A comparison of the release of substance P and some synthetic analogues from micropipettes by microiontophoresis or pressure

Characterization of the techniques of pressure ejection and microiontophoresis using in vivo electrochemistry

Catecholamine levels in frontal cortex were determined in vivo by electrochemical detection after the local application of dopamine (DA) from multibarrel micropipettes by pressure ejection or microiontophoresis. Tissue DA levels were linearly related to microapplication doses with either technique and reached steady state with longer application times. Furthermore, the plateau DA tissue concentrations were clearly related to ejection pressure or iontophoretic current. Using either microapplication technique, the tissue DA levels decreased as distance between the recording electrode and the tip of the drug pipette increased. However, pressure ejected and iontophoretically applied drug differed in their concentration versus time dynamics. Thus, although similar tissue concentrations of drug can be generated by the two techniques, the time dynamics of the drug effects may not be comparable. The quantitative use of these drug application techniques requires a minimal amount of variance in release between pipettes in order to effectively measure small sensitivity differences. Although the 10-fold variance with microiontophoresis does not appear resolvable at present, improved pipette construction techniques permit the variability in dosage to be limited to a maximum of 3-fold with pressure ejection. In addition, the present data also suggest that this variance can be further minimized by holding either ejection duration or ejection pressure constant when establishing dose-response relationships.

The excitation by neurotensin of nucleus tractus solitarius neurons induces apneustic breathing

The possible involvement of neurotensin in the regulation of respiratory drive has been tested on single brainstem respiratory related neurons and on the global respiratory output. The neuropeptide was locally applied either by microiontophoresis or by pressure injection in the dorsal and ventral respiratory areas of the anesthetized bivagotomized cat. Effects of neurotensin applications were studied, on the one hand on the firing discharge of respiratory related neurons and on the other hand on the phrenic nerve activity and on arterial blood pressure. An increase of the firing frequency of respiratory related neurons was induced by neurotensin applied by iontophoresis or by pressure injection (0.005-33.5 fmol/s) on single neurons. In the latter case, neurotensin was active at concentration 10(3) times lower than glutamate. A bilateral apneustic pattern was induced on the phrenic nerve activities by microinjection of neurotensin (0.23-0.54 pmol/s) in one ventrolateral nucleus tractus solitarius without alteration of arterial blood pressure. These results suggest that the release of neurotensin in the nucleus tractus solitarius regulates respiratory rhythmogenesis by increasing the inspiratory duration.

Diffusion from an injected volume of a substance in brain tissue with arbitrary volume fraction and tortuosity

When a substance is pressure-injected from a micropipette into the extracellular space of the brain it may either form a cavity or it may infiltrate the extracellular space. In either case subsequent diffusion is governed by the volume fraction and tortuosity of the brain tissue as well as the diffusion coefficient of the substance itself. Appropriate equations, solutions and approximations to these problems are discussed. The results are relevant to the interpretation of studies on neuropharmacology and in situ electrochemistry.

Intracranial application of substances in the unrestrained, awake rat by pressure injection through glass micropipettes

A method is described which allows intracranial injection of drug solutions through glass micropipettes (tip diameter 15 micron) in the freely moving rat by use of air pressure. Compared with conventional injection methods through steel cannulae this method has the advantages of (1) minimal destruction of brain tissue, (2) precise injection of small volumes of solutions (1 nl to several microliters), (3) simultaneous recording of local EEG activity, and (4) usage of multipipette assemblies.

Are the proposed substance P receptor sub-types, substance P receptors?

Recently, a number of laboratories have postulated the existence of receptor sub-types for substance P. This review is intended to represent a critical appraisal of these reports. In the majority of cases, the evidence for the existence of receptor sub-types has been obtained from observed potency differences of agonists. The problems with this approach are discussed. In addition, information obtained through substance P antagonists, binding studies and investigations of second messenger systems is presented and discussed in relation to the above receptor subdivisions. It is concluded that the present results are consistent with the existence of three receptor sub-types; however, it is suggested that substance P is the natural agonist for only one of these receptors, and that substance K and tuftsin may be the transmitters for the other two receptor sub-types.

Pressure injections of fluid in the nanoliter range via micropipettes

The relationship between pressure, ejection duration and volume ejected was experimentally determined in vitro for micropipettes with different external tip diameters. The relationship between ejection duration and ejected volume is linear in the steady state (i.e. with ejection durations of 1 s or longer) and at sufficiently high pressures (above about 100 kPa) and for pipettes with a sufficiently high hydrodynamic conductance (larger than 1 pl s-1 kPa-1 at 230 kPa). In this range, flows were found with low Reynolds numbers (smaller than 10), which is consistent with laminar flows. For all but the largest micropipettes, the relationship between pressure and ejected volume is alinear: the pipettes' apparent hydrodynamic conductances increase with increasing pressure. Micropipettes with apparent hydrodynamic conductances between 0.04 and 1400 pl s-1 kPa-1 (at 230 kPa) were tested. Duration-pressure combinations could be defined where the duration-volume relationship was either linear or monotonic. such duration-pressure combinations were different for pipettes with different apparent hydrodynamic conductances. A quick method is described to measure the pipette's apparent hydrodynamic conductance at the pressure used, corrected for the fluid's viscosity. Measurement of this conductance permits predictable injections of known volumes of fluid in the range of 100 pl to 1 microliter with a precision of 10-20%.