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Electrical stimulation thresholds required to elicit eyeblinks with either pontine or cerebellar interpositus stimulation were measured before and after classical eyeblink conditioning with paired pontine stimulation (conditioned stimulus, CS) and corneal airpuff (unconditioned stimulus, US). Pontine stimulation thresholds dropped dramatically after training and returned to baseline levels following extinction, whereas interpositus thresholds and input-output functions remained stable across training sessions. Learning rate, magnitude of threshold change, and electrode placements were correlated. Pontine projection patterns to the cerebellum were confirmed with retrograde labeling techniques. These results add to the body of literature suggesting that the pons relays CS information to the cerebellum and provide further evidence of synaptic plasticity in the cerebellar network.

Hurley LM, Tracy JA, Bohorquez A. Serotonin 1B receptor modulates frequency response curves and spectral integration in the inferior colliculus by reducing GABAergic inhibition. J Neurophysiol 100: 1656 -1667, 2008. First published July 16, 2008 doi:10.1152/jn.90536.2008. The selectivity of sensory neurons for stimuli is often shaped by a balance between excitatory and inhibitory inputs, making this balance an effective target for regulation. In the inferior colliculus (IC), an auditory midbrain nucleus, the amplitude and selectivity of frequency response curves are altered by the neuromodulator serotonin, but the changes in excitatoryinhibitory balance that mediate this plasticity are not well understood. Previous findings suggest that the presynaptic 5-HT1B receptor may act to decrease the release of GABA onto IC neurons. Here, in vivo extracellular recording and iontophoresis of the selective 5-HT1B agonist CP93129 were used to characterize inhibition within and surrounding frequency response curves using two-tone protocols to indirectly measure inhibition as a decrease in spikes relative to an excitatory tone alone. The 5-HT1B agonist attenuated such two-tone spike reduction in a varied pattern among neurons, suggesting that the function of 5-HT1B modulation also varies. The hypothesis that the 5-HT1B receptor reduces inhibition was tested by comparing the effects of CP93129 and the GABA A antagonists bicuculline and gabazine in the same neurons. The effects of GABA A antagonists on spike count, tuning bandwidth, twotone ratio, and temporal response characteristics mimicked those of CP93129 across the neuron population. GABA A antagonists also blocked or reduced the facilitation of evoked responses by CP93129. These results are all consistent with the reduction of GABA A -mediated inhibition by 5-HT1B receptors in the IC, resulting in an increase in the level of evoked responses in some neurons, and a decrease in spectral selectivity in others. I N T R O D U C T I O NNeuromodulatory signals such as serotonin are broadly released in the brain in response to changes in internal state (Trulson andJacobs 1979, 1981) but transform the response properties of sensory neurons in highly specific ways (Hurley et al. 2004;Mooney et al. 1996;Xiang and Prince 2003). This specificity is achieved through the targeting of classes of neurons by a variety of receptor types that may alter intrinsic neuron properties or target presynaptic neurons and alter the release of neurotransmitter (Hoyer et al. 1994(Hoyer et al. , 2002. One consequence of these effects is that serotonin can alter the balance of different types of inputs within sensory regions. For example, serotonin may alter the balance between ascending versus descending projections (Mooney et al. 1996) or the balance between within-versus between-layer communication in the cortex (Xiang and Prince 2003).In the inferior colliculus (IC), an auditory midbrain nucleus, serotonin modifies a number of response properties, including both spectral and temporal aspects of ev...

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