The distribution of fos immunoreactivity in rat brain following freezing and escape responses elicited by electrical stimulation of the inferior colliculus.

Several sources of evidence indicate that the inferior colliculus also integrates acoustic information of an aversive nature besides its well-known role as a relay station for auditory pathways. Gradual increases of the electrical stimulation of this structure cause in a hierarchical manner alertness, freezing and escape behaviors. Independent groups of animals implanted with bipolar electrodes into the inferior colliculus received electrical stimulation at one of these aversive thresholds. Control animals were submitted to the same procedure but no current was applied. Next, analysis of Fos protein expression was used to map brain areas activated by the inferior colliculus stimulation at each aversive threshold and in the controls. Whereas alertness elicited by stimulation of the inferior colliculus did not cause any significant labeling in any structure studied in relation to the respective control, electrical stimulation applied at the freezing threshold increased Fos-like immunoreactivity in the central amygdaloid nucleus and entorhinal cortex. In contrast, escape response enhanced Fos-like immunoreactivity in the nucleus cuneiform and the dorsal periaqueductal gray matter of the mesencephalon. This evidence supports the notion that freezing and escape behaviors induced by electrical stimulation of the inferior colliculus activate different neural circuitries in the brain. Both defensive behaviors caused significant expression of c-fos in the frontal cortex, hippocampus and basolateral amygdaloid nucleus. This indistinct pattern of c-fos distribution may indicate a more general role for these structures in the modulation of fear-related behaviors. Therefore, the present data bring support to the notion that amygdala, dorsal hippocampus, entorhinal cortex, frontal cortex, dorsal periaqueductal gray matter and cuneiform nucleus altogether play a role in the integration of aversive states generated at the level of the inferior colliculus.

Serotonergic innervation of the auditory brainstem of the Mexican free-tailed bat, Tadarida brasiliensis.

Anatomical and electrophysiological evidence suggests that serotonin alters the processing of sound in the auditory brainstem of many mammalian species. The Mexican free-tailed bat is a hearing specialist, like other microchiropteran bats. At the same time, many aspects of its auditory brainstem are similar to those in other mammals. This dichotomy raises an interesting question regarding the serotonergic innervation of the bat auditory brainstem: Is the serotonergic input to the auditory brainstem similar in bats and other mammals, or are there specializations in the serotonergic innervation of bats that may be related to their exceptional hearing capabilities? To address this question, we immunocytochemically labeled serotonergic fibers in the brainstem of the Mexican free-tailed bat, Tadarida brasiliensis. We found many similarities in the pattern of serotonergic innervation of the auditory brainstem in Tadarida compared with other mammals, but we also found two striking differences. Similarities to staining patterns in other mammals included a higher density of serotonergic fibers in the dorsal cochlear nucleus and in granule cell regions than in the ventral cochlear nucleus, a high density of fibers in some periolivary nuclei of the superior olive, and a higher density of fibers in peripheral regions of the inferior colliculus compared with its core. The two novel features of serotonergic innervation in Tadarida were a high density of fibers in the fusiform layer of the dorsal cochlear nucleus relative to surrounding layers and a relatively high density of serotonergic fibers in the low-frequency regions of the lateral and medial superior olive.