An inexpensive microdrive for chronic single-unit recording

0361-9230/93 $6.00 + .OO

Brain Research Bulletin. Vol. 32, pp. 321-323, 1993 Printed

in the USA.

Copyright

All rights reserved.

0 1993 Pergamon Press Ltd.

An Inexpensive Microdrive for Chronic Single-Unit Recording ERIC

GOLDBERG,

GEOFF

MINERBO

AND

TIM

SMOCK’

Howard Hughes Undergraduate Research Laboratory, Department qfPsychology, University of Colorado at Boulder, Boulder, CO 80309 Received

2 December

1992; Accepted

1 March

1993

GOLDBERG, E.. G. MINERBO AND T. SMOCK. An inexpensive microdrive,for chronic single-unit recording. BRAIN RES BULL 32(3) 32 l-323, 1993.-A microdrive and microelectrode costing about $2.67 is described. The assembly is lightweight,

compact, and versatile and is ideal for chronic single-unit recording from behaving rats. It is easy to assemble and operate and has been used successfully by undergraduates in the teaching and research laboratory.

Chronic electrophysiology

Microdrive

Single-unit recording

netted with the three o’clock tube via a tunnel which can be drilled through the window in the cap. By inserting the drill tip into the window and aiming for the top center hole so that the drill tip emerges out of the center hole in the threaded end, the appropriate tunnel will be created. The three o’clock hole must now be tapped starting on the threaded end of the head cap. Using a tap (No. K93A from Starrett, Athol, MA), tap the hole only until the tip of the tap appears in the window of the cap. Do not tap the hole completely through to the other side of the cap. The 18 ga. cannula should now be glued (using Superglue) into the three o’clock hole in the nonthreaded end allowing approximately 2.5 mm to protrude from the bottom of the head cap. Once the glue has dried, with the 22 ga. cannula removed, use pliers to slightly close the protruding end of the 18 ga. cannula. The 22 ga. should still be able to fit in the 18 ga. and slide with some resistance. A small window must now be carved in one end of the 22 ga. cannula. A small drill bit should be placed perpendicular to the cannula and used to furrow out the window in the shape of a semicircle in the cannula when viewed from the side. The window should be approximately 1 mm from the end of the cannula. The 22 ga. cannula should now be inserted, window end first, into the end of the 18 ga. cannula protruding from the nonthreaded end of the cap. The window in the 22 ga. cannula should now be visible in the window of the cap. A 15 cm length of Teflon coated wire should now be threaded through the head cap. Make a perpendicular kink in one end of the wire approximately 0.5 mm from an end of the wire. Insert the kinked end into the nonwindowed end of the 22 ga. cannula. The kink should appear in the window of the 22 ga cannula. With fine tipped forceps, grab the kink and feed the wire through the tunnel connecting the center and the three o’clock hole making sure that the wire exits the center hole on the threaded end of the cap. The window of

CHRONIC electrophysiology with freely moving animals is a demanding technique and is usually attempted only by full-time researchers with advanced training in neurophysiology. We have developed a low-tech recording method that undergraduates, following a l-semester introductory neuroscience course and a 2-week tutorial on acute recording techniques, can apply to the freely behaving rat. Mastery of all technical aspects, from assembly of the microdrive to single-unit recording, can be acquired in about 3 weeks of part-time training and the technique, because of its low cost, is ideal for a research laboratory facing budgetary constraints. DESIGN

AND APPLICATION

Figure 1 illustrates the microdrive with the Teflon-coated fine-wire electrode. It consists of a male head cap (2) from McIntyre Miniature Connector Company (plug STC-89~1-220, Carleton Univ. Science Workshop, Ottawa, Ontario, Canada), 5 mm of I8 ga. cannula, 12 mm of 22 ga. cannula, approximately 7 cm of Teflon-coated wire from the Medwire Corporation (part no. 3 1655 3t, Medwire Corporation, Mount Vernon, NY), and one amphenol pin. Initially, the two cannulas need to be prepared. The 18 ga. should be cut with a file to the proper length and fully opened at each end. The ends can be opened after cutting with the use of a grinding stone attached to a drill. Simply grind down the cannula until the end is fully opened and circular. A 12 mm length of 22 ga. cannula prepared in the same manner should now easily slide into the 18 ga. cannula. The head cap must be prepared with the use of a small drill bit. A window should be created on the outside of the cap beginning just below the threads and continuing down 4 mm and 2 mm wide along the three o’clock tube (Fig. 1). The center tube needs to be con-

’ To whom requests for reprints should be addressed.

321

322

GOLDBERG,

Drill Window

Top View

Here

lJSE OF IHk

Side View

ti

FIG. 1. Top and side view of completed drive.

the 22 ga. cannula can now be turned towards the inside of the cap to allow for easier movement of the wire through the cap. Now the functional electrode can be created. The tip of the Teflon-coated wire without the amphenol pin should be cut off. It is imperative that this step be executed with great care. The cut should be as square as possible with the length of the wire. A square cut can be obtained by cutting the wire quickly with an extremely sharp instrument such as a new scalpel blade on a hard surface. Once the cut has been made, check the resistance of the electrode. This will provide a reference point resistance. By holding the wire approximately 4 cm from the end which was cut and quickly passing the cut tip through a small flame, the resistance of the wire can be increased significantly. Initially, the tip should be passed through the blue cone in the flame. The flame should be approximately 1.5 cm in height and the blue cone should be about 0.5 cm in height. Several quick initial passes can be made. The resistance should be checked after the initial passes. If an increase in resistance has occurred, proceed cautiously making only one or two passes before checking the resistance. (It is important that the wire be cleaned of any solution used for testing the resistance before passing it through the flame.) If no increase has occurred, make several more passes at a slightly slower rate. After the second attempt, if no increase occurs, cut at least 4 mm off the tip and make another attempt. Electrodes with resistances of up to 45 MB have been obtained. We use electrodes of 9-20 MR for well-isolated chronic single units in the amygdala (Fig. 2). The properly made electrode, if viewed under a microscope, will reveal that the Teflon coating has flowed towards the end of the wire and begun to cover the exposed metal. The amount of exposed metal is inversely proportional to the resistance of the electrode. Once the desired resistance has been obtained, the electrode should be glued into the 22 ga. cannula. leaving approximately 2 mm of the electrode protruding from the nonwindowed end of the 22 ga. cannula, carefully place a drop of Superglue into this end of the cannula, taking care not to get glue on the tip of the electrode. The cannula should accept the glue by capillary action. After allowing the glue to dry, the drive may be completed. About 2-3 mm of the excess wire should remain in the window compartment of the three o’clock hole. This will be the slack which will allow the electrode to be lowered. Cut off the end of the wire with the amphenol pin approximately 3 cm from the threaded end of the cap. This end should now be stripped taking care not to pull the electrode wire out of the cannula. After doubling the wire over, clamp a male amphenol pin on this end and check the resistance of the electrode. (The center hole should be reamed out with an 18 ga. needle.) This pin may now be inserted into the center hole of the cap. The drive is now complete.

MINERBO

AND SMOCK

DRIVE

We use the drive for chronic single-unit recording in the medial amygdaloid nucleus of the rat. Figure 2 shows unit records obtained at various intervals during a l-h experiment in an open field box. Figure 2B shows the increased discharge rate of peptidergic cells in the amygdala upon copulation (1). The following is a brief description of the use of the drive in these experiments. After anesthetizing the animal an incision in the shaved scalp should be made from a point just posterior to a line between the eyes to the posterior part of the skull. After removing the fascia, a hole can be drilled in the skull through which the electrode can be lowered. Holes to accommodate two ground screws and two anchor screws also need to be drilled. The two ground screws consist ofa small jeweler‘s screw with approximately 5 cm of enamel insulated wire soldered on to it and an amphenol pin on the other end of the wire. The two pins should be inserted into two previously reamed holes in the head cap. Before lowering the electrode into the brain, the headstage should be centered over the head. (It is recommended that the electrode be lowered to a point just above the target during surgery and lowered further after surgery.) Once it has been inserted to the desired location, the exposed cannula should be coated with Vaseline and the entire head stage can be secured in place with the use of dental cement. Care should be taken not to allow cement to flow into the window of the headstage. After a recovery period of I week the animal is connected to the recording equipment via an adapter (socket # STC 89Sl220, McIntyre Co.) which has been altered to accommodate the drive screw. The screw is used to lower the electrode in the brain. Using a length of threaded rod (32 threads per inch), grind down one end and notch the other end as for a regular flat head screw. The unnotched end should be smooth so as not to catch on the cannula as it is being driven lower. The screw should be inserted

B

2ms FIG. 2. Single-unit activity in the amygdala of the freely behaving rat. (A) Unit of medial amygdala at start of experiment; (B) unit discharge 10 min later, showing increased activity upon copulation; (C) unit activity 7 min later, showing return tobaseline. Bars = 100 rV, 2 ms.

INEXPENSIVE

323

MICRODRIVE ACKNOWLEIXiEMENTS

into the tapped end of the three o’clock hole until it lightly contacts the 22 ga. cannula. To lower the electrode after implantation, rotate the screw clockwise in fractions of a turn until a

Supported by the Howard Hughes Undergraduate Biomedical Education Initiative. The authors wish to thank Kathi Hoskins for typing

well-isolated

the manuscript.

unit is obtained.

REFERENCES I. Minerbo, G.: Albeck, D.; Goldberg, E.; Lindberg, T.; Nakari, M.: Martinez, C.; Ganitano. J.; Smock, T. Activity of peptidergic neurons in the amygdala during sexual behavior in the mate rat (submitted).

2. Molino, A.: McIntyre, D. C. Another inexpensive headplug for the electrical recording and/or stimulation ofrats. Physiol. Behav. 9:273275: 1972.