**2. Dorsolateral pons in cardiorespiratory hypothalamic defense responses: role of the Parabrachial complex**

Recent data show that neurons located within the PBc play a role in the cardiorespiratory response evoked from HDA. As previously mentioned, the stimulation of cell bodies located within the PBc resembles the cardiovascular response elicited by HDA stimulation, thus evoking tachycardia and hypertension [63].

Neuropharmacological studies show that the inhibition with muscimol of somata located within the main subdivisions of the PBc, lPB and mPB-KF produces two different patterns of cardiorespiratory responses evoked to HDA stimulation [105].

The inhibition with muscimol of neurons located within the mPB-KF reduces the tachycardia and the pressure response evoked by HDA stimulation [105] (**Figure 3A**). It is known that neuronal activity of the parabrachial nuclei can modify the effectiveness of the baroreflex in rat, rabbit and cat [56, 106] and that the PBc is essential for a full expression of the bradycardia that typically accompanies the initial hypotensive response to blood loss and for the normal rate of blood pressure recovery [107, 108].

hypertensive response, although, and probably, the most important factor is the inhibition of the excitatory projections from the PBc to the IML. The most relevant conclusion from this data is the suggestion that the reset of the barorreceptor reflex elicited by HDA activation could be also mediated though a secondary indirect pathway using the PBc of

**Figure 3.** Neuropharmacological interactions between HDA and PBc. From top to bottom, instantaneous respiratory rate

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Cardiorespiratory response evoked to HDA stimulation before (left) and after (right) muscimol microinjection within the mPB-KF (A) and lPB (B). The arrows show the onset of the HDA electrical stimulation. Authors´ figure modified

O), instantaneous heart rate (bpm) and blood pressure (mmHg).

Therefore, the activity of mPB-KF makes an important contribution to the modulation of the intensity of the cardiovascular response evoked on HDA stimulation through an indirect

On the other hand, the inhibition of neurons located within the lPB with muscimol abolishes the respiratory response evoked to HDA stimulation [105]. Similar to mPB-KF inhibition, the increase of blood pressure evoked to HDA stimulation decreases after the microinjection of muscimol within the lPB; however, no significant changes of the heart rate response were

the pons [105].

from Ref. [105].

observed (**Figure 3B**).

pathway to both the IML and the NTS.

(rpm), respiratory flow (ml/s), pleural pressure (cm H<sup>2</sup>

The decrease in the cardiovascular response to HDA stimulation seems to be an indication of a resetting of the baroreceptor reflex. The normal cardiovascular response to hypothalamic stimulation, tachycardia and pressor response is due to direct activation of neurons from the RVLM, which send direct projections to sympathetic preganglionic neurons of the IML. The inhibition or the resetting of the baroreceptor reflex is the origin of the tachycardia observed during the activation of the HDA. This inhibition seems to be partially mediated by GABAA receptors located within the NTS, which produces a hyperpolarization of baroreceptor cells [42, 58].

The reset of the baroreceptor response partially explains the decrease of the tachycardia observed during the stress reaction evoked from the activation of the HDA. It could also explain, through an indirect modulatory pathway, the decrease of the magnitude of the

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and chemical stimulation [65]. The microinjection of muscimol or lidocaine within the A5 region modifies the pattern of the cardiorespiratory responses evoked from PBc stimulation [65]. The expiratory facilitatory response elicited from mPB-KF activation is reversed to an inspiratory facilitatory response. Nevertheless, when the lPB is activated, no changes are observed in the inspiratory facilitatory response. The magnitude of the increase of the pressor response and the tachycardia observed during PBc stimulation decreases significantly after A5 blocking microinjections. Moreover, a high number of extracellularly recorded neurons in the A5 region are activated on electrical stimulation within the mPB-KF nuclei [65]

These functional connections suggest a possible interaction between PBc and A5 pontine regions in mediating the defense response evoked from the HDA. This statement will be dis-

Recent data show that neurons located within the PBc play a role in the cardiorespiratory response evoked from HDA. As previously mentioned, the stimulation of cell bodies located within the PBc resembles the cardiovascular response elicited by HDA stimulation, thus

Neuropharmacological studies show that the inhibition with muscimol of somata located within the main subdivisions of the PBc, lPB and mPB-KF produces two different patterns of

The inhibition with muscimol of neurons located within the mPB-KF reduces the tachycardia and the pressure response evoked by HDA stimulation [105] (**Figure 3A**). It is known that neuronal activity of the parabrachial nuclei can modify the effectiveness of the baroreflex in rat, rabbit and cat [56, 106] and that the PBc is essential for a full expression of the bradycardia that typically accompanies the initial hypotensive response to blood loss and for the normal

The decrease in the cardiovascular response to HDA stimulation seems to be an indication of a resetting of the baroreceptor reflex. The normal cardiovascular response to hypothalamic stimulation, tachycardia and pressor response is due to direct activation of neurons from the RVLM, which send direct projections to sympathetic preganglionic neurons of the IML. The inhibition or the resetting of the baroreceptor reflex is the origin of the tachycardia observed during the activation of the HDA. This inhibition seems to be partially mediated by GABAA receptors located within the NTS, which produces a hyperpolarization of baro-

The reset of the baroreceptor response partially explains the decrease of the tachycardia observed during the stress reaction evoked from the activation of the HDA. It could also explain, through an indirect modulatory pathway, the decrease of the magnitude of the

**2. Dorsolateral pons in cardiorespiratory hypothalamic defense** 

(**Figure 2**).

52 Hypothalamus in Health and Diseases

cussed deeply in the following sections.

evoking tachycardia and hypertension [63].

rate of blood pressure recovery [107, 108].

receptor cells [42, 58].

**responses: role of the Parabrachial complex**

cardiorespiratory responses evoked to HDA stimulation [105].

**Figure 3.** Neuropharmacological interactions between HDA and PBc. From top to bottom, instantaneous respiratory rate (rpm), respiratory flow (ml/s), pleural pressure (cm H<sup>2</sup> O), instantaneous heart rate (bpm) and blood pressure (mmHg). Cardiorespiratory response evoked to HDA stimulation before (left) and after (right) muscimol microinjection within the mPB-KF (A) and lPB (B). The arrows show the onset of the HDA electrical stimulation. Authors´ figure modified from Ref. [105].

hypertensive response, although, and probably, the most important factor is the inhibition of the excitatory projections from the PBc to the IML. The most relevant conclusion from this data is the suggestion that the reset of the barorreceptor reflex elicited by HDA activation could be also mediated though a secondary indirect pathway using the PBc of the pons [105].

Therefore, the activity of mPB-KF makes an important contribution to the modulation of the intensity of the cardiovascular response evoked on HDA stimulation through an indirect pathway to both the IML and the NTS.

On the other hand, the inhibition of neurons located within the lPB with muscimol abolishes the respiratory response evoked to HDA stimulation [105]. Similar to mPB-KF inhibition, the increase of blood pressure evoked to HDA stimulation decreases after the microinjection of muscimol within the lPB; however, no significant changes of the heart rate response were observed (**Figure 3B**).

Similar results are observed with PAG stimulation, thus indicating that the PBc is also a critical relay in mediating dorsal PAG-evoked sympathoexcitation and baroreflex modulation [109]. In addition, neurons localized in the lPB are involved in mediating the defense-like behavior response during the stimulation of the dorsal PAG, modulating the arterial baroreflex [71]. This inhibitory effect is more evident from the mPB-KF than from lPB.

Therefore, the pressor response evoked during the stimulation of the HDA and PAG may involve the recruitment of neurons of both the lPB and mPB-KF subdivisions, which, using an indirect pathway, activate the IML.

Morphological studies have confirmed the presence of reciprocal connections between the PBc and different hypothalamic regions [110]. It has been also described that the PBc projects widely to areas of the forebrain involved in cardiovascular regulation and defense reactions [111]. It also projects, via descending fibers, to brainstem nuclei including the A5 region, the NTS and the IML of the spinal cord [112].

It is important to stand out the complete abolishment of the respiratory response to HDA stimulation after the inhibition of lPB somata with muscimol. The lPB is part of the neuronal pathways involved in the sympathoexcitatory component of the chemoreflex [113]. Fos protein expression studies show that the tachypnea evoked on HDA stimulation is produced by activation of carotid chemoreceptors within neurons of the lPB [94]. Moreover, neuronal recordings show that during chemoreflex stimulation, neurons of the lPB are activated and that this increase in firing precedes the classical hypertensive response to chemoreceptor stimulation, thus showing the relevance of lPB neuronal circuits on the central modulation of chemoreceptor inputs and reflex [114].

There are also indications that HDA stimulation may facilitate the chemoreceptor reflex by means of a group of intrinsic excitatory neurons localized within the NTS [115]. These cells are activated or facilitated by HDA-NTS direct excitatory connections. These neurons are also the main targets of excitatory inputs from the lPB [56]. The inhibition of these lPB excitatory projections with muscimol leads to the abolishment of the tachypneustic response evoked on HDA stimulation.

Electrophysiological studies using neuronal recordings support the above. A significant number of mPB-KF and lPB neurons are affected from HDA stimulation, confirming the importance of the functional correlation between the HDA and these pontine regions. The presence of anti-/orthodromic activations, short and long latency excitations, and inhibitions and excitatory/inhibitory activities gives electrophysiological evidence of reciprocal connections between these regions. It is also an index of the complexity of the different types of synaptic interactions between both areas (**Figure 4**) [105].

Studies related to glutamate receptors suggest that this neurotransmitter plays a crucial role in mediating the functional relation between the PBc and the HDA [116]. Glutamate activates metabotropic and ionotropic (NMDA and non-NMDA) receptors [117]. By employing immunocytochemical and in situ hybridization techniques, studies have demonstrated the presence of both metabotropic and ionotropic receptors in different nuclei of the PBc and KF [118–120]. Activation of vagal afferent fibers releases glutamate within the PBc [121]. An ascending excitatory pathway involving glutamate from the NTS to the PBc has been described [122]. In

**Figure 4.** HDA and PBc neurophysiological interactions. (A) Shows a rate histogram (bin size 2 s) representing the firing of an lPB cell not excited nor inhibited during HDA stimulation that increased the activity during HDA stimulation. (B) Shows a rate histogram (bin size 2 s) of an mPB-KF cell not excited nor inhibited during HDA stimulation showing a decrease of activity during HDA stimulation (0.1 ms given at 1 Hz). (C) The poststimulus time histogram shows spontaneous activity of an lPB neuron and double excitation after HDA stimulation. (D) The poststimulus time histogram shows an inhibition of an mPB neuron after HDA stimulation (100 stimuli, 1 Hz). Authors´ figure modified from Ref. [105].

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Similar results are observed with PAG stimulation, thus indicating that the PBc is also a critical relay in mediating dorsal PAG-evoked sympathoexcitation and baroreflex modulation [109]. In addition, neurons localized in the lPB are involved in mediating the defense-like behavior response during the stimulation of the dorsal PAG, modulating the arterial barore-

Therefore, the pressor response evoked during the stimulation of the HDA and PAG may involve the recruitment of neurons of both the lPB and mPB-KF subdivisions, which, using an

Morphological studies have confirmed the presence of reciprocal connections between the PBc and different hypothalamic regions [110]. It has been also described that the PBc projects widely to areas of the forebrain involved in cardiovascular regulation and defense reactions [111]. It also projects, via descending fibers, to brainstem nuclei including the A5 region, the

It is important to stand out the complete abolishment of the respiratory response to HDA stimulation after the inhibition of lPB somata with muscimol. The lPB is part of the neuronal pathways involved in the sympathoexcitatory component of the chemoreflex [113]. Fos protein expression studies show that the tachypnea evoked on HDA stimulation is produced by activation of carotid chemoreceptors within neurons of the lPB [94]. Moreover, neuronal recordings show that during chemoreflex stimulation, neurons of the lPB are activated and that this increase in firing precedes the classical hypertensive response to chemoreceptor stimulation, thus showing the relevance of lPB neuronal circuits on the central modulation of

There are also indications that HDA stimulation may facilitate the chemoreceptor reflex by means of a group of intrinsic excitatory neurons localized within the NTS [115]. These cells are activated or facilitated by HDA-NTS direct excitatory connections. These neurons are also the main targets of excitatory inputs from the lPB [56]. The inhibition of these lPB excitatory projections with muscimol leads to the abolishment of the tachypneustic response evoked on HDA stimulation. Electrophysiological studies using neuronal recordings support the above. A significant number of mPB-KF and lPB neurons are affected from HDA stimulation, confirming the importance of the functional correlation between the HDA and these pontine regions. The presence of anti-/orthodromic activations, short and long latency excitations, and inhibitions and excitatory/inhibitory activities gives electrophysiological evidence of reciprocal connections between these regions. It is also an index of the complexity of the different types of synaptic

Studies related to glutamate receptors suggest that this neurotransmitter plays a crucial role in mediating the functional relation between the PBc and the HDA [116]. Glutamate activates metabotropic and ionotropic (NMDA and non-NMDA) receptors [117]. By employing immunocytochemical and in situ hybridization techniques, studies have demonstrated the presence of both metabotropic and ionotropic receptors in different nuclei of the PBc and KF [118–120]. Activation of vagal afferent fibers releases glutamate within the PBc [121]. An ascending excitatory pathway involving glutamate from the NTS to the PBc has been described [122]. In

flex [71]. This inhibitory effect is more evident from the mPB-KF than from lPB.

indirect pathway, activate the IML.

54 Hypothalamus in Health and Diseases

NTS and the IML of the spinal cord [112].

chemoreceptor inputs and reflex [114].

interactions between both areas (**Figure 4**) [105].

**Figure 4.** HDA and PBc neurophysiological interactions. (A) Shows a rate histogram (bin size 2 s) representing the firing of an lPB cell not excited nor inhibited during HDA stimulation that increased the activity during HDA stimulation. (B) Shows a rate histogram (bin size 2 s) of an mPB-KF cell not excited nor inhibited during HDA stimulation showing a decrease of activity during HDA stimulation (0.1 ms given at 1 Hz). (C) The poststimulus time histogram shows spontaneous activity of an lPB neuron and double excitation after HDA stimulation. (D) The poststimulus time histogram shows an inhibition of an mPB neuron after HDA stimulation (100 stimuli, 1 Hz). Authors´ figure modified from Ref. [105].

vitro studies also show that glutamate agonists depolarize neurons of the PBc [123], and lPB stimulation causes local glutamate release, which depolarizes lPB neurons through NMDA and non-NMDA receptors [124].

Moreover, the blockade of glutamate receptors and the microinjections of glutamate into the PBc and KF elicit a variety of cardiovascular and respiratory responses indicating that this amino acid is an important neurotransmitter for mediating autonomic functions in these regions [61, 63, 64, 122–127].

The pattern of the cardiorespiratory response evoked from HDA is modified by the microinjection of different glutamate antagonists into the PBc [116]. Kynurenic acid, a nonspecific ionotropic glutamate receptor antagonist, microinjected into the lPB and mPB abolishes the tachycardia and decreased the pressor response to HDA electrical stimulation (**Figure 5A** and **B**). The respiratory response is only abolished when kynurenic acid is microinjected into the lPB (**Figure 5A**) [116]. These results suggest that ionotropic glutamate receptors located within the lPB region are involved in both the respiratory- and the cardiovascular-evoked responses from the HDA, whereas ionotropic glutamate receptors located in mPB seem to be only involved in the modulation of the cardiovascular response.

The effectiveness of the modulation is depending on the distribution of these receptors within the PBc and these findings suggest that lPB appears to exert a more efficient modulation on the cardiovascular response to HDA stimulation compared with mPB. This cardiovascular response seems to be mediated by a direct activation of neurons located within the RVLM, which send direct efferences to sympathetic preganglionic neurons of the IML [128–130]. The activity of the RVLM can be also modulated via indirect projections. The changes in heart rate and blood pressure evoked from "defense" regions of the brain may use separate efferent pathways [51]. The blockade of the PBc attenuates the dorsal PAG-evoked changes in blood pressure [109], thus indicating that the cardiovascular changes observed during the stimulation of the HDA could be partially modulated by "direct" efferences to the RVLM but also by indirect projections, which involve the activation of ionotropic glutamate receptors located in the PBc [116].

It is known that the PBc is crucial mediating the changes of heart rate appearing during baroreceptor reflex activation [105]. The fall in the magnitude of the cardiovascular changes to HDA stimulation observed after the microinjection of kynurenic acid could indicate that neurons of the lPB and mPB exert an inhibition of tonic excitatory inputs, at the level of the NTS, on inhibitory mechanism of the baroreceptor reflex [40]. This hypothesis is also supported by the observation that the blood pressure response also tends to disappear with the decrease and/or the abolishment of tachycardia.

efficacy [131], in calcium currents and deactivation kinetics as well as other single channel characteristics [132]. NMDA receptors of lPB are composed of NR2A and NR2B subunits, which are characterized by high affinity for glutamate and long mean open time. NMDA receptors located within the mPB are composed of NR2D subunits, which exhibit low affinity

**Figure 5.** Neuropharmacological interactions between HDA and PBc, role of glutamate. From top to bottom,

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and blood pressure (mmHg). The cardiorespiratory responses evoked on HDA stimulation before (left) and after (right) kynurenic acid microinjection within the lPB (A) and mPB-KF (B) are shown. The arrows show the onset of the HDA

O), instantaneous heart rate (bpm)

instantaneous respiratory rate (rpm), respiratory flow (ml/s), pleural pressure (cm H<sup>2</sup>

electrical stimulation. Authors´ figure modified from Ref. [116].

In summary, the arterial blood pressor response observed during HDA stimulation could be mediated by the activation of neuronal glutamate ionotropic receptors located in both lPB and mPB somata, which exert an indirect excitation to sympathetic preganglionic neurons at the level of the IML. The inhibitory mechanism of the baroreceptor reflex seems to depend more on the activation of lPB glutamate ionotropic receptors than mPB receptors, because tachycardia associated to the pressor response is only suppressed after lPB microinjections [116].

With respect to the changes of respiratory rate observed during the stimulation of the HDA, we have to highlight that are only abolished when the microinjection of kynurenic acid is delivered within the lPB (**Figure 5A**). Nevertheless, the respiratory response remains unchanged when

for glutamate [119, 132].

Another fact that could explain the more efficient modulation exerted from lPB on the cardiovascular response elicited by HDA stimulation is the specific expression of glutamate subtype receptors located within this region. A very different profile is observed when compared with the mPB or with other subnuclei of the PBc. GluR4 non-NMDA receptor subunits predominate in the internal lPB [118]. These subunits are characterized by a high sensitivity for glutamate. There is also evidence that the external and internal lPB express specific subunits of NMDA receptors, which are different to that of the mPB [119]. NMDA receptors can be quite different with respect to their physiological and pharmacological channel properties, such as differences in glutamate affinity and glycine sensitivity, crucial coagonist for glutamate

vitro studies also show that glutamate agonists depolarize neurons of the PBc [123], and lPB stimulation causes local glutamate release, which depolarizes lPB neurons through NMDA

Moreover, the blockade of glutamate receptors and the microinjections of glutamate into the PBc and KF elicit a variety of cardiovascular and respiratory responses indicating that this amino acid is an important neurotransmitter for mediating autonomic functions in these

The pattern of the cardiorespiratory response evoked from HDA is modified by the microinjection of different glutamate antagonists into the PBc [116]. Kynurenic acid, a nonspecific ionotropic glutamate receptor antagonist, microinjected into the lPB and mPB abolishes the tachycardia and decreased the pressor response to HDA electrical stimulation (**Figure 5A** and **B**). The respiratory response is only abolished when kynurenic acid is microinjected into the lPB (**Figure 5A**) [116]. These results suggest that ionotropic glutamate receptors located within the lPB region are involved in both the respiratory- and the cardiovascular-evoked responses from the HDA, whereas ionotropic glutamate receptors located in mPB seem to be only involved in the modula-

The effectiveness of the modulation is depending on the distribution of these receptors within the PBc and these findings suggest that lPB appears to exert a more efficient modulation on the cardiovascular response to HDA stimulation compared with mPB. This cardiovascular response seems to be mediated by a direct activation of neurons located within the RVLM, which send direct efferences to sympathetic preganglionic neurons of the IML [128–130]. The activity of the RVLM can be also modulated via indirect projections. The changes in heart rate and blood pressure evoked from "defense" regions of the brain may use separate efferent pathways [51]. The blockade of the PBc attenuates the dorsal PAG-evoked changes in blood pressure [109], thus indicating that the cardiovascular changes observed during the stimulation of the HDA could be partially modulated by "direct" efferences to the RVLM but also by indirect projections,

which involve the activation of ionotropic glutamate receptors located in the PBc [116].

It is known that the PBc is crucial mediating the changes of heart rate appearing during baroreceptor reflex activation [105]. The fall in the magnitude of the cardiovascular changes to HDA stimulation observed after the microinjection of kynurenic acid could indicate that neurons of the lPB and mPB exert an inhibition of tonic excitatory inputs, at the level of the NTS, on inhibitory mechanism of the baroreceptor reflex [40]. This hypothesis is also supported by the observation that the blood pressure response also tends to disappear with the decrease

Another fact that could explain the more efficient modulation exerted from lPB on the cardiovascular response elicited by HDA stimulation is the specific expression of glutamate subtype receptors located within this region. A very different profile is observed when compared with the mPB or with other subnuclei of the PBc. GluR4 non-NMDA receptor subunits predominate in the internal lPB [118]. These subunits are characterized by a high sensitivity for glutamate. There is also evidence that the external and internal lPB express specific subunits of NMDA receptors, which are different to that of the mPB [119]. NMDA receptors can be quite different with respect to their physiological and pharmacological channel properties, such as differences in glutamate affinity and glycine sensitivity, crucial coagonist for glutamate

and non-NMDA receptors [124].

56 Hypothalamus in Health and Diseases

regions [61, 63, 64, 122–127].

tion of the cardiovascular response.

and/or the abolishment of tachycardia.

**Figure 5.** Neuropharmacological interactions between HDA and PBc, role of glutamate. From top to bottom, instantaneous respiratory rate (rpm), respiratory flow (ml/s), pleural pressure (cm H<sup>2</sup> O), instantaneous heart rate (bpm) and blood pressure (mmHg). The cardiorespiratory responses evoked on HDA stimulation before (left) and after (right) kynurenic acid microinjection within the lPB (A) and mPB-KF (B) are shown. The arrows show the onset of the HDA electrical stimulation. Authors´ figure modified from Ref. [116].

efficacy [131], in calcium currents and deactivation kinetics as well as other single channel characteristics [132]. NMDA receptors of lPB are composed of NR2A and NR2B subunits, which are characterized by high affinity for glutamate and long mean open time. NMDA receptors located within the mPB are composed of NR2D subunits, which exhibit low affinity for glutamate [119, 132].

In summary, the arterial blood pressor response observed during HDA stimulation could be mediated by the activation of neuronal glutamate ionotropic receptors located in both lPB and mPB somata, which exert an indirect excitation to sympathetic preganglionic neurons at the level of the IML. The inhibitory mechanism of the baroreceptor reflex seems to depend more on the activation of lPB glutamate ionotropic receptors than mPB receptors, because tachycardia associated to the pressor response is only suppressed after lPB microinjections [116].

With respect to the changes of respiratory rate observed during the stimulation of the HDA, we have to highlight that are only abolished when the microinjection of kynurenic acid is delivered within the lPB (**Figure 5A**). Nevertheless, the respiratory response remains unchanged when kynurenic acid is microinjected into the mPB (**Figure 5B**) [116]. The result suggests that only glutamate receptors of the lPB modulate the respiratory response to HDA stimulation.

Some studies in rats have used HDA electrical stimulation to map methodically populations of neurons within the brainstem and other areas, which are excitated by changes in arterial blood pressure [134, 135]. In the A5 region, blood pressure changes cause a specific and con-

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A c-Fos-ir expression is induced during HDA stimulation in both A5 noncatecholaminergic (TH-negative) and A5 catecholaminergic (TH-positive) cells of the pons [136]. This increase in c-Fos expression is higher in noncatecholaminergic than in catecholaminergic neurons [136]. In addition, in both populations of neurons of the A5 region, this activation seems probably to be due to a direct activation from the HDA and not due to a secondary activation to the pres-

This result is further confirmed with neuronal recordings. It is described as the possible role of A5 neurons in respiratory modulation [65, 93]. Moreover, there are electrophysiological evidences of interactions between HDA and A5 catecholaminergic neurons. The importance of the connections between both regions is confirmed with the observation that a significant number of these A5 neurons are activated from HDA stimulation [136]. In the same way as with PBc, antidromic and orthodromic activation are observed in A5 neurons. Cells that are antidromically activated are spontaneously active, while cells orthodromically activated are silent, indicating the origin of the somata (**Figure 6**). After clonidine, A5 cells are active and decrease their frequency of discharge while, in all cases, hypothalamic fibers are silent [136]. The presence of activations or facilitations indicates the existence of polysynaptic pathways acting on the A5 region. The complexity of the different types of synaptic connections is illustrated by the association of these activations with inhibitions

On the other hand, as previously mentioned, the stimulation of cell bodies located within the A5 region resembles the cardiovascular response elicited by HDA electrical stimulation, thus eliciting an increase in heart rate and blood pressure [104] and suggesting the possible interaction between both cardiorespiratory regions. In order to evaluate this possible modulation,

Muscimol microinjection within the A5 region does not produce changes in the respiratory response to HDA electrical stimulation; however, a clear decrease is observed in the cardiovascular response (**Figure 7**). The increase in heart rate and the hypertension evoked to HDA activation involve a direct excitation of neurons located in the RVLM, which send direct projections to the preganglionic neurons of the IML that are responsible for the acute pressor response [137]. Also, the release of adrenaline by a direct activation of the adrenal medulla provides a secondary increase of blood pressure contributing to the hypertensive

Indirect forebrain projections can also modulate the activity of the RVLM. Furthermore, HDA stimulation activates the chemoreceptor reflex by means of the excitation or facilitation of chemoreceptor neurons located in the NTS, in a parallel circuit to the activation of the RVLM and the preganglionic neurons in the IML [38]. An inhibition of the baroreceptor

microinjection of muscimol also has been made into the A5 region [136].

sistent pattern of c-Fos expression.

or disfacilitations.

response.

sure response elicited during stimulation of the HDA.

It has been shown that the lPB is an important part of the neuronal pathways for the modulation of the respiratory response evoked on HDA stimulation. Muscimol microinjections within the lPB have similar effects to kynurenic microinjections [105]; tachypnea observed during HDA stimulation is abolished. This observation gives a role for the described lPB afferent connections from several hypothalamic nuclei involved in the defense reaction [110].

Hayward et al. obtained similar results with the blockade of glutamate receptors with the microinjection of kynurenic acid into the lPB during the dorsal PAG stimulation, one of the so-called secondary brain defense regions, confirming the importance of lPB in the integration of tachypneic responses from supraencephalic regions [133].

There are indications that HDA stimulation may facilitate the chemoreceptor reflex at specific cells located within the NTS [115]. These neurons are activated by HDA-NTS direct excitatory connections and are also the main targets of excitatory inputs from the lPB [56]. Glutamate seems to activate these excitatory inputs. The inhibition of the activation of these lPB projections with kynurenic acid leads to the abolishment of tachypnea evoked on HDA stimulation [116].

According to these observations, the cardiovascular component of the response to HDA stimulation seems to be modulated by glutamatergic neurons located in both the lPB and the mPB, whereas the respiratory component seems to be only mediated by glutamate receptors of the mPB. Moreover, different subnuclei within the lPB are involved in this cardiorespiratory modulation, which includes the crescent, ventral, central and external subnuclei. It is interesting to note that microinjections into the internal subnucleus of the lPB have no effects on this cardiorespiratory response. This result is an indication of the specificity and complexity of this region. Nearby areas, separated only by microns, such as the external and internal subnuclei of the lPB, show very different effects in the cardiorespiratory response to HDA stimulation. In contrast, all mPB microinjections, including external mPB, have an effect. These results give us clear evidence that glutamatergic neurons of the PBc are essential intermediaries for the modulation of the descending pathways for cardiovascular sympathetic and respiratory control mechanisms [116]. The impact of these projections on overall cardiorespiratory function is highly dependent on convergent inputs from specific subnuclei of the lPB region and from alternate pathways outside the PBc. Direct projections to the RVLM are also involved in HDA-evoked changes in arterial pressure [128–130], thus supporting those changes in heart rate and blood pressure evoked from "defense" regions of the brain that may travel via separate pathways [51].
