Chronic cocaine administration switches corticotropin-releasing factor2 receptor-mediated depression to facilitation of glutamatergic transmission in the lateral septum

Abstract

Corticotropin-releasing factor (CRF) and urocortin (Ucn I) are endogenous members among a family of CRF-related peptides that activate two different and synaptically localized G-protein-coupled receptors, CRF1 and CRF2. These peptides and their receptors have been implicated in stress responses and stress with cocaine abuse. In this study, we observed significant alterations in excitatory transmission and CRF-related peptide regulation of excitatory transmission in the lateral septum mediolateral nucleus (LSMLN) after chronic cocaine administration. In brain slice recordings from the LSMLN of control (saline-treated) rats, glutamatergic synaptic transmission was facilitated by activation of CRF1 receptors with CRF but was depressed after activation of CRF2 receptors with Ucn I. After acute withdrawal from a chronic cocaine administration regimen, CRF1 activation remained facilitatory, but CRF2 activation facilitated rather than depressed LSMLN EPSCs. These alterations in CRF2 effects occurred through both presynaptic and postsynaptic mechanisms. In saline-treated rats, CRF1 and CRF2 coupled predominantly to protein kinase A signaling pathways, whereas after cocaine withdrawal, protein kinase C activity was more prominent and likely contributed to the CRF2-mediated presynaptic facilitation. Neither CRF nor Ucn I altered monosynaptic GABA(A)-mediated IPSCs before or after chronic cocaine administration, suggesting that loss of GABAA-mediated inhibition could not account for the facilitation. This switch in polarity of Ucn I-mediated neuromodulation, from a negative to positive regulation of excitatory glutamatergic transmission after chronic cocaine administration, could generate an imbalance in the brain reward circuitry associated with the LSMLN.

Figure 1.

Chronic cocaine administration and its acute withdrawal facilitated LSMLN EPSCs. A, Left, EPSCs increased in amplitude with increasing stimulus intensity after chronic cocaine administration. Right, Chronic cocaine administration and its withdrawal shifted the input-output relationships for LSMLN EPSCs to the left. B, Left, Pairs of EPSCs were elicited at various interstimulus intervals in LSMLN synapses from control/saline- and chronic cocaine-treated rats. Right, Paired-pulse ratios were decreased after chronic cocaine administration, suggesting a presynaptic increase in probability of transmitter release. The asterisk indicates significant differences from control ratios at 100, 150, and 200 msec intervals (p < 0.05).

Figure 2.

After chronic cocaine administration and its acute withdrawal, Ucn I facilitated LSMLN-EPSCs, which is blocked by the CRF₂ antagonist Ast₂B. A, After chronic cocaine administration, CRFr/h still facilitated EPSCs, but with diminished potency. B, Ucn I-induced depression of EPSCs in control neurons was switched to facilitation after chronic cocaine treatment. C, The CRF₂ antagonist Ast₂B blocked the Ucn I-induced facilitation but did not cause reversal to EPSC depression. CRF₁- or CRF₂-specific antagonists alone did not affect EPSCs.

Figure 3.

The Ucn I switch from depression to facilitation of EPSCs after chronic cocaine administration was attributable to changes at presynaptic and postsynaptic sites A, Left, Pairs of enhanced EPSCs were further facilitated by Ucn I at all four paired-pulse intervals tested. Right, Summary of paired-pulse data depicting a downward shift after Ucn I (○). The asterisk indicates significant differences from control ratios at all intervals (p < 0.05). B, Top, Spontaneously released mEPSCs were increased in frequency after the addition of Ucn I. Bottom, Summary of data showed that Ucn I had no effect on mEPSC amplitude but significantly increased mEPSC frequency. The asterisk indicates significant difference from control (p < 0.05).

Figure 4.

CRF₁ and CRF₂ coupling to PKA and PKC was altered differentially after chronic cocaine administration and its acute withdrawal. A, PKA inhibition by H-89 blocks CRF facilitation in neurons from control/saline rats (▦) and after chronic cocaine administration (▪). PKC inhibition with GF 109203X did not affect CRF₁ facilitation or CRF₂ depression of EPSCs in neurons from control/saline animals but also did not alter postsynaptic facilitation mediated through CRF₁ in chronic cocaine animals. B, After chronic cocaine administration, PKC inhibition blocked the switched presynaptic facilitation mediated by CRF₂. The asterisk indicates significant differences from control CRF or Ucn I values before and after chronic cocaine administration (p < 0.05).

Figure 5.

Neither CRF nor Ucn I affected GABA_A-IPSCs. A, B, Control GABA_A-IPSCs recorded in neurons from saline/control rats before, during, and after superfusion with CRF and Ucn I. C, D, GABA_A-IPSCs recorded in neurons from rats treated chronically with cocaine and acutely withdrawn before, during, and after superfusion with CRF and Ucn I.