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VOL. 71 (4), 821-833, 2005 CHOLINERGIC CURRENTS IN XENOPUS...

of human brain where membranes were obtained (3). In some cases,
using acetylcholine, we recorded currents from human muscarinic
receptors with the characteristic oscillatory profile for several
minutes because this profile was caused by fluctuations in
intracellular IP4 which led the opening of calcium activated chloride
channels (15).

The oocytes in which were stimulated by all the agonists tested
were taken as the most representative, with respect to sensitivity to
the agonists. Generally, we recorded differences in the effect of these
agonists in the same oocyte but did not find any significant difference
between the means. However, with DMPP at a holding potential of
–90 mV, the difference was significant and may be due to a greater
incorporation of a7 or a3ß4 receptors, in which DMPP is the most
potent agonist (6).

The distinct kinetics of the currents detected can be explained by
the predominant presence of some subunit combinations and to the
concentration of the agonist dose used. Currents elicited in a3ß4 and
a3ß2 decay substantially faster than those recorded in a2ß4 or a4ß4,
and the currents elicited by the a7 receptor are faster that any other
heteromeric combination (6). The currents that we recorded could
be the addition of the distinct kinds of receptor currents, in which,
in some cases, the prevalence of one of the receptors determines the
profile or the kinetics of the current. We found that sensitivity to a-
bungarotoxin was about 40%, a result which indicated the presence
of the human neuronal nicotinic acetylcholine receptor a7 in the
sample, the only neuronal subunit with sensitivity to a-bungarotoxin.
Sensitivity to d-tubocurarine was variable, with mean inhibition
between 26% and 43% of the current induced by the agonist,
depending on the brain zone from where membranes were obtained.
These results indicate the major presence of a4ß2 and a7 subunits
in the frontal cortex sample, because these receptors are sensitive
to d-tubocurarine of 50% (6). In the currents obtained from
hippocampus this inhibition was lower.

In conclusion, active nicotinic receptors survive for at least several
hours after death. The profiles of the currents of transplanted human
brain nicotinic receptors are very similar to those recorded from
oocytes expressing human mRNA, indicating that the latter model is

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