Dual action of intracellularly released calcium on the quantal mediator secretion

The mice diaphragm muscle and microelectrode technique were used to check the influence of ryanodine (0.5 mcM) on spontaneous and evoked mediator release under conditions of potassium depolarization (8-16 mM [K+]ex or rhythmic (4-100 Hz) stimulation of motor nerve terminals. Weak tonic calcium loading (by muscle exposition to 8 mM [K+]ex) caused a two-fold frequency increase if miniature and plate potentials (MEPPs), which was returned to the basal level by subsequent application of ryanodine. This inhibitory effect of ryanodine was blocked by apamin (500 nM) a blocker of K+(Ca)-channels. A greater calcium load of terminals (in solution with 16 mM [K+]ex) caused a 15-fold increase of MEPPs frequency. Subsequent ryanodine application caused an additional 2-3-fold increase of MEPPs frequency. During rhythmic activity of motor synapses, ryanodine was able to decrease the amplitude of EPP by 60% at plateau phase at short low frequency (4 Hz) of discharges and to increase the amplitude of EPP by 60-150% at high frequency (70-100 Hz) of discharges. It is concluded that rynodine induced calcium release from intraterminal Ca2+-stores can influence dual: excitatory or inhibitory, action on spontaneous and evoked mediator release, due to different intraterminal calcium loads and regimen of synaptic activity.     

Dissociable Neural Response Signatures for Slow Amplitude and Frequency Modulation in Human Auditory Cortex

Natural auditory stimuli are characterized by slow fluctuations in amplitude and frequency. However, the degree to which the neural responses to slow amplitude modulation (AM) and frequency modulation (FM) are capable of conveying independent time-varying information, particularly with respect to speech communication, is unclear. In the current electroencephalography (EEG) study, participants listened to amplitude- and frequency-modulated narrow-band noises with a 3-Hz modulation rate, and the resulting neural responses were compared. Spectral analyses revealed similar spectral amplitude peaks for AM and FM at the stimulation frequency (3 Hz), but amplitude at the second harmonic frequency (6 Hz) was much higher for FM than for AM. Moreover, the phase delay of neural responses with respect to the full-band stimulus envelope was shorter for FM than for AM. Finally, the critical analysis involved classification of single trials as being in response to either AM or FM based on either phase or amplitude information. Time-varying phase, but not amplitude, was sufficient to accurately classify AM and FM stimuli based on single-trial neural responses. Taken together, the current results support the dissociable nature of cortical signatures of slow AM and FM. These cortical signatures potentially provide an efficient means to dissect simultaneously communicated slow temporal and spectral information in acoustic communication signals.     

Differential neural control of intrarenal blood flow

To test whether renal sympathetic nerve activity (RSNA) can differentially regulate blood flow in the renal medulla (MBF) and cortex (CBF) of pentobarbital sodium-anesthetized rabbits, we electrically stimulated the renal nerves while recording total renal blood flow (RBF), CBF, and MBF. Three stimulation sequences were applied 1) varying amplitude (0.5-8 V), 2) varying frequency (0.5-8 Hz), and 3) a modulated sinusoidal pattern of varying frequency (0. 04-0.72 Hz). Increasing amplitude or frequency of stimulation progressively decreased all flow variables. RBF and CBF responded similarly, but MBF responded less. For example, 0.5-V stimulation decreased CBF by 20 +/- 9%, but MBF fell by only 4 +/- 6%. The amplitude of oscillations in all flow variables was progressively reduced as the frequency of sinusoidal stimulation was increased. An increased amplitude of oscillation was observed at 0.12 and 0.32 Hz in MBF and to a lesser extent RBF, but not CBF. MBF therefore appears to be less sensitive than CBF to the magnitude of RSNA, but it is more able to respond to these higher frequencies of neural stimulation.     

The role of cyclic adenosine monophosphate in simple forms of plasticity in the edible snail

cAMP-dependence of synaptic depression and facilitation was investigated in functionally identified synaptic connection in the snail. It was found that 5 mM imidazole (phosphodiesterase activator) as well as 2 mM tolbutamide (inhibitor of cAMP-dependent protein kinase) do not change the rate of EPSPs depression during rhythmic (0.1 Hz) nerve stimulation, and do not affect facilitation. But treatment with both these drugs decreases EPSPs amplitude. Possibility of cAMP-dependent modulation of synaptic effectiveness is discussed.     

Two regions in the isolated brainstem of the frog that modulate respiratory-related activity

Using microinjection techniques, we have explored the isolated, complete midline sectioned brainstem of the frog (Rana catesbeiana) to identify regions that influence the endogenous respiratory-related motor activity. Ten-nanoliter injections of lidocaine (1%), GABA (100 mM) and glutamate (10 and 100 mM) into discrete regions of the rostral and the caudal brainstem produced different effects on the phasic neural discharge. In the rostral site lidocaine, GABA and glutamate injections altered neural burst frequency with little or no effect on burst amplitude. In the caudal site, responses to lidocaine and GABA injections consisted primarily of decreases in neural burst amplitude, often, but not always associated with minor decreases in burst frequency. In this same region, the response to glutamate was characterized by a temporary interruption of the rhythmic neural burst activity. The largest responses to substance injection in both regions were obtained at sites ranging between 200 and 500 m from the ventral surface, in the ventral medullary reticular formation. The results reveal the existence of two areas in the frog brainstem that influence respiratory motor output, one related to the respiratory burst frequency and the other related to the amplitude of the motor output.Abbreviations V trigeminal nerve - VI abducens nerve - VII facial nerve - VIII auditory nerve - X vagal nerve - H hypoglossal nerve - VRG ventral respiratory group - NTS nucleus of the solitary tract     

Eye movements in Daphnia pulex (De Geer).

1. The various types of eye movement exhibited by the cyclopean eye of Daphnia pulex were studied using high speed motion photography. 2. This rudimentary eye, which consists of only 22 ommatidia, can move through approximately 150 degrees in the sagittal plane and 60 degrees in the horizontal plane. 3. Four classes of eye movement were found: (1) a high speed tremor at 16 Hz with an amplitude of 3-4 degrees, which resembles physiological nystagmus, (2) a slow rhythmic scanning movement at 4 Hz, and 5-6 degrees amplitude, (3) large fast eye movements similar to saccadic eye movements and (4) optokinetic nystagmus produced by moving striped patterns. 4. Where the fast tremor occurred concurrently with the slow rhythmic scan, a Fourier analysis revealed that the former was the fourth harmonic of the latter.     

Responses of frog trochlear motoneurons to linear acceleration

Summary Firing behaviour of frog (Rana temporaria) single trochlear motoneurons and multi-unit activity of trochlear nerve were studied during sinusoidal linear accelerations or ramp-hold stimuli (side-up, side-down) in the dark. Frequencies of sinusoidal stimulation ranged from 0.05 Hz to 2 Hz and peak accelerations were between 0.001g and 0.1g.Phase lead of trochlear nerve mass activity relative to imposed acceleration decreased with increasing frequency from 43±10° at 0.1 Hz to 13±6° at 1 Hz for transverse acceleration, and from 37±9° at 0.1 Hz to 1.5±5° at 1 Hz for longitudinal acceleration.The majority of trochlear motoneurons, characterized by short antidromic latencies (<3 ms), had response phases ranging from 52°±6° at 0.05 Hz to 18°±11° at 2 Hz. Their phase behaviour was thus similar to that observed in the nerve multi-unit activity. Response sensitivities typically increased with increasing frequency and showed a clear dependency on stimulus amplitude. In addition, some motoneurons were recorded at antidromic latencies between 3 ms and 6 ms. These units showed less phase lead at frequencies below 0.2 Hz and a rather constant sensitivity in the frequency range tested.The present results together with previous work on primary otolith afferents indicate that information in phasic, phasic-tonic and tonic afferents may be transmitted rather faithfully to motoneurons. This implies that, unlike the case in higher vertebrates, very little central processing occurs in the amphibian maculo-ocular reflex.     

Electromyography of the opercularis muscle of Rana catesbeiana: An amphibian tonic muscle

The opercularis muscle of Rana catesbeiana originates on the suprascapular cartilage of the shoulder girdle and inserts on the otic opercular element. It is part of the levator scapulae musculature and lies dorsomedial to the levator scapulae superior and inferior muscles. Bipolar electrode recordings from all three muscles show electrical activity linked to cyclical firing of the posterior intermandibularis muscle, an important ventilatory muscle. The opercularis muscle shows low amplitude, erratic signals when animals are sumerged. Upon emergence of the snout region, the opercularis muscle shows rhythmic low amplitude activity at twice the rate of buccal pumping. Lung ventilation is synchronized with this rhythm and at ventilation the opercularis muscle shows higher amplitude activity. Upon submergence, opercularis activity again shows low level activity with no rhythmic pattern. Opercularis muscle activity has a major low frequency component (about 30 Hz) that probably corresponds to activity of tonic muscle fibers. Higher frequency signals (about 200–250 Hz) comparable to those of the levator scapulae muscles are also present and probably represent activity of phasic muscle fibers. Activity of the opercularis muscle is correlated with conditions in which aerial respiration is possible, and this pattern of activity supports an opercularis role in aerial hearing and/or detection of substrate vibrations. As far as we know, this is the first report of electromyographic analysis of a vertebrate tonic muscle.     

Involvement of sympathetic nerve activity in skin blood flow oscillations in humans

We have used the wavelet transform to evaluate the time-frequency content of laser-Doppler flowmetry (LDF) signals measured simultaneously on the surfaces of free microvascular flaps deprived of sympathetic nerve activity (SNA), and on adjacent intact skin, in humans. It was thereby possible to determine the frequency interval within which SNA manifests itself in peripheral blood flow oscillations. The frequency interval from 0.0095 to 2 Hz was examined and was divided into five subintervals: I, approximately 0.01 Hz; II, approximately 0.04 Hz; III, approximately 0.1 Hz; IV, approximately 0.3 Hz; and V, approximately 1 Hz. The average value of the LDF signal in the time domain as well as the mean amplitude and total power in the interval from 0.0095 to 2 Hz and amplitude and power within each of the five subintervals were significantly lower for signals measured on the free flap (P < 0.002). The normalized spectral amplitude and power in the free flap were significantly lower in only two intervals: I, from 0.0095 to 0.021 Hz; and II, from 0.021 to 0.052 Hz (P < 0.05); thus indicating that SNA is manifested in at least one of these frequency intervals. Because interval I has recently been shown to be the result of vascular endothelial activity, we conclude that we have identified SNA as influencing blood flow oscillations in normal tissues with repetition times of 20-50 s or frequencies of 0.02-0.05 Hz.     

Slow Rhythmic Oscillations within the Human Cranium: Phenomenology, Origin, and Informational Significance

Slow rhythmic oscillations in the human cranial cavity were studied using two noninvasive methods: the bioimpedance method (volume ratios between liquid media in the cranial cavity) and transcranial ultrasound Doppler echography (variation in the blood flow in the middle cerebral artery). The combination of these methods made it possible to estimate the intracranial hemodynamics. Simultaneous recording of these parameters and their spectral analysis were carried out in healthy subjects and patients with intracranial hypertension syndrome and disturbed cerebrospinal fluid (CSF) flow. The parameters were recorded at rest and immediately after manual (osteopathic) correction. The recording and analysis were performed using a Macintosh-IIsi PC and the Chart-3.52, Cricket Graph-3.32, and Canvas-3.5 software. It was found that slow oscillations of the bioimpedance (BIM) in the frequency range 0.08–0.2 Hz were of intracranial origin and were related to the mechanisms of regulation of the blood supply to and oxygen consumption by cerebral tissue, as well as with the dynamics of the CSF circulation.     

Changes in the EEG Spectrum and Subjective Characteristics of the General State after Stimulation with Variable Frequency Photic Stimuli with Two Types of Organization

Two sinusoidal signals, one with a constant frequency of 13 Hz and the other with a frequency continuously changing from 1 to 6 Hz and back, were presented simultaneously to subjects through spectacles with light-emitting diodes either to both eyes as a product (amplitude modulation of a constant frequency by a variable one) or to each eye separately. Both kinds of variable frequency exposure revealed a rhomboid pattern of the resonance activation of the EEG spectrum, similar to the spectral dynamics of a signal subject to amplitude modulation. This testifies to the key role of EEG amplitude modulation in the responses of the nervous system to variable frequency rhythmic stimuli. Both types of photic stimulation led to a substantial increase in EEG spectral density and improved the subjects' self-rating of the overall state of well-being, activity, and mood. In addition, separate stimulation of each eye led to an improvement in the anxiety and exercise performance indices (the Luscher color test) and a significant correlation between the intensity of EEG responses and changes in the general state. These differences are explained in terms of the involvement of the interhemispheric interaction mechanisms in the processing of complex rhythmic signals by the brain.     

Motor unit recruitment when neuromuscular electrical stimulation is applied over a nerve trunk compared with a muscle belly: quadriceps femoris

Neuromuscular electrical stimulation (NMES) can be delivered over a nerve trunk or muscle belly and both can generate contractions through peripheral and central pathways. Generating contractions through peripheral pathways is associated with a nonphysiological motor unit recruitment order, which may limit the efficacy of NMES rehabilitation. Presently, we compared recruitment through peripheral and central pathways for contractions of the knee extensors evoked by NMES applied over the femoral nerve vs. the quadriceps muscle. NMES was delivered to evoke 10 and 20% of maximum voluntary isometric contraction torque 2-3 s into the NMES (time(1)) in two patterns: 1) constant frequency (15 Hz for 8 s); and 2) step frequency (15-100-15 Hz and 25-100-25 Hz for 3-2-3 s, respectively). Torque and electromyographic activity recorded from vastus lateralis and medialis were quantified at the beginning (time(1)) and end (time(2); 6-7 s into the NMES) of each pattern. M-waves (peripheral pathway), H-reflexes, and asynchronous activity (central pathways) during NMES were quantified. Torque did not differ regardless of NMES location, pattern, or time. For both muscles, M-waves were ～7-10 times smaller and H-reflexes ～8-9 times larger during NMES over the nerve compared with over the muscle. However, unlike muscles studied previously, neither torque nor activity through central pathways were augmented following 100 Hz NMES, nor was any asynchronous activity evoked during NMES at either location. The coefficient of variation was also quantified at time(2) to determine the consistency of each dependent measure between three consecutive contractions. Torque, M-waves, and H-reflexes were most variable during NMES over the nerve. In summary, NMES over the nerve produced contractions with the greatest recruitment through central pathways; however, considering some of the limitations of NMES over the femoral nerve, it may be considered a good complement to, as opposed to a replacement for, NMES over the quadriceps muscle for maintaining muscle quality and reducing contraction fatigue during NMES rehabilitation.     

Influence of "animal hypnosis" on intersignal movements of rabbits during rhythmical defensive dominant

Defensive dominanta was produced in four rabbits with electrodermal rhythmic stimulation of the left fore paw with the frequency of 0.5 Hz. During testing the dominanta with acoustical stimuli, the paw muscles responded with either a decrease in its non-rhythmic activity or rhythmic quiver with the frequency close to that of electrodermal stimulation. In intersignal periods, the paw shuddered with mean intervals of about 2 seconds. After the hypnotization procedure, the intersignal paw shudders in three rabbits became longer (1.5-6 minutes) and more powerful (the amplitude of movements 1.5-2 times increased as compared to that before the hypnotization). The intervals between movements of a paw increased from 2 to 4.5 seconds.     

Nicotinamide adenine dinucleotide is released from sympathetic nerve terminals via a botulinum neurotoxin A-mediated mechanism in canine mesenteric artery

Using high-performance liquid chromatography techniques with fluorescence and electrochemical detection, we found that beta-nicotinamide adenine dinucleotide (beta-NAD) is released in response to electrical field stimulation (4-16 Hz, 0.3 ms, 15 V, 120 s) along with ATP and norepinephrine (NE) in the canine isolated mesenteric arteries. The release of beta-NAD increases with number of pulses/stimulation frequencies. Immunohistochemistry analysis showed dense distribution of tyrosine hydroxylase-like immunoreactivity (TH-LI) and sparse distribution of TH-LI-negative nerve processes, suggesting that these blood vessels are primarily under sympathetic nervous system control with some contribution of other (e.g., sensory) neurons. Exogenous NE (3 micromol/l), alpha,beta-methylene ATP (1 micromol/l), neuropeptide Y (NPY, 0.1 micromol/l), CGRP (0.1 micromol/l), vasoactive intestinal peptide (VIP, 0.1 micromol/l), and substance P (SP, 0.1 micromol/l) had no effect on the basal release of beta-NAD, suggesting that the overflow of beta-NAD is evoked by neither the sympathetic neurotransmitters NE, ATP, and NPY, nor the neuropeptides CGRP, VIP, and SP. Botulinum neurotoxin A (BoNTA, 0.1 micromol/l) abolished the evoked release of NE, ATP, and beta-NAD at 4 Hz, suggesting that at low levels of neural activity, release of these neurotransmitters results from N-ethylmaleimide-sensitive factor attachment protein receptor/synaptosomal-associated protein of 25 kDa-mediated exocytosis. At 16 Hz, however, the evoked release of NE, ATP, and beta-NAD was reduced by BoNTA by approximately 90, 60, and 80%, respectively, suggesting that at higher levels of neural activity, beta-NAD is likely to be released from different populations of synaptic vesicles or different populations of nerve terminals (i.e., sympathetic and sensory terminals).     

Input-size dependence of the baroreflex neural arc transfer characteristics

Static characteristics of the baroreflex neural arc from pressure input to sympathetic nerve activity (SNA) show sigmoidal nonlinearity, whereas its dynamic characteristics approximate a derivative filter where the magnitude of SNA response becomes greater as the input frequency increases. To reconcile the static nonlinear and dynamic linear components, we examined the effects of input amplitude on the apparent linear transfer function of the neural arc. In nine anesthetized rabbits, we perturbed isolated carotid sinus pressure by using binary white noise while varying the input amplitude among 5, 10, 20, and 40 mmHg. With increasing input amplitude, the transfer gain at 0.01 Hz decreased from 1.21 +/- 0.27 to 0.49 +/- 0.28 arbitrary units/mmHg (P < 0.01). Moreover, the slope of the transfer gain between 0.03 and 0.3 Hz decreased from 14.3 +/- 3.7 to 6.5 +/- 2.5 dB/decade (P < 0.01). We conclude that the model consisting of a sigmoidal component following rather than preceding a derivative component explains the observed results and thus can be used as a first approximation of the overall neural arc transfer characteristics.     

Vesicle cycle in mouse diaphragm motor nerve terminals

In our research on mouse diaphragm muscles the dynamic of neurotransmitter secretion and synaptic vesicles recycling (exo-endocytosis cycle) at the long-term rhythmic stimulation (20Hz) are explored using an intracellular microelectrode registration and a fluorescent microscopy. It have been shown, thate change of end plant potentials (EPP) amplitude at the rhythmic training occurs in three phases: initial transient decrease, long amplitude stabilization (1-2 min)--the plateau and secondary slow decrease. After 3 minute stimulations the EPP amplitude recovery observed during several seconds. Loading the synaptic vesicle by fluorescent endocytic dye FM 1-43 had shown that the rhythmic stimulation results to gradual (during 5-6 mines) fluorescence decrease in NT, indicating the synaptic vesicle exocytosis. The quantum analysis of the electrophysiological data and their comparison to the fluorescent researches date has allowed to assume, that mouse motor nerve terminals are characterized by high rate of endocytosis and fast synaptic vesicle reuse (average recycling time about 50 sec) that can provide effective maintenance of synaptic transmission at long high-frequency activity. Sizes of ready releasable and recycling synaptic vesicle pools are quantitatively determined. It is assumed, that vesicle recycling occurs on a short fast way to inclusion in recycling pool. So, in the stimulation protocol that were used the synaptic vesicles from reserve pool remain unused. Thus in our conditions recycling pool vesicles cycle repeatedly without reserve pool release.     

Losartan corrects abnormal frequency response of renal vasculature in congestive heart failure

In congestive heart failure, renal blood flow is decreased and renal vascular resistance is increased in a setting of increased activity of both the sympathetic nervous and renin-angiotensin systems. The renal vasoconstrictor response to renal nerve stimulation is enhanced. This is associated with an abnormality in the low-pass filter function of the renal vasculature wherein higher frequencies (> or =0.01 Hz) within renal sympathetic nerve activity are not normally attenuated and are passed into the renal blood flow signal. This study tested the hypothesis that excess angiotensin II action mediates the abnormal frequency response characteristics of the renal vasculature in congestive heart failure. In anesthetized rats, the renal vasoconstrictor response to graded frequency renal nerve stimulation was significantly greater in congestive heart failure than in control rats. Losartan attenuated the renal vasoconstrictor response to a significantly greater degree in congestive heart failure than in control rats. In control rats, the frequency response of the renal vasculature was that of a first order (-20 dB/frequency decade) low-pass filter with a corner frequency (-3 dB, 30% attenuation) of 0.002 Hz and 97% attenuation (-30 dB) at > or =0.1 Hz. In congestive heart failure rats, attenuation did not exceed 45% (-5 dB) over the frequency range of 0.001-0.6 Hz. The frequency response of the renal vasculature was not affected by losartan treatment in control rats but was completely restored to normal by losartan treatment in congestive heart failure rats. The enhanced renal vasoconstrictor response to renal nerve stimulation and the associated abnormality in the frequency response characteristics of the renal vasculature seen in congestive heart failure are mediated by the action of angiotensin II on renal angiotensin II AT1 receptors.     

Evaluation of respiratory modulation on the pulse wave amplitude in low-birth-weight neonate.

A slow oscillation of sympathetic vasoconstriction (Mayer waves) which is affected by the respiratory movements seems to appear as the fluctuation of pulse wave amplitude (PPG.P-P) in the frequency domain (0.1 Hz). Whether the vasomotor in low frequency has appeared in the pulse wave of the neonate and whether Mayer waves appear as the pulse wave oscillation of the immature low-birth-weight neonate are not fully understood from the point of autonomic nerve regulation mechanism. We therefore analyzed the frequency characteristics of PPG.P-P, respiration wave and its amplitude (RW.P-P) together with the heart rate variability (HRV) to examine the relationships between the frequency spectra.     

Altered Active Zones,Vesicle Pools,Nerve Terminal Conductivity,and Morphology during Experimental MuSK Myasthenia Gravis

Recent studies demonstrate reduced motor-nerve function during autoimmune muscle-specific tyrosine kinase (MuSK) myasthenia gravis (MG). To further understand the basis of motor-nerve dysfunction during MuSK-MG, we immunized female C57/B6 mice with purified rat MuSK ectodomain. Nerve-muscle preparations were dissected and neuromuscular junctions (NMJs) studied electrophysiologically, morphologically, and biochemically. While all mice produced antibodies to MuSK, only 40% developed respiratory muscle weakness. In vitro study of respiratory nerve-muscle preparations isolated from these affected mice revealed that 78% of NMJs produced endplate currents (EPCs) with significantly reduced quantal content, although potentiation and depression at 50 Hz remained qualitatively normal. EPC and mEPC amplitude variability indicated significantly reduced number of vesicle-release sites (active zones) and reduced probability of vesicle release. The readily releasable vesicle pool size and the frequency of large amplitude mEPCs also declined. The remaining NMJs had intermittent (4%) or complete (18%) failure of neurotransmitter release in response to 50 Hz nerve stimulation, presumably due to blocked action potential entry into the nerve terminal, which may arise from nerve terminal swelling and thinning. Since MuSK-MG-affected muscles do not express the AChR γ subunit, the observed prolongation of EPC decay time was not due to inactivity-induced expression of embryonic acetylcholine receptor, but rather to reduced catalytic activity of acetylcholinesterase. Muscle protein levels of MuSK did not change. These findings provide novel insight into the pathophysiology of autoimmune MuSK-MG.     

GCaMP as an indirect measure of electrical activity in rat trigeminal ganglion neurons

While debate continues over whether somatosensory information is transmitted via labeled line, population coding, frequency coding, or some combination therein, researchers have begun to address this question at the level of the primary afferent by using optical approaches that enable the assessment of neural activity in hundreds to even thousands of neurons simultaneously. However, with limited availability of tools to optically assess electrical activity in large populations of neurons, researchers have turned to genetically encoded Ca²⁺ indicators (GECIs) including GCaMP to enable the detection of increases in cytosolic Ca²⁺ concentrations as a correlate for neuronal activity. One of the most widely used GECIs is GCaMP6, which is available in three different versions tuned for sensitivity (GCaMP6s), speed (GCaMP6f), or a balance of the two (GCaMP6m). In order to determine if these issues were unique to GCaMP6 itself, or if they were inherent to more than one generation of GCaMP, we also characterized jGCaMP7. In the present study, we sought to determine the utility of the three GCaMP6 isoforms to detect changes in activity in primary afferents at frequencies ranging from 0.1–30 Hz. Given the heterogeneity of sensory neurons, we also compared the performance of each GCaMP6 isoform in subpopulations of neurons defined by properties used to identify putative nociceptive afferents: cell body size, isolectin B4 (IB4) binding, and capsaicin sensitivity. Finally, we compared results generated with GCaMP6 with that generated from neurons expressing the next generation of GCaMP, jGCaMP7s and jGCaMP7f. A viral approach, with AAV9-CAG-GCaMP6s/m/f, was used to drive GECI expression in acutely dissociated rat trigeminal ganglion (TG) neurons, and neural activity was driven by electrical field stimulation. Infection efficiency with the AAV serotype was high >95 %, and the impact of GCaMP6 expression in TG neurons over the period of study (<10 days) on the regulation of intracellular Ca²⁺, as assessed with fura-2, was minimal. Having confirmed that the field stimulation evoked Ca²⁺ transients were dependent on Ca²⁺ influx secondary to the activation of action potentials and voltage-gated Ca²⁺ channels, we also confirmed that the signal-to-noise ratio for each of the isoforms was excellent, enabling detection of a single spike in>90% of neurons. However, the utility of the GCaMP6 isoforms to enable an assessment of the firing frequency let alone changes in firing frequency of each neuron was relatively limited and isoform specific: GCaMP6s and 6m had the lowest resolution, enabling detection of spikes at 3 Hz in 15% and 32% of neurons respectively, but it was possible to resolve discrete single spikes up to 10 Hz in 36% of GCaMP6f neurons. Unfortunately, using other parameters of the Ca²⁺ transient, such as magnitude of the transient or the rate of rise, did not improve the range over which these indicators could be used to assess changes in spike number or firing frequency. Furthermore, in the presence of ongoing neural activity, it was even more difficult to detect a change in firing frequency. The frequency response relationship for the increase in Ca²⁺ was highly heterogeneous among sensory neurons and was influenced by both the GCaMP6 isoform used to assess it, the timing between the delivery of stimulation trains (inter-burst interval), and afferent subpopulation. Notably, the same deficiencies were observed with jGCaMP7s and 7f in resolving the degree of activity as were present for the GCaMP6 isoforms. Together, these data suggest that while both GCaMP6 and jGCaMP7 are potentially useful tools in sensory neurons to determine the presence or absence of neural activity, the ability to discriminate changes in firing frequency ≥ 3 Hz is extremely limited. As a result, GECIs should probably not be used in sensory neurons to assess changes in activity within or between subpopulations of neurons.