[Modulation of theta-activity in the septo-hippocampal system by the alpha-2-adrenoreceptor agonist clonidine].
The influence of the alpha 2-adrenoreceptors agonist clonidine on the neuronal activity of the medial septal area (MS) and hippocampal EEG was studied in unanaesthetized rabbits. A slight and short-term decrease in the theta-rhythm modulation in the MS neuronal activity and/or EEG was revealed in 30.4% of tests after the bilateral intraventricular injection of a small dose of clonidine (0.5 microgram/5 microliters of water). On the contrary, a high dose of clonidine (5 micrograms/5 microliters) substantially enhanced the theta modulation in 100% of tests. The frequency of the theta bursts in the MS increased, on average, by 1.6 +/- 0.18 Hz (from 5.25 +/- 0.19 to 6.8 +/- 0.17 Hz). The regularity of the theta modulation became almost twice higher: the time constant of damping increased from 0.34 +/- 0.04 to 0.60 +/- 0.08 s. Increase in the neuronal activity in the MS produced by the high dose of clonidine was always accompanied by appearance of continuous stable theta waves in the EEG; the spectral power in the theta range increased, on average, by 480 +/- 98%. This strong effect arose suddenly but was relatively short-lasting (12 +/- 3.5 min) and usually abruptly terminated. It is concluded that the noradrenergic system has a double control over the theta oscillations through the alpha 2-adrenoreceptors agonist. In low concentrations the agonist clonidine acts on the high affinity inhibitory presynaptic autoreceptors reducing the noradrenaline release and suppressing the theta activity. In a high dose clonidine predominantly acts on postsynaptic (low affinity) adrenoreceptors substantially increasing the frequency and regularity of the theta bursts in the activity of septal neurons. Presumably, different types of alpha 2-adrenoreceptors participate in regulation of the theta oscillations in different functional states. It is suggested that the noradrenergic locus coeruleus is a functional synergist of the activating reticular formation participating in the urgent phasic activation of the septohippocampal system during the action of sudden strong stimuli.
Kichigina
V F
VF
Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences, Pushchino.
Kutyreva
E V
EV
D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D006624:Hippocampus / Q000187:drug effects* / Q000502:physiology; D011817:Rabbits
D058666:Adrenergic beta-2 Receptor Agonists; D000318:Adrenergic beta-Agonists; D003000:Clonidine