Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC15
Sympathoexcitation induced by optogenetic stimulation of astrocytic networks within the brainstem is partially mediated by glutamatergic mechanisms.
N. Marina1, M. Figueiredo2, A. G. Teschenmacher2, S. Kasparov2, A. Gourine1
1. Neuroscience, Physiology and Pharmacology, University College London, London, United Kingdom. 2. Physiology and Pharmacology, University of Bristol, Bristol, United Kingdom.
Astrocytes control complex central nervous functions by regulating synaptic strength and information processing (Halassa et al., 2007). Optogenetic activation of astrocytes located in the rostral ventrolateral medulla (RVLM) produces significant elevations in sympathetic nerve activity and blood pressure (Marina et al., 2010). Here, we sought to identify the putative gliotransmitter involved in glial control of central sympathetic tone. Experiments were conducted under the UK Animals (Scientific Procedures) Act 1986. Adult male Sprague Dawley rats were anaesthetised with a mixture of ketamine (60 mg kg-1; i.m.) and medetomidine (250 μg kg-1, i.m.) and were stereotaxically targeted with an adenoviral vector to express mutated channelrhodopsin2 -ChR2(H134R) - in the RVLM under transcriptional control of the enhanced astrocytic-specific GFAP promoter (Liu et al., 2008). Experiments were carried out 10-12 days after viral transduction under α-chloralose anaesthesia (100 mg kg-1 iv, supplemented with 20 mg kg-1 iv as required). Animals were ventilated artificially with O2-enriched air, the ventral surface of the brainstem was exposed and renal sympathetic nerve activity (RSNA) and mean arterial blood pressure (MABP) were recorded. The ventral medullary surface was stimulated unilaterally with pulses of blue light (20ms duty cycle) after application of artificial cerebrospinal fluid (aCSF, i.e., control), PPADS (100 μM), MRS2179 (200 μM) or AP-5 (500 μM) onto the ventral medullary surface. RESULTS: Sympathoexcitatory responses to photostimulation of ChR2-expressing RVLM astrocytes were unaffected by application of purinergic receptor antagonists: RSNA increased by 19.4 ± 1.5 % (aCSF, n=7) vs 19.7 ± 4.8 % (PPADS, n=3) and 24.6 ± 5.6 % (MRS2179, n=4); MABP increased by 13.8 ± 3.1 mmHg (aCSF, n=7) vs 18.5 ± 7.5 mm Hg (PPADS, Ttest=0.4; n=3) and 19.4% ± 8.7 mmHg (MRS2179, Ttest=0.2; n=4). However, sympathoexcitatory responses to optogenetic stimulation of RVLM astrocytes were substantially decreased in the presence of NMDA receptor antagonist: RSNA increased by 48.2 ± 17.4% (aCSF, n=4) vs 21.8 ± 14.8 % (AP-5, n=4). However, pressor responses were not affected by AP-5: MABP increased by 12.2 ± 3 mmHg (aCSF) vs 7.6 ± .05 (AP-5, Ttest=0.1, n=4). Immunohistochemical analysis revealed abundant ChR2-expressing astrocytes located in close proximity to tyrosine hydroxilase-positive neurones within the RVLM. CONCLUSIONS: These data suggest that glutamatergic mechanisms mediate, at least in part, the activation of local pre-sympathetic neuronal networks of the RVLM in response to optogenetic activation of local astrocytic networks. The role of purinergic gliotransmission in sympathoexcitation is still under scrutiny.
Where applicable, experiments conform with Society ethical requirements