Biomedical Engineering Reference
In-Depth Information
neurons osmoregulate, but are vulnerable to sudden osmolarity gradients and rapid
changes, an effect likely due to rapid osmotic swelling or shrinkage [27]. However, for
experiments lasting 8-12 h, exposure to high-osmolarity conditions for experiments
involving seawater dilutions have been successfully performed for marine toxin detection
at the U.S. Naval Research Laboratory. In addition to these common environmental vari-
ables, changes in medium biochemistry associated with long-term culture and recording,
spanning days and weeks, represent a still challenging and relatively unexplored domain.
Here, accumulation of ammonia, changes in concentration of neuroactive lipids, the pres-
ence of serum albumin, and even subtle effects of metal ions can all impact the stability
and response reliability of the network sensor. This is the reason why the native medium
is often changed to a reference medium, of which a large pool is available, if several
medium changes are anticipated. The network responds to the new medium, but reaches
a new level of stable activity that determines the reference level for all subsequent phar-
macological tests. Figure 6.13 depicts the strong, but reversible effects that aged medium
can have on activity. The reduction of activity was attributed to the rise of ammonium ions
resulting from breakdown of glutamine (see Table 6.2).
Confirmation of glutamine problems for long-term experiments with slowly aging
medium is shown with a dual array, closed-chamber experiment where 10 ml of the same
medium was circulated at 50 µl/min (Figure 6.14). Loss of units occurred after 8 days in
30
250
DMEM
5
Test medium
DMEM 5
25
200
20
150
15
100
10
50
5
Spike rate
Burst rate
0
0
0
50
100
150
200
250
300
350
400
450
Time (min)
FIGURE 6.13
Activity reduction in mean network spike and burst rates due to a medium change to an aged medium main-
tained in a closed container for 20 days at 37ºC. Recovery to original activity levels was achieved by a return to
fresh DMEM-5 medium (Dulbecco's modified Eagles medium with 5% heat-inactivated horse serum).
TABLE 6.2
Ionic and Metabolic Components of Fresh and 20-Day-Old Medium
(All Concentrations in mM)
a
Medium Component
Fresh
20 Days
Glucose
25.3
25.3
Glutamine
3.79
0.15
Glutamate
0.00
0.09
Ammonium
0.37
1.50
Sodium
151.2
150.8
Potassium
5.19
4.99
a
Measured with Nova Bioprofile 100 Analyzer.
