No abstract

Dopamine neurons are thought to convey a fast, incentive salience signal, faster than can be mediated by dopamine. A resolution of this paradox may be that midbrain dopamine neurons exert fast excitatory actions. Using transgenic mice with fluorescent dopamine neurons, in which the axonal projections of the neurons are visible, we made horizontal brain slices encompassing the mesoaccumbens dopamine projection. Focal extracellular stimulation of dopamine neurons in the ventral tegmental area evoked dopamine release and early monosynaptic and late polysynaptic excitatory responses in postsynaptic nucleus accumbens neurons. Local superfusion of the ventral tegmental area with glutamate, which should activate dopamine neurons selectively, produced an increase in excitatory synaptic events. Local superfusion of the ventral tegmental area with the D2 agonist quinpirole, which should increase the threshold for dopamine neuron activation, inhibited the early response. So dopamine neurons make glutamatergic synaptic connections to accumbens neurons. We propose that dopamine neuron glutamatergic transmission may be the initial component of the incentive salience signal.

Neurotransmitter glutamate has been thought to derive mainly from glutamine via the action of glutaminase type 1 (GLS1). To address the importance of this pathway in glutamatergic transmission, we knocked out GLS1 in mice. The insertion of a STOP cassette by homologous recombination produced a null allele that blocked transcription, encoded no immunoreactive protein, and abolished GLS1 enzymatic activity. Null mutants were slightly smaller, were deficient in goal-directed behavior, hypoventilated, and died in the first postnatal day. No gross or microscopic defects were detected in peripheral organs or in the CNS. In cultured neurons from the null mutants, miniature EPSC amplitude and duration were normal; however, the amplitude of evoked EPSCs decayed more rapidly with sustained 10 Hz stimulation, consistent with an observed reduction in depolarization-evoked glutamate release. Because of this activitydependent impairment in glutamatergic transmission, we surmised that respiratory networks, which require temporal summation of synaptic input, would be particularly affected. We found that the amplitude of inspirations was decreased in vivo, chemosensitivity to CO 2 was severely altered, and the frequency of pacemaker activity recorded in the respiratory generator in the pre-Bötzinger complex, a glutamatergic brainstem network that can be isolated in vitro, was increased. Our results show that although alternate pathways to GLS1 glutamate synthesis support baseline glutamatergic transmission, the GLS1 pathway is essential for maintaining the function of active synapses, and thus the mutation is associated with impaired respiratory function, abnormal goal-directed behavior, and neonatal demise.

Responses resulting from injury to the trigeminal nerve exhibit differences compared with those caused by lesion of other peripheral nerves. With the aim of elucidating the physiopathological mechanisms underlying cephalic versus extracephalic neuropathic pain, we determined the time course expression of proinflammatory cytokines interleukin-6 (IL-6) and IL-1␤, neuronal injury (ATF3), macrophage/microglial (OX-42), and satellite cells/astrocyte (GFAP) markers in central and ganglion tissues in rats that underwent unilateral chronic constriction injury (CCI) to either infraorbital nerve (IoN) (cephalic area) or sciatic nerve (SN) (extracephalic area). Whereas CCI induced microglial activation in both models, we observed a concomitant upregulation of IL-6 and ATF3 in the ipsilateral dorsal horn of the lumbar cord in SN-CCI rats but not in the ipsilateral spinal nucleus of the trigeminal nerve (Sp5c) in IoN-CCI rats. Preemptive treatment with minocycline (daily administration of 20 mg/kg, i.p., for 2 weeks) partially prevented pain behavior and microglial activation in SN-CCI rats but was ineffective in IoN-CCI rats. We show that IL-6 can upregulate OX-42 and ATF3 expression in cultured microglia and neurons from spinal cord, respectively, as well as in the dorsal horn after acute intrathecal administration of the cytokine. We propose that IL-6 could be one of the promoters of the signaling cascade leading to abnormal pain behavior in SN-CCI but not IoN-CCI rats. Our data further support the idea that different pathophysiological mechanisms contribute to the development of cephalic versus extracephalic neuropathic pain.