Tyramine-like immunoreactivity was identified in neurons of the VIIIth abdominal
ganglion and in axons projecting to the spermatheca of adult females of Locusta migratoria.
Tyramine-like immunoreactive processes were also found throughout all regions of the spermatheca and
tyramine-like immunoreactive bipolar or multipolar neurons were present on the spermathecal sac. HPLC coupled with electrochemical detection revealed more
tyramine than
octopamine present in spermathecal tissue. Electrical stimulation of the ventral ovipositor nerve resulted in a significant increase in
calcium-dependent release of
tyramine from the spermatheca. Both
tyramine and
octopamine increase the frequency and basal tonus of spermathecal contractions in a dose-dependent manner, with
octopamine having a lower threshold. When
tyramine is applied along with a half maximal
octopamine dose, there is an additive effect on contractions of the spermatheca with slight synergistic effects at lower doses of
tyramine. High concentrations of
tyramine (10(-4)M) stimulated increases in
cyclic AMP levels of the spermatheca; an effect blocked by
phentolamine.
Phentolamine has a higher affinity (and thus a lower IC(50) value congruent with 5.6 x 10(-8)M) than
yohimbine (IC(50) congruent with 1.1 x 10(-4)M) in reducing
tyramine-induced spermathecal contractions. Taken together, these results suggest that
tyramine may be a co-transmitter with
octopamine at the spermatheca, with both neuroactive chemicals acting on an
octopamine receptor.