by HCl, sucrose and QHCl. Further analysis of temporal patterns of response to the tastants revealed differences among stimuli in latency and time course. A principal components analysis indicated that time course, apart from magnitude of response, could contribute to neural differentiation of tastants. Information was also derived on neural intensity functions for these basic types of tastants. Additional observations with sodium saccharin revealed minimal neuronal responsivity despite reported evidence of behavioral preference for this tastant by rabbits.

Taste responsivity of amygdaloid units in behaving rabbit: a methodological report.

A technique is described for studying unit electrophysiology of taste in the behaving animal. Preliminary observations on response patterns by amygdaloid units to four putative basic types of taste qualities (represented by sucrose, saline, acid, and quinine), which also vary in motivational properties, revealed evidence of units with exclusive or highly differential response to either sucrose or saline in food-deprived animals. Small quantities (0.3 ml) of these palatable tastants injected intra-orally evoked neural responses lasting 5-8 sec that did not relate directly to the characteristics of the oro-lingual reaction as detected electromyographically; such sensorimotor relations were found in units sampled in the globus pallidus. Some amygdaloid units displayed differential excitatory and inhibitory modes of response to different tastants. Other cells exhibited more phasic patterns of response lasting 1-2 sec to 2 or more tastants, including unpalatable acid and quinine. The results underscore the sensitivity of the technique to stimulus parameters of taste-processing, which is basic to the study of all neurobehavioral functions of taste stimuli.

Effects of hippocampal stimulation on acquisition, extinction and generalization of conditioned suppression in the rat.

Rats implanted with electrodes in the dorsal or ventral hippocampus received posttrial stimulation in training sessions with footshock reinforcement. Afterdischarges without overt seizures were consistently without effect on the rate of acquisition of suppression of licking during an auditory conditioned stimulus (CS), although conditioning was retarded by the delivery of distracting stimuli following footshock. The rate of conditioning remained insensitive to elicitation of dorsal hippocampal afterdischarges (DHAD) despite subsequent alterations of session length, intertrial interval and preexposure to the CS. However, faster extinction of suppression occurred following DHAD, suggesting a limited but essential role of the hippocampus in addressing stored information.

Interrelationship among multiunit activity of the midbrain reticular formation and lateral geniculate nucleus, thalamocortical arousal, and behavior in rats.

Analyses of multiunit activity of midbrain reticular formation (RF) and lateral geniculate nucleus (LG) revealed marked phasic covariation that conformed to characteristics of behavior. Movement patterns appeared to set the range within which RF activity and thalamocortical arousal (as reflected in LG activity) varied. In tests of spontaneous behavior, sensory stimulation, bar pressing for food, and conditioned emotional response. RF activity and thalamocortical arousal conformed to immediate properties of behavior apart from conditions giving rise to the behavior. Other analyses revealed close relationships between RF activity and visual evoked response, but only a general relationship to hippocampal slow-wave rhythms. Our findings emphasize the necessity for refining psychological conceptualizations of arousal processes.

Electrophysiological and behavioral reactivity to photic stimuli following septal lesions and pharmacological treatments in rats.

Analysis of behavioral reactivity and cortical visual evoked response (VER) to photic stimulation revealed 2 patterns of lesion-induced changes. One pattern of VERs reflected a hypoarousal electrophysiological condition. This pattern, present initially under all conditions, could be simulated with administration of scopolamine. A second pattern of VERs developed gradually and appeared to reflect a hyperaroused electrophysiological condition. This pattern could be simulated with d-amphetamine. While both lesion-induced electrophysiological patterns were associated with augmented behavioral reactivity to flashes, the hyperarousal pattern related to more intense conditions of stimulation and more sustained behavioral reactivity. Scopolamine, as opposed to d-amphetamine, reproduced the heightened behavioral reactivity to the flashes. These results were interpreted in terms of a "hypoarousal hypothesis" of sensory hyperreactivity.