The hippocampus is a limbic structure that is involved in the expression of defensive reactions and autonomic changes in rats. The injection of
L-glutamate (L-glu) into the ventral hippocampus (VH) decreases blood pressure and heart rate in anesthetized rats. Activation of
NMDA receptors in the VH increases the production of
nitric oxide (NO), leading to
guanylate cyclase activation. The hypothesis of the present study was that a local
NMDA receptor-NO-
guanylate cyclase interaction mediates the cardiovascular effects of microinjection of L-glu into the VH. Microinjection of increasing doses of L-glu (30, 60 and 200 nmol/200 nL) into the VH of conscious rats caused dose-related pressor and tachycardiac responses. The cardiovascular effects of L-glu were abolished by local pretreatment with: the
glutamate receptor antagonist AP-7 (0.4 nmol); the selective neuronal
NO synthase (nNOS) inhibitor N(ω)-Propyl-
L-arginine (0.04 nmol); the NO scavenger C-
PTIO (2 nmol) or the
guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolol [4,3-a]quinoxalin-1-one (2 nmol). Moreover, these cardiovascular responses were blocked by intravenous pretreatment with: the ganglionic blocker
mecamylamine (2mg/Kg); the nonselective β-
adrenergic receptor antagonist
propranolol (2mg/Kg); the β1-adrenergic receptor selective antagonist
atenolol (1mg/kg). However, pretreatment with the selective α1-adrenergic receptor antagonist
prazosin (0,5mg/kg) caused only a small reduction in the pressor response, without affecting the L-glu evoked
tachycardia. In conclusion, our results suggest that cardiovascular responses caused by L-glu microinjection into the VH are mediated by
NMDA glutamate receptors and involve local nNOS and
guanylate cyclase activation. Moreover, these cardiovascular responses are mainly mediated by cardiac sympathetic nervous system activation, with a small involvement of the vascular sympathetic nervous system.