Effects Of Gastrin-Releasing Peptide On The Electrophysiological Properties Of Rat Hippocampal Neurons

Gastrin-releasing peptide?GRP?is a homologue of bombesin in mammals.In the central nervous system,GRP is expressed and participates in instinctive behaviors.With the development of research,People found that GRP also can regulate the advanced functions of the brain.GRP levels are also associated with the central nervous system diseases.Exogenous GRP administration can alleviate or protect cognitive impairment associated with neurodegenerative disorders.Our previous study proposed that intraperitoneal injection of GRP can improve spatial memory in ischemic model rats.It is well known that hippocampus is an important brain area related to spatial learning and memory,but the mechanisms of GRP on hippocampal neurons is still unclear.In this study,we examined the effects of GRP on excitability of hippocampal CA1 neurons,and further explored the mechanisms of its effects on ion channels and synaptic transmission.Firstly,immunofluorescence staining was used to verify whether GRP receptor was expressed in hippocampal CA1 area.Fresh hippocampal slices?about 400?m?were obtained from 10 to 21 days old Wistar rats.Under whole-cell current clamp mode,we recorded the effects of GRP?250nM?on the excitability of hippocampal CA1 neurons.Voltage-gated Na⁺and K⁺currents were recorded in voltage-clamp mode,which illustrates the ionic mechanism.Moreover,we further explored whether GRP mediates the spontaneous inhibitory postsynaptic currents and the synaptic proteins to elucidate the synaptic transmission mechanism.The main results are as follows:1.The immunofluorescence staining experiment showed that GRP receptor was expressed in rats hippocampal CA1 area.2.GRP reduced the fire frequency of CA1 neurons and decreased the peak amplitude,overshoot and spike half-width,and increased the threshold.3.GRP could reduce the amplitude of I_Na and shifted the inactivation and recovery curves to the hyperpolarization direction,but didn't change the activation curves of I_Na,which illustrates that voltage-gated Na⁺channel more easily inactivated and quicker recovered.4.GRP could increase the amplitudes of I_A and I_K,and the steady-state inactivation curves of I_A were shifted to the right.The slope of I_K activation was decreased significantly,but there was no significant effect on I_A activation and recovery dynamics.5.GRP increased the amplitude and frequency of inhibitory postsynaptic currents significantly;GRP decreased the expression of postsynaptic densities?PSD-95?,but had no significant effect on the expression of synaptophysin?SYP?.According to the results above,we draw conclusion as follows:1.GRP receptor is expressed in rat hippocampal CA1 neurons.2.Acute GRP can reduce the excitability of CA1 neurons,and its mechanism involves both direct and indirect aspects.Direct mechanism:After GRP binding to its receptor,the voltage-gated Na⁺and K⁺channel gated characteristics are changed through intracellular signal transduction pathway,appearing as the decrease of I_Naa amplitude,the increase of I_A and I_K amplitudes,to inhibit the neuron excitability.Indirect mechanism:GRP can also target on the GABAergic interneurons to increase their excitability,thus indirectly reducing the excitability of CA1 neurons.3.GRP has no significant effect on the release of presynaptic neurotransmitters in hippocampus,but decreases the expression of PSD-95,suggesting that GRP could affect excitatory synaptic transmission.In summary,we demonstrate that GRP can inhibit the excitability of hippocampal CA1 area neurons,and clarify the mechanism of ionic channels and synaptic transmission involved in this process,which provide a theoretical basis for GRP to improve animal cognitive function,and new ideas for the treatment of related central nervous system diseases.