New perspectives on beta-adrenergic mediation of innate and learned fear responses to predator odor

Abstract

In the present study, we investigated the role of noradrenergic transmission in unconditioned and conditioned responses to predatory threats. First, we examined the effects of systemically injected beta-blockers on unconditioned and contextual conditioned response to cat odor. The centrally acting beta-blocker (propranolol) was able to impair unconditioned responses, as well as the acquisition of the contextual fear to cat odor; however, the peripherally acting (nadolol) was not effective. Next, we examined the neural substrate underlying the noradrenergic modulation of the defensive response to cat odor and focused on the dorsal premammillary nucleus (PMd), because it represents the hypothalamic site most responsive to predatory threats and, at the same time, presents a dense plexus of noradrenergic fibers. We were able to see that propranolol significantly reduced PMd-Fos expression in response to cat odor and that beta-adrenoceptor blockade in the PMd, before cat odor exposure, reduced defensive responses to the cat odor and to the cat odor-related environment. We have also shown that beta-adrenoceptor blockade in the PMd, before the exposure to cat odor-related context, impaired the contextual conditioned responses. Overall, the present results provide convincing evidence suggesting that central noradrenergic mediation is critical for the expression of unconditioned and contextual conditioned antipredatory responses. We have further shown that the PMd appears to be an important locus to mediate these beta-adrenoceptor effects.

Figure 1.

Experiment 1. Typical recordings of the antagonistic effects of nadolol (A, B) and propranolol (C, D) on heart rate increases after dopamine (18, 36, 72 μg/kg) in anesthetized rats. B and D represent the mean + SEM heart rate increases for all the groups (n = 3). *p < 0.05 compared with the respective control.

Figure 2.

Experiment 2. The effects of propranolol (Prop; 10 mg/kg, i.p.) and nadolol (Nad; 10 mg/kg, i.p.) in the percentage of approach time or hide time and the head-out time in rats confronted to cat odor. The familiarization, conditioning, and context sessions were conducted during 3 consecutive days. Hatched horizontal bars represent the mean and the confidence limits (±95%) for the familiarization data. Subjects received propranolol (n = 18), nadolol (n = 18), or saline (Sal; n = 27) injections 30 min before the conditioning session. In the context session, the subjects received saline and were reexposed to the cat odor-associated context. Histograms represent the conditioning and context data and were expressed as mean + SEM. *p < 0.05 (Newman–Keuls post hoc test) versus saline control group.

Figure 3.

Experiment 3. Effects on the PMd–Fos immunoreactivity of saline (Sal), propranolol (Prop; 10 mg/kg, i.p.), and nadolol (Nado; 10 mg/kg, i.p.) injections 30 min before cat odor exposure. A, Bright-field photomicrographs illustrating the PMd–Fos-positive cells in the saline (Sal)-, propranolol (Prop)-, and nadolol (Nad)-injected groups. fx, fornix; 3V, third ventricle. B, Histogram showing the mean + SEM number of PMd–Fos-positive cells in the Sal-, Prop-, and Nad-treated groups. Data represent means + SEM for five rats per group. *p < 0.05 compared with saline group. One-way ANOVA followed by Newman–Keuls test.

Figure 4.

Experiment 4.1. Effects of atenolol, applied 10 min before the conditioning session on defensive behavior exhibited by rats exposed to a cat odor. The familiarization, the conditioning to the cat odor, and the context sessions were conducted during 3 consecutive days, during which the percentage of approach time, the percentage of hide time, and the head-out time were recorded in 10 min sessions each. Hatched horizontal bars represent the mean and the confidence limits (±95%) for the data obtained during the familiarization session. PBS (n = 9), 10 nmol atenolol (A10; n = 7), and 40 nmol atenolol (A40; n = 7) in the PMd and the A-out group (n = 15) were microinjected (0.2 μl) 10 min before the conditioning to the cat odor. *p < 0.05 compared with PBS group.

Figure 5.

Experiment 4.2. Effects of atenolol, applied 10 min before the first context session, on defensive behavior exhibited by rats exposed to a cat odor. The familiarization, the conditioning to the cat odor, and the two context sessions were conducted during 4 consecutive days, during which the percentage of approach time, the percentage of hide time, and the head-out time were recorded in 10 min sessions each. Hatched horizontal bars represent the mean and the confidence limits (p < 0.05) for the data obtained during the conditioning session. Percentage of approach time, percentage of hide time, and head-out time in the groups microinjected with PBS (n = 6) and 40 nmol atenolol (A40; n = 7) in the PMd and the A-out group (n = 6) 10 min before the first context exposure. *p < 0.05 compared with control group.