MEMRI for visualizing brain activity after auditory stimulation in frogs

Tuned in to communication sounds: Neuronal sensitivity in the túngara frog midbrain to frequency modulated signals

For complex communication signals, it is often difficult to identify the information-bearing elements and their parameters necessary to elicit functional behavior. Consequently, it may be difficult to design stimuli that test how neurons contribute to communicative processing. For túngara frogs (Physalaemus pustulosus), however, previous behavioral testing with numerous stimuli showed that a particular frequency modulated (FM) transition in the male call is required to elicit phonotaxis and vocal responses. Modeled on such behavioral experiments, we used awake in vivo recordings of single units in the midbrain to determine if their excitation was biased to behaviorally important FM parameters. Comparisons of stimulus driven action potentials revealed greatest excitation to the behaviorally important FM transition: a downward FM sweep or step that crosses ~600 Hz. Previous studies using long-duration acoustic exposure found immediate early gene expression in many midbrain neurons to be most sensitive to similar FM. However, those data could not determine if FM coding was accomplished by the population and/or individual neurons. Our data suggest both coding schemes could operate, as 1) individual neurons are more sensitive to the behaviorally significant FM transition and 2) when single unit recordings are analytically combined across cells, the combined code can produce high stimulus discrimination (FM vs. noise driven excitation), approaching that found in behavioral discrimination of call vs. noise.

Input of sensory, limbic, basal ganglia and pallial/cortical information into the ventral/lateral habenula: Functional principles in anuran amphibians

The habenula - a phylogenetically old brain structure present in all vertebrates - is involved in pain processing, reproductive behaviors, sleep-wake cycles, stress responses, reward, and learning. We performed intra- and extracellular recordings of ventral habenula (VHb) neurons in the isolated brain of anurans and revealed similar cell and response properties to those reported for the lateral habenula of mammals. We identified tonic regular, tonic irregular, rhythmic firing, and silent VHb neurons. Transitions between these firing patterns were observed during spontaneous activity. Electrical stimulation of various brain areas demonstrated VHb input of auditory, optic, limbic, basal ganglia, and pallial information. This resulted in three different response behaviors in VHb neurons: excitation, inhibition, or alternating facilitation and suppression of neuronal activity. Spontaneously changing activity patterns were observed to modulate, reset, or suppress the response behavior of VHb neurons, indicating a gating mechanism. This could be a network status or context dependent selection mechanism for which information are transmitted to task relevant brain areas (i.e., sensory system, limbic system, basal ganglia). Furthermore, alternating facilitation and suppression sequences upon auditory nerve stimulation correlated positively fictive motor activities recorded via the compound potential of the vagal nerve. Stimulation of the auditory nerve or the habenula led to facilitation, suppression, or alternating facilitation and suppression of neuronal activity in putative dopaminergic neurons. Due to complex habenula feedback loops with basal ganglia, limbic, and sensory systems, the habenula involvement in a variety of functions might therefore be explained by a modulatory effect on a task-relevant input stream.