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...

Metabotropic serotonin receptors such as 5-HT1A and 5-HT1B receptors shape the level, selectivity, and timing of auditory responses in the inferior colliculus (IC). Less is known about the effects of ionotropic 5-HT3 receptors, which are cation channels that depolarize neurons. In the current study, the influence of the 5-HT3 receptor on auditory responses in vivo was explored by locally iontophoresing a 5-HT3 receptor agonist and antagonists onto single neurons recorded extracellularly in mice. Three main findings emerge from these experiments. First, activation of the 5-HT3 receptor can either facilitate or suppress auditory responses, but response suppressions are not consistent with 5-HT3 effects on presynaptic GABAergic neurons. Both response facilitations and suppressions are less pronounced in neurons with high precision in response latency, suggesting functional differences in the role of receptor activation for different classes of neuron. Finally, the effects of 5-HT3 activation vary across repetition rate within a subset of single neurons, suggesting that the influence of receptor activation sometimes varies with the level of activity. These findings contribute to the view of the 5-HT3 receptor as an important component of the serotonergic infrastructure in the IC, with effects that are complex and neuron-selective.

Cultures of Manduca sexta Johanssen in our laboratory were found to have larvae with missing or deformed mouthparts or antennae. Hypothesizing that these developmental deformities were caused by crowded rearing conditions, we reared larvae in four different population densities and recorded the incidence (% of larvae affected) and types of chemoreceptor deformities. Results showed that the incidence of these deformities was directly proportional to larval population density. Deformities of the maxilla and palp were the most frequent, followed by those of the antenna, epipharynx and maxillary styloconica. Life history traits of larval mass, food consumption, and rate of development were inversely related to larval density for both normal and deformed larvae. We discuss possible causes and mechanisms of these deformities and of changes to life history traits.