Ernest W. Kent

The effects of electrolytic median raphe lesions on two measures of latent inhibition

This study investigates the effects of median raphe lesions on the ability of conditioned stimulus preexposure to impair the acquisition of a shuttlebox avoidance and a taste aversion task. Control rats, who were exposed to the conditioned stimulus prior to its signaling of shock, were impaired relative to nonpreexposed controls in the subsequent acquisition of a shuttlebox avoidance response. In contrast, the performance of rats with median raphe lesions was not affected by preexposure to the conditioned stimulus. Using a taste aversion paradigm, however, lesioned and control subjects showed similar fluid intakes of a flavored solution following a 7-day preexposure period prior to pairing with illness. It is suggested that the appearance of latent inhibition in median raphe-lesioned rats may depend on the saliency of the conditioned stimulus.

Straight alley acquisition and extinction and open field activity following discrete electrolytic lesions of the mesencephalic raphe nuclei.

Open field behavior and the acquisition and extinction of a food-rewarded straight alley task were examined in rats with discrete electrolytic lesions of the midbrain raphe nuclei. Increased open field activity was seen following lesions of either the median or dorsal nucleus of the raphe, although the effect of median lesions was much more pronounced. Acquisition and extinction of a runway task were impaired following lesions of the median, but not dorsal, nucleus when trials were separated by 8 min. Animals with combined lesions of the dorsal and median raphe were behaviorally indistinguishable from those subjects with median raphe lesions alone. In a second experiment, it was determined that the acquisition deficit could be eliminated by a decrease in the intertrial interval, suggesting that the original deficit was not due to a motor impairment following median raphe lesions. Although other investigators have reported raphe involvement in aversively motivated behaviors the current study provides the first demonstration that the median raphe is also involved in the performance of tasks which are appetitively motivated. Additionally, we provide the first evidence that deficits in food-rewarded behaviors similar to those seen following damage to certain limbic structures can be produced by lesions within the midbrain.

Nucleus accumbens lesions reduce amphetamine hyperthermia but not hyperactivity.

Electrolytic lesions of the nucleus accumbens were found to produce an increase in spontaneous locomotion; additionally, these lesions attenuated the hyperthermic, but not the hyperkinetic, actions of d-amphetamine. These results suggest that while dopamine release from the mesolimbic system may underlie amphetamine-induced hyperthermia, other structures must also be involved in amphetamine-induced hypermotility.

Elimination of learned behaviors after transection of fibers crossing the lateral border of the hypothalamus.

Abstract A wire knife operated through a hypodermic needle was used to sever the fibers which cross the lateral border of the diencephalon in rats. Such transections eliminated or significantly impaired the performance of a variety of learned behaviors rewarded by electrical brain stimulation, escape from footshock, or food, and retarded or prevented the acquisition or reacquisition of these behaviors. Many but not all of the animals which showed these effects on instrumental behavior also were aphagic and adipsic. Simple motor or sensory functions were unimpaired. Locomotor activity and exploratory behavior were somewhat increased following the transection. Complex but presumably unlearned behaviors, such as grooming and swimming, were not affected.

Comparison of regional serotonin levels and turnover in the brain of naturally high and low aggressive rats

Rats, matched by weight and goal approach, were tested for aggressiveness using a food-competition paradigm. Those winning and those losing all of their matches were designated as high and low aggressive, respectively. A third group of randomly selected rats never fought and were used as a control for the fighting experience. Half of the rats were used to determine serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels in discrete brain regions. The other half were used to estimate 5-HT turnover in the same brain regions. High and low aggressive rats did not differ with regard to 5-HT or 5-HIAA levels, but high aggressive rats showed faster 5-HT turnover than low aggressive rats. The turnover differences reached statistical significance in only two of the 3 brain regions examined: (a) all structures (minus olfactory bulbs) rostral to the level of the anterior commissure, and (b) the brain stem (hypothalamus-midbrain-medulla). The observed differences cannot be attributed to differing behavioral experiences since the high and low aggressive groups pooled did not differ from the no-fighting control. These findings are discussed with respect to the involvement of serotonin in rat intraspecies aggression.

Simple technique for midline stereotaxic surgery in the rat

A simple technique is described for diverting the superior saggital sinus to allow midline implantation of vertically oriented electrodes or cannulae. This method avoids problems encountered following puncture of the sinus and minimizes cortical trauma.

Intracranial self-stimulation effects along the route of the nigro-striatal bundle

Recent evidence suggesting a possible dopaminergic nigro-striatal substrate of self-stimulation led us to map this route for both self-stimulation and stimulus-bound motor effects. The results of 128 electrode placements show that the route of the nigro-striatal projection supports strong self-stimulation effects from the substantia nigra to the ento-peduncular nucleus. Beyond this level, such effects disappear. Our results indicate that the striatum itself is neutral with regard to reinforcement, and suggest that such apparent neutrality cannot be ascribed to motor or other artifacts. These findings require a reappraisal of the hypothesis of a dopaminergic self-stimulation system, although they are not in conflict with the idea that dopaminergic manipulations may affect self-stimulation through some more general regulatory influence on operant responding.

Impaired patterned responding in rats with electrolytic median raphe lesions

Abstract Rats with electrolytic lesions of the median nucleus of the raphe were severely impaired in the acquisition of a straight alley, single alternation response compared to sham operated controls. This deficit was due to the failure of lesioned animals to reduce their running speeds on non-reinforced trials. As similar results have been obtained following damage to several limbic structures, this study provides behavioral evidence for the concept of a limbic midbrain circuit.

Reticular units: Relations between responses to sensory stimulation and responses to neurohumors

Evidence is presented which supports the hypothesis that mesencephalic reticular neurons which show an increase in rate on presentation of arousing sensory stimuli are inhibited by topical adrenergic stimulation, and that cholinergic effects on these neurons are variable according to local geography, perhaps representing functional groups. The data was obtained from acute curarized rat preparations with microgram quantities of transmitter substances applied in crystaline form to the vicinity of the neurons with implanted cannulae.

An ultrasonic UCS

Abstract A waveform generator is described which produces a triangular wave of continuously varying frequency ranging from 20–30 KHz. Suitably amplified, this sound was shown to be an effective noxious UCS for laboratory rats while inaudible to man.