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FIGURE 5–2. Detection of changes in firing rates and pattern of spike discharge by extracellular recording measurements.Extracellular recording techniques are an effective means of assessing the effects of afferent pathway stimulation or drug administration on neuron activity. On the other hand, the measurements that can be made are typically restricted to changes in firing rates or in the pattern of spike discharge. (A) This firing rate histogram illustrates the response of a substantia nigra–zona reticulata neuron to stimulation of the -aminobutyric acid (GABA)ergic striatonigral pathway. A common method for illustrating how a manipulation affects the firing rate of a neuron is by constructing a firing rate histogram. This is typically done by using some type of electronic discriminator and counter to count the number of spikes that a cell fires in a given time. In this example, the counter counts spikes over a 10-second interval and converts this number to a voltage, which is then plotted on a chart recorder. The counter then resets to zero and begins counting spikes over the next 10-second interval. Therefore, in this firing rate histogram, the height of each vertical line is proportional to the number of spikes that the cell fires during each 10-second interval, with the calibration bar on the left showing the equivalent firing frequency in spikes per second. During the period at which the striatonigral pathway is stimulated (horizontal bars above trace marked "STIM"), the cell is inhibited, as reflected by the decrease in the height of the vertical lines. When the stimulation is terminated, a rebound activation of cell firing is observed. (B) In this figure, a similar histogram is used to illustrate the effects of a drug on the firing of a neuron. (B1) This figure shows the well-known inhibition of dopamine neuron firing rate on administration of the dopamine agonist apomorphine (APO). Each of the filled arrows represents the intravenous administration of a dose of APO. After the cell is completely inhibited, the specificity of the response is tested by examining the ability of the dopamine antagonist haloperidol (HAL [open arrow]) to reverse this response. Typically, drug sensitivity is determined by administering the drug in a dose–response fashion. This is done by giving an initial drug dose that is subthreshold for altering the firing rate of the cell. The first dose is then repeated, with each subsequent dose given being twice that of the previous dose. This is continued until a plateau response is achieved (in this case, a complete inhibition of cell discharge). (B2) The drug is administered in a dose–response manner to facilitate the plotting of a cumulative dose–response curve, with drug doses plotted on a logarithmic scale (i.e., a log dose–response curve). To compare the potency of two drugs or the sensitivity of two cells to the same drug, a point on the curve is chosen during which the fastest rate of change of the response is obtained. The point usually chosen is that at which the drug dose administered causes 50% of the maximal change obtained (i.e., the ED50). As is shown in this example, the dopamine neurons recorded after a partial dopamine depletion (dashed line) are substantially more sensitive to inhibition by APO than the dopamine neurons recorded in control (solid line) rats. (C) In addition to determining the firing rate of a neuron, extracellular recording techniques may be used to assess the effects of drugs on the pattern of spike discharge. This is typically done by plotting an interspike interval histogram. In this paradigm, a computer is connected to a spike discriminator, and a train of about 500 spikes is analyzed. The computer is used to time the delay between subsequent spikes in the train (i.e., the time interval between spikes) and plots this in the form of a histogram, in which the x-axis represents time between subsequent spikes and the y-axis shows the number of interspike intervals that had a specific delay (bin = range of time; e.g., for 1-msec bins, all intervals between 200.0 and 200.99 msec). (C1) The cell is firing irregularly (as shown by the primarily normal distribution of intervals around 200 msec), with some spikes occurring after longer-than-average delays (i.e., bins greater than 400 msec, probably caused by spontaneous inhibitory postsynaptic potentials [IPSPs] delaying spike occurrence). (C2) In contrast, this cell is firing in bursts, which consist of a series of 3–10 spikes with comparatively short interspike intervals (i.e., approximately 70 msec) separated by long delays between bursts (i.e., events occurring at greater than 150-msec intervals). The computer determined that in this case, the cell was discharging 79% of its spikes in bursts, compared with 0% in (C1).Source. (A) Adapted from Grace AA, Bunney BS: "Opposing Effects of Striatonigral Feedback Pathways on Midbrain Dopamine Cell Activity." Brain Research 333:271–284, 1985. Copyright 1985, Elsevier. Used with permission. (B) Adapted from Pucak ML, Grace AA: "Partial Dopamine Depletions Result in an Enhanced Sensitivity of Residual Dopamine Neurons to Apomorphine." Synapse 9:144–155, 1991. Copyright 1991, Wiley. Used with permission. (C) Adapted from Grace AA, Bunney BS: "The Control of Firing Pattern in Nigral Dopamine Neurons: Single Spike Firing." Journal of Neuroscience 4:2866–2876, 1984a. Copyright 1984, Society for Neuroscience. Used with permission.

FIGURE 5–3. Relationship between action potentials recorded intracellularly and those recorded extracellularly from dopamine-containing neurons.(A) During intracellular recordings, an action potential is initiated from a negative resting membrane potential (e.g., ~55 mV), reaches a peak membrane potential (solid arrow), and is followed by a repolarization of the membrane and usually an afterhyperpolarization. An inflection in the rising phase of the spike (open arrow) is often observed. This reflects the delay between the initial segment spike that initiates the action potential (occurring prior to the open arrow) and the somatodendritic action potential that it triggers (occurring after the open arrow). (B) A computer was used to differentiate the membrane voltage deflection occurring in the action potential in (A) with respect to time, resulting in a pattern that shows the rate of change of membrane voltage. Note that the inflection is exaggerated (open arrow), and the peak of the action potential crosses zero (solid arrow), because at the peak of a spike, the rate of change reaches zero before reversing to a negative direction. (C) A trace showing a typical action potential in a dopamine neuron recorded extracellularly. The extracellular action potential resembles the differentiated intracellular action potential in (B). This is because the extracellular electrode is actually measuring the current crossing the membrane during the action potential and is therefore, by definition, equivalent to the absolute value of the first derivative of the voltage trace in (A). The amplitude of the extracellular spike is indicated in volts, because the parameter measured is actually the voltage drop produced across the electrode tip by the current flux and is therefore much smaller than the actual membrane voltage change that occurs in (A).Source. Adapted from Grace AA, Bunney BS: "Intracellular and Extracellular Electrophysiology of Nigral Dopaminergic Neurons, I: Identification and Characterization." Neuroscience 10:301–315, 1983. Copyright 1983, International Brain Research Organization. Used with permission.

FIGURE 5–4. Effects of intracellular manipulations of cGMP levels on basal activity of striatal medium spiny neurons.Intracellular application of selective pharmacological agents enables the investigator to examine the direct effects of these agents on the membrane activity of single neurons as well as to manipulate intracellular second-messenger systems. This figure demonstrates that manipulation of intracellular cyclic guanosine monophosphate (cGMP) levels potently and specifically modulates the membrane activity of striatal medium spiny neurons in a manner that cannot be achieved by extracellular application of drugs. Striatal neurons were recorded after intracellular application (~5 minutes) of either A) vehicle (control), a 0.5% solution of dimethylsulfoxide (DMSO); B) the drug 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), which blocks cGMP synthesis by inhibiting the synthetic enzyme guanylyl cyclase; C) ODQ plus cGMP; or D) the drug zaprinast, which inhibits phosphodiesterase enzymes responsible for degrading cGMP. (A) Left: After vehicle injection, striatal neurons exhibited typical rapid spontaneous shifts in steady-state membrane potential and irregular spontaneous spike discharge. Right: Time interval plots of membrane potential activity recorded from control neurons demonstrated bimodal membrane potential distributions indicative of bistable membrane activity. (B) Left: Striatal neurons recorded after ODQ injection exhibited significantly lower-amplitude depolarizing events compared with vehicle-injected controls and rarely fired action potentials. Right: The depolarized portion of the membrane potential distribution of neurons recorded after ODQ injection was typically shifted leftward (i.e., hyperpolarized) compared with controls. (C) Left: Striatal neurons recorded after ODQ and cGMP coinjection rarely fired action potentials but exhibited high-amplitude depolarizing events with extraordinarily long durations. Right: The membrane potential distribution of neurons recorded after ODQ and cGMP coinjection was similar to that of controls, indicating that cGMP partially reversed some of the effects of ODQ. (D) Left: Striatal neurons recorded after intracellular injection of zaprinast exhibited high-amplitude depolarizing events with extraordinarily long durations. Additionally, all of the cells fired action potentials at relatively high rates (0.4–2.2 Hz). Right: The membrane potential distribution of these neurons was typically shifted rightward (i.e., depolarized) compared with controls. Because zaprinast blocks the degradation of endogenous cGMP, we can conclude that basal levels of cGMP depolarize the membrane potential of striatal neurons and facilitate spontaneous postsynaptic potentials. Arrows indicate the membrane potential at its maximal depolarized and hyperpolarized levels.Source. Adapted from West AR, Grace AA: "The Nitric Oxide–Guanylyl Cyclase Signaling Pathway Modulates Membrane Activity States and Electrophysiological Properties of Striatal Medium Spiny Neurons Recorded In Vivo." Journal of Neuroscience 24:1924–1935, 2004. Copyright 2004, Society for Neuroscience. Used with permission.

FIGURE 5–5. Intracellular staining of neuron recorded intracellularly.During intracellular recordings, the recording pipette is filled with an electrolyte to enable the transmission of membrane voltage deflections to the preamplifier. The electrode may also be filled with substances, such as a morphological stain, for injection into the impaled neuron. In this example, the electrode was filled with the highly fluorescent dye Lucifer yellow. Because this dye has a negative charge at neutral pH, it may be ejected from the electrode by applying a negative current across the electrode, with the result that the Lucifer yellow carries the negative current flow from the electrode and into the neuron. Because this dye diffuses rapidly in water, it quickly fills the entire neuron impaled. The tissue is then fixed in a formaldehyde compound, the lipids clarified by dehydration-defatting or by using dimethylsulfoxide (Grace and Llinás 1985), and the tissue examined under a fluorescence microscope. In this case, a brightly fluorescing pyramidal neuron in layer 3 of the neocortex of a guinea pig is recovered.

FIGURE 5–6. Patch clamp electrophysiology and calcium imaging.By combining patch clamping with injection of selective dyes, the dynamics of calcium can be imaged in real time within isolated neurons. (A) Using infrared differential interference contrast (IR-DIC) microscopy, the image of a patch pipette can be observed attached to a neuron during a whole-cell electrophysiology experiment. Note the relatively large size of the pipette tip (coming from the left side of the image). Calibration bar = 2 m. (B) After filling the neuron with a calcium-sensitive dye, bis-Fura 2, the live neuron can be imaged with a fluorescence microscope. The dye takes about 10 minutes to fill the neuron after rupturing the patch membrane. Calibration bar = 10 m. (C) A unique property of the dye bis-Fura 2 is that it changes its fluorescence properties as it binds calcium. This can be observed by the changes in the fluorescence signal in response to a single (top) or five (bottom) action potentials. The fluorescence traces correspond to two regions, one close to the cell body (red box and red trace) and one farther out in the apical dendrite (orange box, orange trace). In this way, one can observe changes in calcium dynamics and how they correspond to activity states within single neurons.

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