Glutamatergic, GABAergic, and endocannabinoid neurotransmissions within the dorsal hippocampus modulate the cardiac baroreflex function in rats.

The dorsal hippocampus (DH) is involved in the modulation of the cardiac baroreflex function. There is a wide expression of the NMDA and AMPA/Kainate receptors within the DH. Glutamate administration into the DH triggers both tachycardia and pressor responses. Moreover, GABAergic interneurons and endocannabinoid system play an important role in modulation of the activity of glutamatergic neurons within the DH. Therefore, the present work aimed to evaluate the involvement of the glutamatergic, GABAergic, and endocannabinoid neurotransmissions within the DH in cardiac baroreflex function in rats. We have used the technique of vasoactive drugs infusion to build both sigmoidal curves and linear regressions to analyze the cardiac baroreflex function. Bilateral injection into the DH of DL-AP7, a NMDA receptor antagonist (10 or 50 nmol/500 nL), or NBQX, an AMPA/Kainate antagonist (100 nmol/ 500 nL), reduced the cardiac baroreflex function. On the other hand, bilateral injection of Bicuculline, a GABAA receptor antagonist (1 nmol/500 nL), or AM251, a CB1 receptor antagonist (10 or 100 pmol/500 nL), increased the cardiac baroreflex function. Furthermore, 1 nmol/500 nL of the NMDA receptor antagonist, when administrated alone, was ineffective to change baroreflex function, but it was able to inhibit the alteration in the cardiac baroreflex function elicited by the dose of 100 pmol/500 nL of the CB1 receptor antagonist. Taken together, these findings suggest that glutamatergic, GABAergic, and endocannabinoid neurotransmissions interact each other within the DH to modulate the cardiac baroreflex function.

Unravelling cortico-hypothalamic pathways regulating unconditioned fear-induced antinociception and defensive behaviours.

The medial prefrontal cortex can influence unconditioned fear-induced defensive mechanisms organised by diencephalic neurons that are under tonic GABAergic inhibition. The posterior hypothalamus (PH) is involved with anxiety- and panic attack-like responses. To understand this cortical mediation, our study characterised anterior cingulate cortex (ACC)-PH pathways and investigated the effect of ACC local inactivation with lidocaine. We also investigated the involvement of PH ionotropic glutamate receptors in the defensive behaviours and fear-induced antinociception by microinjecting NBQX (an AMPA/kainate receptor antagonist) and LY235959 (a NMDA receptor antagonist) into the PH. ACC pretreatment with lidocaine decreased the proaversive effect and antinociception evoked by GABAA receptor blockade in the PH, which suggests that there may be descending excitatory pathways from this cortical region to the PH. Microinjections of both NBQX and LY235959 into the PH also attenuated defensive and antinociceptive responses. This suggests that the blockade of AMPA/kainate and NMDA receptors reduces the activity of glutamatergic efferent pathways. Both inputs from the ACC to the PH and glutamatergic hypothalamic short links disinhibited by intra-hypothalamic GABAA receptors blockade are potentially implicated. Microinjection of a bidirectional neurotracer in the PH showed a Cg1-PH pathway and PH neuronal reciprocal connections with the periaqueductal grey matter. Microinjections of an antegrade neurotracer into the Cg1 showed axonal fibres and glutamatergic vesicle-immunoreactive terminal boutons surrounding both mediorostral-lateroposterior thalamic nucleus and PH neuronal perikarya. These data suggest a critical role played by ACC-PH glutamatergic pathways and AMPA/kainate and NMDA receptors in the panic attack-like reactions and antinociception organised by PH neurons.