Human cerebrospinal fluid increases the excitability of pyramidal neurons in the in vitro brain slice

Abstract

Key pointsr The cerebrospinal fluid contains numerous neuromodulators at ambient levels but whether, and how, they affect the activity of central neurons is unknown.r This study provides experimental evidence that human cerebrospinal fluid (hCSF) increases the excitability of hippocampal and neocortical pyramidal neurons.r Hippocampal CA1 pyramidal neurons in hCSF displayed lowered firing thresholds, depolarized resting membrane potentials and reduced input resistance, mimicking properties of pyramidal neurons recorded in vivo.r The excitability-increasing effect of hCSF on CA1 pyramidal neurons was entirely occluded by intracellular application of GTPγS, suggesting that neuromodulatory effects were mediated by G-protein coupled receptors.r These results indicate that the CSF promotes spontaneous excitatory neuronal activity, and may help to explain observed differences in the activity of pyramidal neurons recorded in vivo and in vitro.Abstract The composition of brain extracellular fluid is shaped by a continuous exchange of substances between the cerebrospinal fluid (CSF) and interstitial fluid. The CSF is known to contain a wide range of endogenous neuromodulatory substances, but their collective influence on neuronal activity has been poorly investigated. We show here that replacing artificial CSF (aCSF), routinely used for perfusion of brain slices in vitro, with human CSF (hCSF) powerfully boosts spontaneous firing of CA1, CA3 and layer 5 pyramidal neurons in the rat brain slice. CA1 pyramidal neurons in hCSF display lowered firing thresholds, more depolarized resting membrane potentials and reduced input resistance, mimicking properties of pyramidal neurons recorded in vivo. The increased excitability of CA1 pyramidal neurons was completely occluded by intracellular application of GTPγS, suggesting that endogenous neuromodulators in hCSF act on G-protein coupled receptors to enhance excitability. We found no increase in spontaneous inhibitory synaptic transmission by hCSF, indicating a differential effect on glutamatergic and GABAergic neurons. Our findings highlight a previously unknown function of the CSF in promoting spontaneous excitatory activity, and may help to explain differences observed in the activity of pyramidal neurons recorded in vivo and in vitro.

Control of Feeding Behavior by Cerebral Ventricular Volume Transmission of Melanin-Concentrating Hormone

“…In contrast to synaptic transmission, where communication is discrete and dependent on neuroanatomical connections, CSF transmission of neuropeptides and small molecule neurotransmitters represents a neuroendocrine-like method of humoral transmission with the potential for widespread neuromodulation of distal regions of the brain (Veening and Barendregt, 2010). Indeed recent findings show that unidentified neuromodulators are present in human CSF in sufficient quantities to excite neurons, thereby supporting the plausibility of CSF transmission as a mechanism for system-wide modulation of neuronal activity (Bjorefeldt et al, 2015). However, the physiological contribution of CSF transmission to regulating behavior has remained hypothetical and largely understudied, possibly due in part to the technical challenge of separating the effects CSF-mediated signaling from concurrent synaptic transmission.…”

Human cerebrospinal fluid promotes spontaneous gamma oscillations in the hippocampus in vitro

“…We first examined the effect of hCSF on KA‐induced gamma oscillations in CA3 stratum pyramidale (Figure ), a well‐established method for studying these oscillations in vitro (Fisahn, ). Previous work has shown that hCSF increases the excitability of both hippocampal pyramidal cells (Bjorefeldt et al, ) and interneurons (Bjorefeldt et al, ), raising the possibility that synchronized neuronal activity might be promoted. Step application of 25, 35, and 45 nM KA to aCSF (of matched composition with regard to hCSF, see Methods) successively evoked gamma oscillations of increasing power from 1.33 ± 0.11 × 10 −09 V 2 to 4.13 ± 0.11 × 10 −09 V 2 ( n = 9, p < .001, One‐way repeated measures ANOVA with Sidak's multiple comparisons test, Figure c).…”

“…Part of the pooled hCSF was refrozen at a −80°C and kept at the Sahlgrenska Academy in Gothenburg to be used in whole‐cell patch clamp recordings. Electrolyte and glucose concentrations, as well as pH and osmolality, were measured according to methodology described in Bjorefeldt et al (). A matched aCSF was then prepared based on measured hCSF values, and used as control.…”

Spikelets in Pyramidal Neurons: Action Potentials Initiated in the Axon Initial Segment That Do Not Activate the Soma

“…An alternative would be to establish a reliable spikelet model in vitro . We propose to recreate in vitro a state of a pyramidal cell that retains the in vivo properties of sodium channels, for example by prolonged stimulation with fluctuating inputs and/or application of relevant neurotransmitters and neuromodulators naturally present in the cerebrospinal fluid in vivo [48]. Additionally, it might be necessary to record from neurons located in the middle of a slice, to minimize the dendritic loss and the resulting decrease in the somato-dendritic current sink.…”