Dorsal hippocampus cholinergic and nitrergic neurotransmission modulates the cardiac baroreflex function in rats.

Hippocampus is a limbic structure involved in the baroreflex and chemoreflex control that receives extensive cholinergic input from basal forebrain. Hippocampal muscarinic receptors activation by acetylcholine might evoke nitric oxide synthesis, which is an important neuromodulator of cardiovascular responses. Thus, we hypothesize that cholinergic and nitrergic neurotransmission within the DH modulates the baroreflex and chemoreflex function. We have used vasoactive drugs (phenylephrine and sodium nitroprusside), and potassium cyanide infused peripherally to induce, respectively, baroreflex or chemoreflex responses in awake animals. Bilateral injection into the DH of the acetylcholinesterase inhibitor (neostigmine) reduced baroreflex responses. Meanwhile, the non-selective muscarinic receptor antagonist (atropine) or the M1-selective muscarinic receptor antagonist increased baroreflex responses (pirenzepine). Furthermore, the neuronal nitric oxide synthase inhibitor (N-propyl) or the intracellular NO scavenger (carboxy-PTIO) increased baroreflex responses, as well as the selective inhibitor of NO-sensitive guanylyl cyclase (ODQ), increased the baroreflex responses. Besides, bilateral administration of an ineffective dose of a neuronal nitric oxide synthase inhibitor abolished the reduction in the baroreflex responses evoked by an acetylcholinesterase inhibitor. On the other hand, we have demonstrated that hippocampal cholinergic neurotransmission did not influence the chemoreflex function. Taken together, our findings suggest that nNOS-derived nitric oxide in the DH participates in acetylcholine-evoked baroreflex responses.

Cannabidiol increases the nociceptive threshold in a preclinical model of Parkinson's disease.

Medications that improve pain threshold can be useful in the pharmacotherapy of Parkinson's disease (PD). Pain is a prevalent PD's non-motor symptom with a higher prevalence of analgesic drugs prescription for patients. However, specific therapy for PD-related pain are not available. Since the endocannabinoid system is expressed extensively in different levels of pain pathway, drugs designed to target this system have promising therapeutic potential in the modulation of pain. Thus, we examined the effects of the 6-hydroxydopamine- induced PD on nociceptive responses of mice and the influence of cannabidiol (CBD) on 6-hydroxydopamine-induced nociception. Further, we investigated the pathway involved in the analgesic effect of the CBD through the co-administration with a fatty acid amide hydrolase (FAAH) inhibitor, increasing the endogenous anandamide levels, and possible targets from anandamide, i.e., the cannabinoid receptors subtype 1 and 2 (CB1 and CB2) and the transient receptor potential vanilloid type 1 (TRPV1). We report that 6-hydroxydopamine- induced parkinsonism decreases the thermal and mechanical nociceptive threshold, whereas CBD (acute and chronic treatment) reduces this hyperalgesia and allodynia evoked by 6-hydroxydopamine. Moreover, ineffective doses of either FAAH inhibitor or TRPV1 receptor antagonist potentialized the CBD-evoked antinociception while an inverse agonist of the CB1 and CB2 receptor prevented the antinociceptive effect of the CBD. Altogether, these results indicate that CBD can be a useful drug to prevent the parkinsonism-induced nociceptive threshold reduction. They also suggest that CB1 and TRPV1 receptors are important for CBD-induced analgesia and that CBD could produce these analgesic effects increasing endogenous anandamide levels.

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.

Hippocampal subareas arranged in the dorsoventral axis modulate cardiac baroreflex function in a site-dependent manner in rats.

NEW FINDINGS: What is the central question of this study? Classically, areas of the brainstem are involved in the cardiac baroreceptor reflex. However, forebrain areas, such as the hippocampus, may also modulate the cardiac baroreflex function. What is the main finding and its importance? According to the hippocampal subarea recruited dorsoventrally, the baroreflex function can be either facilitated or inhibited. These results are according to the new topographical division proposed for the hippocampus, i.e. it can be divided into functionally and anatomically different regions along its dorsoventral axis. From a neuroanatomical point of view, we may split the hippocampal formation into the dorsal (DH) and ventral hippocampus (VH). Although the basic intrinsic circuitry of the hippocampus seems to be maintained throughout its longitudinal axis, dorsal and ventral portions connect differently with cortical and subcortical areas and express different gene patterns, being functionally distinct. Differential stimulation of the DH or VH can evoke either an increase or a decrease in blood pressure, heart rate and sympathetic activity. However, to the best of our knowledge, specific involvement of the hippocampus and its different subareas in the baroreflex function remains to be investigated. In the present work, therefore, we evaluated the involvement of hippocampal subareas arranged on the dorsoventral axis in cardiac baroreflex modulation. Our results suggest that inhibition of hippocampal subareas by CoCl2 , a calcium-dependent synaptic neurotransmission blocker, differentially affects baroreflex sensitivity; administration of CoCl2 into the DH increased cardiac baroreflex function, whereas it diminished cardiac baroreflex function when administered into the VH. In contrast, administration of CoCl2 into intermediate portions of the hippocampus did not affect the baroreflex response. Our findings suggest that the hippocampus influences baroreflex function according to the hippocampal subarea recruited dorsoventrally.

Ventral hippocampus modulates bradycardic response to peripheral chemoreflex activation in awake rats.

NEW FINDINGS: What is the central question of this study? Does reversible synaptic inactivation by CoCl2 in the dorsal (DH) or ventral (VH) portions of the hippocampus have a modulatory effect on cardiovascular and respiratory responses evoked by chemoreflex activation in awake rats? What is the main finding and its importance? Using i.v. infusion of KCN to activate the peripheral chemoreflex before and after microinjection of CoCl2 into VH, we showed that the bradycardic response was increased, but not the pressor and tachypnoeic responses even if the tidal volume had been increased. Thus, VH but not DH may be involved in the modulation of the parasympathoexcitatory component of the peripheral chemoreflex. In rats, peripheral chemoreflex activation evokes pressor and bradycardic responses as well as a tachypnoeic response. Studies have shown that limbic structures, such as the hippocampus, can modulate autonomic reflexes. Evidence suggests that the dorsal (DH) and the ventral (VH) portions of the hippocampus are structurally and functionally distinct; therefore, in the present study we tested the hypothesis that local neurotransmission of the DH and VH are involved in the neural pathways of the cardiovascular and ventilatory responses to chemoreflex activation. Thus, the goal of the present study was to compare the chemoreflex responses elicited by i.v. injection of KCN (40 µg per rat) in awake rats before and after DH and VH synaptic transmission was temporarily inhibited by bilateral microinjections of 500 nl of the unspecific synapse blocker, CoCl2 (1  mm). Bilateral inhibition of VH, but not DH, 10 min before KCN infusion was able to enhance the bradycardic response (P < 0.05), with no changes in the typical pressor and tachypnoeic responses evoked by chemoreflex activation (P > 0.05). Furthermore, the tidal volume was significantly increased (P < 0.05) even though no other respiratory parameter had been significantly changed (P > 0.05), suggesting that VH can exert a tonic modulatory action on tidal volume. Therefore, the present study reports, for the first time, that DH neurotransmission did not exert an influence on chemoreflex responses, whereas VH mediates, at least in part, the parasympathoexcitatory component of the peripheral chemoreflex.

The medial amygdaloid nucleus modulates the baroreflex activity in conscious rats.

The medial amygdaloid nucleus (MeA) is involved in cardiovascular control. In the present study we report the effect of MeA pharmacological ablations caused by bilateral microinjections of the nonselective synaptic blocker CoCl2 on cardiac baroreflex responses in rats. MeA synaptic inhibition evoked by local bilateral microinjection of 100 nL of CoCl2 (1 mM) did not affect blood pressure or heart rate baseline, suggesting no tonic MeA influence on resting cardiovascular parameters. However, 10 min after CoCl2 microinjection into the MeA of male Wistar rats, the reflex bradycardic response evoked by intravenous infusion of phenylephrine was significantly enhanced when compared with the reflex bradycardic response observed before CoCl2. The treatment did not affect the tachycardic responses to the intravenous infusion of sodium nitroprusside (SNP). Baroreflex activity returned to control values 60 min after CoCl2 microinjections, confirming a reversible blockade. The present results indicate an involvement of the MeA in baroreflex modulation, suggesting that synapses in the MeA have an inhibitory influence on the bradycardic component of the baroreflex in conscious rats.