Predator odor-evoked BOLD activation in the awake rat: modulation by oxytocin and V₁a vasopressin receptor antagonists

Abstract

Modulators of unconditioned fear are potential targets for developing treatments for anxiety disorders. We used blood oxygen level dependent (BOLD) MRI to investigate the pattern of brain activity during the presentation of a predator odor (cat fur) and a repulsive novel odor, butyric acid (BA), to awake rats. We further tested whether odor-evoked BOLD activation involved oxytocin (OT) and vasopressin V(1a) receptors. Animals were subdivided into groups either administered an intracerebroventricular injection of artificial cerebrospinal fluid (CSF), an OT receptor antagonist or a V(1a) antagonist (125 ng/10 μL each) 90 min before studies. BA odor evoked robust brain activation across olfactory, sensory, memory and limbic regions. The magnitude of BOLD activation across these regions was greater for BA than with cat fur. However, blockade of OT and V(1a) receptors differentially modulated odor evoked neural activity, particularly in the amygdala. OT and V(1a) antagonism preferentially modulated BOLD responding to BA in the cortical amygdala. While, OT and V(1a) antagonisms preferentially modulated BOLD responding to cat fur in the central amygdala. The data suggest that although OT receptors modulate BOLD activation in response to a novel and repulsive odor such as BA, vasopressin V(1a) receptors exert a modulatory influence on the neural response to a predator odor.

Fig. 1

Time spent in one of three compartments of a test chamber during no odor, butyric acid (BA) and cat fur exposure. Each odor was presented during a 15-min (900 s) test during which an empty vial or a vial containing an odor was present in one compartment. Ninety minutes before behavioral testing, rats received a 10 uL intracerebroventricular injection of CSF, an oxytocin antagonist (OT-A) or a vasopressin V_1a receptor antagonist. Figure on the left depicts the three-compartment test cage with odor-paired, adjacent, and non-odor paired sides. Data are presented to the right in that same order from top to bottom. Columns represent the three different groups, CSF-treated, OT-A and V_1a-A treated male rats. All data presented as mean±standard error. Asterisks denote significant differences from no odor condition (open columns; two way ANOVA *p<0.05;**p<0.01).

Fig. 2

Butyric acid induced neural activity in the awake rat. Shown are composite maps for significant increases in signal intensity during presentation of air containing 5 uL 170 mM butyrate (airflow rate 1200 cc/min). Ninety minutes before behavioral testing, rats received a 10 uL intracerebroventricular injection of CSF (left column images), an oxytocin antagonist (+OT-A) or a vasopressin V_1a receptor antagonist (+V_1a). Overlay coloring threshold is set between 1% and 10% signal changes. Left column indicates specific regions of interest. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Cat fur induced neural activity in the awake rat. Shown are composite maps for significant increases in signal intensity during presentation of air flowing through a connector vial containing humidified cat fur (airflow rate 1200 cc/min). Ninety minutes before behavioral testing, rats received a 10 uL intracerebroventricular injection of CSF (left column images), an oxytocin antagonist (+OT-A) or a vasopressin V_1a receptor antagonist (+V_1a). Overlay coloring threshold is set between 1% and 10% signal changes. Left column indicates specific regions of interest. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

BOLD signal changes in response to a novel butyric acid odor and cat fur odor. Shown are subnuclei of the amygdala and the lower right panel shows data for the olfactory tubercle. All data presented as mean±standard error. Each panel shows the mean percent change in BOLD signal in response to butyric acid and cat fur for CSF, OT-A and V_1a-A treated animals. *OT-A significantly different from CSF, **V_1a-A significantly different from CSF and ^φV_1a-A significantly different from OT-A (Two way ANOVA p<0.05).

Fig. 5

Temporal profile of the BOLD signal change in response to butyric acid odor. Data are shown for the olfactory tubercle (A) and the cortical amygdala (B). Shaded areas surrounding plot lines indicate±standard error. Arrows show the time of onset of odor delivery.

Fig. 6

BOLD signal changes in response to cat fur odor. All data presented as mean±standard error. Each panel shows the mean percent change in BOLD signal in response to cat fur for CSF, OT-A and V_1a-A treated animals. *OT-A significantly different from CSF and **V_1a-A significantly different from CSF (One way ANOVA p<0.05).