Sensory system development influences the ontogeny of eyeblink conditioning

Disruption of rat deep cerebellar perineuronal net alters eyeblink conditioning and neuronal electrophysiology

The perineuronal net (PNN) is a specialized type of extracellular matrix found in the central nervous system. The PNN forms on fast spiking neurons during postnatal development but the ontogeny of PNN development has yet to be elucidated. By studying the development and prevalence of the PNN in the juvenile and adult rat brain, we may be able to understand the PNN's role in development and learning and memory. We show that the PNN is fully developed in the deep cerebellar nuclei (DCN) of rats by P18. By using enzymatic digestion of the PNN with chondroitinase ABC (ChABC), we are able to study how digestion of the PNN affects cerebellar-dependent eyeblink conditioning in vivo and perform electrophysiological recordings from DCN neurons in vitro. In vivo degradation of the PNN resulted in significant differences in eyeblink conditioning amplitude and area. Female animals in the vehicle group demonstrated higher levels of conditioning as well as significantly higher post-probe conditioned responses compared to males in that group, differences not present in the ChABC group. In vitro, we found that DCN neurons with a disrupted PNN following exposure to ChABC had altered membrane properties, fewer rebound spikes, and decreased intrinsic excitability. Together, this study further elucidates the role of the PNN in cerebellar learning in the DCN and is the first to demonstrate PNN degradation may erase sex differences in delay conditioning.

Changes in membrane properties of rat deep cerebellar nuclear projection neurons during acquisition of eyeblink conditioning

Previous studies have shown changes in membrane properties of neurons in rat deep cerebellar nuclei (DCN) as a function of development, but due to technical difficulties in obtaining viable DCN slices from adult animals, it remains unclear whether there are learning-related alterations in the membrane properties of DCN neurons in adult rats. This study was designed to record from identified DCN cells in cerebellar slices from postnatal day 25-26 (P25-26) rats that had a relatively mature sensory nervous system and were able to acquire learning as a result of tone-shock eyeblink conditioning (EBC) and to document resulting changes in electrophysiological properties. After electromyographic electrode implantation at P21 and inoculation with a fluorescent pseudorabies virus (PRV-152) at P22-23, rats received either four sessions of paired delay EBC or unpaired stimulus presentations with a tone conditioned stimulus and a shock unconditioned stimulus or sat in the training chamber without stimulus presentations. Compared with rats given unpaired stimuli or no stimulus presentations, rats given paired EBC showed an increase in conditioned responses across sessions. Whole-cell recordings of both fluorescent and nonfluorescent DCN projection neurons showed that delay EBC induced significant changes in membrane properties of evoked DCN action potentials including a reduced after-hyperpolarization amplitude and shortened latency. Similar findings were obtained in hyperpolarization-induced rebound spikes of DCN neurons. In sum, delay EBC produced significant changes in the membrane properties of juvenile rat DCN projection neurons. These learning-specific changes in DCN excitability have not previously been reported in any species or task.

Sensory system development influences the ontogeny of hippocampal associative coding and trace eyeblink conditioning

Until recently, it was believed that hippocampal development was the primary rate-limiting factor in the developmental emergence of hippocampal forms of learning, such as trace eyeblink conditioning (EBC). Indeed, hippocampal neuronal activity shows an age-related increase in both complexity and task responsiveness during trace EBC. However, recent work from our laboratory suggests that sensory system development may also play a role. Training with the earlier-developing somatosensory system results in an earlier emergence of trace EBC in rats, suggesting that the development of sensory input to the hippocampus may influence the development of trace EBC. The goal of the current study was to examine the activity of hippocampal CA1 pyramidal cells during acquisition of trace EBC with an early-developing somatosensory CS. Rat pups were trained with a vibration CS on postnatal days (P) 17-19, P21-23, and P24-26 while CA1 pyramidal cell activity was recorded. Results indicated that CA1 neurons show an age-related increase in responsiveness to trial events. Although the magnitude of neuronal responding showed age-related increases in activity, all three age groups demonstrated learning-related increases in firing rate magnitude and peaks in firing rate were evident both at CS onset and offset. These findings suggest that the ontogeny of trace eyeblink conditioning is related to both hippocampal and sensory system development.

Developmental Changes in Hippocampal CA1 Single Neuron Firing and Theta Activity during Associative Learning

Hippocampal development is thought to play a crucial role in the emergence of many forms of learning and memory, but ontogenetic changes in hippocampal activity during learning have not been examined thoroughly. We examined the ontogeny of hippocampal function by recording theta and single neuron activity from the dorsal hippocampal CA1 area while rat pups were trained in associative learning. Three different age groups [postnatal days (P)17-19, P21-23, and P24-26] were trained over six sessions using a tone conditioned stimulus (CS) and a periorbital stimulation unconditioned stimulus (US). Learning increased as a function of age, with the P21-23 and P24-26 groups learning faster than the P17-19 group. Age- and learning-related changes in both theta and single neuron activity were observed. CA1 pyramidal cells in the older age groups showed greater task-related activity than the P17-19 group during CS-US paired sessions. The proportion of trials with a significant theta (4-10 Hz) power change, the theta/delta ratio, and theta peak frequency also increased in an age-dependent manner. Finally, spike/theta phase-locking during the CS showed an age-related increase. The findings indicate substantial developmental changes in dorsal hippocampal function that may play a role in the ontogeny of learning and memory.

Sensory system development influences the ontogeny of trace eyeblink conditioning

The developmental emergence of delay eyeblink conditioning (EBC) is dependent on the development of the sensory system stimulated by the conditioned stimulus (CS). However, trace EBC has traditionally been believed to be dependent on the development of forebrain structures, such as the hippocampus. If hippocampal development alone is limiting the developmental emergence of trace EBC, then using an earlier developing sensory modality should not affect the rate or asymptote of conditioning. The goal of the current study was to investigate whether using a vibration CS would facilitate the ontogeny of trace EBC relative to an auditory CS. Rat pups received six sessions of trace EBC or unpaired training using either a tone or vibration CS on postnatal day (P) 17-18, 21-22, or 24-25. Training with a vibration CS resulted in rapid conditioning as early as P17-18, whereas training with a tone CS did not result in rapid conditioning until after P17-18. The results suggest that the ontogeny of trace EBC depends, at least in part, on sensory system development.

Ontogeny of septohippocampal modulation of delay eyeblink conditioning

The current study investigated the effects of disrupting the septohippocampal theta system on the developmental emergence of delay eyeblink conditioning. Theta oscillations are defined as electroencephalographic (EEG) waveforms with a frequency between 3-8 Hz. Hippocampal theta oscillations are generated by inputs from the entorhinal cortex and the medial septum. Theta activity has been shown to facilitate learning in a variety of paradigms, including delay eyeblink conditioning. Lesions of the medial septum disrupt theta activity and slow the rate at which delay eyeblink conditioning is learned (Berry & Thompson, [1979] Science 200:1298-1300). The role of the septohippocampal theta system in the ontogeny of eyeblink conditioning has not been examined. In the current study, infant rats received an electrolytic lesion of the medial septum on postnatal day (P) 12. Rats were later given eyeblink conditioning for 6 sessions with an auditory conditioned stimulus on P17-19, P21-23, or P24-26. Lesions impaired eyeblink conditioning on P21-23 and P24-26 but not on P17-19. The results suggest that the septohippocampal system comes online to facilitate acquisition of eyeblink conditioning between P19 and P21. Developmental changes in septohippocampal modulation of the cerebellum may play a significant role in the ontogeny of eyeblink conditioning.

Extinction, reacquisition, and rapid forgetting of eyeblink conditioning in developing rats

Eyeblink conditioning is a well-established model for studying the developmental neurobiology of associative learning and memory. However, age differences in extinction and subsequent reacquisition have yet to be studied using this model. The present study examined extinction and reacquisition of eyeblink conditioning in developing rats. In Experiment 1, post-natal day (P) 17 and 24 rats were trained to a criterion of 80% conditioned responses (CRs) using stimulation of the middle cerebellar peduncle (MCP) as a conditioned stimulus (CS). Stimulation CS-alone extinction training commenced 24 h later, followed by reacquisition training after the fourth extinction session. Contrary to expected results, rats trained starting on P17 showed significantly fewer CRs to stimulation CS-alone presentations relative to P24s, including fewer CRs as early as the first block of extinction session 1. Furthermore, the P17 group was slower to reacquire following extinction. Experiment 2 was run to determine the extent to which the low CR percentage observed in P17s early in extinction reflected rapid forgetting versus rapid extinction. Twenty-four hours after reaching criterion, subjects were trained in a session split into 50 stimulation CS-unconditioned stimulus paired trials followed immediately by 50 stimulation CS-alone trials. With this "immediate" extinction protocol, CR percentages during the first block of stimulation CS-alone presentations were equivalent to terminal acquisition levels at both ages but extinction was more rapid in the P17 group. These findings indicate that forgetting is observed in P17 relative to P24 rats 24 h following acquisition. The forgetting in P17 rats has important implications for the neurobiological mechanisms of memory in the developing cerebellum.