Recording Changes in Renal Sympathetic Nerve Activity in Response to Arterial Pressure in Rats

Take an anesthetized rat in the supine position and make an incision in the inguinal region to expose the femoral artery.

Occlude the artery to temporarily restrict blood flow. Create an opening, insert a pressure-sensing catheter to measure arterial blood pressure, and secure the catheter.

Position the rat in the prone position in a stereotaxic frame and make a lateral incision below the ribs.

Using a microscope, identify the renal sympathetic nerves, or RSNs, that regulate blood flow to the kidney.

Lift the nerve onto a recording electrode, add oil to prevent nerve dehydration and record electrical activity.

Intravenously inject a drug that causes arterial narrowing, which elevates systemic blood pressure.

Baroreceptors detect elevated blood pressure and signal the brainstem to inhibit sympathetic nerves, including RSNs.

Remove the oil, apply a gel to secure the electrode for stable recording, and monitor the RSN activity. 

A transient inhibition of RSN activity coinciding with increased arterial pressure confirms drug-induced RSN inhibition.

Before arterial cannulation, inspect the pressure-sensing catheter under high magnification to confirm that the catheter is free of bubbles and debris and has an intact meniscus between the fluid and gel-filled components. Prior to each implantation, refill the gel at the distal tip of the catheter, and use a magnet to turn on the transmitter. Place a loose overhand half knot in the proximal suture to briefly occlude the femoral artery, and holding the catheter introducer in the non-dominant hand, use vessel cannulation forceps to grasp the tip of the cannula of the telemetry unit to avoid displacing the gel from the tip.

Use a bent 22-gauge needle tip to puncture a hole in the artery and insert the cannula into the artery as far as possible. Then, secure the pressure catheter with the proximal and distal suture ties, and tuck the body of the telemetry implant inside the flank adjacent to the incision. Gently move the rat into the prone position in a stereotaxic surgery frame with the head between the ear bars.

To isolate the renal sympathetic nerves, connect a wire Renal Sympathetic Nerve Activity or RSNA electrode to a 10X preamplifier in a microelectrode amplifier, and make a scalpel incision extending from 4 to 5 centimeters below the ribs in the caudal direction, slightly lateral to the spine.

Blunt-dissect to visualize the paraspinal muscles and make a very superficial 1 to 2-centimeter rostrocaudal incision where the fat meets the muscle. Using cotton-tipped applicators, spread the fat away from the muscle to visualize the kidney without entering the peritoneal space, and use retractors to gently separate the kidney from the paraspinal muscles to visualize the renal artery and abdominal aorta, and use high magnification to identify the renal nerves in the incision pocket.

The nerve bundles are most easily visible in the right angle formed by the aorta and the renal artery, closely following the renal artery from the aorta to the kidneys. Then, use micro-dissecting tweezers to gently dissect a segment of the nerve bundle to be placed on the recording electrode from the surrounding tissue. Secure the wire RSNA electrode in a holder and lower the electrode to the level of the nerve segment.

Use a nerve hook to gently lift the segment of the renal nerve onto the electrode without stretching the nerve and fill the incision with mineral oil to keep the exposed renal sympathetic nerve hydrated. Attach a grounding clip to the animal, in the other end to the Faraday cage, and use high and low pass filtering to direct the signal to the amplifiers. Then, adjust the gain up to 10 kilohertz, including an audio monitor, to assess the bursting pattern of the RSNA.

To assess the quality of the RSNA recording, evoke the baroreflex with a 100-microliter bolus intravenous injection of 10 micrograms per milliliter of phenylephrine to increase the blood pressure and to inhibit the renal sympathetic nerve activity, adjusting the position of the electrodes to improve the signal as necessary. Once a clear signal has been obtained, aspirate the mineral oil, and secure the electrode in place with silica gel.