AxoClamp 900A

This week I installed a brand new Axoclamp 900A (Molecular Devices) in my rig.
The new amplifier has several neat features. One can save the setting of the amplifier as a configuration. This may be good for doing different kinds of experiments routinely in one rig.

They changed the headstages. This is really annoying.
The new headstage somewhat resembles that of Axopatch, but shorter. There is no rod and it is designed to be attached directly onto the manipulator. And, of course, it won’t fit on our manipulator. We use Siskyou’s micromanipulator. Siskyou provides a “dove-tail” headstage connector, which costs like $175 for each. This is a rip off.
So, I constructed two headstage holders by myself from scratch.


Photo: The old headstage (blue) and the new headstage (black).
The “dovetail” slide connectors were made from plastic plates cut out from the microelectrode box (Sutter). A wooden rod ($0.87) was bought at Home Depot. It was cut, carved, grooved, and epoxy-glued upon the dovetail plate. The tightness of the dove tail can be adusted by putting something under the headstage. I use a small amount of dental wax. The total cost is less than a dollar + my labor.

Another feature I don't like is that there are no knobs on the front face of the amplifier. You need to control the capacitance compensation using mouse, or type in, like 8 μF through your computer. It has to be all controlled by PC. Bridge balancing is automatic. This sounds good, but actually it is not. In a real E-phys experiment, you don't really balance the bridge that accurately, because I don't trust bridge at all for measuring input resistances. I mostly use the bridge to keep the output within the voltage range of your AD converter.

This was my first day of using it. I might change my mind after getting used to this machine. I don't like it so far, though.

Patch-clamp amplifier vs. intracellular amplifier

People often ask me what the differences are between Axopatch and Axoclamp (both are made by Axon Instruments).

The answer is simple: Axopatch is a highly-sensitive I-V converter (ammeter). Axoclamp is a highly-stable voltmeter.


Axopatch or so-called patch-clamp amplifiers are designed for recording the membrane current, but it can also measure the membrane potential under the "current clamp mode".
On the other hand, Axoclamp is an amplifier designed for recording voltage with high-resistance microelectrodes. You can also measure membrane current by using "voltage-clamp mode." One may think that the continuous single-electrode voltage clamp mode in Axoclamp sounds very similar or almost identical to the whole-cell patch clamp by Axopatch.

Having either one being able do both voltage and current recordings, why do we need both?



The headstage of Axopatch contains a highly sensitive I-V converter, so that it can measure current with better s/n ratio. It can indirectly measure the voltage by calculating from the current being passed and membrane input resistance, but very susceptible to errors due to changes in the electrode resistance. This is why the patch pipette has to be big, with a resistance as low as possible.

Ironically, the large electrode tip makes he recorded potential less accurate, because of the junctional current. Junctional current is produced by the difference between the electrode solution and the cytoplasmic fluid. This current can be minimized by adjusting the ionic content of the electrode solution and reducing the size of the electrode tip (but this will increase the electrode resistance).


The headstage of Axoclamp contains the "voltage follower" circuit. This voltage follower circuit is the "magic" that makes it possible to record voltage with a sharp mega-ohm electrode. A voltage follower produce virtually infinite input resistance, which makes the electrode resistance almost negligible. Thus, the voltage recorded by Axoclamp is more accurate and relieble than Axopatch.
The current measured under voltage clamp mode in Axoclamp is actually the current generated artificially in the feedback circuit. Such indirect measurement is bad in s/n ratio and slower than Axopatch.