A1 noradrenergic input to medial preoptic-medial septal area: an electrophysiological study

The effects of electrical stimulation of the A1 noradrenergic cell group on the extracellular activity of medial preoptic-medial septal (MPO-S) neurons were studied in ovariectomized (OVX) female rats under different hormonal conditions [estrogen and progesterone (E-P)-primed or unprimed] via conventional electrophysiological techniques. In addition, MPO-S neurons responding to the A1 noradrenergic cell group stimulation were characterized by examining their connections with the arcuate nucleus/median eminence (ARC/ME). With respect to stimulation of the A1 noradrenergic cell group, a large proportion of MPO-S neurons exhibited one of the following three types of orthodromic responses: orthodromic excitation (OD+), orthodromic inhibition (OD-), or complex orthodromic response (OD comp). The predominant type of response was OD+. Although E-P-priming did not change the relative proportion of neurons displaying a specific type of orthodromic response, it did increase the overall percentage of MPO-S neurons orthodromically responding to A1 stimulation (39% of MPO-S neurons responded to A1 stimulation in the E-P-primed group whereas only 25% of the neurons did so in the unprimed group). Neurons responsive to A1 stimulation were more likely to respond orthodromically to ARC/ME stimulation than were those neurons which were unresponsive to A1 stimulation. About one fifth of MPO-S neurons identified as projecting to the ARC/ME [putative luteinizing hormone-releasing hormone (LHRH) neurons] orthodromically responded to A1 stimulation. The predominant type of response was OD+. The present study indicates that A1 noradrenergic input to MPO-S neurons is predominantly excitatory.(ABSTRACT TRUNCATED AT 250 WORDS)