Role of the hippocampus, the bed nucleus of the stria terminalis, and the amygdala in the excitatory effect of corticotropin-releasing hormone on the acoustic startle reflex

Abstract

Previously, we demonstrated that transection of the fimbria/fornix blocked the excitatory effect of corticotropin-releasing hormone (CRH) on startle (CRH-enhanced startle), suggesting that the hippocampus and its efferent target areas that communicate via the fimbria may be critically involved in CRH-enhanced startle. The bed nucleus of the stria terminalis (BNST) receives direct projections from the ventral hippocampus via the fimbria/fornix. Therefore, the role of the ventral hippocampus, the BNST, and the amygdala in CRH-enhanced startle was investigated. NMDA lesions of the BNST completely blocked CRH-enhanced startle, whereas chemical lesions of the ventral hippocampus and the amygdala failed to block CRH-enhanced startle. However, the same amygdala-lesioned animals showed a complete blockade of fear-potentiated startle, a conditioned fear response sensitive to manipulations of the amygdala. In contrast, BNST-lesioned rats had normal fear-potentiated startle. This indicates a double dissociation between the BNST and the amygdala in two different paradigms that enhance startle amplitude. Microinfusions of CRH into the BNST, but not into the ventral hippocampus, mimicked intracerebroventricular CRH effects. Furthermore, infusion of a CRH antagonist into the BNST blocked CRH-enhanced startle in a dose-dependent manner. Control studies showed that this blockade did not result from either leakage of the antagonist into the ventricular system or a local anesthetic effect caused by infusion of the antagonist into the BNST. The present studies strongly suggest that CRH in the CSF can activate the BNST, which could lead to activation of brainstem and hypothalamic BNST target areas involved in anxiety and stress responses.

Fig. 1.

Effects of sham lesions and chemical lesions of the bed nucleus of the stria terminalis, ventral hippocampus, and central or basolateral nucleus of the amygdala on mean percent change of startle amplitude after intracerebroventricular infusion of 1 μg of CRH (A) or ACSF (B). Each data point represents the mean percent change of 20 postdrug test trials. C, Smallest (left) and largest (right) lesions of these areas.

Fig. 2.

Mean percent change of startle amplitude after intracerebroventricular infusion of 1 μg of CRH or ACSF. Theopen circles show the behavioral data of the animals that had a blockade of CRH-enhanced startle after NMDA lesions of the ventral hippocampus [group 1 (G1)], and theclosed circles show the behavioral data of the animals that had super CRH-enhanced startle after lesions [Group 2 (G2)]. Notice the difference in the scale.

Fig. 3.

Effects of sham lesions and NMDA lesions of the bed nucleus of the stria terminalis and central or basolateral nucleus of the amygdala on fear-potentiated startle. Each barrepresents the mean startle amplitude over 30 noise alone trials (10 of each at 90, 95, and 105 dB; black bars) or 30 light noise trials (10 of each at 90, 95, and 105 dB noise in the presence of light; white bars). The difference (hatched bars) between the noise and light noise trials indicates the magnitude of fear-potentiated startle.

Fig. 4.

Effects of various doses of CRH infused into the bed nucleus of the stria terminalis (A) or the ventral hippocampus (B), on mean percent change of startle amplitude. Each data point represents the mean percent change of 20 postdrug test trials. C, Histological reconstructions showing placements of cannula tips of the animals included in the data analysis.

Fig. 5.

A, Mean percent change of startle amplitude after fourth ventricle infusion of 0.5 μg of CRH into rats pretreated 5 min earlier with various doses of the CRH antagonist into the BNST. The same rats were subsequently tested with intra-BNST infusion of 160 ng of CRH, and the behavioral data are shown inB. Each data point represents the mean percent change of 20 postdrug test trials. i.c., Intracisternal.C, Histological reconstructions showing placements of intra-BNST cannula tips of the animals included in the data analysis.

Fig. 6.

Mean percent change of startle amplitude after fourth ventricle infusion of 0.5 μg of CRH into rats pretreated 5 min earlier with 6 μg of the CRH antagonist into the lateral ventricles. Each data point represents the mean percent change of 20 postdrug test trials. I.c., Intracisternal; i.c.v., intracerebroventricular.

Fig. 7.

A, Mean percent change of startle amplitude after fourth ventricle infusion of 0.5 μg CRH into rats pretreated 5 min earlier with 6 μg of the CRH antagonist into the CeA. Each data point represents mean percent change of 20 postdrug test trials. I.c., Intracisternal. B, Effects of ACSF or 6 μg of the CRH antagonist infused into the CeA on expression of fear-potentiated startle. Each barrepresents the mean startle amplitude over 30 noise alone trials (10 of each at 90, 95, and 105 dB; black bars) or 30 light noise trials (10 of each at 90, 95, and 105 dB noise in the presence of light; white bars). The difference (hatched bars) between the noise and light noise trials indicates the magnitude of fear-potentiated startle. C, Histological reconstructions showing placements of intra-CeA cannula tips of the animals included in the data analysis.