Ketamine inhibits presynaptic and postsynaptic nicotinic excitation of identified cardiac parasympathetic neurons in nucleus ambiguus.
BACKGROUND
Ketamine increases both blood pressure and heart rate, effects commonly thought of as sympathoexcitatory. The authors investigated possible central nervous system actions of ketamine to inhibit cardiac parasympathetic neurons in the brainstem by inhibiting multiple nicotinic excitatory mechanisms.


METHODS
The authors used a novel in vitro approach to study the effect of ketamine on identified cardiac parasympathetic preganglionic neurons in rat brainstem slices. The cardiac parasympathetic neurons in the nucleus ambiguus were retrogradely prelabeled with the fluorescent tracer by placing rhodamine into the pericardial sac. Dye-labeled neurons were visually identified for patch clamp recording. The effects of ketamine were tested on nicotine-evoked ligand-gated currents and spontaneous glutamatergic miniature synaptic currents (mini) in cardiac parasympathetic preganglionic neurons.


RESULTS
Ketamine (10 microm) inhibited (1) the nicotine (1 microm)-evoked presynaptic facilitation of glutamate release (mini frequency, 18 +/- 7% of control; n = 9), and (2) the direct postsynaptic ligand-gated current (27 +/- 8% of control; n = 9), but ketamine did not alter the amplitude of postsynaptic miniature non-N-methyl-D-aspartate currents. alpha Bungarotoxin, an antagonist of alpha 7 containing nicotinic presynaptic receptors, blocked ketamine actions on mini frequency (n = 10) but not mini amplitude.


CONCLUSIONS
Ketamine inhibits the presynaptic nicotinic receptors responsible for facilitating neurotransmitter release, as well as the direct ligand-gated inward current, but does not alter the nicotinic augmentation of non-N-methyl-D-aspartate currents in brainstem parasympathetic cardiac neurons. Such actions may mediate the decrease in parasympathetic cardiac activity and increase in heart rate that occurs with ketamine.
Irnaten
Mustapha
M
Department of Pharmacology, George Washington University, Washington, DC 20037, USA.
Wang
Jijiang
J
Venkatesan
Priya
P
Evans
Cory
C
K  Chang
Kyoung S
KS
Andresen
Michael C
MC
Mendelowitz
David
D
D000778:Anesthetics, Dissociative / Q000494:pharmacology*; D000818:Animals; D002038:Bungarotoxins / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005260:Female; D018698:Glutamic Acid / Q000494:pharmacology; D006321:Heart / Q000187:drug effects / Q000294:innervation*; D006339:Heart Rate / Q000187:drug effects; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007649:Ketamine / Q000494:pharmacology*; D009538:Nicotine / Q000494:pharmacology; D018722:Nicotinic Agonists / Q000494:pharmacology; D018733:Nicotinic Antagonists / Q000494:pharmacology*; D010275:Parasympathetic Nervous System / Q000187:drug effects*; D011247:Pregnancy; D051381:Rats; D016194:Receptors, N-Methyl-D-Aspartate / Q000187:drug effects; D011978:Receptors, Nicotinic / Q000187:drug effects*; D017661:Receptors, Presynaptic / Q000187:drug effects*
D000778:Anesthetics, Dissociative; D002038:Bungarotoxins; D018722:Nicotinic Agonists; D018733:Nicotinic Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D011978:Receptors, Nicotinic; D017661:Receptors, Presynaptic; D018698:Glutamic Acid; D007649:Ketamine; D009538:Nicotine