Estrogen Differentially Modulates the Cannabinoid- Induced Presynaptic Inhibition of Amino Acid Neurotransmission in Proopiomelanocortin Neurons of the Arcuate Nucleus

Abstract

The present study sought to determine whether cannabinoids inhibit glutamatergic and GABAergic synaptic input onto neurons of the hypothalamic arcuate nucleus (ARC), and whether estrogen modulates this process. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomized female guinea pigs. CB1 receptor activation reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation that were sensitive to ionotropic glutamate receptor antagonists. The CB1 receptor antagonist AM251 increased evoked EPSC (eEPSC) amplitude, and reversed the agonist-induced decrease. CB1 receptor activation similarly decreased the amplitude of evoked inhibitory postsynaptic currents (eIPSCs). The cannabinoid-induced reduction in eEPSC and eIPSC amplitude correlated with a decrease in the frequency of miniature EPSCs (mEPSCs) and IPSCs (mIPSCs) that were abolished by ionotropic glutamate and GABA_A receptor antagonists, respectively. AM251 increased mEPSC frequency, and antagonized the agonist-induced decrease. Compared to neurons obtained from vehicle-treated controls, estradiol benzoate (25 µg; s.c.) given 24 h prior to experimentation increased mEPSC frequency, and markedly decreased the potency of CB1 receptor agonists to decrease mEPSC frequency. Conversely, the steroid potentiated the cannabinoid-induced decrease in mIPSC frequency. These effects were observed in neurons subsequently identified as proopiomelanocortin (POMC) neurons. These data reveal that ARC neurons, including POMC neurons, receive glutamatergic and GABAergic synaptic inputs that are presynaptically inhibited by cannabinoids, and differentially modulated by estrogen. These opposing effects of estrogen on the cannabinoid regulation of amino acid neurotransmission excite POMC neurons, and lend additional insight into the mechanisms underlying estrogen-induced anorexia and negative feedback of the reproductive axis.

Sex Differences in the Cannabinoid Modulation of Appetite, Body Temperature and Neurotransmission at POMC Synapses

“…We have demonstrated previously that CB1 receptor agonists presynaptically modulate ionotropic glutamatergic and GABAergic currents in POMC neurons as evidenced by: (1) a reduced amplitude of evoked EPSCs and IPSCs, (2) an increase in the S2:S1 ratio using the paired pulse paradigm, and (3) a decrease in the frequency, but not amplitude, of mEPSCs and mIPSCs [23, 24]. Given the sex differences in the cannabinoid-induced modulation of feeding behavior that were particularly pronounced for the agonist more so than the antagonist, we wanted to determine if this sexual disparity could be attributed in part to alterations in synaptic input onto anorexigenic POMC neurons.…”

“…Whole-cell patch recordings in hypothalamic slices were performed as previously described [23, 24]. Briefly, during the recording session, slices were maintained in a chamber perfused with a warmed (35°C), oxygenated aCSF that was identical to that described above, the exception being an increase in the CaCl 2 concentration to 2 m M .…”

“…Synaptic transmission in the ARC was evaluated as described previously [23, 24]. We recorded mEPSCs and mIPSCs in the presence of TTX (500 n M ) and the appropriate receptor antagonists from holding potentials of –75 and –30mV, respectively.…”

“…Moreover, endogenous cannabinoids retrogradely inhibit GABAergic neurotransmission at orexigenic melanin-concentrating hormone (MCH) synapses in the LHA [19], which would account for the cannabinoid-induced depolarization and the accompanying increase in firing rate of these cells [20]. Furthermore, cannabinoids increase neuropeptide Y (NPY) release from the ARC [21], and decrease glutamatergic synaptic input onto anorexigenic pro-opiomelanocortin (POMC) neurons from transgenic mice [22] and guinea pigs [23, 24]. Finally, the adipostat leptin decreases hypothalamic concentrations of 2-arachidonyl glycerol [25], and physiologically antagonizes glucocorticoid stimulation of endogenous cannabinoid biosynthesis and release in the PVN [26].…”

Endocannabinoids and prostaglandins both contribute to GnRH neuron-GABAergic afferent local feedback circuits

“…5B). For example, estradiol can block cannabinoid-induced presynaptic inhibition of fast synaptic transmission in pro-opiomelanocortin neurons (Nguyen and Wagner 2006). Estradiol reduced the ability of neuronal activity to increase intracellular calcium levels in astrocytes in hippocampal cultures (Rao and Sikdar, 2006).…”