By means of the Golgi-Cox and Nissl methods we investigated the cyto- and fiberarchitecture as well as the morphology of neurons in the subcortical auditory structures of the Mongolian gerbil (Meriones unguiculatus), a frequently used animal model in auditory neuroscience. We describe the divisions and subdivisions of the auditory thalamus including the medial geniculate body, suprageniculate nucleus, and reticular thalamic nucleus, as well as of the inferior colliculi, nuclei of the lateral lemniscus, superior olivary complex, and cochlear nuclear complex. In this study, we 1) confirm previous results about the organization of the gerbil's subcortical auditory pathway using other anatomical staining methods (e.g., Budinger et al. [2000] Eur J Neurosci 12:2452-2474); 2) add substantially to the knowledge about the laminar and cellular organization of the gerbil's subcortical auditory structures, in particular about the orientation of their fibrodendritic laminae and about the morphology of their most distinctive neuron types; and 3) demonstrate that the cellular organization of these structures, as seen by the Golgi technique, corresponds generally to that of other mammalian species, in particular to that of rodents.

During deep anesthesia, the electroencephalographic (EEG) signal of the brain alternates between bursts of activity and periods of relative silence (suppressions). The origin of burst-suppression and its distribution across the brain remain matters of debate. In this work, we used functional magnetic resonance imaging (fMRI) to map the brain areas involved in anesthesia-induced burst-suppression across four mammalian species: humans, long-tailed macaques, common marmosets, and rats. At first, we determined the fMRI signatures of burst-suppression in human EEG-fMRI data. Applying this method to animal fMRI datasets, we found distinct burst-suppression signatures in all species. The burst-suppression maps revealed a marked inter-species difference: in rats, the entire neocortex engaged in burst-suppression, while in primates most sensory areas were excluded—predominantly the primary visual cortex. We anticipate that the identified species-specific fMRI signatures and whole-brain maps will guide future targeted studies investigating the cellular and molecular mechanisms of burst-suppression in unconscious states.

We investigated the frequency-related topography of connections of the primary auditory cortical field (AI) in the Mongolian gerbil with subcortical structures of the auditory system by means of the axonal transport of two bidirectional tracers, which were simultaneously injected into regions of AI with different best frequencies (BFs). We found topographic, most likely frequency-matched (tonotopic) connections as well as non-topographic (non-tonotopic) connections. AI projects in a tonotopic way to the ipsilateral ventral (MGv) and dorsal divisions (MGd) of the medial geniculate body (MGB), the reticular thalamic nucleus and dorsal nucleus of the lateral lemniscus, and the ipsi- and contralateral dorsal cortex of the inferior colliculus (IC) and central nucleus of the IC. AI receives tonotopic inputs from MGv and MGd. Projections from different BF regions of AI terminate in a non-tonotopic way in the ipsilateral medial division of the MGB (MGm), the suprageniculate thalamic nucleus (SG) and brachium of the IC (bic), and the ipsi- and contralateral external cortex and pericollicular areas of the IC. The anterograde labeling in the intermediate and ventral nucleus of the lateral lemniscus, parts of the superior olivary complex, and divisions of the cochlear nucleus was generally sparse; thus a clear topographic arrangement of the labeled axons could not be ruled out. AI receives non-tonotopic inputs from the ipsilateral MGm, SG, and bic. In conclusion, the tonotopic and non-tonotopic corticofugal connections of AI can potentially serve for both conservation and integration of frequency-specific information in the respective target structures.

Motivated by the increasing evidence that auditory cortex is under control of dopaminergic cell structures of the ventral midbrain, we studied how the ventral tegmental area and substantia nigra affect neuronal activity in auditory cortex. We electrically stimulated 567 deep brain sites in total within and in the vicinity of the two dopaminergic ventral midbrain structures and at the same time, recorded local field potentials and neuronal discharges in cortex. In experiments conducted on three awake macaque monkeys, we found that electrical stimulation of the dopaminergic ventral midbrain resulted in short-latency (~35 ms) phasic activations in all cortical layers of auditory cortex. We were also able to demonstrate similar activations in secondary somatosensory cortex and superior temporal polysensory cortex. The electrically evoked responses in these parts of sensory cortex were similar to those previously described for prefrontal cortex. Moreover, these phasic responses could be reversibly altered by the dopamine D1-receptor antagonist SCH23390 for several tens of minutes. Thus, we speculate that the dopaminergic ventral midbrain exerts a temporally precise, phasic influence on sensory cortex using fast-acting non-dopaminergic transmitters and that their effects are modulated by dopamine on a longer timescale. Our findings suggest that some of the information carried by the neuronal discharges in the dopaminergic ventral midbrain, such as the motivational value or the motivational salience, is transmitted to auditory cortex and other parts of sensory cortex. The mesocortical pathway may thus contribute to the representation of non-auditory events in the auditory cortex and to its associative functions.

The TW concept opens up a new approach for MRI of medium-sized animals in horizontal UHF scanners.