Eye-blink conditioning is associated with changes in synaptic ultrastructure in the rabbit interpositus nuclei

Abstract

Eye-blink conditioning involves the pairing of a conditioned stimulus (usually a tone) to an unconditioned stimulus (air puff), and it is well established that an intact cerebellum and interpositus nucleus, in particular, are required for this form of classical conditioning. Changes in synaptic number or structure have long been proposed as a mechanism that may underlie learning and memory, but localizing these changes has been difficult. Thus, the current experiment took advantage of the large amount of research conducted on the neural circuitry that supports eye-blink conditioning by examining synaptic changes in the rabbit interpositus nucleus. Synaptic quantifications included total number of synapses per neuron, numbers of excitatory versus inhibitory synapses, synaptic curvature, synaptic perforations, and the maximum length of the synapses. No overall changes in synaptic number, shape, or perforations were observed. There was, however, a significant increase in the length of excitatory synapses in the conditioned animals. This increase in synaptic length was particularly evident in the concave-shaped synapses. These results, together with previous findings, begin to describe a sequence of synaptic change in the interpositus nuclei following eye-blink conditioning that would appear to begin with structural change and end with an increase in synaptic number.

Classical eyeblink conditioning using electrical stimulation of caudal mPFC as conditioned stimulus is dependent on cerebellar interpositus nucleus in guinea pigs

“…It is consistent with the findings of the previous studies using peripheral CS in eyeblink conditioning that cerebellar interpositus nucleus plays an essential role in CR expression [4,27,29,31,66,70,71] . Steinmetz [72] speculated that the cerebellum may only be involved in simple, discrete, aversive, and somatic associative learning that occurs with a relatively short interstimulus interval.…”

Axonal Sprouting and Formation of Terminals in the Adult Cerebellum during Associative Motor Learning

“…The air puff signal (US) is relayed by climbing fibers from the medial part of the dorsal accessory inferior olive and/or dorsomedial group of the principal olive to PCs in the C3 or D0 zones of the cerebellar cortex (Mostofi et al, 2010). These olivary parts also give rise to collaterals to the lateral part of the AIN and DLH, respectively (Ruigrok and Voogd, 2000;Pijpers et al, 2005;Sugihara and Shinoda, 2007). Moreover, these CN regions have been shown to influence those parts of the facial nucleus that control the eyelid muscles (Morcuende et al, 2002;Gonzalez-Joekes and Schreurs, 2012).…”

“…Repeated pairings of tone and air puff gradually lead to the development of a timed eyelid closure in response to the tone, which is called the conditioned response (CR). The US is relayed by climbing fibers from specific parts of the inferior olive to Purkinje cells (PCs) in distinctive eyeblink controlling zones of the cerebellar cortex (Hesslow, 1994;Mostofi et al, 2010) and, by way of climbing fiber collaterals, to the lateral part of the anterior interposed nucleus (AIN) of the cerebellar nuclei (CN), including its dorsolateral hump (DLH) (Ruigrok and Voogd, 2000;Pijpers et al, 2005;Sugihara and Shinoda, 2007). The CS is relayed to the same PC zones by mossy fibers originating from the lateral part of the basilar pontine nuclei (BPN) (Kandler and Herbert 1991;Leergaard and Bjaalie, 2007;Halverson and Freeman, 2010;Mostofi et al, 2010).…”

Learning-related long-term potentiation of inhibitory synapses in the cerebellar cortex

“…Several lines of experimental data demonstrate that, in addition to its major functional role in the regulation of fine motor control, the cerebellum is crucially involved in other important functions such as sensory-motor learning and memory, e.g., conditioned eye-blink responses (20)(21)(22).…”

“…Recently, it has been shown that the time window for multiple input coincidence detection by hippocampal neurons is unchanged after in vitro LTP only when both excitatory and inhibitory synapses are potentiated (19). Thus, the temporal fidelity for spike generation within a neuronal network requires adequate levels of excitation and inhibition to be maintained.Several lines of experimental data demonstrate that, in addition to its major functional role in the regulation of fine motor control, the cerebellum is crucially involved in other important functions such as sensory-motor learning and memory, e.g., conditioned eye-blink responses (20)(21)(22).Recent studies have shown that the cerebellum also is involved in fear-related memory (23, 24), and it is activated by a sensory stimulus predicting a painful stimulation after conditioning (25,26). In addition, in the cerebellar cortex fear conditioning is accompanied by LTP of the excitatory synapses formed between parallel fibers (PFs) and Purkinje cells (PCs) (5).…”