Defensive behaviors and brain regional activation changes in rats confronting a snake
Introduction
Recent attention to defense systems in rodents suggests that these involve a common set of structures, including the amygdala, hypothalamus, and midbrain periaqueductal gray, but with specific differences related to the type of threat stimulus and situation [[1], [2], [3], [4], [5]]. In particular, exposure to a carnivore predator (i.e., a cat), or to an aggressive conspecific, elicits activation in parallel circuits in the medial amygdala (MeA), the ventromedial nucleus of the hypothalamus (VMH), the dorsal premammillary nucleus (PMD) and the periaqueductal nucleus of the midbrain (PAG). In each nucleus, however, the specific areas activated appear to be different: For a cat predator, the posteroventral portion of the MeA (MeApv); the dorsomedial portion of the VMH (dmVMH); the ventrolateral portion of the PMD (PMDvl), and the dorsolateral portion of the PAG (PAGdl) [1] are involved, while the presence of an attacking conspecific involves activation in each of these same nuclei; but in different portions, posterodorsal MeA, ventrolateral VMH, dorsomedial PMD, and dorsomedial and lateral PAG (see Ref. [1]).
Silva et al. [4] utilized pharmacogenetic neural inhibition tools to inhibit neurons in the VMH, for mice confronted by these two types of threat, a rat predator or an aggressive conspecific. Inhibition of cells in the dmVMH sharply reduced defensive behaviors in the presence of a predator, without altering defensiveness to the aggressive conspecific, whereas inhibition of cells in the VMHvl selectively reduced defensiveness to the conspecific. Foot shock, in the same situation in which the predator and aggressive conspecific were presented, elicited little activity in the VMH, and selective activation of these portions of the VMH did not alter defensiveness to the shock [4].
On the other hand, conditioned stimuli previously paired with pain are associated with activation in the central nucleus of the amygdala (CeA) and in the vlPAG [1,4]. Viellard et al. [5] further examined the systems associated with shock-based contextual fear conditioning, utilizing two very different situations; confinement inside the shock chamber itself, or, placement in an apparatus outside the shock chamber, but with access to it. This difference produced a profound alteration in the defensive behaviors seen, with freezing in the shock chamber, and very little freezing but high levels of risk assessment, exploration, and piling of substrate against the shock chamber, in the animals placed outside it. This difference is compatible with a view of risk assessment as a major defensive response to less intense threatening stimuli, particularly when an element of ambiguity is involved [6]. Moreover, when the animals were placed in the apparatus outside the shock chamber, but with access to it, there was a Fos upregulation in the dorsomedial PMD and dorsomedial and lateral PAG, in a way similar to what has been described for animals exposed to aggressive conspecifics.
These findings indicate that different classes of threat stimuli, although involving some or many of the same structures, nonetheless show differences that suggest they involve separate circuits. How may these specific systems be related to potential differences in the behaviors associated with defense? Silva et al. [4] found that a predator (rat), an attacking conspecific, and foot shock produced increased stretch postures and immobility, as well as reduced locomotion, in mice. However, Viellard et al. [5] reported that behaviors to threat were very different in the two conditioned fear contexts (inside or outside with access to, the shock chamber). These findings suggest that an adequate analysis of the behavioral and brain system correlates of defense may require different threat stimuli, and a wider range of test situations and behaviors measured.
The current study involves a snake predator, a larger test chamber with a number of objects in which the rat subjects can shelter, or, on which they can climb. The use of a living snake to elicit defensive behaviors has been validated in a number of laboratory rodents, including rats, mice, and hamsters [[7], [8], [9], [10], [11]]. Although the specific behaviors measured and reported are virtually identical to those used previously, additional spatial data were intended to capture a flight-hiding dimension that has not been possible in many previous studies of neural systems involved in fear/defense related behaviors. As with the other types of threat that have been used, this study includes analysis of activation patterns of septal, amygdalar, hypothalamic and brainstem sites to search a possible neural substrate for the snake-induced defensive behavior.
Section snippets
Animals
The threat stimulus was a wild rattler snake (Crotalis durissus terrificus), a species native to Brazil, weighing approximately 1.6 kg. The snake was maintained in captivity in a snake pit in the animal house of the Ribeirão Preto Medical School of the University of São Paulo (FMRP-USP). It was fed every 24 h with rodents previously killed by CO2 inhalation.
Subjects were 10 adult male Long-Evans laboratory rats (R. norvegicus) weighing 250−350 g. from the animal facility of the Department of
Behavior of the snake stimulus
The Brazilian rattler was very active and made strikes at each of the 5 rats that encountered it. For 3 of these rats (Rs2, Rs3 and Rs5), the strike did make contact, but the contact was extremely brief and no injury to any of these rats was apparent.
Behavior of the rats
Individual behaviors for control and rattler-exposed rats are presented in Table 1.
Controls: The rats confronting the toy (rubber) snake appeared to treat it as an object of some interest but showed little defensiveness toward it. Each of them
Discussion
Several snake species, including Crotalis durissus terrificus, elicit a range of defensive reactions in laboratory rodents (e.g. [14,15,7,11]). These include a stretch-attend or “flat-back-approach” behavior involving orientation to the threat or potential threat along with a stretched-out body; freezing or defensive immobility; and escape and/or avoidance of the threat. With animate threats such as predators or attacking conspecifics, sniffing at the threat when it is nearby is also common.
Acknowledgements and Funding
This research was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Research Grants #2014/05432-9 (NSC), # 2017/11855-8 (NCC) and #2017/12881-1 (DCB) - Conselho Nacional de Desenvolvimento Científico e Tecnológico (awarded to M.V.C.B., N.C.C. and N.S.C) - Pro-Rectory of the University of São Paulo (USP) Research grant (awarded to N.C.C). J. Mendes-Gomes was supported by FAPESP Post-doctoral fellowship #2013/13398-2); R.P. Bindi was supported by FAPESP Doctoral fellowship
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