Early postnatal neuronal glutamine transport blockade sex‐specifically shifts excitatory synaptic phenotypes in the ventrolateral ventromedial hypothalamic neurons of adult rats: Part I: Abolition of female phenotypes at basal synaptic activity

Abstract

Excitatory synapses in neurons of the ventrolateral ventromedial nucleus of the hypothalamus (vlVMH) use glutamine (Gln) provided by the Gln–glutamate cycle (GGC) to synthesize the glutamate required to sustain glutamatergic synaptic transmission in rat pups. Depending on neuronal activity, these synapses exhibit sexual dimorphic phenotypes. Female pups use Gln to sustain both basal and high‐level neuronal activity, whereas male pups use Gln to sustain only high‐level neuronal activity. Early postnatal GGC blockade alters glutamatergic synaptic transmission in adult female rats and shifts vlVMH neuronal morphology toward that of males. It further attenuates reproductive behavior expression in female rats, but not male rats. Because sexually dimorphic Gln‐dependent synaptic phenotypes differ between sexes at basal synaptic activity, we hypothesized that early postnatal GGC blockade would shift GGC (Gln)‐dependent excitatory synaptic phenotypes in adult female vlVMH neurons toward a male pup‐like phenotype. Sprague‐Dawley rats in the early postnatal period were treated with the neuronal Gln transport blocker alpha‐(methylamino) isobutyric acid (MeAIB; 5 mM; intracerebroventricular injection) or vehicle (distilled water) and raised until adulthood. Brain slices from the treated rats were subjected to whole‐cell patch‐clamp recording of miniature excitatory postsynaptic currents (mEPSCs) to assess unitary (quantal) excitatory synaptic transmission in vlVMH neurons. Furthermore, the sensitivity of mEPSCs to acute MeAIB perfusion was examined. Early postnatal MeAIB treatment reduced quantal and synaptic cleft glutamate levels in adult female rats and increased mEPSC frequency in adult male rats. In vehicle‐treated male and female rats, acute MeAIB perfusion reduced synaptic cleft glutamate levels and mEPSC amplitudes, but not mEPSC kinetics, highlighting a Gln‐dependent phenotype in adult male and female rats, which differs from that observed in male pups. In MeAIB‐treated adult female rats, acute MeAIB perfusion did not further reduce synaptic cleft glutamate level or mEPSC amplitude, suggesting that early postnatal MeAIB treatment induced long‐term impairment of Gln‐dependent excitatory synapses. Conversely, in MeAIB‐treated male rats, acute MeAIB perfusion reduced mEPSC amplitude and increased mEPSC rise time but exerted no significant effect on synaptic cleft glutamate level. Early postnatal neuron Gln transport blockade by MeAIB sex‐specifically disrupts Gln‐dependent excitatory synaptic responses in the vlVMH neurons of adult rats. In female rats, early postnatal MeAIB administration impairs Gln‐dependent synaptic responses by reducing vesicular glutamate content and glutamate release, resulting in a male pup‐like phenotype. By contrast, in male rats, early postnatal MeAIB administration partially preserves the female‐like Gln‐dependent excitatory synaptic phenotype, despite exerting mild effects on postsynaptic receptor sensitivity during adulthood.