In Vivo Electrophysiology
In vivo electrophysiology measures neuronal activity in the brain as local field potentials or single unit neuronal activity. Brain regions can be targeted precisely, and the effects of compounds can be evaluated directly by local iontophoresis or systemically, via IV injection. This allows for the assessment of different animal models of neurodegeneration and together with simultaneous blood sampling, in vivo recordings can be used to generate data on bioavailability and real-time pharmaco-dynamic measures.
Recordings in Anesthetized and Head-Fixed Rodents
Anesthetized recordings involve head fixing an animal in a stereotaxic apparatus and recording from predetermined brain regions of interests. Single unit activity can be acquired as well as local field potentials (LFP, aggregate activity of a group of neurons). Once activity levels and patterns have been established in those brain regions, compounds can be administered via a number of different avenues (IV, IP, stereotaxically infused, etc.) to observe their effect on neuronal firing. Below, a stimulating electrode was placed in the globus pallidus (GP) in order to induce activity being recorded in the substantia nigra (SN). Once baseline activity was established, a compound was introduced to produce a pharmacological change.
Figure 1: Stimulation of Globus Pallidus (GP) briefly inhibits rhythmically firing neurons of the Substantia Nigra pars reticulata (SN). (A) Placement of stimulus (red) and recording (blue) electrodes in the GP and SN respectively. (B) Rhythmically active cells in the SN are briefly inhibited following high intensity stimulation in GP (100 Hz, 5 pulses. 1 mA, arrow). (C) (Top Panel) Raster plot and histogram of 120 repeated stimulations. Firing of SN neurons is inhibited immediately and briefly following GP activation. (Bottom Panel) IV application of Chlodiazepoxide (CDP, 5mg/kg), a positive allosteric modifier of the GABAA receptor, augments suppression of firing in the SN by sub-maximal GP stimulation (red lines, compare top and bottom).
Below is another example of a compound that changed neural signature. NMDA administration increased neuronal firing in the prefrontal cortex and administration of an NMDA antagonist blocked activity in the same region.
Figure 2: NMDA-induced activation of a single neuron in rat prefrontal cortex is blocked by MK-801. (A) Local iontophoretic administration of NMDA (black boxes) increases the firing frequency of the neuron. (B) shows the data binned in 2s intervals. (Right Panel) MK-801 (NMDA antagonist 2mg/kg) administered intravenously blocks the NMDA induced activity.
Recording in Freely Moving Rodents
While anesthetized recordings are ideal for investigating spontaneous and evoked activity from a brain region and testing compound efficacy, it does not allow the coordination of behavior with neuronal firing. Recordings in freely moving animals enable real-time measurement of brain activity when animals are awake and/or performing behavioral tasks. Fixed or movable electrode arrays (up to 16-channel recording) are available to pinpoint neural activity in a given brain region and can be placed in multiple regions to observe possible coordinated activity. Below, CA1 hippocampal neurons are recorded in the freely moving rat. In addition, similar to experiments in the anesthetized animal, compounds can be administered to modify neuronal activity.
Figure 3: Depiction of neural activity of a CA1 hippocampal neuron in a freely-moving rat during extracellular recordings. The multi-electrode array used in here simultaneously recorded spike activity of an isolated neuron, as well as aggregate activity of groups of neurons (local field potentials).
Figure 4: CX546 and Diazepam increased and decreased spike activity of CA1 hippocampal neurons of a freely-moving rat, respectively, in a 5-month old male Long Evans rat. CX546 is a positive allosteric modulator of the AMPA receptor, and Diazepam is a positive allosteric modulator of the GABA receptor. Data illustrates percentages of spontaneous activity relative to baseline during the initial 15-min period after injection of either vehicle or compound. As compared to vehicle, treatment with CX546 (7.5 mg/kg, i.p.) resulted in an increase of firing rate [n=3; Repeated Measures ANOVAs, F(1,4) = 10.81, p < 0.05] while treatment with Diazepam [3 mg/kg, i.p.] resulted in a decrease of firing rate [n=5; Repeated Measures ANOVAs, F(1,4) = 26.26, p < 0.005]. Data are presented as mean ± SEM.
Bioanalysis & Microdialysis