Disruption of rabbit (Oryctolagus cuniculus) nictitating membrane conditioning by posttrial electrical stimulation of hippocampus

GABAergic neurons in the medial septum-diagonal band of Broca (MSDB) are important for acquisition of the classically conditioned eyeblink response

The medial septum and diagonal band of Broca (MSDB) influence hippocampal function through cholinergic, GABAergic, and glutamatergic septohippocampal neurons. Non-selective damage of the MSDB or intraseptal scopolamine impairs classical conditioning of the eyeblink response (CCER). Scopolamine preferentially inhibits GABAergic MSDB neurons suggesting that these neurons may be an important modulator of delay CCER, a form of CCER not dependent on the hippocampus. The current study directly examined the importance of GABAergic MSDB neurons in acquisition of delay CCER. Adult male Sprague-Dawley rats received either a sham (PBS) or GABAergic MSDB lesion using GAT1-saporin (SAP). Rats were given two consecutive days of delay eyeblink conditioning with 100 conditioned stimulus-unconditioned stimulus paired trials. Intraseptal GAT1-SAP impaired acquisition of CCER. The impairment was observed on the first day with sham and lesion groups reaching similar performance by the end of the second day. Our results provide evidence that GABAergic MSDB neurons are an important modulator of delay CCER. The pathways by which MSDB neurons influence the neural circuits necessary for delay CCER are discussed.

Why trace and delay conditioning are sometimes (but not always) hippocampal dependent: a computational model

A recurrent-network model provides a unified account of the hippocampal region in mediating the representation of temporal information in classical eyeblink conditioning. Much empirical research is consistent with a general conclusion that delay conditioning (in which the conditioned stimulus CS and unconditioned stimulus US overlap and co-terminate) is independent of the hippocampal system, while trace conditioning (in which the CS terminates before US onset) depends on the hippocampus. However, recent studies show that, under some circumstances, delay conditioning can be hippocampal-dependent and trace conditioning can be spared following hippocampal lesion. Here, we present an extension of our prior trial-level models of hippocampal function and stimulus representation that can explain these findings within a unified framework. Specifically, the current model includes adaptive recurrent collateral connections that aid in the representation of intra-trial temporal information. With this model, as in our prior models, we argue that the hippocampus is not specialized for conditioned response timing, but rather is a general-purpose system that learns to predict the next state of all stimuli given the current state of variables encoded by activity in recurrent collaterals. As such, the model correctly predicts that hippocampal involvement in classical conditioning should be critical not only when there is an intervening trace interval, but also when there is a long delay between CS onset and US onset. Our model simulates empirical data from many variants of classical conditioning, including delay and trace paradigms in which the length of the CS, the inter-stimulus interval, or the trace interval is varied. Finally, we discuss model limitations, future directions, and several novel empirical predictions of this temporal processing model of hippocampal function and learning.

Hippocampus, space, and memory

We examine two different descriptions of the behavioral functions of the hippocampal system. One emphasizes spatially organized behaviors, especially those using cognitive maps. The other emphasizes memory, particularly working memory, a short-term memory that requires iexible stimulus-response associations and is highly susceptible to interference. The predictive value of the spatial and memory descriptions were evaluated by testing rats with damage to the hippocampal system in a series of experiments, independently manipulating the spatial and memory characteristics of a behavioral task. No dissociations were found when the spatial characteristics of the stimuli to be remembered were changed; lesions produced a similar deficit in both spatial and nonspatial test procedures, indicating that the hippocampus was similarly involved regardless of the spatial nature of the task. In contrast, a marked dissociation was found when the memory requirements were altered. Rats with lesions were able to perform accurately in tasks that could be solved exclusively on the basis of reference memory. They performed at chance levels and showed no signs of recovery even with extensive postoperative training in tasks that required working memory. In one experiment all the characteristics of the reference memory and working memory procedures were identical except the type of memory required. Consequently, the behavioral dissociation cannot be explained by differences in attention, motivation, response inhibition, or the type of stimuli to be remembered. As a result of these experiments we propose that the hippocampus is selectively involved in behaviors that require working memory, irrespective of the type of material (spatial or nonspatial) that is to be processed by that memory.

Partial hippocampal pyramidal cell loss alters behavior in rats: implications for an animal model of schizophrenia

A putative biological substrate of schizophrenia involves cellular pathology within the hippocampus. While hippocampal dysfunction is associated with impaired learning and memory, schizophrenics have been observed to acquire simple conditioned reflexes at rates superior to controls. The present study evaluates the acquisition of shuttlebox avoidance responses in animals with partial damage to hippocampus. Intraventricular microinjections of kainic acid (0.5 or 1.5 nM) were utilized to partially destroy the pyramidal cell population. Animals in the high dosage group acquired the response at rates superior to controls; the low dosage group performed at an intermediate level. Consequently, partial loss of pyramidal neurons may be sufficient to significantly alter simple acquisition. Results are discussed in reference to the "embryological hypothesis" of schizophrenia and mechanisms for induction of schizophrenic behavior in intractable seizure disorders are considered.

Altered activity in the hippocampus is more detrimental to classical conditioning than removing the structure

Hippocampal ablation has no effect on the acquisition of the rabbit's classically conditioned nictitating membrane response. Systemic administration of scopolamine, which alters hippocampal neuronal activity, severely retards acquisition of the conditioned response in normal animals and those with cortical ablations. In animals with hippocampal ablations, however, scopolamine has no effect on conditioning. These findings suggest that altered neuronal activity in the hippocampus is more detrimental to conditioning than removing the structure.

The nictitating membrane response: an electrophysiological study of the abducens nerve and nucleus and the accessory abducens nucleus in rabbit

The nictitating membrane response to periocular electrostimulation was investigated in anesthetized and paralyzed rabbits. Recordings from the abducens nerve, which carries the fibers innervating retractor bulbi muscles that are primarily involved in this reflex, showed two distinct volleys to effective stimulation: a short duration volley with a minimum latency of approximately 4 ms and a longer duration volley beginning approximately 10 ms after stimulus onset and lasting up to 25 ms. Recordings of antidromically evoked field potentials via microelectrodes indicated large responses to abducens nerve stimulation in the vicinity of the accessory abducens nucleus. Single unit recordings from the accessory abducens nucleus produced spike trains with minimal latencies of 3.7-5 ms to eyeshock. The latency of spike discharge was inversely related to stimulus current. Units in the abducens nucleus did not show stimulus-elicited spiking, suggesting that the accessory abducens, but not the abducens, is primarily involved in the reflex pathway. Transverse knife cuts which separated caudal areas of the sensory trigeminal complex from the accessory abducens nucleus did not attenuate the efferent volley to suprathreshold stimulation, suggesting that more rostral components of the trigeminal complex are primarily involved in the reflex pathway.

Augmentation of latent inhibition by electrical stimulation of hippocampus

The effects of low-level hippocampal stimulation on the development of latent inhibition were investigated employing classical conditioning of the nictitating membrane response of rabbits. Four groups were given either (a) no preexposure of the to-be-CS, (b) preexposure, (c) preexposure overlapped by hippocampal stimulation or (d) preexposure overlapped by cortical stimulation, followed by 300 conditioning trials for each group. After conditioning, the hippocampal group was divided into two groups designated HS-1 and HS-2 by means of a post-hoc test for stimulation effects. Conditioning was found to be retarded in all three preexposed groups, with significantly greater retardation (augmentation of latent inhibition) in Group HS-2. The results support a general conception that the hippocampal stimulation produced its effects through the modulation of sensory input.

Hippocampal cellular plasticity during extinction of classically conditioned nictitating membrane behavior

Hippocampal unit responses were recorded during extinction of classically conditioned nictitating membrane (NM) behavior in the rabbit. Prior studies have shown that during the acquisition phase of nictitating membrane conditioning, the frequency of hippocampal cell firing increases at a faster rate (across trials) than learned behavior. Results reported here show that, during the early phases of extinction, conditioned hippocampal unit responses decrement at a faster rate than learned NM behavior. Furthermore, only certain components of the conditioned hippocampal unit response display robust spontaneous recovery across successive days of extinction training. In all, results show that changes in the activity of hippocampal neurons predict changes in learned behavior over trials of conditioning and extinction, and demonstrate that hippocampal cellular activity is particularly sensitive to stimulus configurations or environmental contingencies that produce changes in behavior.

Attenuation of latent inhibition by electrical stimulation of hippocampus

The effects of hippocampal stimulation on the development and expression of latent inhibition were investigated employing classical conditioning of the nictitating membrane response of rabbits. Hippocampal stimulation overlapping conditioned stimulus preexposure produced attenuation of latent inhibition, as did stimulation presented during conditioning after preexposure. However, hippocampal stimulation during conditioning in the absence of preexposure had little effect. The results support the hypotheses that in classical conditioning a major function of the hippocampus is the modulation of sensory input, especially input involved in the learned irrelevance of stimuli.

Stimulation of abducens nucleus supports classical conditioning of the nictitating membrane response

The acquisition and terminal performance of a classical conditioning group compared with a control group indicated that extension of the nictitating membrane elicited by direct electrical stimulation of the abducens nucleus was successfully conditioned to a previously neutral stimulus. The conditioning so obtained was associative and not due to such nonassociative factors as sensitization, pseudo-conditioning, or alteration in base-rate responding.