**3. Ventrolateral pons in cardiorespiratory hypothalamic defense responses: role of the A5 region**

As previously mentioned, there are data suggesting the functional connections between the HDA and the A5 region. Fos protein expression studies, neuronal recording and neuropharmacological experiments confirm this hypothesis [23, 65, 104].

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 consistent pattern of c-Fos expression.

kynurenic acid is microinjected into the mPB (**Figure 5B**) [116]. The result suggests that only

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

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].

As previously mentioned, there are data suggesting the functional connections between the HDA and the A5 region. Fos protein expression studies, neuronal recording and neurophar-

**3. Ventrolateral pons in cardiorespiratory hypothalamic defense** 

macological experiments confirm this hypothesis [23, 65, 104].

**responses: role of the A5 region**

glutamate receptors of the lPB modulate the respiratory response to HDA stimulation.

of tachypneic responses from supraencephalic regions [133].

58 Hypothalamus in Health and Diseases

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 pressure response elicited during stimulation of the HDA.

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 or disfacilitations.

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

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 response.

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

The inhibition of A5 neurons with muscimol microinjections attenuates the cardiovascular response elicited by the stimulation of the HDA (**Figure 7**) [136]. This attenuation can be an indication of an incomplete resetting of the baroreceptor reflex. This effect can explain the decrease in the magnitude of the tachycardia and the hypertension, through an indirect pathway. But the most relevant aspect of this response is probably the inhibition of the excitatory projections from the A5 region to the IML. These findings suggest that an indirect pathway through the A5 region could also mediate the resetting of the baroreceptor reflex evoked by HDA stimulation. The activity of neurons of the A5 region modulates the intensity of the cardiovascular response evoked on HDA stimulation through an indirect pathway to both

**Figure 7.** Neuropharmacological interactions between HDA and A5 region. Instantaneous respiratory rate (upper trace,

Role of the Dorso- and Ventrolateral Pons in Cardiorespiratory Hypothalamic Defense Responses

showing the cardiorespiratory response evoked on HDA stimulation before (left) and after (right) the microinjection of

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

http://dx.doi.org/10.5772/intechopen.72625

61

In summary, the A5 region seems to be an important component of those brainstem pathways known to be involved in mediating autonomic changes associated with the defense response elicited from the PeF and the DMH. This response involves also the integrity of the circuits located within the PBc. It is not possible to separate the activity of the PBc and the A5 region; thus, dorso- and ventrolateral pons act together to mediate the cardiorespiratory response

Amelia Díaz-Casares, Manuel Víctor López-González and Marc Stefan Dawid-Milner\*

the IML and the NTS.

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

muscimol in the A5 region. Authors´ figure from Ref. [136].

evoked on HDA stimulation.

\*Address all correspondence to: msdawid@uma.es

Department of Physiology, School of Medicine, Málaga, Spain

**Author details**

**Figure 6.** HDA and A5 neurophysiological interactions. Extracellular recordings (superimposed sweeps) of four putative cells recorded form the A5 region. (A) Silent neuron (upper trace) with constant latency responses to the HDA (lower trace). The cell was demonstrated to be orthodromically activated from the HDA. (B) Spontaneously active cell (upper trace) excitated with short and long latency responses from HDA stimulation (lower trace). (C) Spontaneously active cell (upper trace) inhibited from HDA stimulation (lower trace). (D) Recording of respiratory flow, pleural pressure, neuronal activity and blood pressure of a putative respiratory-modulated A5 cell with respiratory flow (ml/s, inspiration downwards) and HDA-triggered histograms (lower trace). This respiratory putative A5 neuron shows no modulation from the HDA. Authors´ figure from Ref. [136].

response is also produced, in another parallel pathway, by the inhibition or disfacilitation of baroreceptor neurons located within the NTS [42, 58], inhibition that seems to be mediated through GABAergic interneurons in the NTS [42].

In conscious rats, stress produces tachycardia and hypertension together with a resetting, rather than an inhibition, of the baroreceptor reflex. Thus, heart rate control is reset to higher levels of blood pressure without decrease in the gain of the reflex [54, 138].

The activation of A5 somata with glutamate also produces tachycardia and hypertension [104]. The increase in heart rate, blood pressure and sympathetic vasomotor activity at the same time indicates a baroreceptor reflex reset but without reduction in sensitivity of the reflex.

Role of the Dorso- and Ventrolateral Pons in Cardiorespiratory Hypothalamic Defense Responses http://dx.doi.org/10.5772/intechopen.72625 61

**Figure 7.** Neuropharmacological interactions between HDA and A5 region. Instantaneous respiratory rate (upper trace, rpm), respiratory flow (ml/s), pleural pressure (cm H<sup>2</sup> O), instantaneous heart rate (bpm) and blood pressure (mmHg) showing the cardiorespiratory response evoked on HDA stimulation before (left) and after (right) the microinjection of muscimol in the A5 region. Authors´ figure from Ref. [136].

The inhibition of A5 neurons with muscimol microinjections attenuates the cardiovascular response elicited by the stimulation of the HDA (**Figure 7**) [136]. This attenuation can be an indication of an incomplete resetting of the baroreceptor reflex. This effect can explain the decrease in the magnitude of the tachycardia and the hypertension, through an indirect pathway. But the most relevant aspect of this response is probably the inhibition of the excitatory projections from the A5 region to the IML. These findings suggest that an indirect pathway through the A5 region could also mediate the resetting of the baroreceptor reflex evoked by HDA stimulation. The activity of neurons of the A5 region modulates the intensity of the cardiovascular response evoked on HDA stimulation through an indirect pathway to both the IML and the NTS.

In summary, the A5 region seems to be an important component of those brainstem pathways known to be involved in mediating autonomic changes associated with the defense response elicited from the PeF and the DMH. This response involves also the integrity of the circuits located within the PBc. It is not possible to separate the activity of the PBc and the A5 region; thus, dorso- and ventrolateral pons act together to mediate the cardiorespiratory response evoked on HDA stimulation.

## **Author details**

response is also produced, in another parallel pathway, by the inhibition or disfacilitation of baroreceptor neurons located within the NTS [42, 58], inhibition that seems to be mediated

**Figure 6.** HDA and A5 neurophysiological interactions. Extracellular recordings (superimposed sweeps) of four putative cells recorded form the A5 region. (A) Silent neuron (upper trace) with constant latency responses to the HDA (lower trace). The cell was demonstrated to be orthodromically activated from the HDA. (B) Spontaneously active cell (upper trace) excitated with short and long latency responses from HDA stimulation (lower trace). (C) Spontaneously active cell (upper trace) inhibited from HDA stimulation (lower trace). (D) Recording of respiratory flow, pleural pressure, neuronal activity and blood pressure of a putative respiratory-modulated A5 cell with respiratory flow (ml/s, inspiration downwards) and HDA-triggered histograms (lower trace). This respiratory putative A5 neuron shows no modulation

In conscious rats, stress produces tachycardia and hypertension together with a resetting, rather than an inhibition, of the baroreceptor reflex. Thus, heart rate control is reset to higher

The activation of A5 somata with glutamate also produces tachycardia and hypertension [104]. The increase in heart rate, blood pressure and sympathetic vasomotor activity at the same time indicates a baroreceptor reflex reset but without reduction in sensitivity of the

levels of blood pressure without decrease in the gain of the reflex [54, 138].

through GABAergic interneurons in the NTS [42].

from the HDA. Authors´ figure from Ref. [136].

60 Hypothalamus in Health and Diseases

reflex.

Amelia Díaz-Casares, Manuel Víctor López-González and Marc Stefan Dawid-Milner\*

\*Address all correspondence to: msdawid@uma.es

Department of Physiology, School of Medicine, Málaga, Spain
