CHOLINERGIC MECHANISM IN THE HYPOTHALAMIC SUPRAMAMILLIARY NUCLEUS MODULATE PHYSIOLOGICAL ACTIVATION OF DORSAL HIPPOCAMPUS FIELD CA1

Abstract

Evidence suggests that reticular- and peripheral noxious-stimulation activate an ascending system that modulates hippocampal neural responses during behavioral arousal. Indeed, the two stimuli and behavioral arousal elicit hippocampal theta activation and the suppression of intra-hippocampal synaptic transmission between dorsal hippocampal fields CA3 to CA1. The suppression of synaptic transmission is marked by a decrease in the amplitude of CA1 population spike (PS) evoked on CA3 stimulation. Interestingly, such neural responses in CA1 are also elicited by microinjection of the cholinergic agonist carbachol into the hypothalamic supramammillary nucleus (SuM). Accordingly, the present study explored the hypothesis that cholinergic mechanisms in the SuM underpin the modulation of the neural activity in the dorsal hippocampus field CA1 to reticular stimulation and peripheral application of a noxious stimulus.

Investigations were carried out in vivo in anaesthetized rat using electrophysiological and immunocytochemical techniques. Pharmacological investigations showed that intra-SuM microinjection of carbachol evoked both theta activation and PS suppression at short latency that is consistent with a previous report from the laboratory. In addition, it was found that these effects of carbachol were antagonized by prior microinjection of the cholinergic receptor antagonists, namely atropine and mecamylamine which are antagonists¿ at muscarinic and nicotinic receptors, respectively. The effect of the antagonists was selective insofar that they did not antagonize the CA1 theta activation and PS suppression evoked by intra-SuM glutamate.

Subsequent investigations revealed that microinjection of mecamylamine, but not atropine, into the lateral SuM selectively attenuated the responses elicited in CA1 by stimulation in the region of the reticular pontis oralis (i.e. reticular stimulation) or on nociceptive activation with hind paw injection of formalin (5%, 0.05ml); whereas, microinjection of mecamylamine into the medial SuM did not affect the hippocampal responses elicited by the two stimuli. Furthermore, application of mecamylamine into the lateral SuM attenuated the CA1 responses induced by injection of formalin into the contralateral, but not the ipsilateral hind paw. The lateralization of drug effect is consistent with the predominant unilateral anatomical connections between the SuM and the septohippocampal region.

Immunocytochemical investigations showed that microinjection of carbachol alone into the lateral SuM, but not the medial SuM, elicited the induction of c-Fos protein, especially in the ipsilateral dentate gyrus granule cell layer. On the other hand, c-Fos expression was relatively low in the ipsilateral fields CA3 and CA1. c-Fos protein is a transcription protein that is expressed in neurons on synaptic excitation. Double labeling study provided evidence that intra-SuM carbachol induced c-Fos in GABAergic inhibitory interneurons of the hippocampus as well.

Taken together, the results provide novel evidence that nicotinic cholinoceptive neurons in the lateral SuM are common elements of the neural drive(s) to the hippocampus on reticular activation and noxious stimulation. Furthermore, cholinergic stimulation of the lateral SuM provides a strong excitatory drive impinging onto the dentate granule cells leading to c-Fos molecular plasticity and also activates population of GABAergic inhibitory hippocampal interneurons. The latter might be elements of the neural drive that modulate the excitability of principal neurons, including those of hippocampal pyramidal cells.