Pontine stimulation overcomes developmental limitations in the neural mechanisms of eyeblink conditioning

Abstract

Pontine neuronal activation during auditory stimuli increases ontogenetically between postnatal days (P) P17 and P24 in rats. Pontine neurons are an essential component of the conditioned stimulus (CS) pathway for eyeblink conditioning, providing mossy fiber input to the cerebellum. Here we examined whether the developmental limitation in pontine responsiveness to a CS in P17 rats could be overcome by direct stimulation of the CS pathway. Eyeblink conditioning was established in infant rats on P17-P18 and P24-P25 using pontine stimulation as a CS. There were no significant age-related differences in the rate or level of conditioning. Eyeblink conditioned responses established with the stimulation CS were abolished by inactivation of the ipsilateral cerebellar nuclei and overlying cortex in both age groups. The findings suggest that developmental changes in the CS pathway play an important role in the ontogeny of eyeblink conditioning.Eyeblink classical conditioning has been used as a model system for examining developmental changes in the neural mechanisms underlying motor learning . It is well established that eyeblink conditioning in adult mammals depends on cerebellar interactions with its afferent and efferent brainstem nuclei (Mauk and Donegan 1997;Christian and Thompson 2003). The cerebellum receives input stimulation from an auditory conditioned stimulus (CS) through the pontine mossy fiber projection (Steinmetz et al. , 1987(Steinmetz et al. , 1989Steinmetz 1990;Steinmetz and Sengelaub 1992;Tracy et al. 1998). Input stimulation from an air puff or face shock unconditioned stimulus (US) reaches the cerebellum through the climbing fiber projection from the inferior olive (McCormick et al. 1985;Mauk et al. 1986;Steinmetz et al. 1989). The convergence of mossy and climbing fiber activation of cerebellar neurons is thought to result in changes in synaptic efficacy that constitute the substrate of learning and drive the production of the eyeblink conditioned response (CR). Cerebellar output produces inhibitory feedback to the inferior olive and excitatory feedback to the pons (Sears and Steinmetz 1991;Clark et al. 1997;Kim et al. 1998;Bao et al. 2000;Medina et al. 2002). The feedback connections are thought to be important for acquisition and maintenance of CRs (Medina et al. 2002).Eyeblink conditioning emerges ontogenetically between postnatal days (P) P17 and P24 in rats (Stanton et al. 1992). Neurophysiological, neuropharmacological, and neuroanatomical analyses of the ontogeny of eyeblink conditioning in rats indicate that the developmental emergence of eyeblink conditioning is due to developmental changes in the CS and US pathways . The most substantial developmental change in the US pathway is an increase in the magnitude of cerebellar inhibitory feedback to the inferior olive (Nicholson and Freeman Jr. 2003a,b). The developmental change in inhibitory feedback is due to an increase in inhibitory synapses within the inferior olive (Nicholson and Freeman Jr. 2003a). Developmental addition of inhibitory syn...

Eyeblink conditioning in 12-day-old rats using pontine stimulation as the conditioned stimulus

“…Infusion of muscimol into the cerebellar interpositus nucleus reversibly abolished eyeblink CRs in pups given paired training, suggesting that the expression of associative learning established with pontine stimulation depended on cerebellar activity. These findings are consistent with previous studies using adult and juvenile animals (5,14). Various synaptic plasticity mechanisms have been demonstrated in vitro by using cerebellar preparations from the first 3 postnatal weeks (9-12).…”

“…The detailed surgical methods have been described (13,14). Isoflurane (1-3%) was used for anesthesia during surgery.…”

“…Stimulation of the pontine mossy fibers may be able to overcome the developmental limitations in eyeblink conditioning in younger rats by providing sufficient mossy fiber input to the cerebellum (14). The current study pushed the limits of this hypothesis by examining whether PN stimulation could be an effective CS in 12-day-old rats.…”

Central cannabinoid receptors modulate acquisition of eyeblink conditioning

“…The shock US (2 -3.5 mA, DC constant current) was delivered through a stimulus isolator (Model number 365A, World Precision Instruments). EMG activity was recorded differentially, filtered (500-5000 Hz) and integrated by equipment (JSA Designs) as described in other reports (Nicholson and Freeman 2002;Freeman et al 2005).…”

“…The anterior interpositus nucleus and associated areas of the cerebellar cortex are necessary for acquisition of dEBC (Kim and Thompson 1997;Steinmetz 2000;Christian and Thompson 2003;Thompson and Steinmetz 2009). Moreover, the cerebellar networks mediating this form of associative learning appear to be conserved across species (Woodruff-Pak et al 1996;Rogers et al 2001;Freeman et al 2005).…”