The midbrain periaqueductal grey has no role in the generation of the respiratory motor pattern, but provides command function for the modulation of respiratory activity☆
Introduction
The midbrain periaqueductal grey (PAG), in conjunction with superior and inferior collicular structures, is implicated in initiating coordinated alterations in respiratory, cardiovascular and thermoregulatory parameters. These alterations presumably facilitate behaviours that are simultaneously elicited from the PAG in response to stressful stimuli (Bandler et al., 2000, Dampney et al., 2013).
The PAG is organised into anatomically and functionally distinct columns (Carrive, 1993), that upon stimulation produce specific patterns of autonomic and somatic motor activity. Some of these are implicated in the modulation of respiratory motor patterns corresponding to vocalisation (Carrive et al., 1989a, Carrive et al., 1989b, Carrive et al., 1987, Hayward et al., 2004, Subramanian et al., 2008) or the alteration of ventilation in anticipation of exercise (Green et al., 2007, Paterson, 2014). There is also limited evidence that the PAG may modulate airway defence reflexes such as cough (Jakus et al., 2008, Sessle et al., 1981).
Much attention has been focussed on the anatomical projections of various PAG regions in an effort to grasp the means by which these neurons modulate autonomic and somatic activity. Many studies focus on the modulation of cardiovascular parameters and/or consider respiration purely in terms of vocalisation (Bandler and Carrive, 1988, Carrive et al., 1989a, Carrive et al., 1989b). However, both the magnitude and nature of changes in the respiratory pattern evoked following activation of the dorsomedial (dmPAG), lateral (lPAG) and dorsolateral (dlPAG) PAG show marked variation while changes in sympathetic nerve discharge remain very similar throughout (Iigaya et al., 2010). Similarly, stimulation of the caudal portion of the dmPAG and dlPAG produces changes in respiratory parameters which are greater than those evoked more rostrally, while cardiovascular responses remained consistent (Iigaya et al., 2010, Zhang et al., 2007).
A wide variety of respiratory and vocalisation-related modulations of the respiratory motor pattern have been reported in cats (Subramanian et al., 2008, Subramanian et al., 2007, Zhang et al., 1994). Stimulation or disinhibition of the dmPAG in anaesthetised rats produces increased respiratory frequency and increased tonic drive to the diaphragm (Hayward et al., 2003). Similarly, tachypnoeic responses have been observed on stimulation of the dorsolateral PAG in rats (Huang et al., 2000). Importantly, differentiation in the character of the respiratory response has also been observed in different PAG columns, with stimulation of the dlPAG producing a larger effect upon the respiratory rate and stimulation of the lPAG producing a larger effect upon the amplitude of phrenic nerve discharge (Iigaya et al., 2010).
In the current study, we report that activation of the PAG produces region-specific respiratory patterns in the decerebrate in situ perfused brainstem preparation of the rat. In contrast to previous studies, responses were successfully evoked in the absence of the hypothalamus, indicating that these responses were produced via direct projections to the pontomedullary respiratory rhythm and pattern generator. We propose that this preparation, which is unanaesthetised, provides a novel setting for the study of PAG circuits and the means by which these circuits modulate changes in the respiratory motor pattern. Additionally, we investigated whether the PAG provides a contribution to the generation or shaping of the respiratory pattern.
Section snippets
Materials and methods
All experimental procedures were performed in accordance with the Australian code of practice for the care and use of animals for scientific purposes and conform to the principles of international regulations. This study was approved by and carried out in accordance with guidelines put in place by the ethics committee of the Florey Institute of Neuroscience and Mental Health (AEC 12-004).
Results
Microinjections of 30–60 nl l-glutamate into the PAG (n = 29) in 12 perfused brainstem preparations of rat reliably evoked a variety of site-specific respiratory responses. In contrast, local inhibition of the same PAG sites by injection of the GABA receptor agonist isoguvacine (n = 24 in 9 preparations) never produced any consistent change in ongoing respiratory activity. As previously reported for the perfused brainstem preparation (Abdala et al., 2009), AbNA was either absent or showed weak tonic
Discussion
The primary finding of the current study is that distinct, site-specific patterns of activity in respiratory motor outputs can be evoked by stimulation of the PAG in the decerebrate perfused brainstem preparation of the rat. While some of these responses correspond with patterns evoked in previous studies in vivo (Huang et al., 2000, Iigaya et al., 2010), activation or inhibition of the same sites in the perfused brainstem preparation also produced responses that differed significantly. Thus,
Summary
In summary, we conclude that marked changes in the respiratory motor pattern are reproducibly evoked from the midbrain PAG in the unanaesthetised and decerebrate perfused brainstem preparation of the rat. In contrast to previous studies, these responses were evoked in the absence of the dorsomedial hypothalamus and are most likely mediated via direct projections to the brainstem, the pontine parabrachial complex and possibly the RTN/pFRG. Inhibition of responsive sites within the PAG did not
Author contribution
DF, TB, MD performed perfused brainstem experiments and analysed data. SJ provided histology and anatomical verification of the injection sites. DF TB SJ DS and MD interpreted the results and wrote the manuscript.
Acknowledgements
The study was supported by a future fellowship of the Australian Research Council to MD (FT120100953) and a start-up fund of the Florey Institute of Neuroscience and Mental Health. We acknowledge the support of the Victorian Government through the Operational Infrastructure Scheme.
References (46)
- et al.
Integrated defence reaction elicited by excitatory amino acid microinjection in the midbrain periaqueductal grey region of the unrestrained cat
Brain Res.
(1988) - et al.
Central circuits mediating patterned autonomic activity during active vs. passive emotional coping
Brain Res. Bull.
(2000) - et al.
Defense reaction induced by microinjections of bicuculline into the inferior colliculus
Physiol. Behav.
(1988) The periaqueductal gray and defensive behavior: functional representation and neuronal organization
Behav. Brain Res.
(1993)- et al.
Somatic and autonomic integration in the midbrain of the unanesthetized decerebrate cat: a distinctive pattern evoked by excitation of neurones in the subtentorial portion of the midbrain periaqueductal grey
Brain Res.
(1989) - et al.
Viscerotopic control of regional vascular beds by discrete groups of neurons within the midbrain periaqueductal gray
Brain Res.
(1989) - et al.
Excitation of neurones in a restricted portion of the midbrain periaqueductal grey elicits both behavioural and cardiovascular components of the defence reaction in the unanaesthetised decerebrate cat
Neurosci. Lett.
(1987) - et al.
Role of dorsolateral periaqueductal grey in the coordinated regulation of cardiovascular and respiratory function
Auton. Neurosci. Basic Clin.
(2013) - et al.
Pulmonary and upper airway afferent influences on the motor pattern of vocalization evoked by excitation of the midbrain periaqueductal gray of the cat
Brain Res.
(1993) - et al.
Inhibitory synaptic mechanisms regulating upper airway patency
Respir. Physiol. Neurobiol.
(2002)
Periaqueductal gray matter projection to vagal preganglionic neurons and the nucleus tractus solitarius
Brain Res.
Periaqueductal gray matter input to cardiac-related sympathetic premotor neurons
Brain Res.
Connections of the rostral ventral respiratory neuronal cell group: an anterograde and retrograde tracing study in the rat
Brain Res. Bull.
Roles of periaqueductal gray and nucleus tractus solitarius in cardiorespiratory function in the rat brainstem
Respir. Physiol.
Brainstem circuitry of tracheal–bronchial cough: c-fos study in anesthetized cats
Respir. Physiol. Neurobiol.
Respiratory pattern in awake rats: effects of motor activity and of alerting stimuli
Physiol. Behav.
The lateral respiratory neurones of the medulla: their associations with nucleus ambiguus, nucleus retroambigualis, the spinal accessory nucleus and the spinal cord
Brain Res.
A working heart–brainstem preparation of the mouse
J. Neurosci. Methods
Suppressive influences from periaqueductal gray and nucleus raphe magnus on respiration and related reflex activities and on solitary tract neurons, and effect of naloxone
Brain Res.
Characterizations of eupnea, apneusis and gasping in a perfused rat preparation
Respiration Physiology
Identification of different types of respiratory neurones in the dorsal brainstem nucleus tractus solitarius of the rat
Brain Res.
Respiratory responses elicited by rostral versus caudal dorsal periaqueductal gray stimulation in rats
Auton. Neurosci. Basic Clin.
Phox2b-expressing neurons of the parafacial region regulate breathing rate, inspiration, and expiration in conscious rats
J. Neurosci. Off. J. Soc. Neurosci.
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This paper is part of a special issue entitled “Non-homeostatic control of respiration”, guest-edited by Dr. Eugene Nalivaiko and Dr. Paul Davenport.