A method for the intracranial delivery of reagents to voltammetric recording sites

Abstract

Carbon fiber microelectrodes are widely used for electrochemical monitoring in the intact brain. The local delivery of reagents to the recording site is often desirable. The approach of co-implanting a micropipette near the microelectrode presents some limitations that are overcome by the use of double-barreled devices. One barrel supports the carbon fiber and the other barrel serves as a pipet for local reagent delivery. Some studies have used iontophoretic delivery but here we consider the alternative approach of pressure ejection. However, placing the pipet so close to the electrode raises the risk that reagent can leak into the recording site. This problem is easily solved. We filled the tip of the pipet with vehicle solution, the barrel with a reagent solution, and separated the two solutions with an air gap to prevent their mixing. With this approach, reagent is delivered only after ‘priming’ pressure pulses: we show in two examples that unintended reagent delivery (leakage) prior to the priming pulses is non-detectable.

Introduction

Voltammetry in conjunction with carbon fiber microelectrodes (Michael and Borland, 2007) provides unique insights into the neurochemistry of the intact brain. For example, voltammetry has elucidated significant details of the activity of dopamine (DA) in the rat striatum and nucleus accumbens (Kita et al., 2007, May and Wightman, 1989a, May and Wightman, 1989b, Moquin and Michael, 2009, Rodriguez et al., 2006, Zachek et al., 2010). Combining voltammetry with intracranial, rather than systemic, delivery of reagents to the recording site is a powerful enhancement of the technique. For example, the broadly distributed dopamine D2 receptor (D2R) plays a central role in regulating DA (Benoit-Marand et al., 2001, Cass and Gerhardt, 1994, Herr et al., 2010, Moquin and Michael, 2011). Local delivery of D2R agonists and antagonists to the recording site allows the role of D2Rs in DA terminals fields to be distinguished from that of D2Rs in the midbrain. Local reagent delivery also aids the in vivo calibration of microelectrodes (Garguilo and Michael, 1993, Herr et al., 2008) and the ultrastructural analysis of the recording sites (Peters et al., 2004).

The co-implantation of glass (Cass et al., 1993, Cragg et al., 2001, Daws and Toney, 2007, Makos et al., 2009, Wang et al., 2010) and fused silica (Kulagina et al., 1999) pipets has been used to deliver reagents near recording sites. Co-implantation is somewhat cumbersome, involves two penetrations of the brain, and limits the precision of the delivery site relative to the recording site. Sufficient reagent to assure delivery across the distance of separation (typically 100–200 μm) is necessary (Sabeti et al., 2002). It is difficult to assess the geometrical symmetry of the delivery, which further limits precision of the method. And, in some cases the reagent is delivered into the pipet track rather than into the tissue itself (Garguilo and Michael, 1996).

A double-barreled electrode (DBE) solves these limitations (Herr et al., 2008, Moquin and Michael, 2011). One barrel contains the carbon fiber while the other barrel remains open and serves as the delivery pipet. This eliminates the need for double penetration of the brain and assures that reagent is always delivered to the recording site: this is especially valuable when the recording site is changed during an experiment, for example, to optimize the site (Garris et al., 1993). However, the close proximity of the pipet tip to the electrode raises the risk (see Section 3) that the reagent can affect the electrochemical recording by leaking from the pipet tip: here, we show that this is easily solved.

The leak problem is easily solved by introducing an air gap into the pipet to separate the fluid in the pipet tip from that in the pipet barrel. This does not prevent the leak per se, but it does prevent the leak from carrying reagent into the recording site.