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

A neural model of hippocampal-striatal interactions in associative learning and transfer generalization in various neurological and psychiatric patients

Building on our previous neurocomputational models of basal ganglia and hippocampal region function (and their modulation by dopamine and acetylcholine, respectively), we show here how an integration of these models can inform our understanding of the interaction between the basal ganglia and hippocampal region in associative learning and transfer generalization across various patient populations. As a common test bed for exploring interactions between these brain regions and neuromodulators, we focus on the acquired equivalence task, an associative learning paradigm in which stimuli that have been associated with the same outcome acquire a functional similarity such that subsequent generalization between these stimuli increases. This task has been used to test cognitive dysfunction in various patient populations with damages to the hippocampal region and basal ganglia, including studies of patients with Parkinson's disease (PD), schizophrenia, basal forebrain amnesia, and hippocampal atrophy. Simulation results show that damage to the hippocampal region-as in patients with hippocampal atrophy (HA), hypoxia, mild Alzheimer's (AD), or schizophrenia-leads to intact associative learning but impaired transfer generalization performance. Moreover, the model demonstrates how PD and anterior communicating artery (ACoA) aneurysm-two very different brain disorders that affect different neural mechanisms-can have similar effects on acquired equivalence performance. In particular, the model shows that simulating a loss of dopamine function in the basal ganglia module (as in PD) leads to slow acquisition learning but intact transfer generalization. Similarly, the model shows that simulating the loss of acetylcholine in the hippocampal region (as in ACoA aneurysm) also results in slower acquisition learning. We argue from this that changes in associative learning of stimulus-action pathways (in the basal ganglia) or changes in the learning of stimulus representations (in the hippocampal region) can have similar functional effects.

Oscillatory brain states and learning: Impact of hippocampal theta-contingent training

Eyeblink classical conditioning is a relatively simple form of associative learning that has become an invaluable tool in our understanding of the neural mechanisms of learning. When studying rabbits in this paradigm, we observed a dramatic modification of learning rate by conducting training during episodes of either hippocampal theta or hippocampal non-theta activity as determined by on-line slow-wave spectral analysis. Specifically, if animals were given trials only when a computer analysis verified a predominance of slow-wave oscillations at theta frequencies (3-8 Hz), they learned in half as many trials as animals trained during non-theta hippocampal activity (58 vs. 115). This finding provides important evidence from awake, behaving animals that supports recent advances in our knowledge of (i) brain sites and neurobiological mechanisms of learning and memory, specifically hippocampus and theta oscillations, (ii) the biological plausibility of current models of hippocampal function that posit important roles for oscillatory potentials, and (iii) the design of interfaces between biological and cybernetic (electronic) systems that can optimize cognitive processes and performance.

Hippocampal theta oscillations and classical conditioning

Studies are reviewed that support a hypothesized role for hippocampal theta oscillations in the neural plasticity underlying behavioral learning. Begun in Richard F. Thompson's laboratory in the 1970s, these experiments have documented a relationship between free-running 3- to 7-Hz hippocampal slow waves (theta) and rates of acquisition in rabbit classical nictitating membrane (NM) conditioning. Lesion and drug manipulations of septohippocampal projections have affected NM and jaw movement conditioning in ways consistent with a theta-related brain state being an important modulator of behavioral acquisition. These findings provide essential empirical support for the recently developed neurobiological and computational models that posit an important role for rhythmic oscillations (such as theta) in cellular plasticity and behavioral learning.

Eyeblink classical conditioning: hippocampal formation is for neutral stimulus associations as cerebellum is for association-response

Extensive evidence has been amassed that the cerebellum, hippocampus, and associated circuitry are activated during classical conditioning of the nictitating membrane/eyeblink response. In this article, the authors argue that the cerebellum is essential to all eyeblink classical conditioning paradigms. In addition, the septohippocampal system plays a critical role when the classical conditioning paradigm requires the formation of associations in addition to the simple association between the conditioned and unconditioned stimuli. When only a simple conditioned stimulus--unconditioned stimulus association is needed, the septohippocampal system has a more limited, modulatory role. The neutral stimulus association versus simple association-response distinction is one of the ways in which declarative or relational memory can be separated from nondeclarative or nonrelational memory in classical conditioning paradigms.

Scopolamine disruption of behavioral and hippocampal responses in appetitive trace classical conditioning

Twelve young rabbits (3-6 months; Oryctolagus cuniculus) were classically conditioned in a trace jaw movement paradigm (300 ms tone, 450 ms trace, 200 ms intraoral water) after implantation of electrodes into area CA1 of dorsal hippocampus. Rabbits were divided into two groups and administered either 0.5 mg/kg scopolamine hydrobromide (HBr) or 0.5 mg/kg scopolamine methylbromide (MBr) subcutaneously before daily training sessions. Rabbits given HBr took significantly more trials to reach a behavioral criterion of eight conditioned responses in any nine consecutive trials than rabbits given MBr (P = 0.03). Conditioned, but not unconditioned, rhythmic jaw movement responses of the HBr group were of a lower frequency (Hz) than those of MBr rabbits (P = 0.02). The magnitude of hippocampal conditioning-related responses across the first 3 days of training was significantly smaller for HBr rabbits than for MBr rabbits (P = 0.02). These effects of central cholinergic blockade are similar to those reported for undrugged aging rabbits trained in the same paradigm (Seager MA, Borgnis RL, Berry SD. Neurobiol. Aging 1997;18(6):631 639).

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 lesion's effects on the conditioned stimulus/unconditioned stimulus association and not to its effects on unconditioned responding to the conditioned stimulus or unconditioned stimulus alone.

Vasopressin and ACTH analogs affect orienting but not activity in rabbits

Two experiments were conducted in which albino rabbits were tested for cardiac Orienting Reflexes to novel tones and open field locomotor activity in a novel environment. In Experiment I subcutaneous doses of desglycinamide-arginine8-vasopressin (DGAVP; 25 micrograms/kg) and ACTH fragment 4-10 (250 micrograms/kg) altered tonic heart rate but did not affect bradycardiac Orienting Reflexes, activity, habituation or retention of habituation. In Experiment II a lower dose of DGAVP (5 micrograms/kg) enhanced the magnitude of cardiac orienting reflexes when administered 60 min before an initial orienting test. Both DGAVP and a lower dose of ACTH4-10 (50 micrograms/kg) administered before the first orienting test enhanced cardiac orienting and delayed habituation during a second (retention) test conducted following saline treatment; neither peptide had any effect on orienting when administered before the retention test. The lower doses of the peptides also failed to affect activity, habituation of activity or retention of habituation. These data suggest that low doses of DGAVP and ACTH4-10 affect stimulus processing and attention, but not more generalized responses to environmental novelty, in rabbits.

Comparison between the behavioural effects of septal and hippocampal lesions: a review

The literature on the behavioural effects of septal and hippocampal lesions is classified according to behavioural paradigm. The effects of the two kinds of lesion are summarized and compared to each other. A 'septo-hippocampal syndrome,' consisting of the effects common to both lesions, is delineated, and divergences between the effects of the two lesions are noted.

Hippocampal lesions and Pavlovian cardiovascular conditioning

New Zealand albino rabbits received either sham, cortical control, or hippocampal lesions and were subjected to differential Pavlovian conditioning in which tones of different frequencies served as conditional stimuli (CSs), and a brief paraorbital electric shock train served as the unconditional stimulus. Heart rate (HR), blood pressure (BP), and electromyographic (EMG) conditional responses (CRs) were recorded. Animals with cortical and hippocampal damage or animals with cortical damage alone revealed attenuated bradycardiac CRs, but HR CRs of the former two groups did not significantly differ. BP changes were minimal; reliable differences were not obtained between CS+ and CS-. However, these changes consisted of small but reliable depressor responses, which were not affected by either cortical or hippocampal lesions. Few EMG CRs were obtained. These data, combined with those of previous experiments, suggest that forebrain structures may modulate higher level processing of stimulus information, perhaps in terms of assessing the biological significance of such stimulation.

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.

Forebrain norepinephrine and serotonin concentrations and cardiac conditioning in normal rabbits and rabbits with septal lesions

Septal lesioned or sham operated rabbits were subjected to two days of differential Pavlovian (classical) heart rate (HR) conditioning in which tones served as the conditioned stimuli and paraorbital electric shock served as the unconditioned stimulus. After completion of training, norepinephrine (NE) and serotonin (5-HT) concentrations were determined in the hippocampus and neocortex to determine if lesion-induced amine depletion was related to changes in HR conditioned response (CR). The magnitude of the bradycardiac CR was increased by septal lesions during the initial session but during the second session. several lesioned animals revealed accelerative HR changes resulting in an attenuated HR CR in the septal damaged group. The more accelerative HR responding in the septal lesioned animals was accompanied by increased EMG activity which appeared to be related to damage to more posteroventral areas of the septum. Septal lesions produced a depletion of forebrain NE of approximately 30%, but NE concentrations did not appear to be specifically related to lesion-induced changes in the HR CR. However, both NE and 5-HT concentrations were correlated with the magnitude of the HR CR in intact, but not in lesioned, animals.

Lateral hypothalamic lesions: effects on Pavlovian cardiac and eyeblink conditioning in the rabbit

Rabbits received either bilateral lesions of the far lateral hypothalamus (LH), or sham-operate lesions. After recovery from surgery the animals were exposed to Pavlovian conditioning in which eyeblink (EB), electromyographic (EMG), and heart rate (HR) conditioned responses (CRs) were recorded. Adaptation, acquisition and extinction training were studied. Subsequent to conditioning, free-field activity, paraorbital electric shock thresholds, and HR unconditioned responses (URs) were measured. At the end of behavioral testing forebrain norepinephrine (NE), dopamine (DA), an serotonin (5-HT) concentrations were assessed. Animals with LH lesions did not differ from controls on EB responding during either adaptation or acquisition. However, lesioned animals revealed more EB CRs during extinction than control animals. EMG CRs were minimal in both groups of animals. Lesioned animals showed smaller HR changes than control animals during all phases of training, although HR CRs tended to consist of bradycardia in all animals. No differences were observed in either shock thresholds or open field activity as a result of LH lesions. Cortical and hippocampal 5-HT and NE concentrations were significantly depleted in lesioned as compared to control animals. These findings were interpreted within the framework of the phasic control of attention by midbrain modulation of forebrain structures; however, the interruption of downward going fibers which mediate brain stem cardiac control cannot be ruled out.

Adrenal demedullation and peripheral 6-OHDA administration in the rabbit: effects on body weight, general activity and cardiovascular responsivity

Two experiments were undertaken to determine the effects of systemic 6-hydroxydopamine (6-OHDA) administration and adrenal enucleation on body weight, brain and heart norepinephrine, general activity, and cardiovascular responsivity in the rabbit. 6-Hydroxydopamine produced long lasting, dose related decreases in baseline blood pressure (BP) and in pressor responses following electric shock. Partial reversal of the BP changes was observed in nonenucleated animals tested 20 days after 6-OHDA administration but not in enucleated animals tested 5, 10 or 20 days post-injection or in control animals tested 5, 10 or 15 days after vehicle injection. Although 6-OHDA administration produced transient weight loss and activity decrements, no long term weight or activity changes were observed.

Effect of avian basal forebrain lesions, including septum, on heart rate conditioning

The possible involvement of basal telencephalic structures in visually conditioned heart rate change (established by pairing light and foot-shock) was studied in 156 pigeons by evaluating conditioning performance following lesions of the septum or lobus parolfactorius. Extensive destruction of the septal complex had no effect on either the orienting response or the development of the conditioned response. Lesions of the lobus parolfactorius did not affect the orienting response or overall conditioned response levels, but it did slightly prolong the latency of the conditioned heart rate change. It is concluded that the septum, despite its being cardioactive, is not involved in conditioned heart rate change and that the lobus parolfactorius participates minimally. Thus, of the principal limbic structures of the avian telencephalon, only the amygdalar homologue appears critical in this defensive conditioning task [3].