N-methyl-D-aspartate receptors play important roles in acquisition and expression of the eyeblink conditioned response in glutamate receptor subunit delta2 mutant mice

The neural circuitry and molecular mechanisms underlying delay and trace eyeblink conditioning in mice

Classical eyeblink conditioning (EBC), a simple form of associative learning, has long been served as a model for motor learning and modulation. The neural circuitry of EBC has been studied in detail in rabbits. However, its underlying molecular mechanisms remain unclear. The advent of mouse transgenics has generated new perspectives on the studies of the neural substrates and molecular mechanisms essential for EBC. Results about EBC in mice differ in some aspects from those obtained in other mammals. Here, we review the current studies about the neural circuitry and molecular mechanisms underlying delay and trace EBC in mice. We conclude that brainstem-cerebellar circuit plays an essential role in DEC while the amygdala modulates this process, and that the medial prefrontal cortex (mPFC) as a candidate is involved in the extra-cerebellar mechanism underlying delay eyeblink conditioning (DEC) in mice. We propose the Amygdala-Cerebellum-Prefrontal Cortex-Dynamic-Conditioning Model (ACPDC model) for DEC in mice. As to trace eyeblink conditioning (TEC), the forebrain regions may play an essential role in it, whereas cerebellar cortex seems to be out of the neural circuitry in mice. Moreover, the molecular mechanisms underlying DEC and TEC in mice differ from each other. This review provides some new information and perspectives for further research on EBC.

Olivary climbing fiber alterations in PN40 rat cerebellum following postnatal ethanol exposure

Developmental ethanol exposure in rats during postnatal days (PN) 4-6 is known to cause significant loss of the cerebellar Purkinje cells. It is not known what happens to the surviving neurons as they continue to develop. This study was designed to quantify the interactions between the olivary climbing fibers and the Purkinje cells when the cerebellar circuits have matured. Rat pups were treated with a daily dose of ethanol (4.5g/kg body weight) delivered by intragastric intubation on PN4, PN4-6, or PN7-9. The interactions between the climbing fibers and the Purkinje cells were examined on PN40 using confocal microscopy. Mid-vermal cerebellar sections were stained with antibodies to calbindin-D28k (to visualize Purkinje cells) and vesicular glutamate transporter 2 (VGluT2, to visualize climbing fibers). Confocal z-stack images were obtained from Lobule 1 and analyzed with Imaris software to quantify the staining of the two antibodies. The VGluT2 immunostaining was significantly reduced and this was associated with alterations in the synaptic integrity, and synaptic number per Purkinje cell with only a single exposure on PN4 enough to cause the alterations. Previously, we demonstrated similar deficits in climbing fiber innervation when analyzed on PN14 (Pierce, Hayar, Williams, and Light, 2010). The present study confirms that these alterations are sustained and further identifies the decreased synaptic density as well as alterations to the general morphology of the molecular layer of the cerebellar cortex that are the result of the binge ethanol exposure.

Developmental alterations in olivary climbing fiber distribution following postnatal ethanol exposure in the rat

Ethanol exposure during postnatal days (PN) 4-6 in rats alters cerebellar development resulting in significant loss of Purkinje cells. There is little knowledge, however, on what happens to the neurons that survive. In this study, rat pups were treated with a daily dose of ethanol (either 3.6 or 4.5 g/kg body weight) delivered by intragastric intubation on PN4, PN4-6, or PN7-9. Then the interactions between climbing fibers and Purkinje cells were examined on PN14 using confocal microscopy. Mid-vermal cerebellar sections were stained with antibodies to calbindin-D28k (to visualize Purkinje cells) and vesicular glutamate transporter 2 (VGluT2, to visualize climbing fibers). Confocal z-stack images were obtained from Lobule 1 and analyzed with Imaris software to quantify the staining of the two antibodies. The VGluT2 immunostaining was significantly reduced in the PN4 and PN4-6 ethanol groups for the 4.5 g/kg dose level, compared to controls, indicating that the cerebellar circuitry was significantly altered following developmental ethanol exposure. Not only were there fewer Purkinje cells following ethanol exposure, but the surviving neurons had significantly fewer VGluT2-labeled synapses. These alterations in the synaptic integrity were both dose dependent and temporally dependent.

Unimpaired trace classical eyeblink conditioning in Purkinje cell degeneration (pcd) mutant mice

Young adult Purkinje cell degeneration (pcd) mutant mice, with complete loss of cerebellar cortical Purkinje cells, are impaired in delay eyeblink classical conditioning. In the delay paradigm, the conditioned stimulus (CS) overlaps and coterminates with the unconditioned stimulus (US), and the cerebellar cortex supports normal acquisition. The ability of pcd mutant mice to acquire trace eyeblink conditioning in which the CS and US do not overlap has not been explored. Recent evidence suggests that cerebellar cortex may not be necessary for trace eyeblink classical conditioning. Using a 500 ms trace paradigm for which forebrain structures are essential in mice, we assessed the performance of homozygous male pcd mutant mice and their littermates in acquisition and extinction. In contrast to results with delay conditioning, acquisition of trace conditioning was unimpaired in pcd mutant mice. Extinction to the CS alone did not differ between pcd and littermate control mice, and timing of the conditioned response was not altered by the absence of Purkinje cells during acquisition or extinction. The ability of pcd mutant mice to acquire and extinguish trace eyeblink conditioning at levels comparable to controls suggests that the cerebellar cortex is not a critical component of the neural circuitry underlying trace conditioning. Results indicate that the essential neural circuitry for trace eyeblink conditioning involves connectivity that bypasses cerebellar cortex.