Dorsal and ventral hippocampus modulate autonomic responses but not behavioral consequences associated to acute restraint stress in rats

Abstract

Recent evidence has suggested that the dorsal (DH) and the ventral (VH) poles of the hippocampus are structurally, molecularly and functionally different regions. While the DH is preferentially involved in the modulation of spatial learning and memory, the VH modulates defensive behaviors related to anxiety. Acute restraint is an unavoidable stress situation that evokes marked and sustained autonomic changes, which are characterized by elevated blood pressure (BP), intense heart rate (HR) increases, skeletal muscle vasodilatation and cutaneous vasoconstriction, which are accompanied by a rapid skin temperature drop followed by body temperature increases. In addition to those autonomic responses, animals submitted to restraint also present behavioral changes, such as reduced exploration of the open arms of an elevated plus-maze (EPM), an anxiogenic-like effect. In the present work, we report a comparison between the effects of pharmacological inhibition of DH and VH neurotransmission on autonomic and behavioral responses evoked by acute restraint stress in rats. Bilateral microinjection of the unspecific synaptic blocker cobalt chloride (CoCl2, 1mM) into the DH or VH attenuated BP and HR responses, as well as the decrease in the skin temperature, elicited by restraint stress exposure. Moreover, DH or VH inhibition before restraint did not change the delayed increased anxiety behavior observed 24 h later in the EPM. The present results demonstrate for the first time that both DH and VH mediate stress-induced autonomic responses to restraint but they are not involved in the modulation of the delayed emotional consequences elicited by such stress.

Figure 1. A diagrammatic representation based on the rat brain atlas of Paxinos and Watson (1997) indicating injections sites of vehicle or CoCl₂ (closed circle) into the Dorsal Hippocampus (DH) and Ventral Hippocampus (VH).

cc- corpus callosum; IA- Inter aural.

Figure 2. Time-course of bilateral microinjection of 200 nL of vehicle (n=6/ DH and VH) or 1 mM of CoCl₂ (Cobalt, n=6/ DH and VH) administered into DH or VH on changes in mean arterial pressure (∆MAP), heart rate (∆HR) and cutaneous temperature (∆Temp) of animals submitted to 60 min of restraint stress.

Symbols represent the means and bars the SEM. *P<0.05, Bonferroni’s post-hoc test.

Figure 3. Infrared digital images of representative rats which received either CoCl2 (indicated by A) or vehicle (indicated by B) into DH, in its home cage and during the first minute, thirty and sixty minutes of restraint.

Note the drop in cutaneous tail temperature during the restraint in vehicle treated animal and the absence of this drop in CoCl2 treated animal. The same effects were observed in VH treated animals. All images use the same color-coding for temperature.

Figure 4. Effects of bilateral microinjection of 200 nL of vehicle (n=6/ DH and VH) or 1 mM of CoCl₂ (n=6/ DH and VH) administered into DH or VH immediately before a 1-h restraint period on behavior observed 24 h later in the elevated plus-maze (EPM).

A non-stressed group was used as control. Columns represent the means and the bars the SEM. *P<0.05, Bonferroni’s post-hoc test.