Lesions of the midline thalamic nuclei impair classical conditioning under partial but not continuous reinforcement conditions.

Rabbits with lesions of the midline thalamic nuclei were compared to rabbits with sham lesions on classical eyeblink (EB) and heart rate (HR) conditioning. Separate groups of sham and lesioned animals received either 25, 50 or 100% reinforcement with a periorbital shock unconditioned stimulus. Animals with lesions showed slightly impaired EB conditioning, compared to sham animals, under the partial but not continuous reinforcement schedules. Midline lesions also reduced the magnitude of the decelerative HR conditioned response under partial but not continuous reinforcement conditions. These findings suggest that the midline thalamic nuclei, like the mediodorsal nucleus of the thalamus, process information required for response selection under non-optimal learning conditions.

Controversies in clinical endodontics: Part 2. Single-appointment vs multiple-appointment treatment.

Endodontics is no stranger to controversies. For most of the first 50 years of the 20th century, a cloud hung over endodontic therapy of any kind, as problems with the focal infection theory were attributed to pulpless teeth causing a wide variety of maladies. Even recently, endodontic treatment has been attacked as being responsible for many chronic and acute illnesses, despite a multitude of information to the contrary. The controversies we discuss here, however, have nothing to do with the decision to treat, where we have no doubt as to the efficacy for therapy, but rather how such treatment should be rendered. Controversies have raged in the past, such as silver points vs gutta-percha as the canal filling material of choice, and to culture or not to culture, to name just two. The subjects discussed in this series of articles are those that have come up much more recently or older topics that have returned as areas for disagreement: (1) lateral canals: filling and significance; (2) single-appointment vs multiple-appointment treatment; (3) filling from the open position; and (4) calculation of working length. This article addresses single-appointment vs multiple-appointment treatment.

Amygdala-prefrontal interactions and conditioned bradycardia in the rabbit.

Damage to the amygdala or medial prefrontal cortex (mPFC) produces deficits in learned autonomic adjustments. Whether connections between these 2 structures are necessary for acquiring classically conditioned heart rate (HR) decelerations was studied. Connections between them were interrupted by unilateral lesions of the mPFC and amygdala, but on opposite sides of the brain. One experiment focused on the mPFC and amygdala central nucleus and a second on the mPFC and amygdala basolateral complex. Bilateral lesions of either structure disrupted the HR conditioned response. The response of the 2 crossed lesioned groups also was smaller than that of the sham-operated and unilateral lesioned groups, but significant conditioned stimulus-evoked bradycardia occurred in both, suggesting that although interactions between the amygdala and mPFC may normally occur during associative emotional responding, these connections are not necessary for the acquisition of conditioned bradycardia.

Lesions of the mediodorsal nucleus of the thalamus and classical eyeblink conditioning under less-than-optimal stimulus conditions: role of partial reinforcement and interstimulus interval.

Rabbits received lesions of the mediodorsal nucleus of the thalamus (MD) or sham lesions and were subjected to classical eyeblink (EB) and heart rate (HR) conditioning. Separate groups of sham and lesioned animals received either 50% or 25% reinforcement with a periorbital shock unconditioned stimulus. Other groups received an interstimulus interval of either 1.0 or 1.5 s. Animals with MD lesions acquired the EB conditioned response (CR) more slowly than sham-lesioned animals with either the 1.5-s ISI or with the 25% reinforcement schedule. The lesions had no significant effect on the HR CR, however. Results suggest that information processed by MD is relayed to the prefrontal cortex and is required for somatomotor response selection under nonoptimal learning conditions.

Mediodorsal thalamic lesions impair acquisition of an eyeblink avoidance response in rabbits.

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD) or sham lesions. These animals were compared on four sessions of instrumental avoidance conditioning, during which an eyeblink (EB) response during the presentation of a tone-conditioned stimulus prevented the occurrence of a paraorbital electric shock unconditioned stimulus. Lesions of MD retarded acquisition of the EB avoidance response, but did not affect asymptotic performance. Concomitant heart rate (HR) changes were little affected by MD lesions, although there was some evidence that such changes were slightly larger in MD-lesioned animals. These results suggest that MD participates in some general aspect of the learning process, perhaps by affecting other behavioral processes such as 'attention' or 'arousal'.

Multiple- and single-unit activity in area 32 (prelimbic region) of the medial prefrontal cortex during Pavlovian heart rate conditioning in rabbits.

Data from both brain lesion and brain stimulation experiments suggest that area 32 of the medial prefrontal cortex (prelimbic area) participates in the acquisition and/or expression of conditioned bradycardia. This report describes experiments designed to determine whether cells in this area of the brain exhibit changes in neural activity during classical conditioning that can be related to these learned heart rate changes. Thus, multiple- and single-unit activity was recorded from area 32 in rabbits during Pavlovian heart rate (HR) conditioning. In the first experiment, neuronal discharge recorded from chronically implanted multiple-unit electrodes in the superficial and deep layers of area 32 increased systematically in response to the presentation of tone conditioned stimuli (CS) paired with paraorbital electric shock as the unconditioned stimulus (US). This tone-evoked increase in multiple-unit activity (MUA) closely paralleled the acquisition of the decelerative HR conditioned response in animals that received paired CS/US presentations, but was of smaller amplitude in animals given unpaired CS/US presentations. Tone-evoked increases in MUA also occurred during tone-alone presentations prior to training, but this activity declined over trials. During extinction, CS-evoked MUA increases also declined over trials. These findings suggest that the CS-evoked increase in neuronal activity in area 32 was associatively produced. A second experiment examined the CS-evoked response of single units (n = 98) in area 32 during differential Pavlovian conditioning, in which one tone (CS+) was consistently followed by the paraorbital shock US and a second tone (CS-) was not. Four types of cells were found: (1) 32% of the cells studied showed no change in response to either CS+ or CS- presentation, (2) 48% showed CS-evoked increases to either CS+ or CS-, (3) 14% showed CS-evoked decreases to CS+ or CS-, and (4) 7% showed a biphasic response, in which an increase was followed by a decrease in activity. These cell types are similar to those previously found in the more dorsal anterior cingulate cortex (area 24) and frontal eyefields (area 8). However, a major difference between the present findings and those of our previous studies of areas 8 and 24 is that in several instances changes in CS-evoked activity in area 32 were significantly greater in response to the CS- than to the CS+, suggesting that some cells in the prelimbic region may code the absence of aversive stimulation, that is, a period of relative "safety."

Neuronal activity in the midline thalamic nuclei during Pavlovian heart rate conditioning.

Multiple unit activity (MUA) was recorded from chronically implanted electrodes in the midline nuclei of the thalamus in rabbits receiving either Pavlovian heart rate (HR) conditioning, or explicitly unpaired presentations of the conditioned stimulus (CS) and unconditioned stimulus. Animals receiving paired training demonstrated increases in CS-evoked MUA compared to animals receiving unpaired training; however, these increases were small and only weakly correlated with the HR conditioned response, suggesting that the midline nuclei are only minimally, if at all, involved in associative learning.

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.

Differential and reversal Pavlovian conditioning in rabbits with mediodorsal thalamic lesions: assessment of heart rate and eyeblink responses.

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD), or sham lesions. These animals were compared on acquisition and reversal of a Pavlovian conditioned discrimination task in which tones were the conditioned stimuli and paraorbital electric shock was the unconditioned stimulus. Eyeblink and heart rate conditioned responses were assessed. Lesions of MD impaired the reversal, but not original acquisition, of the eyeblink discrimination. Heart rate discrimination was somewhat impaired during both acquisition and reversal. There were no differences between lesion and sham animals on control measures assessing general activity, somatomotor shock thresholds, or heart rate unconditioned responses.

Mediodorsal thalamic lesions and Pavlovian conditioning of heart rate and eyeblink responses in the rabbit.

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD) or sham lesions. These animals were compared on 4 sessions of Pavlovian eyeblink and heart rate conditioning, in which a tone was the conditioned stimulus and a paraorbital electrical shock was the unconditioned stimulus. Lesions of MD retarded acquisition of the eyeblink conditioned response and abolished the late-occurring tachycardiac component of the heart rate conditioned response. The data are compatible with previous experiments suggesting that MD participates in the sympathetic control associated with somatomotor learning.

Neuronal activity in the mediodorsal and intralaminar nuclei of the dorsal thalamus during classical heart rate conditioning.

Multiple unit activity (MUA) was recorded from chronically implanted electrodes in either the mediodorsal (MD) or the intralaminar (IL) nuclei of the dorsal thalamus in separate groups of rabbits during (a) habituation of the cardiac orienting reflex, (b) Pavlovian heart rate (HR) conditioning, and (c) extinction of the HR conditioned response (CR). Other animals with similar recording electrodes received explicitly unpaired presentations of the conditioned stimulus (CS) and unconditioned stimulus (US). The cardiac orienting reflex and the HR CR consisted of bradycardia. However, tone-evoked tachycardia was obtained in animals that received CS/US unpaired presentations. MUA evoked by the CS consisted of a short latency (20-40 ms) increase under all conditions, which reached its maximum 200-300 ms after CS onset. This response habituated greatly during tone-alone pretraining, but was considerably greater in the paired than unpaired group during the later trials of conditioning in animals with MD, but not IL, placements. Instead, a longer latency increase (greater than 500 ms) in MUA occurred in the paired but not in the unpaired animals in the IL group. The MUA increases in both instances, including the early, short latency increase in the MD group, and the longer latency increase in the IL group, were trial-related, and declined to pretraining levels during extinction, indicating that these neuronal changes had an associative basis. These findings suggest that neuronal activity in both MD and IL is related to the early events involved in Pavlovian conditioning, but that the relative roles of these two closely related thalamic nuclei in associative learning must be somewhat different.

Role of the prefrontal--thalamic axis in classical conditioning.

The major conclusion to be drawn from the above-described research on the role of the PFCag in classical conditioning is obviously that it plays a primary and perhaps necessary role in the establishment of visceral cues associated with exposure to classical conditioning contingencies. Specifically, these visceral changes appear to be of an inhibitory character. This is significant, since we have postulated that inhibitory cardiac changes invariably accompany initial processing of sensory stimuli for informational value. Such visceral changes are thus not epiphenomena associated with other simultaneously occurring physiological events. A variety of lesion experiments implicate the PFCm as a central structure in this process, since damage to this area greatly attenuates, and in the case of hypothalamic knife cuts, completely eliminates learned bradycardia. Neuroanatomical tract-tracing experiments revealed that the PFCm and lag have direct projections to the NTS and DVM in the dorsomedial medulla and the nucleus ambiguous in the ventral medulla, all of which provide medullary output control of visceral activities. The nucleus ambiguous and DVM have been specifically implicated in vagal control in the rabbit (Ellenberger et al., 1983). Electrical stimulation of the PFCm provides additional evidence that this area of the brain participates in parasympathetic activities, including cardiac inhibition, since stimulation of the entire MD projection cortex, including the PFCm, produces HR decelerations accompanied by depressor responses. However, since lesions of the Iag produced relatively little effect on conditioned bradycardia, this part of the PFCag does not appear to play a major role in the development of conditioned bradycardia. Electrophysiological recording studies, including both multiple unit as well as extracellular single unit studies reinforce these conclusions. A short latency (40-180 msec) CS-evoked increase in MUA was recorded from cells in both the dorsomedial as well as central PFCm. The magnitude of these CS-evoked neuronal changes (a) was correlated with the magnitude of concomitantly occurring conditioned bradycardia; (b) was trial-related; (c) was not obtained in a similar pseudoconditioning group; and (d) declined to pretraining levels during subsequent experimental extinction. Similar, but not identical, CS-evoked changes in neuronal activity were recorded from MD. Although tone-evoked increases in MUA were also obtained from the Iag, this activity did not show the characteristics of associative learning. Single unit analysis also suggests the importance of the PFCm in elicitation of conditioned bradycardia.(ABSTRACT TRUNCATED AT 400 WORDS)

Prefrontal projections to the medial nuclei of the dorsal thalamus in the rabbit.

Retrograde transport of horseradish peroxidase (HRP) from the mediodorsal (MD), ventromedial (VM), ventroposterior (VP) and intralaminar (IL) nuclei of the dorsal thalamus revealed a topographical pattern of efferents from the frontal cortex. MD injections labeled the midline and insular regions of the prefrontal cortex (Pfc), including the anterior limbic, and most ventral part of the precentral agranular Pfc, as well as the agranular insular cortex. VM injections labeled only the most dorsomedial part of the granular insular cortex, whereas IL and VP injections labeled the dorsal precentral agranular Pfc and a strip of cortex that extended laterally across the superior aspect of the forceps minor. The IL injections also labeled cells that extended ventrally into the granular and agranular insular areas.

Parasagittal thalamic knife-cuts and cardiac changes.

Rabbits received knife-cuts either medial or lateral to the mediodorsal nucleus of the thalamus (MD), and had stimulating electrodes implanted in MD. These animals were compared to animals with sham knife-cuts for (1) the cardiac component of the orienting reflex (OR) to novel tone stimuli; (2) differential pavlovian heart rate (HR) conditioning, utilizing tones of different frequencies as CSs and paraorbital shock as the US; and (3) heart rate, blood pressure, electromyographic activity, and respiration changes elicited by electrical stimulation in MD. Medial knife-cuts led to faster habituation of the OR, and to somewhat attenuated HR conditioning, but had no effect on stimulation-elicited changes. Lateral knife-cuts, on the other hand, enhanced the stimulation-elicited HR response, but had little effect on the HR OR. The HR CR was unaffected by lateral cuts during an initial conditioning session, but was attenuated in both knife-cut groups during a second session, compared to sham control animals.

Leg flexion conditioning in the rat: its advantages and disadvantages as a model system of age-related changes in associative learning.

Twelve- and 28-month-old Fischer 344 rats of both sexes received five 60-trial sessions of Pavlovian conditioning in which the CS was a 75 dB, 10,000 Hz tone, and the US was a 0.5-mA, 0.5-sec duration footshock. Right foreleg flexion was measured as the conditioned response (CR). Other animals received a random sequence of unpaired tones and footshock and served as pseudoconditioning control groups. Interstimulus intervals (ISIs) of 1.5 and 3.5 sec were studied. The longer ISI resulted in higher rates of responding in both the conditioning and pseudoconditioning groups. However, with the exception of the young males, all animals showed significantly higher levels of responding in the conditioning groups. Females also showed faster acquisition and higher levels of responding than males. A significant sex by age by sessions interaction occurred, suggesting that old males may be somewhat retarded in acquiring the leg flexion CR compared to the other groups of animals. Old males were also slower to reach a criterion of 5 successive CRs than either young males or young or old females.

Parasagittal thalamic knife cuts retard Pavlovian eyeblink conditioning and abolish the tachycardiac component of the heart rate conditioned response.

Rabbits received parasagittal knife cuts lateral to the mediodorsal nucleus of the thalamus (MD), severing afferents and efferents to and from the prefrontal cortex. These animals were compared to sham animals in a Pavlovian eyeblink and heart rate conditioning experiment in which a tone was the conditioned stimulus (CS) and paraorbital electrical shock was the unconditioned stimulus (US). Knife cuts retarded acquisition of the eyeblink conditioned response (CR), and abolished the late-occurring tachycardiac component of the heart rate CR. These data are compatible with previous experiments which suggest that MD participates in the sympathetic control associated with somatomotor learning.

Age-related changes in associative learning: studies in rabbits and rats.

Two experimental models for studying age-related changes in associative learning are described. One involves classical (Pavlovian) conditioning of eyeblink and heart rate in the rabbit. The second involves Pavlovian leg flexion and heart rate conditioning in the rat. Advantages and disadvantages of each model are discussed. Results with both models suggest differential effects of aging on acquisition of autonomic and somatomotor responses, thus underlining the utility of assessing multiple response systems to adequately characterize age-related changes in learning and memory.

Classically conditioned cardiac responses in "old" and "young" Fischer 344 rats.

Male and female Fischer 344 rats, 12 or 26-28 months of age, received two sessions of Pavlovian heart rate conditioning, and were compared with same-sex and same-age controls receiving unpaired presentations of the tone conditional stimulus (CS) and the shock unconditional stimulus (US). Older rats of both sexes demonstrated slower acquisition of the heart rate (HR) conditioned response (CR), and smaller magnitude changes than did the younger animals. Control experiments in 6-, 12-, 24-, and 30-month-old animals indicated that these differences were not due to an impaired sensitivity to the CS or US in the older animals. Results are discussed in terms of their implications for use of this animal model in investigations of age-related deficits in associative learning.

Mediodorsal thalamic lesions impair differential Pavlovian heart rate conditioning.

Rabbits with bilateral ibotenic acid or vehicle injections into the mediodorsal nucleus of the thalamus (MD) received differential Pavlovian heart rate (HR) conditioning in which tones of different frequencies served as conditioned stimuli (CSs) and a paraorbital electric shock train was the unconditioned stimulus (US). Rabbits with ibotenic acid lesions of MD demonstrated impaired differential HR conditioning, due primarily to enhanced responding to the non-reinforced CS-. The magnitude of the HR conditioned response (CR) was somewhat enhanced in lesioned animals compared to vehicle control animals, as was the HR component of the orienting response to non-reinforced tones. These results are consistent with a role for MD in some aspect of association formation, possibly through its mediation of the late-occurring sympathetic component of the conditioned HR response. These data also demonstrate that MD does not participate in the vagal drive that produces Pavlovian conditioned bradycardia.