Shirley L. Buchanan

Efferent connections of the medial prefrontal cortex in the rabbit

The different cytoarchitectonic regions of the medial prefrontal cortex (mPFC) have recently been shown to play divergent roles in associative learning in rabbits. To determine if these subareas of the mPFC, including areas 24 (anterior cingulate cortex), 25 (infralimbic cortex), and 32 (prelimbic cortex) have differential efferent connections with other cortical and subcortical areas in the rabbit, anterograde and retrograde tracing experiments were performed using the Phaseolus vulgaris leukoagglutinin (PHA-L), and horseradish peroxidase (HRP) techniques. All three areas showed local dorsal-ventral projections into each of the other areas, and a contralateral projection to the homologous area on the other side of the brain. All three also revealed a trajectory through the striatum, resulting in heavy innervation of the caudate nucleus, the claustrum, and a lighter projection to the agranular insular cortex. The thalamic projections of areas 24 and 32 were similar, but not identical, with projections to the mediodorsal nucleus (MD) and all of the midline nuclei. However, the primary thalamic projections from area 25 were to the intralaminar and midline nuclei. All three areas also projected to the ventromedial and to a lesser extent to the ventral posterior thalamic nuclei. Projections were also observed in the lateral hypothalamus, in an area just lateral to the descending limb of the fornix. Amygdala projections from areas 32 and 24 were primarily to the lateral, basolateral and basomedial nuclei, but area 25 also projected to the central nucleus. All three areas also showed projections to the midbrain periaqueductal central gray, median raphe nucleus, ventral tegmental area, substantia nigra, locus coeruleus and pontine nuclei. However, only areas 24 and the more dorsal portions of area 32 projected to the superior colliculus. Area 25 and the ventral portions of area 32 also showed a bilateral projection to the parabrachial nuclei and dorsal and ventral medulla. The dorsal portions of area 32, and all of area 24 were, however, devoid of these projections. It is suggested that these differential projections are responsible for the diverse roles that the cytoarchitectonic subfields of the mPFC have been demonstrated to play in associative learning.

Concomitant heart rate and corneoretinal potential conditioning in the rabbit (Oryctolagus cuniculus): effects of caudate lesions.

Abstract Rabbits received either bilateral, unilateral, or sham caudate lesions and were subjected to Pavlovian conditioning training. Corneoretinal potential (CRP), electromyographic (EMG), and heart rate (HR) CRs were assessed. Lesions which destroyed the anteromedial two thirds of the head of the caudate nucleus greatly impaired the acquisition of the CRP response during both simple and differential Pavlovian conditioning. However, the magnitude of the HR CR and the HR discrimination were unaffected by these lesions. Measures of free field activity, electromyographic activity, and CRP thresholds to shock revealed no evidence of a motor or sensory deficit in the lesioned animals. These data thus demonstrate a deficit in a specific learned somatomotor response but no impairment in the autonomic changes which usually accompany somatomotor conditioning.

Autonomic responses are elicited by electrical stimulation of the medial but not lateral frontal cortex in rabbits.

Conscious rabbits received electrical stimulation of the anterior midline frontal cortex or lateral somatosensory and motor cortex, through chronically-implanted electrodes. Active sites for cardiovascular responses were found in the anterior midline cortex, but stimulation of the frontolateral sensory-motor cortex either did not elicit cardiovascular changes or elicited only small and variable changes when stimulated. The heart-rate response elicited was, in all cases, bradycardia. All blood pressure changes consisted of depressor responses. Stimulation of the lateral frontal cortex almost always resulted in increases in EMG activity, although many placements were observed in the medial frontal cortex that were unaccompanied by movement. In all cases in which depressor responses and bradycardia were elicited, increases in respiration rate and decreases in depth also occurred. The active area from which bradycardia and depressor responses were elicited forms the medial portion of the cortical projection area of the mediodorsal nucleus of the thalamus and thus may be involved in the autonomic accompaniments of the behavioral activities, i.e. learning and memory processes, associated with this nucleus.

Autonomic-somatic relationships in the rabbit (Oryctolagus cuniculus): Effects of hippocampal lesions

Rabbits received sham, cortical control, or dorsal hippocampal lesions and were subjected to simple Pavlovian conditioning. Eyeblink (EB), electromyographic (EMG), and heart rate (HR) CRs were assessed. Shock thresholds, HR URs, and free-field activity were also measured in selected animals. The acquisition of the EB and EMG CRs was not impaired in hippocampal lesioned animals, although hippocampal lesioned animals revealed impaired extinction performance on these measures. The magnitude of the HR CR was enhanced in hippocampectomized animals relative to control animals. Free-field activity was also greater in hippocampal lesioned animals, but shock thresholds and HR URs were unaffected by hippocampectomy. These findings suggest that “orienting” mechanisms may be impaired in hippocampal lesioned animals, resulting in an enhanced visceromotor response to stimulation which, under certain conditions, may affect somatomotor behaviors.

Divergencies in Pavlovian conditioned heart rate and eyeblink responses produced by hippocampectomy in the rabbit (Oryctolagus cuniculus).

An overhead conveyor includes an I-beam having a web and a pair of side flanges. A pair of opposed interconnected brackets are adapted to support a load, and a trolley wheel assembly is journalled upon each of the brackets and movably mounted upon the I-beam. The wheel assembly comprises a cylindrical shell having an annular shoulder flange at one end and an axial shaft extending from its other end and projected through the upper end of a bracket and secured thereto. A wheel having an axial bore and an internal annular flange has snugly fitted therein a pair of graphite alloy sleeve bearings which bear against the wheel flange, the wheel bearings being loosely disposed upon the shell. The shell flange loosely and retainingly engages one of the bearings and a washer on the shell shaft loosely and retainingly engages the other of the bearings against axial endwise movements respectively.

Cingulate damage attenuates conditioned bradycardia

New Zealand albino rabbits received either sham, midline cortical, lateral cortical, or combined medial-lateral cortical lesions, and were subjected to differential pavlovian conditioning in which tones of different frequencies served as the conditioning stimulus and a brief paraorbital electric shock train served as the unconditioned stimulus. Heart rate conditioned responses were recorded.d Damage to medial but not lateral neocortex abolished the heart rate conditioned response. These data suggest that midline cortical structures may modulate higher level processing of stimulus information.

Age-related changes in pavlovian conditioning: Central nervous system correlates

Two groups of New Zealand Albino rabbits, average age 6 and 40 months, were subjected to Pavlovian conditioning in which eyeblink (EB) and heart rate (HR) responses were assessed. At the end of conditioning biogenic amine content was assessed in several brain areas; dopamine receptor binding was assessed in caudate nucleus. Older animals revealed an impairment in EB conditioning relative to the young animals; the conditioned decelerative HR response was increased in magnitude in the old animals. Significant deficits in cortical norepinephrine and serotonin were observed in the old subjects, but in both old and young animals the magnitude of the bradycardia was positively correlated with forebrain norepinephrine and inversely correlated with serotonin concentration. Receptor binding experiments revealed that 3H-spiroperidol receptor density was decreased in caudate nuclei of old animals; however, receptor affinity was unaffected by age.

Orienting and classical conditioning in the rabbit (Oryctolagus cuniculus): effects of septal area lesions.

Rabbits with either sham or septal lesions recieved differential classical conditioning training in which tones of different frequencies served as CSs and paraorbital shock served as the UCS. Eyeblink (EB), EMG, and heart rate (HR) CRs were concommitantly assessed. Free field activity was also studied in selected animals. Animals with septal lesions revealed an impaired EB discrimination which resulted from increased responding to the CS-. These animals showed HR CRs of greater magnitude than sham animals. The HR discrimination was, however, unimpaired by septal lesions. Differential EMG activity also occurred, but was unaffected by septal destruction. Septal lesions resulted in greater free-field activity associated with visual stimulation, but no differences were obtained between septal and sham animals in the dark. It is suggested that these results were due to an enhanced reactivity to environmental stimulation, possibly related to dysfunctions of the orienting reflex.

CLASSICAL (PAVLOVIAN) CONDITIONING MODELS OF AGE-RELATED CHANGES IN ASSOCIATIVE LEARNING AND THEIR NEUROBIOLOGICAL SUBSTRATES

In the present paper, we will evaluate classical conditioning in both human and non-human animals as possible models for assersing the neurobiological mechanisms that underlie the learning and memory changes that occur in senescent organisms

Electrical stimulation of insular cortex elicits cardiac inhibition but insular lesions do not abolish conditioned bradycardia in rabbits

Conscious rabbits received electrical stimulation of insular and more posterior perirhinal cortex through chronically implanted electrodes. Active sites for cardiovascular responses were found in both anterior and posterior insular cortex as well as more posterior perirhinal regions. Although differential response topographies occurred related to anterior versus posterior insular cortex, all heart rate responses consisted of bradycardia. Pharmacological manipulations revealed that this bradycardia was due to a combination of vagal and sympatho-inhibitory mechanisms. Some posterior sites yielded pressor responses, and bradycardia which was sensitive to phentolamine, suggesting that the bradycardia in these instances was due to activation of the baroreceptor reflex. All other blood pressure changes were depressor responses. In a second experiment two different groups of rabbits with lesions of either anterior or posterior agranular insular cortex were compared with a third group of animals with sham lesions in a differential Pavlovian conditioning experiment. No lesion completely abolished the classically conditioned bradycardia associated with tone/shock contingencies. However, anterior insular lesions attenuated the magnitude of the conditioned bradycardia compared to the posterior and sham lesions. Control experiments suggested that this attenuation was due to the lesions effects on the conditioned stimulus/unconditioned stimulus association and not to its effects on unconditioned responding to the conditioned stimulus or unconditioned stimulus alone.