Genetic disruption of dopamine β-hydroxylase dysregulates innate responses to predator odor in mice

Abstract

In rodents, exposure to predator odors such as cat urine acts as a severe stressor that engages innate defensive behaviors critical for survival in the wild. The neurotransmitters norepinephrine (NE) and dopamine (DA) modulate anxiety and predator odor responses, and we have shown previously that dopamine β-hydroxylase knockout (Dbh -/-), which reduces NE and increases DA in mouse noradrenergic neurons, disrupts innate behaviors in response to mild stressors such as novelty. We examined the consequences of Dbh knockout on responses to predator odor (bobcat urine) and compared them to Dbh-competent littermate controls. Over the first 10 min of predator odor exposure, controls exhibited robust defensive burying behavior, whereas Dbh -/- mice showed high levels of grooming. Defensive burying was potently suppressed in controls by drugs that reduce NE transmission, while excessive grooming in Dbh -/- mice was blocked by DA receptor antagonism. In response to a cotton square scented with a novel "neutral" odor (lavender), most control mice shredded the material, built a nest, and fell asleep within 90 min. Dbh -/- mice failed to shred the lavender-scented nestlet, but still fell asleep. In contrast, controls sustained high levels of arousal throughout the predator odor test and did not build nests, while Dbh -/- mice were asleep by the 90-min time point, often in shredded bobcat urine-soaked nesting material. Compared with controls exposed to predator odor, Dbh -/- mice demonstrated decreased c-fos induction in the anterior cingulate cortex, lateral septum, periaqueductal gray, and bed nucleus of the stria terminalis, but increased c-fos in the locus coeruleus and medial amygdala. These data indicate that relative ratios of central NE and DA signaling coordinate the type and valence of responses to predator odor.

Keywords: Dopamine; Dopamine β-hydroxylase; Norepinephrine; Predator; Toxoplasma gondii.

Fig. 1

Effects of DBH knockout on behavioral responses to predator odor. Dbh +/− (n = 7) and Dbh −/− (n = 7) mice were exposed to water or bobcat urine for 10 min, and the amount of time engaged in defensive burying (A) and grooming (B) were assessed. Shown are mean +/− SEM. ***p < 0.001, ****p < 0.0001.

Fig. 2

Effects of adrenergic drugs on behavioral responses to predator odor in control mice. Dbh +/− (n = 8) mice were pretreated with vehicle, the α1AR antagonist prazosin (0.5 mg/kg), the α2AR antagonist atipamezole (1 mg/kg), or the βAR antagonist propranolol (5 mg/kg) and exposed to bobcat urine 30 min later for 10 min. Shown is the mean +/− SEM time engaged in defensive burying (A–C) and grooming (D–F). *p < 0.05.

Fig. 3

Effects of pharmacological DBH inhibition on behavioral responses to predator odor. Dbh+/− mice (n = 10) were pretreated with vehicle or the DBH inhibitor nepicastat (100 mg/kg) and exposed to bobcat urine for 10 min, and the amount of time engaged in defensive burying (A) and grooming (B) were assessed. Shown are the mean +/− SEM. *p < 0.05, **p < 0.01.

Fig. 4

Effects of dopaminergic drugs on predator odor-induced grooming and locomotor activity in Dbh −/− mice. Dbh −/− mice were pretreated with vehicle (n = 7–13), the non-selective DA receptor antagonist flupenthixol (0.25 mg/kg; n = 7), the D2 antagonist L-741,626 (10 mg/kg; n = 8), or the D1 antagonist SCH-23390 (0.03, 0.01, or 0.006 mg/kg; n = 8), or and exposed to bobcat urine 30 min later for 10 min. Shown is the mean +/− SEM time engaged in grooming (A-C) and distance travelled (D). ****p < 0.0001, ***p < 0.001, **p < 0.01, ns = not significant.

Fig. 5

Effects of DBH knockout on arousal and nestlet shredding during predator odor exposure. Dbh+/− and Dbh −/− mice were exposed to lavender oil (A-B; n = 7,8) or bobcat urine (C-D; n = 8,9) for 90 min, and the fraction of animals that shredded their cotton nesting material (A,C) and fell asleep (B,D) were assessed. *p < 0.05, **p < 0.01.

Fig. 6

Regional c-fos responses to predator odor in Dbh+/− and Dbh −/− mice. Dbh +/− (n = 8) and Dbh −/− (n = 9) mice were exposed to bobcat urine for 90 min. Shown are (A) representative images and (B) the mean +/− SEM number of c-fos+ (magenta) cells in the locus coeruleus (LC), anterior cingulate cortex (ACC), dorsal bed nucleus of the stria terminalis (dBNST), periaqueductal gray (PAG), paraventricular nucleus of the hypothalamus (PVN), lateral septum (LS), and medial amygdala (MeA). Images are counterstained with the nuclear marker DAPI (blue) and the noradrenergic terminal marker norepinephrine transporter (NET; green). *p < 0.05, ***p < 0.001, ****p < 0.0001.

Fig. 7

Proposed noradrenergic circuit for defensive behavioral responses to predator odors. The PVN hypothalamus is activated by ancient odorant pathways, in turn exciting the LC by releasing CRH (black arrow). The LC projects to a constellation of brain regions implicated in defensive behavior, where it releases NE in DBH-sufficient animals (blue arrows). Loss of NE results in hypoactivity in PAG, ACC, LS, and dBNST (regions without circles). Interestingly, loss of NE did not affect neuronal activity in the PVN (green circle), suggesting this pathway remains intact. Further, NE deficiency resulted in hyperactivity in the LC and the MeA (magenta circles). Image created with Biorender.