Anesthetics Change the Intrinsic Excitability of Neurons

Gamma oscillations in the brain are hypothesized to be involved in consciousness (Gray, 1994). Interestingly, general anesthetics are known to change both the incidence and frequency of gamma oscillations in the hippocampus. They are also known to increase the amplitude and decay time-constant of postsynaptic inhibitory currents (Whittington et al., 1996). No causal relationship, however, has been established between these network effects and cellular effects. In an effort to begin this description, I have measured the frequency-current relationships in CA1 pyramidal cells both under control conditions and in the presence of the anesthetic propofol.

Methods:

Using the whole-cell patch-clamp method, I delivered a series of input currents to the neurons. Whole-cell patch-clamp is a method where a finely-tipped glass micropipette is filled with an artificial intracellular fluid and brought down to the surface of a cell. Negative pressure (sucking) allows the cell to form a seal with the pipette and rupture the section of membrane just under the pipette tip. The intracellular fluid of the cell becomes continuous with the artificial intracellular fluid contained in the pipette. An electrode is inserted into the back end of the pipette, which can be used to record from and manipulate the cell electrically.

Each cell received the current steps with and then without the anesthetic drug propofol.

Results:

The most significant result that I saw was a change in the gain of the neuron (as evidenced by a change in the slope of the f-I curve) in response to propofol. I recorded the firing rates of the neurons in response to increasing steps of currents (current steps are shown in Figure 1).

Figure 1. Current steps.

Firing rates were separated into initial rates (rate during the first 0.3 seconds of pulse times) and steady-state rates (rate during the last third of pulse times). For this study, I was most interested in the steady-state firing rates. First, I recorded the f-I curves from neurons first without and then with propofol added (Figure 2; n=9). The process took about 30 minutes. There was a modest change in the gain of the neurons in response to the propofol treatment.

Figure 2. Steady-state f-I curve for the control (blue) and with the addition of propofol (red).

However, as I was doing the experiments I noticed that the gain of the neuron would change over time, independent of the addition of propofol. Therefore, I wanted to eliminate the effects of time from my analysis of the effects of propofol. I again followed the same protocol as I did for Figure 1, but without adding propofol for the second f-I curve. This second recording I called a "delayed control" (Figure 3; n=10). Here I also noticed a change in gain, but in the opposite direction.

Figure 2. Steady-state f-I curve for the control (blue) and delayed control (red).

When I compared the gain of the neurons with propofol added with the delayed recordings without propofol, significant differences were seen (Figure 4; p=0.021, unpaired t-test).

Figure 4. Bar graph of changes in gain (slope) as a result of the addition of propofol.

This is indicative of a change in the intrinsic excitability of the neurons as a result of the propofol treatment. This change in intrinsic neuronal excitability may, in addition to the synaptic effects of propofol, lead to changes in network behavior and contribute to propofol-induced anesthesia.

The entire poster for this study can be seen here: Utah BME 2011 Conference Poster.

References

Gray, CM (1994). Synchronous oscillations in neuronal systems: mechanisms and functions. Journal of Computational Neuroscience, 1(1-2), 11–38.

Whittington, MA, Jefferys, JG, & Traub, RD (1996). Effects of intravenous anaesthetic agents on fast inhibitory oscillations in the rat hippocampus in vitro. British Journal of Pharmacology, 118(8), 1977–86.

Posted December 7th, 2012 in Neuroscience, Science.