Acetazolamide potentiates the afferent drive to prefrontal cortex in vivo

Abstract

The knowledge on real-time neurophysiological effects of acetazolamide is still far behind the wide clinical use of this drug. Acetazolamide - a carbonic anhydrase inhibitor - has been shown to affect the neuromuscular transmission, implying a pH-mediated influence on the central synaptic transmission. To start filling such a gap, we chose a central substrate: hippocampal-prefrontal cortical projections; and a synaptic phenomenon: paired-pulse facilitation (a form of synaptic plasticity) to probe this drug's effects on interareal brain communication in chronically implanted rats. We observed that systemic acetazolamide potentiates the hippocampal-prefrontal paired-pulse facilitation. In addition to this field electrophysiology data, we found that acetazolamide exerts a net inhibitory effect on prefrontal cortical single-unit firing. We propose that systemic acetazolamide reduces the basal neuronal activity of the prefrontal cortex, whereas increasing the afferent drive it receives from the hippocampus. In addition to being relevant to the clinical and side effects of acetazolamide, these results suggest that exogenous pH regulation can have diverse impacts on afferent signaling across the neocortex.

Keywords: Carbonic anhydrase; hippocampus; prefrontal cortex; single‐unit activity; synaptic plasticity.

Figure 1

Experimental design and electrode placements. (A) Chronic electrophysiological timeline, undertaken once per rat after a 5‐ to 7‐day post‐surgical recovery. Vehicle (Veh) and acetazolamide (AZ) solutions were administered via the intraperitoneal route while keeping the rat plugged into the electrophysiological cable. The medial prefrontal cortex (mPFC) was recorded while stimulating the intermediate hippocampal formation CA1/subiculum area (CA1/sub) throughout the session. (B) Top: coronal sections (Paxinos and Watson 2007) with electrolytic lesions across rats indicated by red circles. Bottom: Nissl‐stained specimens from two representative subjects. The arrows point to electrolytic lesions.

Figure 2

Acetazolamide (AZ) enhanced the hippocampal‐prefrontal cortical paired‐pulse facilitation (PPF). (A) Top: heat plot with field postsynaptic potential (fPSP) sweeps along dimension 1 (Y) and perievent time in dimension 2 (X). Paired‐pulse electrical stimuli were delivered once every 10 s, for a total of 1080 sweeps (180 basal, 180 post‐vehicle, Veh, 720 post‐AZ). fPSP voltage values were Z‐scored (color bar) against the 20 msec prepulse period. Red and black triangles situate paired‐pulse stimuli and injections, respectively. Heat‐plot data are from averaging across rats (N = 8). Bottom: average basal and post‐AZ fPSP voltage traces from the heat‐plot data. Gray areas illustrate the time windows from which amplitudes were measured. (B) Top left: fPSP1 (blue) versus fPSP2 (orange) amplitudes. Each time point represents 18 fPSP, that is, 3 min. Black lines indicate Tukey's post hoc differences after two‐way repeated measures ANOVA. Bottom left: PPF, that is, ratios between fPSP2 and fPSP1. The asterisk informs an effect of time, after one‐way repeated measures ANOVA. Right: same data, but shown as ratios from baseline means. (C) Data from the left graphs of panel B, but averaged across four periods: baseline, post‐Veh, initial 30 min post‐AZ, and final 30 min post‐AZ. The asterisk in the inferior graph informs an effect of time, after one‐way repeated measures ANOVA. All data are shown as mean ± standard error.

Figure 3

Acetazolamide (AZ) effects on medial prefrontal cortical (mPFC) firing combined net inhibition with increased paired‐pulse responsivity in a minor portion of neurons. (A) Top: heat plot with single units (i.e., their nonperievent rate histograms) along dimension 1 (Y) and time bins (3 min width) in dimension 2 (X). Spike counts were Z‐scored (color bar) against the initial 15 min baseline period. Black triangles represent injections. Bottom: Mean ± standard error from the heat‐plot data (n = 22). The asterisk indicates an effect of time, after one‐way repeated measures ANOVA. (B) Two representative neurons: their perievent raster plots (top), and correspondent 3 msec‐binned histograms (bottom). Red triangles situate paired‐pulse stimuli, and post‐AZ firing rates are from the initial 30 min post‐AZ period. A subjacent voltage trace from Figure 2 (left histogram) illustrates the timing between field and unit responses to hippocampal stimuli. (C) Top: firing responses to conditioning (Resp1) and test pulses (Resp2) after counting spikes within 15–42 poststimulus latencies. Bottom: ratios between Resp2 and Resp1 firing rates. The asterisk informs an effect of time, after one‐way repeated measures ANOVA. Data were averaged across four periods: baseline, post‐Veh, initial 30 min post‐AZ, and final 30 min post‐AZ. All data are shown as mean ± standard error.