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 rate 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 neurones 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 stimulaton 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 predominatly excitatory. Furthermore, the findings that (1) the proportion of neurons which responded to A1 stimulation was increased by E-P-priming, a treatment used to induce a surge in luteinizing hormone (LH) secetion in an OVX rat, (2) the neurons responding to A1 stimulation had a tendency to receive input from the ARC/ME, a brain site intimately involved in the control of LH release, and (3) the activity of putative LHRH neurons was able to be affected by A1 stimulation suggest that A1 input to MPO-S area may be involved in the control of LH secretion.