Reflex Regulation of Autonomic Influences upon the Oxytocin-Induced Contractile Response of the Mammary Gland in the Anesthetized Rat*

Abstract

Intramammary pressure (IMP) from an abdominal mammary gland, generated in response to intrajugular injections of oxytocin, were recorded before, during, and after 120–140 sec of electrical stimulation of the cut central end of a contralateral mammary nerve in the urethane-anesthetized lactating rat. The stimulation parameters were 5- to 30-V 1-msec pulses at a frequency of 10/sec. The amplitude of the IMP responses to a given dose of oxytocin was reduced by about 50% when tested after 60 and 120 sec of electrical stimulation and recovered to prestimulus heights 3–10 min after cessation of the stimulus. The depressant effect of nerve stimulation then could be repeated. Bilateral adrenal ligature, iv injection of 0.3 mg propranolol, or section of the spinal cord above the level of entry of the mammary nerve completely blocked the depressant effect of nerve stimulation. In fact, the slope and amplitude of the IMP response eventually increased above that found before mammary nerve stimulation after each of these procedures. The depressant effect of nerve stimulation upon the oxytocin-induced rise in IMP also could be rapidly offset by adding an additional 0.2–0.6 ml milk intraductally to the gland whose pressure response to oxytocin was being monitored. When a small amount (0.04–0.06 ml) of milk was introduced intraductally into a distant thoracic gland, a gradual rather than rapid blockade of the depressant action of electrical stimulation ensued, requiring 35–40 min to become maximally effective. Thus, it would appear that afferent information from mammary mechanoreceptors can reflexly antagonize the sympathetic adrenal system which has been aroused earlier.