Chemosensitivity of hypophysiotropic neurons to the microelectrophoresis of biogenic amines

The purpose of this study was to determine whether norepinephrine (NE), dopamine (DA), serotonin (5-HT), acetylcholine (ACh) and glutamate (Glut) when applied in the immediate vicinity and in minute quantities to single neurons within the preoptic-hypothalamic complex via microelectrophoresis, could modulate their electrical activity. The action of these putative neurotransmitters on medial-preoptic (MPO) and arcuate-ventromedial (ARC-VM) neurons was studied in the urethane anesthetized, ovariectomized female rat. Multi-barrelled glass microelectrodes were used for extracellular recording and for microelectrophoresis of the putative transmitters. Four specific neuron types were identified by median eminence electrical stimulation; namely, antidromically identified (AI) MPO neurons, uninvaded MPO neurons, AI ARC-VM neurons and uninvaded ARC-VM neurons. The major findings based on responsive neurons (i.e., those neurons which reacted to the chemical application by an increase or decrease in spontaneous activity) are summarized. The data demonstrate that NE had a pronounced excitatory effect on uninvaded and AI ARC-VM neurons and an inhibitory influence on AI MPO neurons. The influence of dopamine was not as clearly defined, although there was a slight trend towards excitation in uninvaded MPO neurons and inhibition in AI MPO neurons. Results based on the testing with 5-HT showed it to have an inhibitory action on uninvaded MPO neurons. Furthermore, ACh and Glut were shown to have an excitatory action on the majority of responsive neurons tested. The remaining neurons tested with either NE, DA or 5-HT displayed a similar number of excitatory and inhibitory responses. The findings obtained provide positive support for a catecholaminergic and cholinergic regulation or neurons projecting towards the median eminence which are presumably involved in pituitary function.