Auditory trace fear conditioning requires perirhinal cortex

The hippocampus is well-known to be critical for trace fear conditioning, but nothing is known about the importance of perirhinal cortex (PR), which has reciprocal connections with hippocampus. PR damage severely impairs delay fear conditioning to ultrasonic vocalizations (USVs) and discontinuous tones (pips), but has no effect on delay conditioning to continuous tones. Here we demonstrate that trace auditory fear conditioning also critically depends on PR function. The trace interval between the CS offset and the US onset was 16s. Pre-training neurotoxic lesions were produced through multiple injections of N-methyl-D-aspartate along the full length of PR, which was directly visualized during the injections. Control animals received injections with phosphate-buffered saline. Three-dimensional reconstructions of the lesion volumes demonstrated that the neurotoxic damage was well-localized to PR and included most of its anterior-posterior extent. Automated video analysis quantified freezing behavior, which served as the conditional response. PR-damaged rats were profoundly impaired in trace conditioning to either of three different CSs (a USV, tone pips, and a continuous tone) as well as conditioning to the training context. Within both the lesion and control groups, the type of cue had no effect on the mean CR. The overall PR lesion effect size was 2.7 for cue conditioning and 3.9 for context conditioning. We suggest that the role of PR in trace fear conditioning may be distinct from some of its more perceptual functions. The results further define the essential circuitry underlying trace fear conditioning to auditory cues.

Bicuculline administration into ventromedial hypothalamus: effects on fear and regional brain monoamines and GABA concentrations in rats

The effects of bicuculline methiodide administration into ventromedial hypothalamus (15 ng per site, bilaterally) on fear behavior and monoamines (NA, DA, 5-HT) and GABA in structures of the brain defensive system (hypothalamus, midbrain gray matter, amygdala, hippocampus and frontal cortex) were studied. Fear behavior was examined in the modified version of light-dark transition test. The time out from the illuminated compartment of chamber, the time spent there and number of returns to the illuminated compartment was measured. Additionally motor activity, i.e., number of crossings and rearings in dark as well as in the illuminated part of compartment, was registered. Blockade of GABAA receptors in the ventromedial hypothalamus resulted in increased fear behavior, i.e. decrease of time out from illuminated compartment and decrease of the time spent there. Motor behavior remained unchanged. HPLC analysis showed reduction of GABA concentration in all investigated brain structures. An increase of NA concentration in all examined structures with exception of the hypothalamus without effect on MHPG/NA was observed as well. Dopamine level remained unchanged, but DOPAC/DA ratio increased in all structures, except frontal cortex. Also HVA/DA ratio increased in the hypothalamus and midbrain. 5-HT concentration increased only in midbrain, 5-HIAA increased in midbrain and in frontal cortex, and 5-HIAA/5-HT ratio increased only in frontal cortex. These results indicate that GABA-ergic and monoaminergic systems remain in functional interactions and that these interactions may play an important role in the neurochemical regulation of fear behavior. The possible mechanism of GABA--monoaminergic interactions is discussed.